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Chapter 1 Zoonotic disease history.


After completing this chapter, the learner should be able to

* List classic examples of zoonoses

* Describe how zoonoses became prevalent in human society

* Differentiate between biological and mechanical vectors

* Describe ways that zoonotic disease can be transmitted from animals to humans

* Differentiate between endemic, enzootic, sporadic, epidemic, epizootic, pandemic, and panzootic diseases

Key Terms

acute disease

biological vector

chronic disease



contagious disease

direct transmission

emerging zoonoses

endemic disease

enzootic disease

epidemic disease



epizootic disease





indirect transmission

infectious disease

latent disease

mechanical vector

morbidity rate

mortality rate



pandemic disease

panzootic disease



reverse zoonoses


source of infection

sporadic disease

subacute disease



vertical transmission



Animals, both domestic and wild, are subject to a great variety of diseases. Many of these diseases are confined to a particular animal species; however, some animal disease agents may cause human disease. Diseases that are naturally transmitted from animals to humans are termed zoonoses (singular is zoonosis). Zoonotic infections are indigenous, or native, to a particular animal and they may or may not produce clinical illness in the animal.
Zoonoses are diseases that are naturally
transmitted from animals to humans.
The term reverse zoonoses or anthroponoses
refer to diseases naturally
transmitted from humans to animals.

More than 150 zoonoses are known, but fewer than half of them are clinically significant. Classic examples of zoonoses are the plague, also known as "The Black Death" during the Middle Ages, and tuberculosis, also known as consumption because this disease consumes one's entire body. In addition to well-established zoonoses, new zoonoses continue to be discovered including Lyme disease (discovered in the late 1970s) and hantavirus pulmonary syndrome (discovered in the United States in 1993). The West Nile virus outbreak in New York City in the summer of 1999 infected a large number of birds (mostly crows) and humans who developed flu-like symptoms from the bite of virus-infected mosquitoes.

Emerging zoonoses are zoonotic diseases caused either by apparently new agents or by previously known agents appearing in places or in species in which the disease was previously unknown. For example, closely-related strains of some viruses may mutate into "new" viruses that cause human disease. Such is the case with influenza, a virus that causes human respiratory disease. Closely-related strains of the same virus infect birds, horses, and pigs. When these viral strains replicate in different animal hosts, different selective pressures may favor different mutant forms of the virus. When diverse strains of a virus infect the same animal, genetic recombination can produce a "new" virus that might have enhanced virulence (or ability to cause disease) for humans. Scientists have found that a single gene may have been responsible for the devastating virulence of the virus that caused the 1918 Spanish influenza outbreak that killed an estimated 20 to 40 million people. Public health professionals continue to be concerned about the role that existing and emerging zoonoses play on the human and animal populations.


To understand animal origins of human disease the evolution of infectious organisms needs to be considered. Diseases that have infected humans can be traced to our ancient ancestors. These diseases followed humans through the evolutionary process and continue to infect humans today. Prior to the agricultural revolution (about 10,000 years ago) humans lived in small, hunter-gatherer groups. These groups moved frequently and were isolated from neighboring groups of people. Their diets consisted mainly of plants and small amounts of animal protein. The nomadic nature of these people limited their accumulation of sewage and garbage that in turn limited the number of rodents and the number of disease-causing organisms to which hunter-gatherers were exposed.

When people altered their lifestyles with the domestication of plants and animals, the spread of disease became easier. Food production and the ability to store food for later use allowed people to cease roaming and to live in communities. The domestication of animals by 6000 B.C. resulted in the close contact of animals and people. Crowded barnyards contained animal feed and animal waste, which were optimal environments for insects and rodents that in turn lead to fruitful places for the breeding and transmission (transfer) of diseases. These barnyards were the first breeding grounds for diseases such as rinderpest and cowpox in cattle as well as sources of disease transmission such as tuberculosis from consumption of unpasteurized milk.

