epidemiology and cycle of microbial diseases

Epidemiology and cycle of Microbial Diseases

Dr. Chhaya SawantShri C. B. Patel Research centre, Mumbai, India

Epidemiology means?

The science that evaluates patterns of disease occurrence, frequency, determinants,

distribution, and control of health and disease in a defined human population.

Epidemiology leads to the identification of causal and preventive factors in human disease.

Epidemiology Defined• “The study of epidemics”• Greek roots

epi = upon (as in “epidermis”)

demos = the people (as in demography)

ology = “to speak of”, “to study”• Modern definitions of epidemiology refer to

– distributions in populations (statistical)– determinants (pathophysiology, biology, chemistry,

psychology)– control of health problems (biological, social,

economic, political, administrative, legal)

Epidemiology

• Also referred to as “Medical Ecology”. Why?

- Study of a disease in its natural environment.

Study of the clinical aspects and ecological aspects of a given disease are important for the public health measures to control the diseases to be effective

The Black Plague

• 1330’s, outbreak of plague in China.

• Spread to Italy and the rest of Europe.

• 25 million people died in just under five

years between 1347 and 1352 – 1/3 of the

population of Europe.

Influenza – 1918

Killed 50 to 100 million people worldwide, in a single year.

The First Indications of Person-to-Person Spread of an Infectious Disease

Edward Jenner (1749-1823) - developed a vaccine against smallpox using cow pox (160 years before virus was identified)

In 1773 Charles White, an English surgeon and obstetrician, published his “Treatise on the Management of Pregnant and Lying-In Women” with Puerperal fever.

Puerperal fever is an acute febrile condition that can follow childbirth and is caused by streptococcal infection of the uterus and/or adjacent regions.

In 1795 Alexander Gordon, a Scottish obstetrician, published his “Treatise on the Epidemic Puerperal Fever of Aberdeen” which demonstrated for the first time the

contagiousness of the disease.

In 1843 Oliver Wendell Holmes, a noted physician and anatomist in the United States, published a paper entitled “On the Contagiousness of Puerperal Fever”.

Hungarian physician Ignaz Phillip Semmelweis (1847 and 1849 ), the first person to realize that a pathogen could be transmitted from one person to another.

The pioneer of antisepsis in obstetrics – Saviors of Mothers. 32 years before causal agent was discovered

John Snow (1813-1858) described the association between dirty water and cholera (44 years before vibrio was identified

Brief History of EpidemiologyClassical Nutritional Epidemiology

–James Lind (1716-1794) • conducted an experiment which showed that scurvy could

be treated and prevented with limes, lemons, and oranges– ascorbic acid was discovered 175 years later

–Joseph Goldberger (1874-1927)• identified that pellagra was not infectious but nutritional in

origin and could be prevented by increasing the amount of animal products in the diet and substituting oatmeal for corn grits

– niacin was discovered 10 years later

Objectives of Epidemiology in Emergencies

To identify the priority health problems in the affected community

To determine the extent of disease existing within a community

To identify the causes of disease and possible risk factors

To determine the priority health interventions

To determine the extent of damage and capacity of local infrastructure

To monitor health trends of the community

To evaluate the impact of health programs

Role of an Epidemiologist• Traces the spread of a disease in a population

- To identify its origin

- Mode of Transmission • With the help of Data Obtained from

- Clinical studies

- Disease reporting surveys

- Insurance questionnaires

- interviews with patients

This will help him define common factors that constitute a disease.

• Attack rate : The proportional number of cases developing in the population that was exposed to the infectious agent

• Communicable disease : An infectious disease that can be transmitted from one host to another

• Non-communicable disease : A disease that is not transmitted from one host to another

• Herd immunity : A phenomenon that occurs when a critical concentration of immune hosts prevents the spread of an infectious agent

• Incidence : The number of new cases of a disease in a population at risk during a specified period of time

• Index case : The first identified case of a disease in an outbreak or epidemic• Outbreak : A cluster of cases occurring during a brief time interval and affecting a

specific population; an outbreak may herald the onset of an epidemic• Portal of entry : Surface or orifice through which a disease-causing agent enters the

body• Portal of exit : Surface or orifice from which a disease-causing agent exits and

disseminates• Prevalence : The total number of cases in a given population at risk at any point in

time• Reservoir : The natural habitat of a disease-causing organism

Important Terms

Infectious disease: the unique factor

Historical approach : Infectious diseases can be spread from human to human

(or animal to human)

Modern Approach : Alcohol and drug abuse, cancer, mental conditions, acts of

violence and exposure to lead paint.

