Epidemiologyã The Trustee of the Wellcome Trust 1998Reviewed by: Dr S R A Huttly, Maternal and Child Epidemiology Unit, London School of Hygiene and Tropical Medicine and Dr W A M Cutting, Department of Child Life and Health, The University of Edinburgh, UKPicture: Contaminated water being collected for use in the household puts these women and their families at risk of diarrhoea.Copyright Copyright holder unknown. Image from Hartley J. In: Diarrhoea, a major public health problem. Save the Children Fund.Image references ################ .\IMAGES\T23778.jpg Contents 3 Objectives 4Introduction 5Definition of Key Terms 10Assessment 11 Causes and Effects of DiarrhoealDiseases Worldwide 15 Assessment 16 Modes of Transmission of Diarrhoea 19 Transmission Factors Related tothe Pathogen 23 Assessment 24 Transmission Factors Related tothe Environment 29 Assessment 30 Transmission Factors Related to the Host 37 Assessment 38 Modes of Transmission of Diarrhoea: aSummary 41 Practical Epidemiology 45 Assessment 46 Tutorial Assessment 47 Summary Click on the underlined text to jump tothat screen. Underlined text is interactive. Click on underlined text to view extra information or to jump to another screen. An Indian child with severe dehydration due to acute watery diarrhoea.Copyright Copyright Image from United Nations Children's Fund, India.Image references ################ .\IMAGES\T23159c.jpg Objectives At the end of this tutorial you should be able to: 1. state the definition of diarrhoea and its clinical types 2. know how to describe the impact of diarrhoea on a population 3. show that enteric pathogens are a major global cause of morbidity and mortality 4. indicate the main modes of transmission of diarrhoeal disease 5. describe how transmission is affected by factors related to the pathogen, environment and host 6. summarize the collection of epidemiological data on diarrhoea and the main types of epidemiological study Image references ################ Introduction Diarrhoea Diarrhoea is: · an increase in the number, volume and water content of stools · a global cause of much illness and death· a major factor in childhood malnutrition This tutorial is about the epidemiology of diarrhoea. This is central to the prevention and control of the disease. For related tutorials. Picture: Bottle feeding rather than breast feeding greatly increases this Bangladeshi child’s risk of morbidity and death from diarrhoea. Copyright Image from J Ebrahim Memorial Trust. Diarrhoea morbidity and mortalityEach year there are: · approximately 3.3 million deaths due to diarrhoea, 80% in children under 2 years of age · over 1 billion episodes of diarrhoea, most in the developing world · 5 - 10 million travellers affected by diarrhoeaRelated tutorialsRefer to the tutorial Diarrhoeal Diseases: Prevention and Control. Image references ################ .\IMAGES\T37748.jpg Definition of Key TermsDefinition of Key TermsImage references ################ Definition of Key Terms - 1 Definition of diarrhoea Commonly used definitions in epidemiological studies are: · diarrhoea is the passage of three or more loose or watery stools in 24 hours · an episode of diarrhoea is considered new when it follows a period of at least 2 days in a row without diarrhoea What is the usual number and nature of stools? There are three clinical types of infectious diarrhoea (see table).Clinical Types of Infectious Diarrhoea Clinical typeDefinition Acute wateryLoose or watery stools without visible blood diarrhoea Duration less than 14 days1 Acute bloodyLoose or watery stools with visible red blood2 diarrhoeaDuration less than 14 days1 PersistentLoose or watery stools with or without visible diarrhoea blood Duration 14 days or more1.Most episodes of acute diarrhoea last less than 5 - 7 days. 2.A history of bloody stools reported by the mother is sufficient for a diagnosis.Three or more stoolsThe definition does not apply tobabies in whom breast milk is a major component of their diet. Such babies often pass more than three soft stools each day even when healthy. Loose or watery stool Picture: A watery stool from a patient with diarrhoea due to enterotoxigenic E. coli infection.Image from Tubbs HR. A loose stool is defined as one that takes on the shape of the container into which it is passed.Usual number and nature of stoolsA ‘normal’ stool is difficult to define because the number and nature of normal stools vary greatly with age and diet. For example: · a breast fed infant may have a soft stool after almost every feed, 4 - 6 times per day - this is normal for that baby · an older child on a low residue diet may have only 2 stools per week - this is normal for that child A mother usually first notices a change in the child’s stools and makes the initial diagnosis. Mothers may use local words for different types of diarrhoea - the health worker should be familiar with these words. Image references ################ .\IMAGES\T22792p.jpg Definition of Key Terms - 2 Epidemiological indices Measures of the impact of diarrhoea on a community are the: · incidence · incidence rate · prevalence · prevalence rate Picture: A representation of the relationship between incidence and prevalence. Incidence Prevalence Reduction in prevalence Copyright Image from The Wellcome Trust. Water flowing out of bucketWater lost from the bucket represents a fall in prevalence by: · water lost down the drain - death of the patient · water captured in the cup - recovery of the patientIncidence rateIncidence rate is: · the number of new cases of a disease (eg. diarrhoea) in a given period of time relative to the population size · often expressed as the number of new cases per 100,000 population or episodes per child in that periodPrevalence Prevalence is: · the number of cases of a disease at a particular point in time · of most use for assessing chronic diseases (eg. tuberculosis), rather than acute diseases such as diarrhoeaPrevalence rate Prevalence rate is: · the number of cases of a disease (eg. diarrhoea) at a particular point in time relative to the population size · often expressed as a percentage of the populationIncidenceIncidence is the number of new cases of a disease (eg. diarrhoea) in a given period of time.Water in the bucketWater within the bucket represents the amount of disease (prevalence) in a community.Water flowing into bucketWater flowing into the bucket represents the incidence (new cases). Image references ################ .\IMAGES\Water.gif Definition of Key Terms - 3 Epidemiological indices Measures of the impact of diarrhoea on an individual are the: · case fatality rate · severity of diarrhoea · duration of diarrhoea Picture: The ‘two dimensions’ of the impact of diarrhoeal disease. For an interpretation.Copyright Image from The Wellcome Trust. Case fatality rate The case fatality rate: · is the number of deaths from diarrhoea relative to the size of the diseased population · is expressed as a percentage of the population with diarrhoea · usually rises from the community, to the clinic and hospital as patients with progressively more serious illness are seenSeverity Measures of the clinical severity of diarrhoea vary. The most commonly used are the: · stool frequency · stool volume · risk of dehydrationDurationDuration of illness is: · an important measure of morbidity · affected by the speed of recovery and the rapidity of death · the link between incidence and prevalence where disease is endemic Prevalence = Incidence x DurationInterpretation of the pictureA reduction in mortality can be achieved relatively easily by better case management, eg: ·rehydration therapy with early feeding ·appropriate antimicrobials when indicated A reduction in morbidity is usually harder to achieve, often depending on:· socioeconomic development · behaviourial change, eg. improved personal hygiene · preventative measures such as improved sanitation or water supply Refer to the tutorial Diarrhoeal Diseases: Prevention and Control. Image references ################ .