As surpluses in agricultural products such as grain and meat began to accumulate, worldwide trade routes developed. Worldwide trade routes became avenues for the spread of disease. Diseases prevalent in Europe, such as the plague, were spread to Asia by the transit of flea-infested materials. When Christopher Columbus reached the New World his men brought diseases such as smallpox and measles to the native people of the West Indies and the Americas. The spread of pathogenic organisms no longer depends upon explorers searching for new territories to conquer. International flights, transcontinental railroads, and other modes of international travel allow tourists, business people, military personnel, and refugees to potentially carry diseases from one locale to another. The shipment of goods via ships and trucks can transport rodents and insects carrying disease-causing organisms from one country to the next. International trade of laboratory animals, pets, and livestock also creates the potential for pathogenic organisms to travel from one place to another.

With this background, the story of zoonotic disease and its impact on animal and human disease unfolds. The importance of understanding how animals play a role in the transfer of pathogens, how they serve as a source of infection for people, how they harbor microbes in the environment, and how they can serve as a disease sentinel (a domestic animal host for a particular disease that is placed at various locations to determine the potential for human exposure to a particular disease) are all key concepts to understanding zoonoses and their role in human health.


Infectious agents need a place to reside if they are going to survive and be spread within an environment. A reservoir is an animate or inanimate object that serves as a long-term habitat and focus of dissemination for an infectious agent. Animals can serve as reservoirs for certain human pathogens, although soil, water, and plants can also serve as reservoirs. A reservoir is not the same as a source of infection. A source of infection is an individual or object from which an infection is actually acquired. In some cases the reservoir and the source of infection are the same (as is the case with syphilis in which the reservoir and the source of infection are the human body) and in other cases the reservoir and the source of infection are different (as is the case with Cryptosporidium parvum in which the reservoir may be cattle and the source of infection is water).

Types of Reservoirs

Reservoirs may be living or nonliving. Living reservoirs include humans, animals, and arthropods (insects, arachnids, and crustaceans). Humans are the most important reservoirs of human infectious diseases. Infected humans may show symptoms of the disease and pass it to another human or they may harbor a microbe, but not show symptoms of disease; however, they have the ability to transfer the microbe to another human/animal (carrier).

Wild and domestic animals also serve as reservoirs of human disease. Animals provide an environment for microbes quite similar to the human body. For some zoonotic pathogens, several animal species can serve as a reservoir. For example, most mammals, including cats, dogs, bats, skunks, and raccoons, can be a reservoir for rabies. Humans become infected with the rabies virus when they come into contact with the saliva of a rabid animal, usually through an animal bite. Pathogens can also pass from animal reservoir to humans when people consume or handle contaminated animals or animal products as is the case with the consumption of pork contaminated with the worm Trichinella spiralis.

Arthropods can also transmit zoonotic diseases. Arthropods consist of insects (flies, fleas, and mosquitoes), arachnids (ticks, mites, and spiders), and crustaceans (crayfish and crabs). A classic example of an arthropod spreading disease is biting insects that have previously bitten an infected animal passing along a zoonotic disease. Yersinia pestis, the bacterium that caused the plague, was harbored in rodents, but was transmitted to humans by fleabites. When arthropods transmit disease they are referred to as vectors. The term vector is any live animal that transmits an infectious agent from one host to the next; however, this term is typically used to describe arthropods. Vectors are placed into two categories: biological and mechanical.
A vector is an organism that does not
cause disease itself but which spreads
infection by conveying pathogens from
one host to another.

* Biological vectors actively participate in a pathogen's life cycle, serving as a place where the pathogen multiplies or completes its life cycle. Biological vectors spread infectious agents to its host by biting (injecting infected saliva into blood, defecating around the bite wound, or regurgitating blood into the bite wound), aerosol formation, or touch. Mosquitoes, fleas, ticks, and tsetse flies are good examples of biological vectors.

* Mechanical vectors are not necessary to the life cycle of the pathogen and are passive participants in the transmission of disease. Mechanical vectors may spread disease when their external body parts become contaminated through contact with the pathogen. The pathogen is subsequently transferred to the human or animal indirectly by an intermediate such as contaminated food or directly by contact between the contaminated body part and a mucous membrane or skin surface. Houseflies and cockroaches are good examples of mechanical vectors.

