Zoonoses: Overview

Zoonoses (zoh-ah-NO-seez) are infections that humans contract from animals. The singular form is zoonosis (zoh-uh-NO-sus).

Zoonoses are infections caused by parasites * , bacteria * , fungi * , or viruses * that pass from animals to humans. There are more than 140 diseases that can be transmitted from animals to humans, including plague * (transmitted from rats), avian influenza * (also called bird flu), tapeworm (transmitted from eating pork from infected pigs), canine influenza (transmitted from dogs), cat scratch disease (transmitted from cats), fish tank granuloma (transmitted from fish), prion diseases (transmitted by eating meat from infected cows), monkey virus (transmitted from infected monkeys), psittacosis (sih-tuh-KO-sis; transmitted from infected birds), rabies (transmitted from infected dogs, raccoons, wolves, and other animals), and Salmonella (sal-muh-NEH-luh; transmitted from infected turtles). Most people contract zoonotic (zoh-uh-NAH-tik) infections from pets, farm animals, and other animals with which they are in contact. Places where such contact can occur include petting zoos, nature parks, circuses, and pet stores as well as such special events as state or county fairs, carnivals, and rodeos.

In some cases, there is an intermediate substance, usually food or water, that conveys the disease organism from the animal to humans. Many food-borne illnesses result from consuming eggs, meat, or milk from infected animals, or from drinking water contaminated by animal urine or fecal matter. In some cases, the intermediate substance is fur from an infected animal or cloth made from its wool. Anthrax, a potentially fatal disease, is caused by bacterial spores carried on animal hair that can enter the body through breaks in the skin, by ingestion (swallowing), or by inhalation. Wild animals and insects can be the source of disease, too, particularly for diseases spread by the bite of a tick, mosquito, or fly. Such animals as wild rodents, raccoons, and bats also can carry diseases that may be harmful to humans.

Zoonoses can cause either minor or serious illnesses. In some cases, the organisms involved may infect humans, but the people do not become ill. Other zoonoses can be very dangerous to people, especially anyone with an immune system weakened by age or illness. In addition, some people are at increased risk of contracting a zoonosis because of their occupation: hunters and fishers, farmers and ranchers, zookeepers, foresters, veterinarians, laboratory technicians, and field biologists.

Most of these infections do not spread directly from person to person or do so only in rare instances. Usually they spread from animals to humans in the following ways:

Anthrax is a potentially fatal illness caused by contact with the spores of a bacterium called Bacillus anthracis. The spores are extremely difficult to kill by heat, drying, or most disinfectants, which is why they have been used as agents of bioterrorism. Anthrax spores can enter the human body by breathing, by eating meat from an infected animal, or through breaks in the skin during direct contact with the animal. Once inside the body, the spores multiply and release toxins that can cause the death of tissues and other severe symptoms. Anthrax that affects the skin typically produces black-colored areas of dead skin tissue. If inhaled, the spores produce flulike symptoms, heavy sweating, and chest pains. If swallowed, anthrax spores produce nausea and bloody vomiting, diarrhea, abdominal cramping, fever and chills, and painful swallowing. About 25 percent of people who develop anthrax from swallowing the spores die if untreated. Anthrax can, however, be treated with penicillin or other antibiotics when diagnosed in time.

Borrelia burgdorferi (buh-REEL-e-uh burg-DOR-feree) bacteria inside an infected tick can cause Lyme (LIME) disease in humans after a tick attaches to the skin and feeds on a person's blood. Ticks pick up the bacterium by feeding on the blood of infected deer and mice, which serve as reservoirs for the organism. Lyme disease can produce a number of symptoms, including extreme tiredness, muscle aches, and swollen painful joints. Patients often describe the symptoms as being flulike and pay a visit to the doctor's office because no one else they know has the flu. At the site of the tick bite, some (but not all) people develop a bull's-eye rash, a red rash surrounded by rings that resembles a bull's-eye target. A person with Lyme disease usually is treated with antibiotics.

Plague is a bacterial infection caused by Yersinia pestis (yer-SIN-e-uh PES-tis). People can contract plague through the bite of a flea that has become infected through contact with an infected rodent, such as a rat. The disease causes symptoms such as fever, headache, extreme weakness, abdominal pain, and swollen lymph nodes. In some cases the infection spreads through the blood and can infect the lungs. If this happens, plague can spread from person to person through coughing or sneezing. Plague was the cause of epidemics * in Europe and Asia during the Middle Ages, and it is still seen in the 21st century in many developing countries. It occurs in many developed countries too, including the United States, although not as many cases occur. Most cases in the United States occur in the Four Corners region of the Southwest (Arizona, Colorado, New Mexico, and Utah), California, Oregon, and Nevada. The disease can be fatal if it is not treated with antibiotics. Like anthrax spores, the plague bacterium has been used as a weapon of biological warfare.

Rabbit fever, or tularemia, is a disease that can be spread to people by rabbits, hares, and rodents. Tularemia is caused by the bacterium Francisella tularensis. There are several ways in which the disease can be transmitted, including being bitten by infected ticks, mosquitoes, horseflies, and deer flies; inhaling (breathing in) infected dirt or plant material; direct contact with infected animals through a break in the skin; drinking contaminated water; or, more rarely, eating meat from an infected animal.