Cycle of Microbial Disease

Portal of exit

Portal of entry

Agent

Susceptible Host

Mode of transmission

ReservoirPerson to person transmission

Chain of transmissionReservoir

Human

Person with symptomatic illness

Carriers:

Asymptomatic

Incubating

Convalescent

Chronic

Animal: zoonosis

Environmental: soil, plant, water

Chain of transmission

Human/animal

Respiratory tract

Genito-Urinary tract

Faeces

Saliva

Skin (exanthema, cuts, needles, blood-sucking arthropods)

Conjunctival secretions

Placenta

Environmental

Cooling towers

Portal of exit


Direct

Direct contact

Secretions, Blood, Faeces/urine

Droplet spread

Indirect

Food/water

Aerosol

Animal vectors

Fomites

Medical devices and treatments

Mode of Transmission

Transmission• Transmission of a disease from its reservoir to the

next susceptible host. • Through contact, ingestion of food or water, or via a

living agent such as an insect is called horizontal transmission.

• Vertical transmission, the transfer of a pathogen from a pregnant woman to the fetus, or from a mother to her infant during childbirth.

e.g. Congenital syphilis, Group B streptococci, can infect the newborn as it passes through the birth canal

Human/animal

Respiratory tract

Genito-Urinary tract

Faeces

Saliva

Skin (exanthema, cuts, needles, blood-sucking arthropods)

Conjunctival secretions

Placenta

Portal of entry


Frequency and distribution based Classification

• Sporadic level: occasional cases occurring at irregular intervals

• Endemic level: persistent occurrence with a low to moderate level

• Hyper Endemic level: persistently high level of occurrence

• Epidemic : occurrence clearly in excess of the expected level for a given time period

• Pandemic: epidemic spread over several countries or continents, affecting a large number of people

A Graph Illustrating Three Epidemics The solid blue line indicates the expected number of endemic cases. The connected red dots indicate the actual number of cases. Epidemics (marked by brackets) are sharp increases in the number of cases of a disease above that which is normally expected (solid line).

Disease Progression• Infection • Incubation period • Prodromal stage • Period of illness • Convalescence

Factors that Influence the Epidemiology of Disease

• The Dose• The Incubation Period• Population Characteristics

- Immunity to the pathogen.

- General health.

- Age.

- Gender

- Religious and cultural practices.

- Genetic background.

Measuring Frequency

• Three important statistical measures of disease frequency

– morbidity rate– prevalence rate– mortality rate

Morbidity rate :

Measures the number of individuals that become ill due to a specific disease within a susceptible population during a specific time interval.

Morbidity rate =Number of new cases during a specific time

Total number of individuals in population

• E.g. 700 new cases of influenza per 100,000 individuals is 0.7%.

Prevalence Rate

The total number of individuals infected in a population at any one time no matter when the disease began

• Mortality rateNumber of deaths from a disease per number of cases of the disease

• Mortality rate =Number of deaths due to given disease

size of total population with disease

If 500 people in a town of 100,000 become infected with HIV and 100 die, the mortality rate is…

Recognition of an Infectious Disease in a Population

1. Generation of morbidity data from case reports

2. Collection of mortality data from death certificates

3. Investigation of actual cases

4. Collection of data from reported epidemics

5. Field investigation of epidemics

6. Review of laboratory results: surveys of a population for antibodies against the agent and specific microbial serotypes, skin tests, cultures, stool analyses, etc.

