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When people think about diseases they can get from animals, probably one of the first diseases that comes to mind is rabies. Indeed, rabies is a serious disease that has been around since before recorded history, but the chances of someone in North America developing rabies is pretty low. There are far more common diseases that we get from animals that do not get the attention or generate headlines as does rabies. Cat scratch fever affects over 20,000 people per year in the United States. Between 4% and 20% of children in America are infected with roundworms that have been transmitted from small animals. Compare that with the zero to four cases of rabies seen in the United States each year. Granted, unprotected people will die if they develop rabies, but cat scratch fever and roundworms can also cause death and have serious economic effects.

Zoonoses (singular: zoonosis; adjective: zoonotic) are diseases transmitted among humans and other vertebrates. The key word here is among. Zoonotic diseases can be transmitted from nonhuman animals (henceforth referred to as animals) to humans or from humans to animals. In this book the term zoonotic disease will refer to diseases transmitted from animals to humans.

Zoonotic diseases are not rare:

• All known microbial and parasitic categories include at least one zoonotic agent. From the smallest virus to the largest parasitic worm, there are many agents of disease, called pathogens, that can pass among people and animals (Box 1).

• A majority of infectious diseases are zoonotic. There are more than 150 diseases that are known to be zoonotic.

• Many human diseases probably started out as zoonotic diseases. Although we cannot be sure which diseases made the leap from animals to humans, there is evidence that measles, smallpox, and diphtheria came to us from animals. There is also compelling evidence that AIDS started in monkeys.

• About three quarters of emerging diseases are zoonotic, and when they appear, they can spread rapidly. The West Nile virus appeared in New York City in the United States in 1999 and moved through the country in the summer of 2002, causing human and equine disease and death.

• Most of the animals we come in contact with every day can be sources of zoonotic diseases (Box 2).

BOX 1 Zoonotic diseases grouped by etiology

VIRAL DISEASES

Eastern equine encephalitis

Hantavirus pulmonary syndrome

Influenza

La Crosse encephalitis

Lymphocytic choriomeningitis

Rabies

St. Louis encephalitis

West Nile virus infections

Western equine encephalitis

BACTERIAL DISEASES

Anthrax

Botulism

Brucellosis

Campylobacteriosis

Cat scratch disease

Colibacillosis

Ehrlichiosis

Leptospirosis

Listeriosis

Lyme disease

Mycobacterial infections

Pasteurellosis

Plague

Psittacosis

Q fever

Rat-bite fever

Rocky Mountain spotted fever

Salmonellosis

Staphylococcosis

Tularemia

Vibriosis

Yersiniosis

FUNGAL DISEASE

Dermatomycosis

PARASITIC DISEASES

Arthropod infestation

Scabies

Protozoan infections

Babesiosis

Cryptosporidiosis

Giardiasis

Toxoplasmosis

Round worm infections

Cutaneous larva migrans

Heartworm infection

Hookworm infection

Roundworm infection

Trichinellosis

Visceral larva migrans

Tapeworm infections

Diphyllobothriasis

Dipylidiasis

Echinococcosis

PRION DISEASE

Bovine spongiform encephalopathy

BOX 2 Zoonotic diseases grouped by host

DOGS

Brucellosis

Campylobacteriosis

Cryptosporidiosis

Giardiasis

Hookworm infection

Leptospirosis

Lyme disease

Q fever

Rabies

Ringworm infection

Roundworm (Toxocara) infection

Rocky Mountain spotted fever

Salmonellosis

Tapeworm (Dipylidium) infection

CATS

Campylobacteriosis

Cat scratch disease (Bartonella henselae infection)

Cryptosporidiosis

Hookworm infection

Leptospirosis

Plague (Yersinia pestis infection)

Q fever

Rabies

Ringworm infection

Roundworm (Toxocara) infection

Salmonellosis

Tapeworm (Dipylidium) infection

Toxoplasmosis

CATTLE

Anthrax

Bovine spongiform encephalopathy

Brucellosis

Campylobacteriosis

Cryptosporidiosis

Colibacillosis

Q fever

Rabies

Ringworm infection

Salmonellosis

HORSES

Campylobacteriosis

Cryptosporidiosis

Leptospirosis

Rabies

Ringworm infection

Salmonellosis

SWINE

Q fever

Ringworm infection

Yersiniosis

SHEEP AND GOATS

Anthrax

Q fever

BIRDS

Campylobacteriosis

Psittacosis (Chlamydophila psittaci infection)

Salmonellosis

RODENTS

Hantavirus pulmonary syndrome

Lymphocytic choriomeningitis

Rat-bite fever

Salmonellosis

SNAKES

Salmonellosis

FISH

Salmonellosis

WILDLIFE

Anthrax

Brucellosis

Giardiasis

Hantavirus pulmonary syndrome

Lymphocytic choriomeningitis

Plague (Yersinia pestis infection)

Rabies

Roundworm infection, racoon (Baylisascaris infection)

Tuberculosis (Mycobacterium infection)

Tularemia (Francisella tularensis infection)

Reverse zoonoses are diseases that people can give to animals. Among the more common diseases are Staphylococcus aureus infection, Streptococcus infection, and tuberculosis.

