Animal Borne Diseases

Animal Borne Diseases

Also called: Animal Carried Infections, Pet Transmitted Infections, Pet Carried Infections, Animal Transmitted Infections

Reviewed By:
Vikram Tarugu, M.D., AGA, ACG


Animal-borne diseases, or zoonoses, are any type of animal disease that can be transmitted to humans. These diseases may be caused by a variety of pathogens (e.g., viruses, bacteria, parasites, fungi) and can be present, but cause no symptoms in animals.

The plague, flu and tuberculosis are all well-known animal-borne diseases that have killed millions of people. They are caused by different pathogens, transmitted in different manners and cause different symptoms that can range from mild to deadly.

For most people, there is little risk of being infected by an animal-borne disease. This is despite the presence of cats and dogs in more than half of all households in the United States, and the possibility of contact with animals on farms, in petting zoos or in the wild. People who are at most at risk for contracting an animal-borne disease are patients with some form of immunosuppression. A weakened immune system can result from age, medical conditions (e.g., HIV/AIDS) or medical treatments (e.g., organ transplant, certain medications). For those who have an increased risk of contracting a zoonosis, certain types of animals (e.g., stray cats or dogs, reptiles, primates) are not recommended as pets.

Animal-borne diseases can be transmitted to humans through many routes. Bites or scratches can penetrate the skin and deposit pathogens. Inadvertent hand-to-mouth transmission can also occur after contact with an animal or their excretions. Following excretion, pathogens in dried can become airborne and be inhaled.

If any of these modes of transmission allows an infection to occur, a wide range of disease symptoms can follow. The symptoms experienced depend on the type of pathogen involved, the patient’s overall health, and the severity of the infection. Some infections may clear without medical treatment. If treatment is required, a physician will need to know about all animals with which a patient has had contact, the nature of the contact and when it occurred. In addition, blood tests and tissue samples may be necessary for a definitive diagnosis.

Many animal-borne diseases are treatable. Medications such as antibiotics, antivirals or antihelminthics may be prescribed. In addition, supportive treatments will help reduce the patient’s discomfort until the infection clears.

About animal-borne diseases

An animal-borne disease is any disease that infects animals and may be transmitted to humans. Animals with these infectious diseases may or may not develop signs of illness. Animal-borne diseases are also referred to as zoonoses, from the Greek words zoon (animal) and nosos (disease). The animals involved may include wild animals (e.g., rodents, birds, nonhuman primates) and domestic animals (e.g., dogs, cats, horses), in addition to insects (e.g., mosquitoes) and insect-like animals (e.g., ticks).

Animal-borne diseases may be transmitted to people in a number of ways. Many are transmitted through insect or tick bites. Other methods of transmission include direct physical contact (e.g., dog bite, cat scratch, handling of contaminated feces), inhaling airborne pathogens, and consuming food or water contaminated with material from an infected animal (e.g., feces).

In some cases, an animal source of a human disease may be suspected but not known. In other cases, only rare strains of the disease may be transmitted from animals to humans. For example, most cases of malaria are transmitted between people by mosquitoes. One strain is transmitted through mosquitoes from monkeys to humans. Sometimes, the source of the disease may be the same animal by which infection is transmitted (e.g., rabies). Often, the disease is spread through an insect (e.g., mosquito) that bites an infected animal and then a human. After an infection has been transmitted from one species to another, such as from animals to humans, it may more easily spread from person to person (e.g., Ebola hemorrhagic fever).

In many animals, these diseases may be present without signs of infection. This is a result of the relationship between certain animals and certain pathogens that have developed over long periods of time, allowing the animal to develop immunity to the disease caused by that pathogen. However, when these pathogens are transmitted to humans, the immune system may not have established defenses. In some people, this may lead to life-threatening disease.

In the United States, the opportunities for exposure to potentially infected animals are many. According to a recent national survey, 71.1 million households in the United States own a pet, according to the American Pet Products Manufacturers Association (APPMA). These include dogs, cats, fish, birds, reptiles, horses, rabbits and more.