7. Population surveys using valid statistical sampling to determine who has the disease

8. Use of animal and vector disease data

9. Collection of information on the use of specific biologics—antibiotics, antitoxins, vaccines, and other prophylactic measures

10. Use of demographic data on population characteristics such as human movements during a specific time of the year

11. Use of remote sensing and geographic information systems

• Recognition of an Epidemic• Two types of epidemics

– common source epidemic– propagated epidemic

• Common source epidemic• Reaches a peak within a short period of time -1 to 2 weeks)• A moderately rapid decline in the number of infected patients • A single common contaminated source - food (food poisoning)

or water (Legionnaires’ disease).

• Propagated epidemic (Host to Host)• A relatively slow and prolonged rise and then a gradual decline

in the number of individuals infected. • A single infected individual into a susceptible population. • E.g. Increase in mumps or chickenpox cases, Spread of HIV

infection.

Seasonal Cycles of diseases

• The season of the year in which the epidemic occurs may also be significant.

• Respiratory diseases including Influenza, Respiratory Syncytial

Virus infections, and the common cold are more easily transmitted in crowded indoor conditions during the winter.

• Conversely, vector- and food-borne diseases are more often transmitted in warm weather when people are more likely to be exposed to mosquitoes and ticks, or eating picnic food that has not been stored properly.

Seasonal Cycles of diseases

Seasonal Occurrence of Respiratory Infections Caused by

Respiratory Syncytial Virus

Seasonal Occurrence of Gastrointestinal Diseases

Herd Immunity. Resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population

The kinetics of the spread of an infectious disease and the effect of increasing the number of immune individuals in the population in limiting the disease.

On day 1, a single infected individual enters the population.

The incubation period is 1 day, and recovery occurs in 2 days.

The number of susceptible individuals is the total populationon day 1.

Diagrammatic representation of the spread of an imaginary propagated epidemic. The lower curve represents the number of cases and the upper curve the number of susceptible individuals. Notice the coincidence of the peak of the epidemic wavewith the threshold density of susceptible individuals.

The number of infected and recovered are illustrated in the two graphs.

Herd immunity• Resistance of a population to infection and to spread

of an infectious organism because of the immunity of a large percentage of the population

• Level can be altered by introduction of new susceptible individuals into population

• Level can be altered by changes in pathogen– antigenic shift – major change in antigenic character of

pathogen (recombination in birds, pigs totally new antigen types)

– antigenic drift – smaller antigenic changes (point-mutational changes)

Virulence and the Mode of TransmissionEvidence suggests correlation between mode of transmission and degree of virulence– direct contact less virulent– vector-borne highly virulent in human host;

relatively benign in vector– greater ability to survive outside host more virulent

The host community:• Infection --- immunity --- coexistence• Host-pathogen• co evolution: the case of the rabbit-myxoma virus

– Transmitted by mosquitoes

– Rabbit mortality:

susceptibility of new born rabbits to a moderately virulent strain of the virus

• – Virus virulence: infection of lab rabbits with wild virus

1950 - 1980: Pre introduction levels of rabbit

infestation Introduction of a second virus

(RHDV; host-to-host transmission; instant

killing) --- co evolution.

Emerging and Reemerging Infectious Diseases and Pathogens

By the 1990s, the idea that infectious diseases no longer posed a serious threat to human health was obsolete.

It is now clear that globally, humans will continually be faced with both new infectious diseases and the reemergence of older diseases once thought to be conquered (e.g., tuberculosis, dengue hemorrhagic fever, yellow fever)

Emerging and Reemerging Infectious Diseases and Pathogens

Infectious Disease Mortality in the United States Decreased Greatly during Most of the Twentieth Century.

The insert is an enlargement of the right-hand portion of the graph and shows that the death rate from infectious diseases increased between 1980 and 1994.

Systematic epidemiology

The increased importance of emerging and reemerging infectious diseases has stimulated the establishment of a field called

systematic epidemiology

Which focuses on the ecological and social factors that influence the development of these diseases.