\IMAGES\Dirattkr.gif Definition of Key Terms - 4 The spread of the seventh cholera pandemic.Data from Shears and Hart 1996.Occurrence of disease The pattern of occurrence of a disease is important for planning prevention and control. Different levels of occurrence are: · sporadic · endemic · epidemic · pandemic (see map)1960s 1970s 1980s 1990s Copyright Image from The Wellcome Trust modified from Shears P, Hart CA. Gastrointestinal bacteria. In: Cook GC, ed. Manson’s tropical diseases. 20th ed. London: WB Saunders, 1996:824-48.Sporadic diarrhoeaThis is defined as occasional attacks originating from reservoirs (see screen 27) such as: · humans · animals · the environmentEndemic diarrhoeaThis is defined as continuing, often low level, transmission of diarrhoea within a population.Epidemic diarrhoeaEpidemic diarrhoea: · is a rise in disease incidence above what is normal for the season · can be due to a common cause if a group is suddenly affected at once, eg. an outbreak of food poisoning in those who have shared a mealPandemic diarrhoeaThis is defined as an international epidemic that can continue for years. Image references ################ .\IMAGES\Wrldchlr.gif Definition of Key Terms: AssessmentDiarrhoea in a Class of School Children Weeks of the school term 12 3 4 5 6 7 8 9 10 11 12 A DD B C D D E F G H D I D J K D LM D N DOD P Q DDD R Weekly total1020 2000 5 011 What was the incidence of diarrhoea (D) during the term in the table? For the answer. What do you notice about children A and Q? For the answer.What happened in week 9? For the answer.What was the incidence rate of diarrhoea? For the answer. To return to the start of the section. Answer: Incidence The incidence of diarrhoea was 12 new episodes. This is found by adding all the weekly totals together: 1 + 2 + 2 + 5 + 1 + 1 = 12 Answer: Children A and Q Children A and Q: · each had more than one episode of diarrhoea · may be particularly susceptible or live in a high risk environment Answer: Week 9 In week 9 an outbreak (epidemic) occurred in the class.Five of the 18 children were affected. This situation requires investigation into the cause of the outbreak. Incorrect Answer: Incidence rate The incidence rate was: · 12 episodes in 18 children in 12 weeks or · 12/18 x 52/12 = 2.9 episodes per child per year Note: Nine of the 18 children had no episode of diarrhoea. The distribution of diarrhoea is often skewed in this way. Image references ################ Causes and Effects of Diarrhoea WorldwideCauses and Effects of Diarrhoea Worldwide Image references ################ Causes and Effects of Diarrhoea Worldwide - 1 Diarrhoea morbidity and mortality In developing countries, it is estimated that each year there are: · over 1 billion episodes of diarrhoea in children · an average of 2.6 diarrhoea episodes per child · 3.3 million deaths from diarrhoea, 80% occurring in the first 2 years of life Graph: The prevalence of diarrhoea in different developing countries. Data from Rohde and Northrup 1986. Why do countries differ in the prevalence of diarrhoea?Percentage of days with diarrhoea 20 15 10 5 0 Bangladesh Guatemala Indonesia 39152127333945 Age (months) Copyright Image from The Wellcome Trust modified from Rohde J, Northrup RS. Diarrhoea is a nutritional disease. In: Ladislaus-Sanei L, Scully PE, eds. ICORT II proceedings - second international conference on oral rehydration therapy,Washington, DC, 10-13 December 1985. Washington, DC: Agency for International Development, 1986:30-41.Diarrhoea episodesIn some highly contaminated environments, such as shanty towns, children can have up to 10 episodes of diarrhoea each year.Deaths from diarrhoeaDiarrhoea causes about 30% of all infant deaths in many developing countries.Variation between countriesVariation between countries is explained by differences in factors such as: · diarrhoea pathogens · levels of sanitation · access to clean water · infant feeding practices These and other factors affecting transmission are described on screens 16 - 36. Image references ################ .\IMAGES\Diaprev.gif Causes and Effects of Diarrhoea Worldwide - 2Episodes DeathsImportance of the clinical types of diarrhoea The three clinical types of diarrhoea vary in: · incidence · prevalence · clinical features · aetiology Pie charts: Annual numbers of episodes of diarrhoea and deaths from diarrhoea worldwide. Data from WHO 1995 and Bern et al 1992. Click on each part of the lower pie chart for details.200 million200 million1.4 billion1.2 million480,0001.6 millionAcute wateryAcute bloody Persistent Copyright Image from The Wellcome Trust modified from Bern C et al. Bull World Health Organ 1992;70:705-14 and World Health Organization. The World Health Report 1995. Bridging the gaps. Geneva: WHO, 1995.Persistent diarrhoeaPersistent diarrhoea: · is very closely linked to malnutrition (persistent diarrhoea-malnutrition syndrome) · can cause associated non-intestinal infections, eg. pneumonia · is associated with a range of pathogens similar to those of acute diarrhoea, but especially with: - enteroaggregative E. coli- Cryptosporidium - Shigella - Giardia lamblia Acute bloody diarrhoeaAcute bloody diarrhoea (dysentery): · can cause extraintestinal complications (eg. haemolytic-uraemic syndrome) and malnutrition · is caused by: - Shigella in over 50% of episodes (80% of deaths) - Campylobacter jejuni (data may include Campylobacter coli) - enteroinvasive E. coli- non-typhoid Salmonella - Entamoeba histolytica (in adults) Acute watery diarrhoeaAcute watery diarrhoea: · can rapidly cause dehydration · is caused mainly by: - rotavirus and other enteric viruses - enterotoxigenic E. coli (ETEC) - Vibrio cholerae O1 - Cryptosporidium - Shigella - Campylobacter jejuni (data may include Campylobacter coli) Deaths from diarrhoeaThe data shown probably underestimate the mortality due to persistent diarrhoea, which recent studies suggest causes 45% of all diarrhoea deaths. Different countries vary as to which type of diarrhoea causes most deaths. For example, in: · Bangladesh and Senegal most diarrhoeal deaths are due to acute watery diarrhoea · Brazil most diarrhoeal deaths are due to persistent diarrhoea Image references ################ .\IMAGES\Diapies.gif Causes and Effects of Diarrhoea Worldwide - 3Pathogens in the stools ofyoung children with acute diarrhoea in developing countries. Data from WHO 1992.Pathogens in the stools of young children with diarrhoea Epidemiological surveys have determined the pathogens associated with acute diarrhoea in young children (see pie chart). The bacterium Vibrio cholerae O1 is not shown in the pie chart but is important because it causes: · serious epidemics · 5 - 10% of all cases of acute diarrhoea worldwide For a table of other commonly isolated pathogens. Rotavirus 15 - 25 % ETEC 10 - 20% No pathogen found 20 - 30% C. jejuni 10 - 15% Shigella 5 - 15% Cryptosporidium 5 - 15% No pathogen found Viruses Bacteria ProtozoaCopyright Image from The Wellcome Trust modified from World Health Organization. Readings on diarrhoea. Student manual. Geneva: WHO, 1992.Less Common Pathogens in Childhood Acute DiarrhoeaGroup Species Viruses Small round structured viruses, eg. Norwalk agent Enteric adenoviruses Bacteria Other types of E. coli Other types of V. cholerae,eg. V. cholerae O139Non-typhoid Salmonella Protozoa Giardia lamblia Entamoeba histolyticaIsolation of a pathogen from the stool is not proof that the pathogen caused that episode of diarrhoea.Pathogens in young children These agents are the most important causes of diarrhoea worldwide because most diarrhoea morbidity and mortality is in this age group.Image references ################ .