1. Epidemiology and WSH Infectious Diseases
Lecture 4
ENVR 890-2
Mark D. Sobsey

2. Epidemiology - Definition
The logic of observation and the methods to quantify these observations in populations (groups) of individuals.
The study of the distribution of health-related states or events in specified populations and the application of this study to the control of health problems.
Epidemiology includes:
1) methods for measuring the health of groups and for determining the attributes and exposures that influence health;
2) study of the occurrence of disease in its natural habitat rather than the controlled environment of the laboratory (exception: clinical trials); and
3) methods for the quantitative study of the distribution, variation, an determinants of health-related outcomes in specific groups (populations) of individuals, and the application of this study to the diagnosis, treatment, and prevention of these states or events.

3. Infectious Disease Epidemiology: Classical Epidemiology
the study of epidemics
the study of the dynamic factors involved in the transmission of infectious agents in populations
the natural history of infectious disease
how a disease spreads through groups or a population
how a case of that disease develops in an individual

4. Basic Epidemiological Concepts and Terms
Incidence: # of new cases of disease/total # at risk.
Incidence rate: Incidence/unit of time.
Prevalence: # cases (or # with defined condition) existing at one time.
Prevalence rate: # of such cases/total # at risk.
Epidemic:
# cases in excess of expected # for population
the uncontrolled spread of a disease (or condition) in a community.
Immunity: Inherited, acquired, or induced resistance to infection by a specific pathogen
Acquired resistance due to previous infection is from protective cellular and antibody responses in the host
Herd immunity: cumulative # of immune persons in population or % of population immune.

5. Outbreaks or Epidemics
A disease or condition at involves many or an excessive number of people at the same time and the same place
The occurrence of a disease or condition at a frequency that is unusual or unexpected
increase above background or endemic level
Requirements for an outbreak or epidemic:
(i) presence of an infected host or other source of infection.
(ii) adequate number of susceptibles
(iii) an effective method of contact for transmission to occur.

6. Transmission/Exposure Routes of Infectious Agents: Entry to and/or Exit From the Body
Sites or Portals of Exit or Entry:
Respiratory
Enteric or Gastrointestinal
Skin: especially if skin barrier is penetrated
Genitourinary
Eye

7. Routes or Methods of Entry
Direct Personal Contact: Person (animal)-to-Person
Indirect Personal Contact: Droplet, Fomites, Other Vehicles
Water and Food (Gastrointestinal Tract)
Vector-borne: often insects
Intrauterine or Transplacental
Organ Transplants, Blood and Blood Products

8. Transmission Routes of Infectious Agents

9. Infectious Diseases and the Process of Infection
Infection: the growth/multiplication of a microbe in a host
Infection does not always result in injury of the host (disease)
Two main classes of infection by site:
localized
generalized (disseminated; systemic)
Some infections are usually localized but can sometimes spread to another site
Example: Amoebic dysentery occurs in the intestines (colon); sometimes it spreads to the liver causing liver abscess

10. Localized Infections
Organism enters the body and reaches target site of infection
Organism adheres to or enters host cells and multiplies at site of infection
Infection spreads within the site (e.g., respiratory tract; intestines)
Symptoms of illness appear
Organism does not spread through the lymphatic system or reach the bloodstream
Infection subsides due to host defenses (e.g., immunity)
Agent eliminated from the body; infected cells replaced; "cure"

11. Generalized Infections
Organism enters the body and reaches target site of initial infection
Organism adheres to or enters host cells and multiplies at initial site of infection
Infection spreads within site and to other sites via tissues, lymphatic system, bloodstream (bacteremia, viremia, etc.) and possibly other routes
Symptoms of illness may appear
Organisms infect other organs, tissues and cells; more spread via bloodstream
Symptoms of illness become severe
Host defenses eliminate organisms leading to cure or disease continues, possibly leading to irreversible damage or death

13. Factors Influencing Exposure and Infection: Agent (Microbe) Factors
Sources, Reservoirs, Transport and Persistence (in the Environment)
Ability to Enter a Portal in the Human or Other Host
Ability to Reach and Proliferate at Site(s) of Infection in the Host
Excretion of the Agent from the Host
Quantity and "Quality" (including virulence) of the Infectious

14. Factors Influencing Exposure and Infection: Environmental Factors
Reservoirs: where organisms can live, accumulate or persist outside of the host of interest; could be another organism or the inanimate environment.
Vehicles: inanimate objects/materials by which organisms get from one host to another; includes water, food, objects (called fomites) and biological products (e.g., blood).
Amplifiers: Types of reservoirs where organisms proliferate; often applied to organisms transmitted by the airborne route.
Vectors: Living organisms bringing infectious organisms to a host.
Mechanical vectors: Microbes do not multiply in the vector
ex: biting insects infected with the infectious organism
Biological vectors: Microbes must propagate in the vector before they can be transmitted to a host.