Factors characteristic of the modern world favoring the development and spread of these microorganisms and their

diseases

1. Unprecedented worldwide population growth, population shifts (demographics), and urbanization

2. Increased international travel

3. Increased worldwide transport (commerce), migration, and relocation of animals and food products

4. Changes in food processing, handling, and agricultural practices

5. Changes in human behavior, technology, and industry

6. Human encroachment on wilderness habitats that are reservoirs for insects and animals that harbor infectious agents

7. Microbial evolution (e.g., selection pressure) and the development of resistance to antibiotics and other antimicrobial drugs (e.g., penicillin-resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant enterococci)

Factors characteristic of the modern world favoring the development and spread of these microorganisms and their

diseases

8. Changes in ecology and climate

9. Modern medicine (e.g., immunosuppression)

10. Inadequacy of public infrastructure and vaccination programs

11. Social unrest and civil wars

12. The possibility of bioterrorism

13. Virulence-enhancing mechanisms of pathogens (e.g., the mobile genetic elements—bacteriophages, plasmids, transposons)

Some Examples of Emerging and Reemerging Infectious Diseases. Although diseases such as HIV are indicated in only one or two significant locations, they are very widespread and a threat in many regions.

Public Health System

The public health system comprises of a

network of clinical microbiologists, nurses,

physicians, epidemiologists, and infection control

personnel who supply epidemiological

information to a network of local, state, national,

and international organizations.

The Role of the Public Health System

• Control of Epidemics (Three Types)• First Type : Directed toward reducing or

eliminating the source or reservoir of infection

1. Quarantine and isolation of cases and/or carriers

2. Destruction of an animal reservoir of infection

3. Treatment of sewage to reduce water contamination

4. Therapy that reduces or eliminates infectivity of the

individual

Control of Epidemics (Three Types)

• The second type : designed to break the connection between the source of the infection and susceptible individuals. (general sanitation measures - Transmission)

1. Chlorination of water supplies

2. Pasteurization of milk

3. Supervision and inspection of food and food handlers

4. Destruction of vectors by spraying with insecticides

Control of Epidemics (Three Types)

• The Third type : reduces the number of susceptible individuals and raises the general level of herd immunity by immunization.

1. Passive immunization to give a temporary immunity following exposure to a pathogen or when a disease threatens to take an epidemic form

2. Active immunization to protect the individual from the pathogen and the host population from the epidemic

The most important is Surveillance – Observation, recognition and reporting of Diseases as they occur.

Precautions to be taken by individuals to prevent travel-related infectious diseases.

1. If one is traveling to an area where malaria is endemic, weekly prophylaxis before entering the area and after leaving the area is recommended.

2. Travelers should recall the benefits of abstinence or protective sexual practices, especially the use of condoms. Hepatitis B vaccine should be administered if it is indicated.

3. Travelers should avoid uncooked food, non bottled water and beverages, and unpasteurized dairy products. Use bottled water for drinking, making ice cubes, and brushing teeth.

4. Wash hands with soap and water frequently, especially before each meal.

Precautions to be taken by individuals to prevent travel-related infectious diseases.

5. To prevent respiratory infections, avoid excessive outdoor activities in areas of heavy air pollution during hot or humid parts of the day. Consider tuberculin skin testing before and after travel.

6. Minimize skin exposure and use repellents to prevent arthropod-borne illnesses (e.g., malaria, dengue, yellow fever, Japanese encephalitis).

7. Avoid skin-perforating procedures (e.g., acupuncture, body piercing, tattooing, venipuncture, sharing of razors).

8. Do not pet or feed animals, especially dogs and monkeys.

9. Avoid swimming or wading in non-chlorinated freshwater.

Nosocomial Infections• Nosocomial diseases - caused by bacteria, most of which are

noninvasive and part of the normal microbiota; • Viruses, protozoa, and fungi are rarely involved.

• Source of Hospital Infection• Endogenous sources - Patient’s own microbiota. • Exogenous sources are other than the patient’s own

microbiota .

• In either case the pathogen colonizing the patient may subsequently cause a nosocomial disease.