Nonliving reservoirs include air, soil, dust, food, milk, water, and fomites (objects that are able to transfer disease organisms). Fomites (fomes and fomite are singular forms of the term) are also referred to as vehicles. Direct contact with a zoonotic microbe by way of a nonliving reservoir can transmit animal diseases to humans. Consider the transmission of Toxoplasma gondii by the handling of cat feces in a litterbox. Contaminated water, such as that containing the protozoan Cryptosporidium parvum, is another route of zoonotic disease transmission via a nonliving reservoir. Air contaminated with Bacillus anthracis endospores from animal hides serves as another example of transmission of zoonoses to humans by nonliving reservoirs.

The Effect of Animal Reservoirs

An animal reservoir greatly affects the pattern of disease. New forms of a disease can develop in an animal reservoir. Animal reservoirs can make diseases harder to control. An illustration of how animal reservoirs complicate disease eradication is seen with yellow fever. A virus that is spread through mosquito bites causes yellow fever. Yellow fever ravaged the continents of North America, South America, and Africa until it was discovered that mosquitoes transmitted the virus. Knowing how the disease was spread in these continents allowed for its eradication through the establishment of mosquito control programs. The same approach to eradication of yellow fever in Panama failed in Central America during the construction of the Panama Canal because the virus was found in another animal reservoir, the monkey. Monkeys in the jungle surrounding the Panama Canal site constituted an immense animal reservoir population that made eradication of yellow fever impossible for this region.


The routes of disease transmission are many and varied. Zoonotic pathogens can be transmitted between animals and humans by either direct or indirect transmission (Figure 1-1). Direct transmission is the immediate transfer of an agent from a reservoir to a susceptible host. Indirect transmission is the transfer of an infectious agent carried from a reservoir to a susceptible host. Disease transmission can be further classified as horizontal or vertical. Horizontal transmission is the spread of disease through a population from one infected individual to another. Horizontal transmission can be either direct or indirect. Vertical transmission means the disease is spread from parent to offspring via the placenta, sperm, milk, or ovum. Vertical transmission from parent to offspring is always direct; however, a pregnant woman can contract a zoonotic disease directly from an animal then pass that disease to the fetus vertically (indirect transmission from the original animal; however, the disease is spread directly from mother to fetus). Figure 1-1 summarizes the types of zoonotic disease transmission and provides examples of each type.


Direct and indirect transmission of zoonotic disease can occur by a variety of mechanisms. These mechanisms include contact transmission, airborne transmission, placental transmission, fomite transmission, and arthropod transmission:

* Contact transmission occurs through touch and may be either direct or indirect in its presentation. For example, a human touching the skin lesions of a ringworm-positive cat frequently transmits ringworm. If the person touching the cat's lesions develops ringworm lesions, it is an example of direct contact transmission. An example of indirect contact transmission occurs when a person touches the skin of a ringworm-positive cat then touches another cat transferring the fungal spores to the second cat that then contracts ringworm (the person serves as an intermediary). Contamination from an animal carcass is another example of contact transmission.

* Airborne droplets of respiratory secretions may contain pathogens and have the ability to spread disease via aerosol. Airborne transmission can be direct via a sneeze or cough from the infected animal to an uninfected animal. Newcastle disease, a viral infection of poultry, is spread by inhalation of infectious aerosols. Airborne transmission can also be indirect via a sneeze that contaminates a ventilation system that eventually infects an animal/human. Inhalation of Legionella pneumophila bacteria that have contaminated air-handling systems is an example of indirect airborne transmission.

* Placental transmission occurs from mother to offspring and is a form of direct transmission. The placenta is an organ formed by maternal and fetal tissue that separates the blood of the fetus from the mother, yet permits diffusion of dissolved nutrients and gases to the fetus. Because the fetal and maternal circulation is separated to some degree, the placenta is a relatively effective barrier against microbes entering the fetal circulation from the mother's circulation. However, some microbes can cross the placenta, enter the umbilical vein, and spread by fetal circulation to fetal tissue. The result of microbes crossing the placenta depends upon the gestation stage and the microbe involved. Serious neurologic complications can occur with placental transmission of zoonotic organisms such as Listeria monocytogenes and Toxoplasma gondii in which the mother contracted the disease from an animal and placentally transmitted it to her fetus.