In addition, people are exposed to livestock, animals at public display facilities (e.g., petting zoos, aquarium touch pools) and wild animals. While people are often regularly exposed to animals and the pathogens they may carry, there is usually a low risk of disease transmission to otherwise healthy individuals. People who are infected by animal-borne diseases may have some form of immunosuppression due to disease (e.g., HIV/AIDS) or medical treatment (e.g., organ transplant, chemotherapy, certain medications). The routes of transmission of animal-borne diseases vary greatly. An injury, such as a bite or scratch, may allow pathogens carried in the animal’s saliva to enter the injured body.

Many types of viruses, bacteria and parasites are expelled in animal feces and urine. If the same animal or another has contact with these excretions, the pathogens may transfer to the animal’s skin or fur. Anyone who has direct physical contact with the animal may then become infected. Pathogens released in feces may also become airborne after the waste dries. This allows them to be inhaled and potentially cause infection. A number of diseases may also be transmitted by consuming meat, milk or other products from infected animals, or by drinking water contaminated with the waste of infected animals.

Types and differences of animal-borne diseases

The number of animal-borne diseases that can infect people is difficult to estimate. Many new pathogens have been discovered in recent decades. Others, long known to science, were not thought to be zoonoses. Some are only known to cause disease in people with immune systems suppressed by disease (e.g., HIV/AIDS) or medical treatment.

The large number of animal-borne diseases may be classified by whether the responsible pathogen is a virus, bacterium, fungus, protozoa or parasitic worm (helminth).

Viruses are among the smallest and simplest disease-causing agents. During an infection, viruses take over cells and use them to reproduce. The viruses then take over additional cells and eventually create spores to be released in the environment. Outside the body, the spores are often inactive until another host can be infected. Some of the more common animal-borne diseases caused by viruses include:

  • Flu. While many flu viruses only infect people, others may be transmitted from other mammals and birds. Viruses found in swine, birds (e.g., bird flu) and ferrets have also caused disease in people. They are spread by direct contact with materials contaminated with animal eye or nasal secretions, feces or urine. The viruses may also become airborne in respiratory droplets spread by coughs and sneezes.

  • Arboviral encephalitis diseases. There are many different animal-borne diseases that can produce encephalitis (inflammation of the brain) and are transmitted to humans by arthropods (e.g., mosquitoes, ticks). The viruses that cause these diseases typically infect birds and many types of mammals (e.g., squirrels, horses, bats).

  • Rabies. This disease has a nearly global distribution. It is transmitted through saliva and can infect most species of mammals. The diseases typically result from bites and scratches by infected animals. Vaccination programs for dogs, cats and ferrets have largely eradicated the disease from pets in the United States and other industrialized nations. Human cases in these countries are now rare and most occur from exposure to wild animals (e.g., bats, raccoons).

  • Hantavirus pulmonary syndrome (HPS). Wild rodents spread HPS viruses through their bite and in their feces, which may become airborne when dried. This disease was first identified during an outbreak in United States in 1993. Further study has identified additional cases dating back to 1959. It has also been found throughout much of North and South America.

  • Lymphocytic choriomeningitis. Typically found in rodents (e.g., hamsters, guinea pigs, mice) throughout much of the world. The virus that causes this disease is excreted with feces and may become airborne when dried. Infection causes inflammation of the brain and spinal cord and their membranes (meningitis).

  • Monkeypox. Most cases of this disease occur in central and western Africa where it is transmitted from many small mammals. Human outbreaks in the United States have resulted from exposure to infected pet prairie dogs. It is transmitted through bites, contact with body fluids, or contact with a skin rash on the animal. Person-to-person transmission of the disease has also been recorded.

  • B virus infection. This disease is commonly found in macaque monkeys, which are used in research laboratories and are available as pets. Transmission to humans is rare, with only 26 well-documented cases of infection between 1932 and 2002 according to the U.S. Centers for Disease Control and Prevention (CDC).