15. Environmental Factors Influencing Survival or Proliferation of Infectious Agents
Physical: temperature, relative humidity, sunlight, moisture content or water activity, climate and weather, etc.
Chemical and Nutritional: Antimicrobial chemicals, nutrients for microbial proliferation.
Biological: Antagonistic activity by other organisms: antimicrobial agents, parasitism, etc.; presence and state of a vector

16. Factors Influencing Exposure and Infection: Host Factors and Host Susceptibility
Opportunities for host exposure
transmission routes
host availability
Susceptibility factors
Dosage (quantity) and "quality" of infectious organisms, including their "virulence";
age
immunity
nutritional status
immunocompetence and health status,
genetics
behavior (personal habits) of host.

17. Infectivity of Pathogenic Microorganisms and Risk of Infection, Illness and Death
Illness
 Sequelae
Death
Secondary Spread

18. Dose-Response and Infectious Dose (ID)
Probability of infection is dose-dependent
Higher dose → higher probability of infection/illness; dose-response relationship
Microbes differ in infectivity
Enteric and respiratory viruses: infectious at very low doses
1 cell culture ID50 has high probability of infecting an exposed human (~1 HID50)
Norovirus HID50 is about 1 virion
Most enteric bacteria: infective at moderate (10s-100s cells) to high (1,000 cells) doses
Protozoa: can be infective at low doses
ID50 = 1-10 cysts of Giardia lamblia or oocysts of Cryptosporidium parvum

19. Outcomes of Infection
Microbes differ in their ability to produce the different outcomes of infection:
(i) infection without illness;
(ii) infection with illness; (with or without long-term sequelae) and
(iii) infection, illness and then death

20. The Iceberg Concept: As Applied to Virus Infections

21. Transmission Dynamics of Infectious Diseases: Host States in Relation to Pathogen Transmission
Pathogen Exposure
Susceptible
Infected
Resistant λ3 λ1 λ2
λ = the rate or probability of movement from one state to another

22. Mortality Rates for Different Viruses in Healthy, Immunocompetent Humans (Rates Higher in the Immunocompromised)
VIRUSES: % Mortality
Adenovirus %
Enteroviruses (average)
Coxsackievirus B
Echoviruses
Hepatitis A virus 0.3
Norwalk virus
Rotavirus

23. Mortality Rates for Different Pathogens in Healthy, Immunocompetent Humans (Rates Higher in the Immunocompromised)
BACTERIA: % Mortality
Campylobacter jejuni 0.1
E. coli
Salmonella spp
Shigella spp
PARASITES:
Giardia lamblia
Entamoeba histolytica 0.3

24. Transmission Categories of Water-Related Diseases
Water-borne
Water-washed
Water-based
Water-related/Insect vector-borne

25. Waterborne
Caused by ingestion of water contaminated by human or animal feces or urine containing pathogenic bacteria or viruses
Mostly enteric diseases transmitted by the fecal-oral route
Bacterial: cholera, typhoid, amoebic and bacillary dysentery diseases
Viral: Infectious hepatitis
Protozoan parasitic: amoebic dysentery 
Some are due to organisms NOT fecally associated that proliferate in water
example: Legionellosis (Legionella bacteria) via aerosols and droplets

26. Water-washed or Water Hygiene Diseases
Caused by poor personal hygiene and skin or eye contact with contaminated water
Diseases whose exposure is reduced by the use of water for personal and domestic hygiene:
washing: clothes, floors, other household chores
bathing and other personal hygiene
cleaning of cooking and eating utensils
Includes:
many enteric organisms
diseases of the skin and eyes (ex: trachoma)
insect infestations
Scabies caused by mites
Pediculosis caused by lice
Tick-borne diseases

27. Water-based
Caused by parasites found in intermediate organisms living in contaminated water
Exposure by skin contact with infested water
Schistosomiasis
free‑living larvae released from aquatic snails (the intermediate host) invade the skin
Dracunculiasis (Guinea Worm disease)
Other helminths 

28. Water-related, Insect Vector-borne
Caused by insect vectors, especially mosquitoes, that breed in water; water habitat "insect vector" diseases
Insect vectors breed in or near water
Examples:
Dengue (virus)
Filariasis (nematode worms)
Malaria (protozoan)
Onchocerciasis; river blindness (filarial worm)
Trypanosomiasis (parasite)
Yellow Fever (virus) 