* Fomites (or vehicles) such as animal bedding that have become contaminated by blood, saliva, urine, feces, vomit, exudates, respiratory secretions, or milk may be a source of indirect transmission of infectious agents. Bedding contaminated with Escherichia coli 0157:H7 can be a route of indirect fomite transmission. Fecal-contaminated food and water and contaminated blood products or parenteral injections (substances given directly into the bloodstream) are other examples of indirect fomite transmission of organisms.

* Arthropods such as fleas, mosquitoes, lice, ticks, mites, and flies can indirectly transmit infectious agents. A variety of viral encephalitis diseases such as West Nile encephalitis and Eastern equine encephalitis are spread via mosquitoes and are examples of indirect arthropod transmission (also called vector transmission).


Every disease, whether zoonotic or not, can also be classified based on its severity and longevity. Acute diseases are diseases that develop rapidly but last only a short time. The common cold in humans is an example of an acute disease. Chronic diseases are diseases that develop slowly, usually with less severe clinical signs, and are continual or recurrent. Tuberculosis and leprosy are examples of chronic diseases. Subacute diseases have severities and durations somewhere between acute and chronic diseases (they do not come on as rapidly as acute diseases and do not persist as long as chronic diseases). Examples of subacute diseases are cytomegalovirus in people and some forms of anthrax in animals. Latent diseases are diseases in which a pathogen remains inactive for long periods of time before becoming active as is the case with herpes viral infections of animals and humans.

Diseases can also be classified based on their degree of infectivity. Infectious disease is a disease that is acquired from an infected host. Infectious diseases are also known as communicable diseases. Examples of communicable diseases are tuberculosis and influenza. Easily transmitted communicable diseases are known as contagious diseases as is the case with chickenpox in people. Noncommunicable diseases are not spread from one host to another and diseased individuals do not serve as a source of contamination for others. Tooth decay and tetanus are examples of noncommunicable diseases.


The effects of diseases on a community involve the field of epidemiology. An epidemiologist studies the factors that determine the frequency and distribution of disease within populations. Some factors epidemiologists study include microbe virulence, portals of microbe entry and exit, and the course of disease. Epidemiologists are also concerned about disease surveillance. Disease surveillance involves the statistical analysis (collecting, analyzing, and reporting) of data on the rates of disease prevalence, mortality, and morbidity. Table 1-1 defines statistical terms used in the field of epidemiology.
Epidemiologists are concerned with the
who, what, where, when, why, and how
of infectious disease.

The Frequency of Disease

Epidemiologists monitor statistics to determine the frequency of a particular disease in a given population. There are several stages of disease in a population: endemic, sporadic, epidemic, and pandemic. Endemic means native to the population; therefore, endemic diseases are diseases that are always present within a population of a particular geographic area. Enzootic diseases are conditions affecting animals of a specific geographic area. An enzootic disease is constantly present in a specific animal community, but only occurs in a small number of cases. In some parts of the United States, plague is enzootic in prairie dogs and rodents but is not endemic in people. Plague is only seen occasionally in humans in the United States. When there are a few isolated cases of a disease, such as plague in humans, seen in widespread areas in an unpredictable manner the disease is termed sporadic. Epidemic means visiting the population; therefore, an epidemic disease is a disease with a sudden onset and widespread outbreak within a group. When an epidemic disease enters a new population that has few defenses against it, the disease causes acute signs in all ages. In the early 1980s the annual incidence of measles in the United States was between 1,000 and 3,000 cases. By the late 1980s the number of cases of measles had reached nearly 25,000 and was considered to be an epidemic. Widespread disease in populations of animals other than humans is referred to as being epizootic. Epizootic diseases spread rapidly, simultaneously affecting a large number of animals in a region. Foot-and-mouth disease (FMD) is an example of a disease that can be epizootic. Pandemic means all the population; therefore, a pandemic disease is a disease that is a widespread epidemic and generally involves the spread across continents. An example of a pandemic disease is the influenza outbreak of 1918 in which 20 to 40 million people died worldwide, including 500,000 people in the United States. Panzootic disease is widespread epizootic disease. An example of a panzootic disease is the H5N1 avian influenza outbreak in birds that began in 2003 in parts of Southeast Asia and spread to birds in other parts of the world.