Many viral diseases that can be transmitted from animals to humans have occurred only in certain parts of the world. Animal-borne diseases that are mostly found outside the United States include hemorrhagic fevers (e.g., Rift Valley fever, yellow fever, Ebola hemorrhagic fever, dengue hemorrhagic fever) that occur in the tropics and subtropics such as in Africa or South America. Severe acute respiratory syndrome (SARS) was first identified in 2002 in China. Within months of being identified, SARS spread to four continents and infected more than 8,000 people (including eight in the United States). The virus that causes SARS is believed to have evolved from an animal virus.

Bacteria are single-celled organisms that are more complex than viruses. Most can reproduce without taking over another cell. Some bacteria can survive in the environment, while others take inactive forms (e.g., spores) until an animal that can be infected is contacted. Some of the more common animal-borne diseases caused by bacteria include:

  • Tuberculosis. This wide-ranging and well-known disease typically results from person-to-person transmission of the bacteria Mycobacterium tuberculosis. The zoonotic form of this disease is caused by Mycobacterium bovis and typically causes the same symptoms. It is believed to cause up to 3 percent of tuberculosis cases in the United States. It is most often spread through unpasteurized milk and cheese, but may also result from exposure to infected cattle. It has also been identified in wild badgers, opossums and voles.

  • Salmonella infection. The bacteria responsible for salmonellosis are shed in the feces of infected animals. They are carried by many reptiles and are also often found in the feces of baby chickens and ducks. While the bacteria most often cause only mild gastroenteritis that passes after a week, the infection can also spread to the blood and nervous system.

  • Lyme disease. Ticks spread this disease among many types of mammals, including humans. Symptoms often begin with a skin rash that resembles a bull’s eye. Without treatment, symptoms may progress to include inflammation of the joints and neurological symptoms. Since 1998, between 15,000 and 25,000 human cases of Lyme disease have been reported each year to the CDC.

  • Rickettsial diseases. A number of bacterial diseases can be passed from animals (e.g., deer, dogs, rodents) to humans through tick, flea and louse bites. They include ehrlichiosis, Rocky Mountain spotted fever and typhus.

  • Capnocytophaga infection. While dog bites are often associated with rabies, infection with Capnocytophaga canimorsus bacteria is the most common disease transmitted by dog bites in the United States. The bacteria are also carried by cats and can be transferred by scratches or bites. Infection may quickly move into the bloodstream and can cause meningitis.

  • Leptospirosis. Many types of wild and domestic animals (although rarely cats) carry Leptospira interrogans bacteria, which are transmitted in urine. Human infection most often results from consuming contaminated food or water, but may also follow contact with fluid from the mucous membranes (e.g., eyes, mouth, nose) of an infected animal or an injury. Leptospirosis may cause systemic infection, leading to kidney damage, meningitis, liver failure or respiratory distress. 

Additional animal-borne diseases caused by bacteria include brucellosis, campylobacteriosis, cat scratch disease, pasteurella multocida infection, Q fever, anthrax, southern tick-associated rash illness (STARI), plague, tularemia, yersiniosis, psittacosis and rat bite fever.

Protozoa are single-celled animals that are larger and more complex than bacteria. They contain their genetic material in a nucleus and many do not require an animal host to live. Those that do may act as parasites, living off the host. 

Some of the more common animal-borne diseases caused by protozoan parasites include:

  • Toxoplasmosis. This disease that occurs throughout much of the world is caused by Toxoplasma gondii. The parasite is believed to infect up to 40 percent of cats in the United States, although most show no symptoms. Infected cats pass the parasite eggs in their feces, which may be transmitted to people through hand-to-mouth transfer.

  • Cryptosporidiosis. The protozoa responsible for cryptosporidiosis can be found in almost all types of vertebrates (e.g., mammals, reptiles, birds, fish) and have a nearly global distribution. Human infection is most likely to occur in those who are immunosuppressed. Most cases result from drinking contaminated water, but it is also considered a zoonosis because it can be spread through contact with animal feces.

  • Giardiasis. A gastrointestinal disease caused by Giardia lamblia. These protozoa are common in the United States and transmitted through contact with the urine or feces of infected animals. Like cryptosporidiosis, most cases of giardiasis result from exposure to contaminated water.