There are diseases that people and animals share but that they do not get from each other. For example, blastomycosis and coccidioidomycosis are nonzoonotic mycotic (fungal) diseases that people and animals develop from exposure to contaminated soil but not from exposure to each other.

And then there are the diseases that people think they can get from animals, but it just does not happen. These pseudozoonotic diseases include feline leukemia; feline immunodeficiency disease, also known as feline AIDS; pinworms; colds; and sore throats.

HOSTS

People are often accidental and dead-end hosts for zoonotic diseases. The definitive host of a pathogen is also the natural host of the pathogen. In the case of zoonotic diseases, the definitive hosts are animals. Sometimes definitive hosts become ill from the pathogens, and sometimes they do not. If the animal does not become ill but is still capable of transmitting the disease to people, it is said to be a carrier, or reservoir host. Carriers can also be inanimate objects, such as water or food.

There are a number of methods used to transmit diseases from animals to people. The two major categories of transmission are direct transmission and indirect transmission (Table 1).

TABLE 1 Zoonotic diseases identified by typical means of transmission

DIRECT TRANSMISSION

Contact between the infected animal and the susceptible person can result in direct transmission of a zoonotic disease. This can take place by touching the animal or from droplet infection through the animal’s coughing or sneezing. A person must be within 1 meter in front of an animal for direct droplet transmission to occur. The pathogen may stay on the person’s skin, enter the body through breaks in the skin or mucosal surfaces, be ingested, or be inhaled. Plague is an example of a disease contracted via direct transmission.

INDIRECT TRANSMISSION

Indirect transmission includes any method of transmission in which the infected animal and susceptible person do not actually come in direct contact. Some of these methods include:

• Transmission that involves contact between the person and some inanimate object known as a fomite. Animals contaminate objects with pathogens, and people become infected when they come in contact with the contaminated objects. One example is dermatophytes, fungi that cause skin infections that can be transmitted to people via contact with contaminated bedding, grooming tools, etc. Another example is dust particles contaminated with a pathogen that can enter a person through the respiratory tract, such as with Hantavirus infection. As with direct transmission, any pathogen acquired through indirect transmission may stay on the person’s skin, enter the body through breaks in the skin or mucosal surfaces, be ingested, or be inhaled. Examples of fomites include grooming utensils, blankets, clothing, toys, and dust particles.

• Indirect transmission can also involve vectors. There are two types of vectors—biological vectors and mechanical vectors:

• Biological vectors are animals in which the pathogen must go through part of its life cycle before being passed on to a person. Examples of biological vectors are fleas, ticks, flies, and mosquitoes. They can be reservoir hosts for a pathogen, as in babesiosis.

• Mechanical vectors are animals that carry pathogens to people but are not themselves affected by the pathogens. Examples of mechanical vectors are mosquitoes, ticks, flies, and pets. Yes, insects and arthropods can be either biological or mechanical vectors. They can also be carrier hosts for a pathogen. For example, flies can spread salmonellosis with their feet.

• Vehicles of transmission are neither fomites nor vectors. They are substances that are normally brought into the body upon which a pathogen has hitched a ride. Vehicles include water, air, and food. When water is a vehicle of transmission, it is usually contaminated with feces from an infected animal, as in giardiasis. Air can be a vehicle if the person is standing more than 1 meter away from an infected animal when it sneezes or coughs and the pathogen becomes airborne on dust particles or in droplets. Food transmission usually results in food poisoning.

PREVENTING ZOONOTIC DISEASES

Except for smallpox vaccination, few tools for protecting people and animals from zoonotic diseases were available before the early 1900s. Methods such as thoroughly cooking meat, boiling milk, and quarantining sick animals were used to control diseases. In the 1920s commercial pasteurization became an effective way to prevent zoonotic diseases that are spread through raw milk or products made from raw milk. Insecticides came into use in the 1940s and helped protect against vector-borne diseases. Vaccines are available to protect animals and people against some zoonotic diseases but not all of them. In the 1950s, the United States started mandatory vaccination of dogs for rabies, and that has been a primary contributing factor in the decline in the number of rabies cases seen in people.

PREVENTING ZOONOTIC DISEASES

ABOUT THE DISEASES

In the next section of this book, you will be introduced to many of the known zoonotic diseases. The diseases are listed in alphabetical order by their most commonly used names. Sometimes that means using a common name (cat scratch disease) instead of the etiologic name (bartonellosis). Other times we use the etiologic name (listeriosis) rather than the common name (silage disease) because it is more frequently used. Each disease section contains a description of the disease’s etiology; its hosts; its mode(s) of transmission; methods for diagnosis, treatment, and prevention; and other useful information.