Public health agencies strive to prevent the spread of zoonotic disease and to identify and eliminate any that do occur. These agencies educate the public about how the diseases are transmitted and explain proper sanitation procedures, identify and attempt to eliminate reservoirs and sources of infection, carry out measures to isolate sick animals/humans, and help in the treatment of ill animals/people. These public health agencies have helped to eradicate smallpox and greatly reduce the incidence of poliomyelitis in many parts of the world. Examples of these agencies include:

* The World Health Organization (WHO) is a specialized agency within the United Nations. WHO was founded in 1948 and its missions are to promote cooperation for health care among nations, carry out disease control and eradication programs, and improve the quality of human/animal life. When epidemics occur, WHO sends out teams of epidemiologists to investigate the outbreak and to assist in bringing the outbreak under control. WHO works at the control (reducing the incidence and/or prevalence of a disease), elimination (reduction of case transmission to a predetermined very low level), and eradication (achieving a status where no further cases of that disease occur anywhere and where continued control measures are unnecessary) of diseases.

* The Centers for Disease Control and Prevention (CDC) is a United States federal agency under the control of the U.S. Department of Health and Human Services and its function is to assist state and local health departments in all aspects of epidemiology. The CDC was first established as the Communicable Disease Center in Atlanta, Georgia in 1946 and its focus was on communicable diseases (at that time malaria and typhus). The current role of the CDC is "to promote disease prevention and health promotion goals that will foster a safe and healthful environment where health is protected, nurtured, and promoted." One branch within the CDC is the National Center for Infectious Disease (NCID) and its mission is "to prevent illness, disability, and death caused by infectious disease in the United States and around the world." Some zoonotic diseases, known as nationally notifiable diseases, must be reported to the CDC and include examples such as salmonellosis, shigellosis, tuberculosis, and Lyme disease. The CDC also prepares a weekly publication entitled Morbidity and Mortality Weekly Report (MMWR), which contains information and statistics about infectious outbreaks in the United States and other parts of the world.

* The National Institutes of Health (NIH), a division of the U.S. Department of Health and Human Services, is the primary federal agency for conducting and supporting medical research. The NIH was founded in 1887 with the creation of the Laboratory of Hygiene at the Marine Hospital in Staten Island, New York. NIH publishes guidelines for the care of animals in research facilities as well as the prevention of disease transmission among animals and between animals and humans. NIH funds research on a variety of disease topics including zoonoses and ways to prevent their spread. NIH also educates the public and physicians about the role of zoonoses and human health through providing information such as the role of pets and the immunocompromised person.


It is imperative that veterinary professionals be aware of the different types of zoonotic disease and ways to prevent their spread. The Occupational Safety and Health Administration (OSHA), a division of the U.S. Department of Labor, was developed to "make the place of employment free from recognized hazards that are causing or are likely to cause death or serious physical harm." The most common injury among workers at veterinary clinics is animal bites that occur when trying to restrain animals. Animal bites can transmit zoonoses; therefore, proper technique when handling animals is critical.

Contact with blood, stool, laboratory cultures, and infected animals may all serve as zoonotic disease sources. OSHA requires each workplace to conduct a hazard assessment for each job to determine the level of protective equipment and training required for each task. Examination gloves, masks, safety goggles, aprons, work shoes or boots, eyewash stations, and other protective equipment may be needed when handling certain specimens. Specific safety equipment needed for a particular zoonotic disease will be covered under that section.

Veterinary professionals are also required to report diseases that may affect human public health. Being able to recognize the transmission of zoonoses and to identify means of prevention as well as the education about zoonoses to the public are important functions of the veterinary staff. Both federal and state laws regulate which diseases are reportable and the government agency to which they must be reported. At the federal level, the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) requires licensed veterinary personnel to report diseases that pose a significant threat to human health. Similar guidelines may exist at the state level depending upon the state involved. Individual state guidelines are available through the state's Department of Food and Agriculture or similar agency.

