Epidemiology and WSH Infectious Diseases

Lecture 4

ENVR 890-2

Mark D. Sobsey

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.

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

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.

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.

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

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

Transmission Routes of Infectious Agents

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

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"

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

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

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.

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

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.

Infectivity of Pathogenic Microorganisms and

Risk of Infection, Illness and Death

Infection

Illness

Sequelae

Death

Secondary Spread

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

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

The Iceberg Concept: As Applied to Virus Infections

Transmission Dynamics of Infectious Diseases: Host States in Relation to Pathogen Transmission

Susceptible Infected Resistantλ1 λ2 λ3

Pathogen Exposure

λ = the rate or probability of movement from one state to another

Mortality Rates for Different Viruses in Healthy, Immunocompetent Humans

(Rates Higher in the Immunocompromised)

VIRUSES: % Mortality• Adenovirus 0.01%• Enteroviruses 0.001 (average)

– Coxsackievirus B 0.59-0.94– Echoviruses 0.28

• Hepatitis A virus 0.3• Norwalk virus 0.0001• Rotavirus 0.01

Mortality Rates for Different Pathogens in Healthy, Immunocompetent Humans

(Rates Higher in the Immunocompromised)

BACTERIA: % Mortality• Campylobacter jejuni 0.1• E. coli 0.2• Salmonella spp. 0.1• Shigella spp. 0.2

• PARASITES:• Giardia lamblia 0.0001• Entamoeba histolytica 0.3

Transmission Categories of Water-Related Diseases

• Water-borne

• Water-washed

• Water-based

• Water-related/Insect vector-borne

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

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

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 

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) 

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 factorIntervention 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

Types of Epidemiological Studies: EcologicalDescription:

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

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

Types of Epidemiological Studies: Case-ControlDescription:• 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

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

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.

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

Types of Epidemiological Studies: Prospective CohortApplied 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)

Types of Epidemiological Studies: InterventionDescription:• 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

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): 221-228