\IMAGES\Agendiar.gif Causes and Effects of Diarrhoea Worldwide: AssessmentAre the following statements about diarrhoeal disease true or false?To return to the start of the section. Click on the True or False button for each statement. 1. Diarrhoea kills about 300,000 children each year.2. The most important causes of diarrhoea worldwide are enterohaemorrhagic E. coli, Entamoeba histolytica and Giardia lamblia.3. Rapid or severe dehydration is most closely associated with acute watery diarrhoea. 4. Persistent diarrhoea is defined as an episode that lasts more than 7 days.Correct Text explaining the answer (11-pt plain blue) Incorrect Diarrhoea kills over 3 million children each year, mostly: · in developing countries · under 5 years of age (especially under 2 years)Incorrect Text explaining the answer (11-pt plain blue) Correct Diarrhoea kills over 3 million children each year, mostly: · in developing countries · under 5 years of age (especially under 2 years)Correct Text 11 pt Arial dark blue goes here Incorrect These pathogens may be locally significant, but are globally far less important than: · rotavirus · enterotoxigenic and enteroaggregativeE. coli · Vibrio cholerae O1 · Shigella · Campylobacter jejuni · CryptosporidiumIncorrect Text 11 pt Arial dark blue goes here Correct These pathogens may be locally significant, but are globally far less important than: · rotavirus · enterotoxigenic and enteroaggregativeE. coli · Vibrio cholerae O1 · Shigella · Campylobacter jejuni · CryptosporidiumCorrect Very rapid or severe dehydration is most closely associated with acute watery diarrhoea, eg. rotaviral diarrhoea or cholera. However, both dysentery and persistent diarrhoea can cause dehydration.Incorrect Very rapid or severe dehydration is most closely associated with acute watery diarrhoea, eg. rotaviral diarrhoea or cholera. However, both dysentery and persistent diarrhoea can cause dehydration.Incorrect Very rapid or severe dehydration is most closely associated with acute watery diarrhoea, eg. rotaviral diarrhoea or cholera. However, both dysentery and persistent diarrhoea can cause dehydration.Correct Very rapid or severe dehydration is most closely associated with acute watery diarrhoea, eg. rotaviral diarrhoea or cholera. However, both dysentery and persistent diarrhoea can cause dehydration.Correct Incorrect Persistent diarrhoea by definition lasts at least 14 days. This definition is somewhat arbitrary, although most episodes of acute diarrhoea last less than5 - 7 days.Incorrect Text explaining the answer (11-pt plain blue) Correct Persistent diarrhoea by definition lasts at least 14 days. This definition is somewhat arbitrary, although most episodes of acute diarrhoea last less than5 - 7 days. Image references ################ Modes of Transmission of DiarrhoeaModes of Transmission of Diarrhoea Image references ################ Modes of Transmission of Diarrhoea - 1 Food Drinking water Person to person Faecal-oral transmission Diarrhoea pathogens are generally spread by faecal-oral transmission (see picture) through: · contamination of food · faecal contamination of drinking water · direct person to person spreadAre any other modes of transmissionseen? Faecal contaminationof hands and clothes Endogenous pathogens Contaminated with faeces Not cooked thoroughly Contact, fingers putin mouth Picture: Modes of faecal-oral transmission. Copyright Image from The Wellcome Trust. Contamination of foodFood can come to contain pathogens through two main routes. These are: · contamination with faeces containing diarrhoea organisms · natural contamination by organisms infecting animals used as human food Contaminated food is important in the transmission of: · V. cholerae O1, eg. seafood, vegetables · C. jejuni, eg. chicken and pork · non-typhoid Salmonella, eg. poultry and eggs · enterohaemorrhagic E. coli (EHEC), eg. beefburgersContamination of waterWater becomes contaminated with faeces containing diarrhoea organisms through: · defaecation into or near a water source, eg. a river · rainwater washing faeces into a water source, eg. an unprotected well · a dirty hand or utensil put into stored domestic water Contaminated water is important in the transmission of: · V. cholerae O1 · Cryptosporidium · Giardia lambliaPerson to person spreadPerson to person contact involves faecal contamination of: · hands · clothes · fomites, eg. a towel Person to person contact is important in the transmission of: · Shigella · rotavirus · Entamoeba histolyticaOther modes of transmissionThere is now reasonably good evidence that rotavirus can be transmitted from person to person as an aerosol. Image references ################ .\IMAGES\Foraltsm.gif Modes of Transmission of Diarrhoea - 2Factors affecting transmission Many factors affect the transmission of diarrhoeal disease. It is convenient to group these as most closely related to the: · pathogen · environment· host This is a simplification because most transmission factors relate to more than one of these three.For an example.Picture: A slum in Bangladesh, where conditions favour the transmission of diarrhoea. In general, diarrhoea is a disease of low socioeconomic status affecting mostly people in developing countries, poorer communities and low income households. Copyright Image from Tomkins AM. Multiple transmission factors Transmission from cooked food that has become contaminated may be affected by the: · pathogen - different pathogens grow differently on food · environment - dirty conditions will favour contamination of the meal · host’s behaviour - poor food hygiene will favour transmissionLow socioeconomic statusThere are important exceptions to these generalizations (see screen 40). Image references ################ .\IMAGES\T25669.jpg Transmission Factors Related to the PathogenTransmission Factors Related to the Pathogen Image references ################ Transmission Factors Related to the Pathogen - 1Number of pathogens excreted by infected individuals The concentration of diarrhoea pathogens in the stool varies greatly between different: · pathogens (see table) · individuals · stages of disease Transmission of diarrhoea is favoured by: · excretion of large numbers of pathogens· long term excretion from symptomless carriers Number of Pathogens Excreted per Gram of Stool1 Pathogen Active infectionAsymptomatic carriage Shigella 105 - 108102 V. cholerae O1 107 - 108See Note 2 1. The total stool volume can be up to 10 litres/day for V. cholerae O1. 