Review Questions

Multiple Choice

1. Zoonoses

a. have always been a problem for humans because of our close association with animals.

b. became more of a problem when the nomadic nature of humans increased.

c. increased with the alteration of human lifestyles with the domestication of plants and animals.

d. are easily contained within one geographic area as a result of the isolation of one country to the next.

2. A domestic animal that serves as a host for a particular disease and is placed at various locations to determine the potential for human exposure to a particular disease is a

a. reservoir.

b. source.

c. carrier.

d. sentinel.

3. The bubonic plague is caused by the bacterium Yersinia pestis. This bacterium is found in rodents. When fleas take a blood meal from infected rodents, the bacteria multiply in the flea. What type of vector is the flea in this case?

a. biological

b. mechanical

c. horizontal

d. vertical

4. The effects of diseases on a community involve what field of study?

a. invertebrate disease transmission

b. biology

c. entomology

d. epidemiology

5. What is the name of the weekly CDC publication that contains information and statistics about infectious outbreaks in the United States and other parts of the world?

a. Morbidity and Mortality Weekly Report (MMWR)

b. CDC Weekly Updates (CDC-WU)

c. U.S. Department of Health and Human Services Weekly (US-DHHSW)

d. Incidence of Mortality Weekly (IMW)

6. The statistic that tells about the general health of a population is the

a. ratio.

b. incidence.

c. prevalence.

d. mortality rate.

Zoonotic Disease History

7. The statistic that provides a snapshot view of a particular disease is the

a. ratio.

b. incidence.

c. prevalence.

d. mortality rate.

8. The immediate transfer of an agent from reservoir to a susceptible host is known as

a. direct transmission.

b. indirect transmission.

c. horizontal transmission.

d. vertical transmission.

9. When a person gets infectious agents from a drinking glass, the drinking glass is known as a

a. biological vector.

b. reservoir.

c. vector.

d. fomite.

10. An animate or inanimate object that serves as a long-term habitat and focus of dissemination for an infectious agent is known as a

a. reservoir.

b. source of infection.

c. vector.

d. fomes.

11. -- zoonoses                  A. diseases that are always present
                                 within a population of a particular
                                 geographic area
12. -- vector                    B. a live animal that transmits an
                                 infectious agent from one host to
                                 the next
13. -- sporadic diseases         C. conditions affecting or are
                                 peculiar to animals of a specific
                                 geographic area
14. -- vertical transmission     D. diseases naturally transmitted from
                                 animals to humans
15. -- horizontal transmission   E. diseases with a sudden onset and
                                 widespread outbreak within a
16. -- endemic diseases          F. widespread epizootic disease
17. -- enzootic diseases         G. isolated cases of a disease seen in
                                 widespread areas in an unpredictable
18. -- epidemic diseases         H. spread of disease from one parent
                                 to offspring
19. -- epizootic diseases        I. spread of disease through a
population from one infected
individual to another
20. -- panzootic diseases        J. widespread diseases in populations
of animals other than humans

Case Studies

Identify the following case studies as endemic, enzootic, sporadic, epidemic, epizootic, pandemic, or panzootic.

21. In spring 1993, cryptosporidiosis (a diarrheal disease caused by the protozoan Cryptosporidium parvum) affected more than 400,000 people in Milwaukee, Wisconsin. The oocysts (thick-walled structures in which protozoa develop and are transferred to new hosts) of Cryptosporidium parvum were present in cattle feces that were washed off Wisconsin dairy farms into Lake Michigan. The water of Lake Michigan provides Milwaukee with its drinking water. Although the lake water had been treated with chlorine the tiny oocysts were not killed and passed through the filters that were being used in Milwaukee. Thus the oocysts were present in the city's drinking water and people who drank city water became infected. This outbreak caused the death of more than 100 immunocompromised people.