Signs of rabbit fever usually develop three to five days after exposure and vary depending on how the bacteria enter the person's body. The person may have fever, chills, headache, joint stiffness, muscle pains, shortness of breath, sweating, weight loss, a red spot on the skin (usually at the site of the bite) that becomes sore, and in some cases, pink eye (conjunctivitis). Some people may develop pneumonia. Although rabbit fever is rarely fatal, it has been used as an agent of bioterrorism because it can severely incapacitate people and cause public panic. Rabbit fever is usually treated with antibiotics.

A virus that is carried in the saliva of infected animals can cause rabies when transmitted through a bite or, less commonly, through contact with saliva. Rabies in the United States is most often associated with raccoons, followed by bats and skunks, but any bite produced by an animal, whether domestic, stray, or wild, should be reported immediately to a local animal control agency. Symptoms include fever, difficulty swallowing, delirium * , seizures * , and coma * . If treatment does not begin very soon after the bite, death can result. Treatment includes intensive care in a hospital. A series of vaccinations * started at the time of a bite from a possibly infected animal can prevent the person from developing the disease.

Human fatalities from rabies in the United States are rare, about one or two per year as of 2016, compared to over a hundred per year in 1900. Widespread vaccination of domestic pets like cats and dogs is the major reason why most cases of rabies in the United States are now diagnosed in wild animals, mostly bats, raccoons, and foxes. Worldwide, about 26,000 people die of rabies each year, most of them in Africa and Asia.

Eating contaminated meat or having contact with the feces of an infected cat can put a person at risk of toxoplasmosis (toxo-plaz-MO-sis). This zoonosis is caused by a parasite called Toxoplasma gondii, and can produce such symptoms as swollen lymph nodes, muscle aches, headaches, and sore throat in a healthy person, and life-threatening brain infections in people with weakened immune systems, especially those who have HIV * /AIDS * Trichinosis

How Do Doctors Diagnose and Treat Zoonoses?

Can Zoonoses Be Prevented?

• Having pets regularly examined and vaccinated by a veterinarian.
• Avoiding contact with stray, unfamiliar, or wild animals.
• Cleaning litter boxes daily and animal cages frequently to prevent the growth of bacteria, fungi, and parasites.
• Having someone who has a normal immune system and is not pregnant empty pet litter boxes, bathe pets, clean pet cages, and pick up pet feces.
• Cooking meat from pigs, game animals, and wild boars until it is no longer pink inside and the juices run clear.
• Washing hands with soap and warm water after handling animals and before eating.
• Clearing brush from around the house and other areas that rodents might find attractive, and hiring a professional exterminator to inspect and treat the house, barn, or other structures that show evidence of rodent infestation.
• Avoiding storing food or trash in areas where it could attract animals, particularly when camping or hiking in wilderness locations.
• Wearing long sleeves and long pants when outdoors, especially in wooded areas, to discourage tick and mosquito bites.
• Using insect and mosquito repellent.
• Examining the body and pets for ticks after spending time outside in areas where ticks are known to live.

* parasites (PAIR-uh-sites) are organisms such as protozoa (one-celled animals), worms, or insects that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. Parasites live at the expense of the host and may cause illness.

* bacteria (bak-TEER-ee-a) are single-celled microorganisms, which typically reproduce by cell division. Some, but not all, types of bacteria can cause disease in humans. Many types can live in the body without causing harm.

* fungi (FUN-eye) are a group of organisms that reproduce by spores; they include yeasts, molds, mushrooms, and mildew.

* viruses (VY-rus-sez) are tiny infectious agents that can cause infectious diseases. Viruses can reproduce only within the cells they infect.

* plague (PLAYG) is a serious bacterial infection that is spread to humans by infected rodents and their fleas.

* influenza (IN-floo-EN-za) is a contagious viral infection that attacks the respiratory tract, including the nose, throat, and lungs. Also known as the flu.

* feces (FEE-seez) is the waste products excreted from the gastrointestinal tract.

* lymph nodes (LIMF) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

* abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

* epidemics (eh-pih-DEH-miks) are outbreaks of diseases, especially infectious diseases, in which the number of cases suddenly becomes far greater than usual. Usually, epidemics that involve worldwide outbreaks are called pandemics.

* delirium (dih-LEER-e-um) is a condition in which a person is confused, is unable to think clearly, and has a reduced level of consciousness.

* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

* coma (KO-ma) is an unconscious state, like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.

* vaccinations (vak-sih-NAYshunz), also called immunizations, are the giving of doses of vaccines, which are preparations of killed or weakened germs, or a part of a germ or product it produces, to prevent or lessen the severity of the disease that can result if a person is exposed to the germ itself.

* HIV, or human immunodeficiency virus (HYOO-mun ih-myoono-dih-FIH-shen-see), is the virus that causes AIDS (acquired immunodeficiency syndrome).

* AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).