2. Asymptomatic carriage of V. cholerae O1 for more than a few days is not common, hence the importance of environmental reservoirs (see screen 27).Large numbers of pathogensExcretion of large numbers of enteric pathogens increases contamination of the environment.Asymptomatic carriersPeople who are infected but have no symptoms of diarrhoea are particularly important for transmission, as they will generally not know that they are infectious.Image references ################ Transmission Factors Related to the Pathogen - 2 Persistence of diarrhoea pathogens in a favourable environment Persistence of excreted pathogens The ability of a pathogen to survive and multiply in the environment is important in transmission because: · robust organisms become widely dispersed and are more likely to infect new hosts · organisms that persist for a short time may survive long enough only for direct person to person transmission Which factors affect pathogen survivaltime?Entamoeba histolytica V. cholerae O1 Shigella G. lamblia Enteric viruses (?) Salmonella E. coli < 1 month< 6 months< 1 year Increasing persistenceCopyright Image from The Wellcome Trust. Entamoeba histolytica The trophozoites of Entamoeba histolytica are short lived, but the cysts are capable of longer survival. V. cholerae O1The El Tor biotype of V. cholerae O1: · survives in water for longer than the classical biotype (10 versus 5 days) · may have spread more widely than the classical biotype as a resultFactors Affecting Pathogen Survival Time Favourable Unfavourable Environment Warm, dark,High temperature,humid, some sunlight, dry, nutrientslow pH1 ParasiteResistant form, Vulnerable form, stage2 eg. a cyst eg. a trophozoite 1. The acidic (pH 2 - 3) environment of the stomach kills many ingested pathogens. 2. Entamoeba histolytica trophozoites die rapidly in gastric acid but the cysts are resistant and can cause infection.Image references ################ .\IMAGES\Fig723.gif Transmission Factors Related to the Pathogen - 3 Approximate doses of organisms required to cause diarrhoea Infective dose The number of pathogens needed to cause disease in a new host (infective dose): · varies with the pathogen (see graph) and between individuals · is relatively high for most bacteria The infective dose is estimated by giving a known number of pathogens to a volunteer, who is usually: · an adult · well nourished · from a non-endemic area How does the value obtained relate to children in developing countries?Diarrhoea pathogens V. cholerae E. coli Salmonella Shigella G. lamblia E. histolytica Enteric viruses 102104106 Infective dose For each pathogen there is much variation in infective dose between individual hosts. Copyright Image from The Wellcome Trust based on data from Benenson AS, ed. Control of communicable diseases manual. 16th ed. Washington, DC: American Public Health Association, 1995.Experimental studies of infective doseThe infective doses shown cannot reliably be extrapolated to children: · from an endemic area · in a highly contaminated environment · with malnutrition and impaired immunity For example, the infective dose of V. cholerae O1 is: · 1011 in adult volunteers when given in buffered saline · 106 in adult volunteers when given with food · 102- 103 in Bangladeshi children Image references ################ .\IMAGES\Fig9b.gif Do the factors shown below tend to favour or limit the transmission of diarrhoeal disease? Click your mouse on a Factor box below.Hold the mouse down and drag the box to the correct column.To return to the start of the section. Factor Favour transmissionLimit transmission Transmission Factors Related to the Pathogen: AssessmentExcretion of a small number of pathogens Long term asymptomatic carriage and excretion Survival of the pathogen at low (acidic) pH Very high infective dose Yes. That's right. Yes. That's right. Yes. That's right. Yes. That's right. No. That's wrong. Try again. Well done. You have now finished this assessment.Image references ################ Transmission Factors Related to the EnvironmentTransmission Factors Related to the Environment Image references ################ Transmission Factors Related to the Environment - 1Sanitation Poor sanitation means that faeces cannot be disposed of hygienically. This causes contamination of: · sources of drinking water · the domestic and general environment Transmission of diarrhoea rises progressively as the level of sanitation falls: · a flush or pour-flush toilet · a pit latrine or bucket latrine· an open hole in the ground ornearby bushesPicture: This child playing near an open sewer is exposed to diarrhoea pathogens.Sanitation falling, diarrhoea transmission rising. Copyright Image from Cutting WAM. Disposal of faecesHygienic disposal of the faeces of babies, young children and people with diarrhoea is especially important.Image references ################ .\IMAGES\T23720.jpg Transmission Factors Related to the Environment - 2 Water supply Diarrhoea transmission is favoured by contaminated drinking water and by an inadequate supply of water for personal and domestic hygiene. Diarrhoea is associated with: · exposed sources of drinking water, eg. rivers and unprotected wells (see picture) · a lack of clean piped water in the homePicture: An unprotected well in Tanzania. Rainwater can easily wash faeces into the well, contaminating the supply of drinking water.Copyright Image from Lengeler C. Exposed drinking water sourcesFaecal contamination of exposed community water sources can be by: · people or animals defaecating in, or close to, a pond or river · rainwater washing faeces into a poorly designed or badly maintained wellLack of clean piped waterIf water has to be collected from a communal tap, pump or well and stored at home, this may lead to: · contamination by a dirty hand or utensil put into the storage container · contamination by animals · use of too little water for proper personal and domestic hygiene Image references ################ .\IMAGES\T44192.jpg Transmission Factors Related to the Environment - 3Reservoirs of infection 1. Human reservoirs of infection are symptomless carriers of enteric pathogens. Such reservoirs are: · a significant source of infection · important in maintaining transmission and starting outbreaks 2. Animal reservoirs of infection are usually infected domestic animals. 3. Environmental reservoirs of infection include: · water sources contaminated with sewage · estuarine water and organisms (see picture) · frozen foods, eg. poultry2. 1. 3. Click on each reservoir for details.Picture: Reservoirs of selected pathogens. Copyright Image from The Wellcome Trust. Domestic animal reservoirs Animal reservoirs: · include pigs, cattle, poultry and pets · are called zoonotic sources of infection, ie. carry pathogens that normally infect animals but can spread to humans · spread pathogens to humans by: - contact with animal faeces in food - consumption of foodborne organisms - direct ‘animal to person’ contactHumans Humans are the only known reservoirs of: · ETEC · Shigella · rotavirus Humans are also important reservoirs (but not the only ones) for the protozoa: · Entamoeba histolytica· Giardia lambliaAnimalsAnimals are probably the most important reservoir for: · non-typhoid Salmonella (eg. S. enteritidis) - especially poultry · Campylobacter - chickens and pigs · EHEC - beef cattleEnvironmental (eg. estuaries)Estuarine environments contain these probable reservoirs of pathogens such as V. cholerae: · cyanobacteria - cholera epidemics are linked to algal blooms · plankton · algae · crustaceans · oysters · fish Image references ################ .