  • Babesiosis. Caused by the Babesia protozoan parasite. Tick bites may spread this disease to humans from rabbits, dogs and cats. Once in the human body, the protozoa infect red blood cells and may lead to anemia. Some cases of babesiosis occur in conjunction with Lyme disease.

  • Leishmaniasis. Sand flies transmit the parasites that cause this disease from infected mammals to humans. Occurrence is rare in the United States and most people who are infected show no symptoms. 

In addition to protozoa, animals may also be infected with multicellular worms (helminths) that act as parasites. Some of the more common animal-borne diseases caused by parasitic helminths include:

  • Hookworm infection. These parasites occur throughout much of the world and are common in animals, including dogs and cats. They are expelled in animal feces and can survive for long periods in the soil. The larvae are able to penetrate animals’ foot pads and the bare skin of people. There are only limited occurrences of the disease in the United States.

  • Toxocariasis. This is a type of roundworm parasite that can move from animal hosts to people. Infection typically results from consuming foods contaminated with the parasite’s eggs. Visceral larva migrans is a form of toxocariasis that causes swelling of organs and abdominal pain. Ocular larva migrans is the term for toxocariasis that moves to the eyes and may cause blindness.

  • Tapeworm infections. These infections are usually transmitted to humans through the consumption of food or water contaminated with the eggs of the tapeworm. It may also occur due to the accidental consumption of an infected flea. Certain tapeworms commonly infect cats and dogs in the United States.

  • Heartworm infection. These parasites are transmitted through mosquito bites and occasionally infect humans. Most cases of infection affect the lungs or skin. They may form a blockage in the lungs that must be removed surgically. They may also die in the skin after the mosquito bite, which may cause hives.

A fungus is an organism that gets energy from the organic material in which it grows. Fungi are often found in rich, highly organic soil and appear plant-like, but some forms live on or in animals. Animal-borne diseases caused by fungi include:

  • Cryptococcosis. This fungus is often transmitted through bird feces or in soil in areas where they are kept. It has a nearly global distribution and may be inhaled when dried feces become airborne with dust. Infection may cause pneumonia or meningitis in people and usually occurs in patients with some form of immunosuppression.

  • Ringworm. Despite its name, ringworm is actually a fungus that causes skin infections in people. It may be transmitted from many types of mammals, including dogs, cats, cows, goats, pigs, horses and hedgehogs. It may cause a ring-shaped rash or a bald spot when it occurs on the scalp.

Mad cow disease, or bovine spongiform encephalopathy (BSE), is a type of transmissible spongiform encephalopathy (TSE) that only affects cattle. Thus, it is not considered a zoonosis because BSE is not transmitted to humans. However, when food from BSE-infected cattle is consumed by humans, it is believed to cause another type of TSE that only affects humans, called variant Creutzfeldt-Jakob disease (vCJD). BSE and vCJD can cause progressive nerve and brain damage and are always fatal. They are most common in the United Kingdom but have also occurred in several other countries, including the United States. 

Risk factors and causes of animal-borne diseases

While animal-borne diseases are present throughout the world, in general there is a very low risk of an otherwise healthy patient being infected. Certain conditions increase the odds that a person may develop one of these diseases. People at greatest risk have some form of immunosuppression. They include:

  • Infants and children younger than 5 years old
  • Elderly people
  • Pregnant women
  • Patients undergoing treatments for cancer
  • Patients who have received organ transplants
  • Patients on medications that suppress immune system functioning
  • Patients with HIV or AIDS

Owning a pet or having regular contact with animals increases the risk of infection with an animal-borne disease. The greatest risk is with new pets, which may have an incomplete vaccination history or be young with an immature immune system. In addition, certain animals are more likely to expose people to infectious diseases. They include:

  • Stray animals
  • Reptiles (e.g., turtles, lizards, snakes)
  • Chicks and ducklings
  • Puppies younger than six months old and kittens younger than a year
  • Pets with diarrhea
  • Primates (e.g., apes, monkeys)

Signs and symptoms of animal-borne diseases

A diverse group of diseases are caused by animal-borne pathogens. Though some infections may be present in patients without causing disease, they may also result in severe signs and symptoms. In general, animal-borne diseases may produce symptoms similar to other infections,most of which are the common reactions of the immune system as it attempts to fight the infection. After an incubation period, the patient may experience varying symptoms of fever, nausea, vomiting and headaches.