The Hospital Epidemiologist

• The services provided by the hospital epidemiologist should include

1. Research in infection control 2. Evaluation of disinfectants, rapid test systems, and other products3. Efforts to encourage appropriate legislation related to infection control,

particularly at the state level4. Efforts to contain hospital operating costs, especially those related to fixed

expenses such as the DRGs (diagnosis related groups)5. Surveillance and comparison of endemic and epidemic infection frequencies6. Direct participation in a variety of hospital activities relating to infection

control and maintenance of employee health7. Education of hospital personnel in communicable disease control and

disinfection and sterilization procedures8. Establishment and maintenance of a system for identifying, reporting,

investigating, and controlling infections and communicable diseases of patients and hospital personnel

9. Maintenance of a log of incidents related to infections and communicable diseases

10. Monitoring trends in the antimicrobial drug resistance of infectious agents

The Emerging Threat of Bioterrorism

• The Centers for Disease Control and Prevention recently defined bioterrorism as

“The intentional or threatened use of viruses, bacteria, fungi, or toxins from living organisms to produce death or disease in humans, animals, and plants.”

• a few kilograms of anthrax can kill as many people as a Hiroshima-size nuclear bomb

The goal of bioterrorism is to produce fear in the population with subsequent disruption of

society

History of Bioterrorism

• Biological warfare (BW) employed as far back as 6th century BC.

• Examples of past BW:14th Century: Mongols catapulted corpses with bubonic plague over walls into Crimea.

15th Century: Pizarro presented native South Americans with smallpox-contaminated clothing.

1940: Japan’s “Unit 731” dropped plague-infected fleas over Manchuria & China.

1984 : Rajneeshee Cult contaminated restaurant salad bars with Salmonella typhimurium.

1995: Aum Shinrikyo cult attempted unsuccessfully to disperse BW agents in aerosol form; sarin gas attack in Tokyo.

2001: Anthrax-contaminated letters to U.S. media and government offices.

• In 1998, the U.S. government launched the first national effort to create a biological weapons defense. The initiatives include

• The first ever procurement of specialized vaccines and medicines for a national civilian protection stockpile;

• investment of more time and money in genome sequencing, new vaccine research, and new therapeutic research;

• development of improved detection and diagnostic systems;

• preparation of clinical microbiologists and the clinical microbiology laboratory as members of the “first responder” team, which is to respond in a timely manner to acts of bioterrorism.

Preventions: International treaties, strategic

preparedness (vaccines), make this world a better place for everyone

• Awareness• Laboratory Preparedness• Plan in place• Individual & collective protection• Detection & characterization• Emergency response• Measures to Protect the Public’s Health and Safety• Treatment• Safe practices

Future Challenges to Public Health

• Emergence of new infectious diseases and re-emergence of old ones.

• Inequities in health care.

• Threat of large scale terrorist attacks using nuclear, biological and chemical agents.

• Chronic diseases – coronary heart disease, obesity, diabetes, cancer.

• Continued political instability.

• International coordination of public health efforts.

• Ethics.

• Ignorance.

Types of Epidemiological Studies Undertaken• Descriptive Studies – The Person, The Place, The Time,

• Analytical Studies

- Cross-Sectional Studies : A cross-sectional study surveys a range of people to determine the prevalence of any of a number of characteristics including disease, risk factors associated with disease, or previous exposure to a disease-causing agent.

- Retrospective Studies : A retrospective study is done following a disease outbreak. This type of study compares the actions and events surrounding clinical cases (individuals who developed the disease) against appropriate controls (those who remained healthy).

- Prospective Studies : A prospective study is one that looks ahead to see if the risk factors identified by the retrospective study predict a tendency to develop the disease. Cohort groups, which are study groups that have a known exposure to the risk factor, are selected and then followed over time.

• Experimental Studies

Investigation of a GI illness outbreak

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf



If you were to administer a questionnaire to the church supper participants, what information would you collect?

1. What did you eat?

2. How much did you eat?

3. How long after you ate did you begin to feel sick?

4. How long did it last?

5. Did anyone at the supper show illness prior to the supper?

6. What did you drink?

7. How was the food prepared/ stored?


Without having clinical isolates from the infected individuals for examination, how might you tentatively identify the causative agent?

1. Temperature of storage vs organism?

2. Food vs organism?

3. Homemade vs commercial?

4. Symptoms

5. Culture ice cream.

It the outbreak has already ended, what is the value of working up the case?

epidemiology and cycle of microbial diseases

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