29. Analytical Epidemiology-Types of Studies
Descriptive studies
Intended to describe the distribution of cases of disease in time, place and person
Descriptive studies used in WSH:
Ecological study
Time series study
Analytical studies
Case control
cohort type
In both, individuals/groups are compared on the basis of something, often a risk or risk factor
Intervention studies
experimental studies to observe impacts of a certain intervention (introduced change in/on people & populations) on the risk of illness
Example WSH intervention: POU-HH water treatment

30. Types of Epidemiological Studies: Ecological
Description:
Determines relationship between disease and risk factors
Compares incidence of disease in different communities with varying exposure to risk factors
Advantages/Disadvantages:
Relatively inexpensive to do if data are available on disease rates and risk factors
Data available only for groups, so not known if individuals with disease are exposed to risk factor.
Good for hypotheses generation; can not be used as evidence of epidemiological proof

31. Types of Epidemiological Studies: Time Series
Description:
Determines relationship between disease incidence in population and variation in a risk factor over time.
A kind of ecological study
Advantages/Disadvantages:
As a kind of ecological study, with the same advantages and disadvantages

32. Types of Epidemiological Studies: Case-Control
Description:
Determines the relationship between disease and risk factors
Compares disease incidence in exposed individuals to matched controls
Advantages/Disadvantages:
Relatively inexpensive to carry out
Generates data on individuals exposed to the risk factors in comparison with healthy individuals
Easy to compare diseased and healthy individuals in relation to possible risk factors

33. Retrospective Case-Control Studies
Used to determine if a particular personal characteristic or environmental factor is related to disease occurrence.
Cases: persons who have a specific illness or disease.
Controls, those who do not have the illness or disease
Select both.
Selection may seek to "match" for other variables, such as age, race, sex, wealth, etc.
Query cases and controls to determine if their exposure to environmental hazards have been similar or different
Determine if “cases” were more likely to have been exposed to something that “controls” were not, and that this exposure is consistent with the health effect observed in the cases.
Foodborne outbreak investigations rely on case-control methods
Compute food-specific attack rates for a suspected foodborne illness

34. Retrospective Case-Control Studies
Useful in disease outbreaks where it is possible to determine if certain activities or exposures were related to the disease or illness under investigation.
Example, cases of cholera and their matched controls are asked about their past activity with respect to food consumption, drinking water and swimming events
Results of questioning may show that consuming a certain drinking water source is more likely to have occurred with cholera cases than with controls,
This indicates a potential association between drinking water and the disease.
John Snow’s investigation of cholera in London was partly a case-control study. (It was an intervention study, too - he took off the pump handle and cholera cases stopped)
Snow on Cholera - (If you have not read this, you really must do that soon!)
The linkage between disease and exposure can be determined, but it is seldom possible to determine the magnitude of the exposure.

35. Types of Epidemiological Studies: Cohort
Description:
Compares disease rate in two, or more, populations with different levels of exposure over a specific time period on randomly selected individuals
Advantages/Disadvantages:
Relatively expensive
Generates risk factor data in populations by comparing two or more groups of randomly selected individuals

36. Types of Epidemiological Studies: Prospective Cohort Applied Recreational Water Exposure Illness Risks
Recruit people immediately before or, more commonly, after, participating in recreational water activity (exposure)
Recruit a control group similarly
Follow both cohorts for a period of time, T
he exposure status of the bath.
Acquire data on illness symptoms experienced by the two cohorts
Use questionnaire interviews, in person or by means of telephone
Quantify recreational water quality on the day of exposure
Quantify a relationship between "exposure day" water quality and resulting disease in an exposure-response curve predicting illness from a measure of water quality
Enterococcus spp. bacteria concentration in water relative to risks of GI illness is swimmers (compared/normalized to non-swimmers)

37. Types of Epidemiological Studies: Intervention
Description:
Compares disease rates in two or more groups (cohorts) of randomly chosen individuals after intervening to change the exposure level
Advantages/disadvantages:
Gold standard for epidemiological proof
Can be time consuming and costly
Less costly in developing countries where disease burdens are high and a single type of WSH intervention can be studied for small cohorts

38. Intervention Study: Effect of Household Water Chlorination on Diarrhea Disease Risk
Sobsey,M.D., T. Handzel and L. Venczel (2004) Chlorination and safe storage of household drinking water in developing countries to reduce waterborne disease. Wat. Sci. Tech, 47(3):