\IMAGES\Reservoi.gif Transmission Factors Related to the Environment - 4 Seasonality There are seasonal differences in the peak incidence of diarrhoea due to: · rotavirus - often occurs as a ‘winter epidemic’ in temperate countries (see graph) · endemic bacteria(eg. Shigella, V. cholerae O1) - often occurs all year round with a peak in the hot season Seasonal patterns reflect a complex mix of factors and can be unpredictable. Graph: Average incidence of rotaviral diarrhoea over the year in England and Wales, 1992 - 1995. Data from PHLS, Communicable Disease Surveillance Centre. No. of cases3000 2000 1000 0 12345678910 11 12 13 4 week period (January to December) Copyright Image from The Wellcome Trust based on data from Public Health Laboratory Service, Communicable Disease Surveillance Centre.Rotaviral diarrhoeaThe winter peak incidence of rotaviral diarrhoea in developed countries may reflect: · spread by the aerosol route · increased risk of person to person contact indoors Rotaviral diarrhoea in countries without a long cool season: · shows much less seasonality · is often endemic throughout the yearUnpredictable patternsEndemic cholera for example has peak incidence in: · October to November in Bangladesh but · May to June in India This different seasonality is probably due to washing of faecal material into rivers and wells during the rainy season.Environmental reservoirs such as cyanobacteria (see screen 27) may also play a role.Endemic bacterial diarrhoea The warm weather peak incidence of much bacterial diarrhoea may reflect increased growth of bacteria in contaminated food and water. Image references ################ .\IMAGES\Rotaepi.gif Transmission Factors Related to the Environment: Assessment This picture is of a slum in Indonesia.Make a list of four or five main factors that favour transmission of diarrhoeal diseases in such a setting. For our answers. To return to the start of the section. Copyright Image from United Nations Children's Fund, India. Answer: Transmission factors Diarrhoea in such a setting is favoured by: · lack of adequate sanitation - people defaecating near houses · lack of clean water · animals living close to humans - contamination of the environment · food being prepared in a contaminated environment · people living close together - person to person contact Image references ################ .\IMAGES\T23718a.jpg Transmission Factors Related to the HostTransmission Factors Related to the Host Image references ################ Transmission Factors Related to the Host - 1 Diarrhoeal morbidity rates for children under 5 years in developing countries Age: weanling diarrhoea In developing countries the peak incidence of diarrhoea (see graph) is in the first 2 years of life, especially at around 6 - 12 months. This peak (weanling diarrhoea) is due to: · introduction of complementary foods and liquids (see screen 35) that are: - faecally contaminated - nutritionally inadequate · increased exposure to faeces in the environment as the infant: - begins to crawl- puts contaminated objects into his orher mouthEpisodes/child/year3 2 1 0 Data from Snyder and Merson 1982. 0-56-1112-2324-3536-4748-59 Age (months) For more details. Copyright Image from The Wellcome Trust modified from Snyder JD, Merson MH. Bull World Health Organ 1982;60:605-13.More detailsIn developing countries: · young children may have diarrhoea for 15 - 20% of the time · women are more susceptible during their reproductive years, because of their close contact with young children Image references ################ .\IMAGES\Diarinc.gif Transmission Factors Related to the Host - 2 Nutritional status Malnutrition, particularly severe malnutrition, predisposes to diarrhoea of increased: · duration· severity · case fatality rate · incidence (in some studies) Malnutrition predisposes to diarrhoea because it impairs defence mechanisms such as: · cell-mediated immunity · production of IgA · intestinal secretions· integrity of the bowel mucosa Picture: A child with persistent diarrhoea-malnutrition syndrome. This is the best example of the way that diarrhoea and malnutrition interact to form a vicious circle in which malnutrition predisposes to diarrhoea and diarrhoea causes malnutrition.Copyright Image from Behrens RH. Incidence of diarrhoeaEpidemiological studies of the effect of pre-existing malnutrition on the incidence of diarrhoea: · are methodologically complex to perform and interpret · have not always given consistent results The current consensus is that pre-existing malnutrition: · may predispose to acute diarrhoea, although this is controversial · does predispose to persistent diarrhoea (see picture), because malnutrition is associated with increased duration of each diarrhoea episode Intestinal secretionsSevere malnutrition causes reduced secretion of: · gastric acid (hypochlorhydria) - see screen 21 · mucus · bile · pancreatic juice For details refer to the tutorial Diarrhoeal Diseases: Defence Mechanisms. Image references ################ .\IMAGES\T45355.jpg Transmission Factors Related to the Host - 3Immunological status Remember. Impaired defence mechanisms predispose to diarrhoea. The major causes of impaired immunity worldwide are: · measles · malnutrition (see screen 32) · micronutrient deficiency · concurrent infections, eg. pneumonia · human immunodeficiency virus (HIV) infection (see picture) Picture: A post mortem view of a child with severe malnutrition and HIV infection. Both are risk factors for diarrhoea. For more on diarrhoea in HIV infection.Copyright Image from Lucas SB. RememberAge is an important determinant of immunity. This is because: · breast feeding children acquire maternal antibodies (eg. secretory IgA) in milk · children being weaned are immunologically ‘naive’ and susceptible to infection · elderly people have impaired immune systems and lower gastric acidity (hypochlorhydria)Diarrhoea in HIVHIV infection, especially in sub-Saharan Africa, is often associated with: · diarrhoea lasting more than 1 month and of complex aetiology · weight loss - including severe wastingMeaslesAfter measles there is a period of several weeks when: · cell-mediated immunity is decreased · the risk of severe and persistent diarrhoea is increasedMicronutrient deficiencyPicture: Signs of vitamin A deficiency. The foamy area (B) is Bitot's spot.Image from Sommer A. A Fieldguide to the Detection and Control of Xerophthalmia. Geneva: WHO, 1978. B The micronutrients which are most closely associated with impaired immunity and an increased risk of diarrhoea are: · vitamin A (see picture) · zinc Refer to the tutorial Diarrhoeal Diseases: Prevention and Control.HIVPeople with HIV are at increased risk of persistent diarrhoea due in part to infection by: · Cryptosporidium, Shigella, non-typhoid Salmonella · pathogens uncommon in HIV negative people, eg. Isospora belli, Enterocytozoon bieneusi and Mycobacterium avium-intracellulare Image references ################ .\IMAGES\T43140.jpg .