Symptoms may be more specific to a particular body system. For example, diseases affecting the lungs may produce a cough and diseases affecting the digestive system may produce vomiting or diarrhea. Bites may produce skin symptom such as rash, swelling or redness. Diseases that affect the nervous system may produce dizziness, confusion and seizures.

Diagnosis and treatment of animal-borne diseases

The diversity of animal-borne diseases means that there is also a great variety of methods that a physician may use to diagnose and treat them. Diagnosis will often begin with a medical history and a physical examination. The medical history will likely include questions on symptoms, including their duration, severity and progression. If the physician suspects a possible animal-borne disease, he or she may also ask about recent contact with animals. The physical examination will check for additional signs and symptoms.

If the physician does not ask, the patient should provide information about his or her pets, as well as recent contact with other animals. Any bites or scratches should be reported, including known or suspected insect or tick bites. The patient should also tell his or her physician about recent travel that may have allowed exposure to rare diseases.

The physician is likely to request samples of tissues or excretions to look for signs of disease. These may include:

  • Blood tests. May be used to identify the presence of certain antibodies produced by the patient’s immune system or the pathogen’s genetic material.

  • Stool or urine samples. Many pathogens, especially parasites, may be excreted in feces or urine. Examining these excretions may reveal pathogens or allow the culture of certain bacteria or viruses.

  • Tissue samples. For some diseases, spinal, joint or other bodily fluids may need to be sampled. Biopsies may also be necessary.

Treatment for animal-borne diseases can also vary greatly, if necessary at all. Some of the diseases frequently cause no symptoms or run their course within weeks without treatment. Most of the diseases can be cured with appropriate medications. Antibiotic, antiviral and antihelminthic medications are available to treat bacterial, viral and parasitic infections, respectively.

Prevention methods for animal-borne diseases

Overall, there is very low risk for infection with an animal-borne disease. However, there are certain preventive methods that may further reduce this risk. They include:

  • Wash hands after handling animals or their waste products
  • Select appropriate pets
  • Do not keep wild animals as pets
  • Do not have direct contact with wild animals
  • Avoid rough play with pets to reduce the risk of scratches and bites
  • Use proper flea and tick control on pets
  • Feed pets only high-grade prepared pet food or fully cooked meats and clean water
  • Clip pets’ claws regularly
  • Make regular veterinary visits for pets and keep vaccinations up to date
  • If a pet develops diarrhea, take it to a veterinarian for any necessary treatment
  • Supervise children when around pets
  • Keep a pet’s bedding and environment clean
  • Indoor pets (e.g., cats) present lower risk than those allowed to roam
  • Use repellents when outdoors to prevent mosquito and tick bites

Although they are at greater risk for developing animal-borne diseases, patients with immunosuppression may still keep pets. For those patients, the U.S. Centers for Disease Control and Prevention (CDC) has developed certain recommendations. These include:

  • Do not handle animals that have diarrhea
  • Avoid animals younger than 6 months (a year for cats)
  • Avoid contact with stray animals
  • Do not contact feces (even in litter boxes) without gloves and washing hands
  • Avoid reptiles and exotic pets (e.g., monkeys, ferrets).

Questions for your doctor

Preparing questions in advance can help patients have more meaningful discussions with healthcare professionals regarding their conditions. Patients may wish to ask their doctor the following questions related to animal-borne diseases:

  1. Do my symptoms indicate an animal-borne disease? Which one?
  2. How might I have become infected?
  3. What tests will you perform to diagnose my condition? How do I prepare for these tests?
  4. How will my condition be treated? What are the side effects of this treatment?
  5. For how long will I need to be treated?
  6. Is my condition contagious? Are others in my household likely to be affected?
  7. What is my prognosis?
  8. Am I at risk of contracting another animal-borne disease?
  9. How can I prevent future infections?
  10. Are certain types of pets more likely to expose my family to disease?
  11. If my pet is sick, can I catch its illness?
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