22. The first documented evidence of human immunodeficiency virus (HIV) infection in humans can be traced to an African serum sample collected in 1959 (it is possible that HIV infection occurred before this date). HIV is thought to have been transferred to humans from nonhuman primates. In the 20-year period from 1981 to 2001 WHO stated that acquired immunodeficiency syndrome (AIDS) has become the most devastating disease humankind has ever faced. More than 60 million people have become infected with HIV and an estimated 22 million people have died of AIDS.

23. Foot-and-mouth disease (FMD) is a viral, infectious disease of domestic and wild cloven-hoofed animals. The disease is characterized by vesicular lesions and erosions of the epithelium of the mouth, nares, muzzle, feet, teats, and udder. Morbidity and mortality are the highest in the young; dairy breeds are particularly susceptible to FMD infection. The virus may be spread via aerosols, usually when animals are in close proximity. In 2001, many countries were experiencing outbreaks of FMD. Only the continents of Australia, Antarctica, and North America were free of FMD.


Burton, G., and P. Engelkirk. 2004. Microbiology for the Health Sciences, 7th edition. Philadelphia, PA: Lippincott, Williams & Wilkins, pp. 182-193.

Glossary of epidemiology terms. 2002. (accessed February 28, 2005).

Ingraham, J., and C. Ingraham. 2004. Introduction to Microbiology: A Case History Approach, 3rd edition. Pacific Grove, CA: Thomson Brooks Cole, pp. 364-5.

Kobasa, D., A. Takada, K. Shinya, et al. 2004. Enhanced virulence of influenza A viruses with the haemagglutinin of the 1918 pandemic virus. Nature 431:703-7.

Shapiro, L. 2005. Pathology and Parasitology for Veterinary Technicians. Clifton Park, NY: Thomson Delmar Learning, pp. 99-107.

Talaro, K., and Talaro, A. 1993. Foundations in Microbiology. Dubuque, Iowa: William C. Brown Publishing, pp. 351-354.

Zoonoses: The Natural History of Disease. San Diego Natural History Museum Field Guide, (accessed February 25, 2005).
Table 1-1 Statistical Terms Used in Epidemiology

Term               Definition

Rate               The ratio of the number of individuals in a
                   particular category to the total number of
                   individuals in the population being studied.

Incidence *        The number of new cases of a disease in a defined
(also called the   population over a specific time period as compared
morbidity rate)    to the general healthy population in a certain
                   time period (typically 1 year). Number of new
                   cases / total number of individuals X 100. Think
                   of incidence as the rate of acquiring a disease or
                   condition during a certain period or the general
                   health of a population by giving information about
                   the sick rate.

Prevalence         The total number of existing cases in a population
                   compared to the entire population.

                   Total number of existing cases in a population /
                   total number of individuals in a population X 100.

                   Think of the prevalence as the number of
                   people/animals with a certain disease at a
                   particular time or a snapshot view of the disease.
                   Prevalence is not a rate and is the measure of a
                   disease in a population because it includes all
                   who have been diagnosed in prior years as well as
                   the current year.

Mortality Rate     The ratio of individuals who die from a particular
                   disease compared with those individuals in a

Term               Example

Rate               The pulmonary form of anthrax has a 100% fatality

Incidence *        In a population of 100, 10 people contract
(also called the   ringworm.
morbidity rate)    The incidence is 1 in 10 or 10 / 100 X 100 = 10%.
                   The incidence of ringworm in this population
                   is 10 cases per 100 population.
                   The morbidity rate of stray dogs with
                   leptospirosis is 40%.

Prevalence         In a population of 500 people, 100 individuals
                   have hantavirus infection. The prevalence is
                   100 / 500 X 100 = 20%.

Mortality Rate     The mortality rate in people who contract Listeria
                   monocytogenes from animals is 20%.
                   The mortality rate of pneumonia is 7.5 per 1000
                   animals (if the population is 100,000 than there
                   would be 750 deaths).

* Chronic diseases like diabetes mellitus can have low incidence but
high prevalence because prevalence is a sum of past incidence.
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Author:Romich, Janet Amundson
Publication:Understanding Zoonotic Diseases
Geographic Code:1USA
Date:Jan 1, 2008
Next Article:Chapter 2 Principles of immunity and diagnostic techniques.

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