\IMAGES\T14039p.jpg Transmission Factors Related to the Host - 4 Breast feeding Failure to breast feed exclusively for the first 4 - 6 months of life in developing countries greatly increases the risk of:· death due to diarrhoea (see graph) · severe diarrhoea · morbidity due to diarrhoea The protective effects of breast feeding extend to at least 1 year but are greatest in the first few months of life. Why is breast feeding so protective? Risk of death 20 15 10 5 1 Breast Breast + other milk Other milk only Graph: Adjusted risk of death from diarrhoea in infants according to type of feeding in southern Brazil. Data from Victora et al 1987.Milk diet onlyMilk and solid food Copyright Image from The Wellcome Trust modified from Victora CG et al. Lancet 1987;21:319-22.Why breast feeding is protectiveBreast feeding prevents diarrhoea because it: · shields the infant from enteric pathogens in contaminated: - bottles or teats - milk or other fluids - solid food · gives some immunity by transfer of maternal antibodies, especially secretory IgA · favours good nutritional status Refer to the tutorial Diarrhoeal Diseases: Defence Mechanisms. Exclusive breast feeding Exclusive breast feeding means that up to age 4 - 6 months the infant is given no other fluids or food, such as: · water · teas · fruit juice · animal or formula milk · gruels or other complementary food After 4 - 6 months of age: · exclusive breast feeding does not provide enough nutrition · weaning foods to complement breast feeding should be startedBreast feeding and diarrhoea morbidityGraph: Effect of breast feeding on the incidence of diarrhoea according to age. The relative risks compare no breast feeding with exclusive or partial breast feeding. Image from The Wellcome Trust modified from Feachem RG, Koblinsky M. Bull World Health Organ 1984;62:271-91.Relative risk 3 2 10-33-56-99-11 Age (months)Image references ################ .\IMAGES\Ebdiar.gif .\IMAGES\Brstfedp.gif Transmission Factors Related to the Host - 5 Food hygiene and weaning practice Diarrhoea is associated with: · poor food hygiene in general (see picture) · contaminated weaning food and liquids · nutritionally poor weaning practicePicture: Sources of contamination of food with diarrhoea pathogens. From Motarjemi et al 1993.Contaminated household water Contaminated hands Infected food animal Human and animal excreta Irrigation water Flies Cross contamination Night soil Contaminated food Waste water Dirty pots, cooking utensils Domestic animals Polluted environment Transmission ofdiarrhoea Copyright Image from The Wellcome Trust modified from Motarjemi Y et al. Bull World Health Organ 1993;71:79-92. Food hygieneThe most common causes of food poisoning are: · failure to cook or reheat food sufficiently well to kill foodborne pathogens, eg. Salmonella· transfer of foodborne pathogens from uncooked to cooked meat during storage · leaving cooked food at room temperature for several hours, favouring: 1. faecal contamination of the food, eg. from a dirty hand 2. growth of bacteria, eg. ETEC or ShigellaContaminated weaning food Diarrhoea is associated with introduction of faecally contaminated weaning food and liquids by: · not washing hands before preparing or giving food · using dirty utensils or work surfaces · not cooking food thoroughly enough · leaving cooked food for several hours at room temperature · not using boiling water to prepare formula milkNutritionally poor weaning practiceBad weaning practices can contribute to poor nutritional status and hence to diarrhoea. These practices include: · introducing complementary weaning foods too early (before 4 months) or too late (after 6 months) · giving weaning foods of low energy and nutrient content · not feeding frequently enough · weaning abruptly - often because the mother becomes pregnant again Image references ################ .\IMAGES\Foodcont.gif Transmission Factors Related to the Host - 6 Personal and domestic hygiene Diarrhoea transmisson is favoured by: · poor personal hygiene, such as notwashing hands: - after defaecation - after cleaning the bottoms of babies and young children - before preparing or eating food · poor domestic hygiene, leading to: - contamination of food and drinking water - faecally contaminated utensils and surfacesPicture: A child washing her hands. The use of soap is important, as is a clean cloth or towel to dry the hands.Copyright Image from WHO Photo Library courtesy of Kelly T. Hand washingHand washing with soap or a local substitute: · can reduce the incidence of diarrhoea by up to 35%· may be especially effective against shigellosis Refer to the tutorial Diarrhoeal Diseases: Prevention and Control. Image references ################ .\IMAGES\T34804.jpg Transmission Factors Related to the Host: AssessmentAre the following statements about the transmission of diarrhoea true or false?To return to the start of the section. Click on the True or False button for each statement. 1. The peak incidence of diarrhoea in infancy reflects mainly changes in the developing infant’s immune system.2. Severe malnutrition predisposes to an increased duration of diarrhoea.3. Major causes of impaired immunity worldwide include measles, malnutrition and micronutrient deficiency.4. Exclusive breast feeding in infancy lowers the risk of death from diarrhoea by 40 - 60% compared with bottle feeding only.Correct Text explaining the answer (11-pt plain blue) Incorrect This peak is due largely to: · introduction of contaminated food and liquids at weaning · increased exposure to a contaminated environmentIncorrect Text explaining the answer (11-pt plain blue) Correct This peak is due largely to: · introduction of contaminated food and liquids at weaning · increased exposure to a contaminated environmentCorrect Malnutrition, particularly severe malnutrition, predisposes to diarrhoea of increased: · duration· severity · case fatality rateIncorrect Text explaining the answer (11-pt plain blue) Incorrect Malnutrition, particularly severe malnutrition, predisposes to diarrhoea of increased: · duration· severity · case fatality rateCorrect Text explaining the answer (11-pt plain blue) Correct Other major causes of impaired immunity are: · concurrent infection, eg. pneumonia · HIV infectionIncorrect Text explaining the answer (11-pt plain blue) Incorrect Other major causes of impaired immunity are: · concurrent infection, eg. pneumonia · HIV infectionCorrect Text explaining the answer (11-pt plain blue) Correct Incorrect Exclusive breast feeding in developing countries has a much stronger protective effect than this - the risk of death from diarrhoea is up to 15 times less than with bottle feeding only.Incorrect Text explaining the answer (11-pt plain blue) Correct Exclusive breast feeding in developing countries has a much stronger protective effect than this - the risk of death from diarrhoea is up to 15 times less than with bottle feeding only. Image references ################ Modes of Transmission of Diarrhoea: a SummaryModes of Transmission of Diarrhoea: a Summary Image references ################ Modes of Transmission of Diarrhoea: a Summary - 1 Factors Affecting Faecal-Oral Transmission Favour transmissionLimit transmission Pathogen Pathogen · many excreted · few excreted · survives well in adverse· poor survival, dependence on supportiveenvironment environment · low infective dose· high infective dose· infects animals · infects only humans Environment Environment · faecally contaminated environment · clean environment · unprotected or scarce water supply· clean and plentiful water supply · people living close to animals· no animal reservoirs nearby · crowded living conditions· spacious accommodation HostHost · many susceptibles, especially children· few susceptibles, many immune adults· body defences impaired · healthy and robust hosts · failure to breast feed exclusively · exclusive breast feeding · poor food hygiene, eg. of weaning food · good food hygiene · poor personal and domestic hygiene· good personal and domestic hygieneFactors affecting transmission The table: · summarizes the transmission factors described · lists any other factors not so far discussedImage references ################ Modes of Transmission of Diarrhoea: a Summary - 2 Diarrhoea in developed and developing countries The epidemiology of diarrhoea differs between developed and developing countries. Click on the labels in the map for a summary. Adults Adults Children Adults Children Copyright Image from The Wellcome Trust. Adults in developed countries In adults in developed countries: · diarrhoea is most often of bacterial aetiology, eg. non-typhoid Salmonella, Campylobacter · transmission is often from undercooked or contaminated meat, poultry, etc · travel to different countries is a common cause of diarrhoea · elderly people, especially in institutions, have the highest mortality rate (higher than children)Children in developed countriesIn children in developed countries: · diarrhoea is more commonly of viral aetiology (eg. rotavirus) than in developing countries · transmission of rotavirus can occur by the airborne route · day care facilities are common centres of outbreaks · the average incidence of diarrhoea is often less than 1 - 2 episodes per child per year Adults in developing countries In adults in developing countries: · diarrhoea is most often of bacterial aetiology (eg. ETEC, Shigella) · transmission is mainly by faecal contamination of food and water · diarrhoea from endemic pathogens is uncommon because of acquired immunity (but epidemic cholera or shigellosis kills adults)Children in developing countriesIn children in developing countries: · diarrhoea is more commonly of bacterial aetiology (eg. ETEC, Shigella) than in developed countries (although rotavirus is still a very important cause) · transmission is mainly by faecal contamination of food and water · the average incidence of diarrhoea in some areas, eg. shanty towns, can be up to 10 episodes per child per year Image references ################ .\IMAGES\Wrldmap2.gif Practical Epidemiology Practical EpidemiologyImage references ################ Practical Epidemiology - 1 Incidence of cholera reported to WHO, 1984-1995. Data from WHO 1996. Routine sources of information Assessment of the diarrhoea problem and decision making about control requireinformation about diarrhoea in the community. Such information routinely comes from: · morbidity reports· mortality reports· laboratory reports· individual case reports and autopsy reports· epidemic reports (see graph) None of these sources gives a complete picture of diarrhoea in the community. Why? Cases (1000s) 500 400 300200 100 0 Asia Africa Americas 198419861988199019921994Year What happened in 1991? Copyright Image from The Wellcome Trust modified from World Health Organization. Weekly Epidemiol Record 1996;71:157-63. Mortality reportsMortality reports: · record the numbers of people who die from diarrhoea · are collated from a statutory death certification schemeLaboratory reportsLaboratory reports: · record the pathogens isolated and sometimes their antimicrobial resistance · are prepared by a hospital laboratory or research laboratoryEpidemic reportsEpidemic reports: · record the details of outbreaks (epidemics) of disease · are prepared by public health officials or disease control specialistsCase reports & autopsy reports Case reports and autopsy reports: · record the details of individual patients with diarrhoea · are prepared by doctors and pathologistsMorbidity reports Morbidity reports: · record the numbers of people ill with diarrhoea who present to the health services · are prepared by clinics or hospitalsGraph: AnswerIn 1991 the seventh cholera pandemic spread to South America (notably Peru), causing a huge epidemic.Why no source is completeLimitations of the sources listed include: · morbidity reports exclude people in the community who are ill but do not come to health services · mortality reports may not give an accurate cause of death · laboratory reports may be restricted by available technical facilities · case reports and autopsy reports may be from exceptional cases · epidemic reports may be incomplete, or manipulated for political or financial motives Image references ################ .\IMAGES\Cholcase.gif Practical Epidemiology - 2 Specific epidemiological studies Obtaining more specific data often requires carrying out a systematic survey or study, for which specific objectives must be set. What are the sorts of question that might be addressed? Examples are: 1. How often do children under 5 years get diarrhoea and does their nutritional status affect this? 2. How long does the average episode of diarrhoea last and does nutritional status affect this? 3. Is there an epidemic going on, and which area or group does it affect? What is the likely source or cause? 4. Are there easy ways to help decide which patients need special treatment? 5. Which interventions are effective in preventing diarrhoea? Answering each of these questions requires a different type of study. Different types of study For example: · questions 1 and 2 require a study of diarrhoea in the community · question 3 could be answered from a study of patients in health centres or hospitals · question 5 requires a controlled trial Image references ################ Practical Epidemiology - 3Types of epidemiological study The most important types of epidemiological survey or study are: · prospective incidence study · population sampling · prevalence survey · case-control study · randomized controlled trialPotential subjects Admission criteria Not met Met Admit to study Reject from study Refusers Random allocation Control group Test group Test treatment Control treatment Losses Control outcome Test outcome Comparison of outcomes Conclusion Implementation Picture: The stages of a randomized controlled trial.Copyright Image from The Wellcome Trust modified from Cutting WAM. Prospective incidence studyA prospective incidence study: · measures the incidence of diarrhoea over a period of time by following a group of individuals, ie. it is longitudinal in design · is planned and then carried out to detect new cases This type of study could assess how often children under 5 years of age get diarrhoea.Population sampling or surveillance Population sampling or surveillance: · studies a subset of the population of interest representative by age, social group and geographical area · must avoid selection bias, eg. looking only at sicker hospital patients This type of study could assess whether an epidemic is developing in an area or population.Prevalence survey A prevalence survey: · is a study of a representative sample of homes, schools, etc. · is performed at a set point in time (eg. during one week), ie. is cross sectional in design This type of study could assess the prevalence of diarrhoea among children.Case-control studyA case-control study: · requires two groups of people:1. cases, who have a disease, eg. diarrhoea 2. controls, who do not have the disease · measures the association between a risk factor and diarrhoea This type of study could retrospectively assess the association between improved sanitation or breast feeding and diarrhoea. Randomized controlled trialA randomized controlled trial: · assesses the efficacy of one treatment or intervention against another or against placebo · follows a set protocol (see picture) · requires careful selection of subjects and randomization with respect to the intervention · should if possible be double blind, ie. neither the investigator nor patient knows to which arm of the trial the patient is allocated This type of study is widely used, eg. to compare different oral rehydration salts solutions or anti- microbial agents. Randomized trials are increasingly important in evidence based clinical practice. Image references ################ .