MORBIDITY AND MORTALITY

At the beginning of each disease section, a scale of 1 to 4 plus marks (+ to ++++) indicates the morbidity and mortality for the disease. The morbidity is the likelihood that a person will contract the disease. The mortality is the likelihood that a person who has developed the disease will die as a result.

Introduction to Zoonotic Diseases
- Definition: Zoonoses are diseases transmitted among humans and other vertebrates
- Can be transmitted from animals to humans or vice versa
- More common than often thought
- Examples: rabies, cat scratch fever, roundworm infections
Prevalence and Importance
- All microbial and parasitic categories include at least one zoonotic agent
- Over 150 known zoonotic diseases
- Majority of infectious diseases are zoonotic
- About 75% of emerging diseases are zoonotic
- Many human diseases likely started as zoonotic (e.g., measles, smallpox, diphtheria, AIDS)
Classification of Zoonotic Diseases
- By etiology: Viral, Bacterial, Fungal, Parasitic, Prion
- By host: Dogs, Cats, Cattle, Horses, Swine, Sheep and Goats, Birds, Rodents, Snakes, Fish, Wildlife
Types of Zoonoses
- Reverse zoonoses: Diseases humans give to animals
- Non-zoonotic shared diseases: Diseases shared but not transmitted between humans and animals
- Pseudozoonotic diseases: Diseases people think they can get from animals but can't
Hosts and Transmission
- Definitive host: Natural host of the pathogen (usually animals for zoonotic diseases)
- Carrier/reservoir host: Can transmit disease without showing symptoms
- Direct transmission: Contact, droplets (within 1 meter)
- Indirect transmission: Fomites, vectors (biological and mechanical), vehicles (water, air, food)
Prevention History
- Pre-1900s: Cooking meat, boiling milk, quarantine
- 1920s: Commercial pasteurization
- 1940s: Insecticides for vector-borne diseases
- 1950s: Mandatory dog rabies vaccination in the US
Disease Information in the Book
- Alphabetical listing by common or etiologic name
- Morbidity and mortality ratings (+ to ++++)

Flashcards:

Front: What are zoonoses? Back: Diseases transmitted among humans and other vertebrates
Front: How many known zoonotic diseases are there? Back: More than 150
Front: What percentage of emerging diseases are zoonotic? Back: About 75%
Front: Name three human diseases that likely started as zoonotic. Back: Measles, smallpox, diphtheria
Front: What is a reverse zoonosis? Back: A disease that humans can give to animals
Front: What is a pseudozoonotic disease? Back: A disease people think they can get from animals, but actually can't
Front: What is a definitive host? Back: The natural host of a pathogen, usually animals for zoonotic diseases
Front: What is a carrier or reservoir host? Back: An animal that can transmit a disease without showing symptoms
Front: What are the two major categories of disease transmission? Back: Direct transmission and indirect transmission
Front: What is a fomite? Back: An inanimate object that can transmit pathogens in indirect transmission
Front: What's the difference between biological and mechanical vectors? Back: Biological vectors require the pathogen to complete part of its life cycle; mechanical vectors simply carry the pathogen without being affected
Front: When did commercial pasteurization become widely used to prevent zoonotic diseases? Back: In the 1920s
Front: What does the morbidity rating in the book indicate? Back: The likelihood that a person will contract the disease
Front: What does the mortality rating in the book indicate? Back: The likelihood that a person who has developed the disease will die as a result
Front: Compare the annual cases of rabies to cat scratch fever in the US. Back: Rabies: 0-4 cases; Cat scratch fever: over 20,000 cases

Examples of Zoonotic Diseases by Pathogen Type

1. Viral Diseases: Rabies, West Nile virus, Influenza.

2. Bacterial Diseases: Anthrax, Lyme disease, Cat scratch disease.

3. Fungal Diseases: Ringworm (dermatomycosis).

4. Parasitic Diseases: Roundworm, Toxoplasmosis, Hookworm.

5. Prion Diseases: Bovine spongiform encephalopathy (mad cow disease).

Examples of Zoonotic Diseases by Host

• Dogs: Rabies, Lyme disease, Roundworm infection.

• Cats: Cat scratch disease, Plague, Toxoplasmosis.

• Cattle: Anthrax, Brucellosis, Rabies.

• Horses: Rabies, Leptospirosis.

• Rodents: Hantavirus pulmonary syndrome, Salmonellosis.

• Birds: Psittacosis, Salmonellosis.

• Wildlife: Rabies, Plague, Tuberculosis.

Reverse Zoonoses

•Diseases transmitted from humans to animals, such as Staphylococcus, Streptococcus, and tuberculosis.