\IMAGES\Fig712.gif Which epidemiological studies are needed to address the questions below? To return to the start of the section. Click your mouse on a question box below.Hold the mouse down and drag the box to the type of study that would best address the question. Types of study Questions Randomizedcontrolled trial Epidemic survey Longitudinal prospective incidence study Case-control study Practical Epidemiology: AssessmentIncidence of diarrhoea in a group of schoolchildren Efficacy of a rotavirus vaccine Whether one type of sanitation is associated with dysentery Whether a rise in incidence is due to seasonal variation or an epidemic Yes. That's right. Yes. That's right. Yes. That's right. Yes. That's right. No. That's wrong. Try again. Well done. You have now finished this assessment.Image references ################ Tutorial Assessment The graph shows data from three areas of northeastern Brazil. What is the trend in diarrhoea incidence when the three areas are compared? For the answer. Which factors are likely to help explain this trend? For the answer. Why do all the curves show a peak incidence at 6 - 11 months? For the answer. To return to the start of the tutorial.Episodes per person per year 12 10 8 6 4 2 0 Poor rural Poor urban Better urban < 66-11 12-232-45-910-14 15-19 20-39 40-59 >60 Months Years Age Copyright Image from The Wellcome Trust modified fom Guerrant RL, Kirchhoff LV, Shields DS et al. J Infect Dis 1983;148:986-97. Answer: Trend between areasThe incidence of diarrhoea falls progressively in the sequence: · poor rural - up to 10 episodes per child per year · poor urban - up to 7 episodes per child per year · better urban - less than 2 episodes per child per yearAnswer: Factors in the trendThe incidence of diarrhoea rises as the level of sanitation and access to clean water get worse. This is probably due to: · drinking of faecally contaminated water · failure to dispose of faeces hygienically, leading to faecal contamination of the home · inadequate water supply leading to poor personal and food hygieneAnswer: Peak at 6 - 11 monthsThe peak incidence of diarrhoea at age 6 - 11 months is probably due to: · introduction of complementary foods and liquids that are: - faecally contaminated - nutritionally inadequate · increased exposure to faeces in the environment as the infant: - begins to crawl- puts contaminated objects into his or her mouth · poorly developed defence mechanisms Image references ################ .\IMAGES\Guerran.gif Summary Click on the buttons below for summary information. Picture: Handling uncooked chicken together with these vegetables, which are not to be cooked, is poor food hygiene and might transmit diarrhoea.Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Copyright Section 1 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Section 2 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Section 3 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Section 4 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Section 5 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Section 6 Pop-up text is in blue with a yellow title in 10 pt bold left aligned.The pop up is a display icon within the library, and if the text is too long then a scroll bar should be used, pop-up boxes can be larger. Definition of Key Terms Causes and Effects of Diarrhoeal Diseases Worldwide Modes of Transmission of Diarrhoea Practical Epidemiology Copyright Copyright Image from Skirrow MB. Definition of Key Terms Term Definition Acute watery diarrhoeaLoose or watery stools without visible blood Duration less than 14 days Acute bloody diarrhoeaLoose or watery stools with visible red blood (dysentery) Duration less than 14 days Persistent diarrhoeaLoose, watery or bloody stools Duration 14 days or moreCauses and Effects of Diarrhoeal Disease Worldwide In developing countries, each year there are: · over 1 billion episodes of diarrhoea in children · an average of 2.6 diarrhoea episodes per child · 3.3 million deaths from diarrhoea, 80% occurring in the first 2 years of life Acute watery diarrhoea is most often caused by: · rotavirus · enterotoxigenic E. coli · Vibrio cholerae O1 · Cryptosporidium Acute bloody diarrhoea is most often caused by: · Shigella · Campylobacter jejuni · Entamoeba histolytica (in adults) Persistent diarrhoea is most often associated with: · enteroaggregative E. coli · Cryptosporidium· Shigella · Giardia lambliaModes of Transmission of Diarrhoea Diarrhoea pathogens are spread by faecal-oral transmission through: · contamination of food · faecal contamination of drinking water · direct person to person spread Diarrhoea transmission is associated with the following factors. Pathogen related · excretion of many robust pathogens · low infective dose Environmental · poor sanitation and inadequate supply of clean water · reservoirs of infection Host related · age under 24 (especially 6 - 12) months · malnutrition and impaired immunological status (measles, HIV) · failure to breast feed · poor food hygiene and weaning practice · poor personal and domestic hygienePractical EpidemiologyDefinitions: · incidence is the number of new cases of a disease in a given period of time · prevalence is the number of cases of a disease at a particular point in time · incidence and prevalence rate express these relative to the population size Sources of information: · morbidity and mortality reports · laboratory reports · individual case reports, autopsy reports · epidemic reports Types of epidemiological survey or study: · prospective incidence study · population sampling or surveillance · prevalence survey · case-control study · randomized controlled trial Image references ################ .\IMAGES\T45532s.jpg You have now finished the tutorial Epidemiology ã The Trustee of the Wellcome Trust, 1998 Further reading Further activities Restart tutorial Picture: Seafood, a common source of cholera, on sale at a market in a slum area of Indonesia. Copyright Image from Bryceson ADM. Further reading Bern C, Martines J, de Zoysa I, Glass RI. The magnitude of the global problem of diarrhoeal disease: a ten-year update. Bull World Health Organ 1992;70:705-14. Guerrant RL, Kirchhoff LV, Shields DS et al. Prospective study of diarrheal illnesses in northeastern Brazil: patterns of disease, nutritional impact, etiologies, and risk factors. J Infect Dis 1983;148:986-97. Huilan S, Zhen LG, Mathan MM. Etiology of acute diarrhoea among children in developing countries: a multicentre study in five countries. Bull World Health Organ 1991;69:549-55. Motarjemi Y, Kaeferstein F, Moy G, Quevedo Fl. Contaminated weaning food: a major risk factor for diarrhoea and associated malnutrition. Bull World Health Organ 1993;71:79-92. Sack RB. Epidemiology of acute infectious diarrhea. In: Gracey M, Walker-Smith JA, eds. Diarrhoeal diseases. Nestle nutrition workshop series. Vol. 38. Philadelphia: Vevey/Lippincott-Raven, 1997:75-89. Further activities To look at pictures related to this tutorial, search the image collection using the following keywords: · epidemiology · prevention/controlImage references ################ .\IMAGES\T39751.jpg