Toxoplasmosis

NWDC is currently working on updating this fact sheet. 

Cause

Toxoplasmosis is a disease of wild animals, domestic animals, and humans caused by the single-celled protozoan parasite known as Toxoplasma gondii. T. gondii infection infrequently causes clinical disease, but can occasionally result in severe clinical illness, especially if the person or animal is also infected with another parasite or disease organism. 

Significance

Toxoplasmosis is a common infection in humans. An estimated 60 million people in the United States are infected with this protozoan, though in most people, this infection is asymptomatic. People with a weakened immune system can suffer severe illness, and when women become infected during pregnancy, serious birth defects can result. People become infected when they consume food or water contaminated with eggs from cat feces, or when they consume under-cooked meat containing tissue cysts. 

Species Affected

Cats are the definitive hosts for T. gondii. Feral cats are likely the main source of infective eggs in the environment, but many species of wild cats can also act as definitive hosts. All warm-blooded animals including mammals, birds, and humans are susceptible to infection and can act as intermediate hosts. Toxoplasmosis is known to occur in white-tailed deer, mule deer, elk, moose, pronghorn antelope, bison, wild boar, rabbits, bobcats, Domestic livestock such as sheep, goats, and pigs can become infected. caribou, black bears, polar bears, mink, red foxes, raccoons, skunks, and many bird and rodent species. Marine mammals such as sea lions, seals, sea otters, and dolphins are also susceptible. 

Distribution

T. gondii has a worldwide distribution.

Transmission

Domestic and wild cats are the only known definitive host for T. gondii, meaning the protozoa will only reproduce and create new eggs within these hosts. Eggs are released through the feces of infected cats and begin to develop and become capable of infecting a new host within 1 to 5 days. Intermediate hosts become infected when they ingest soil or vegetation contaminated with infective T. gondii eggs (also called oocysts). Oocysts may also be inhaled through dust contaminated with infected feces. Following ingestion, the eggs develop into trachyzoites and they begin to rapidly multiply. The tachyzoites enter neural and muscle tissue where they become bradyzoites (they multiply more slowly) and become encased within tissue cysts. Cats become infected by feeding on animals containing tissue cysts or by directly ingesting infective eggs. Other carnivorous species and humans can become infected by consuming infective eggs or by eating meat of animals containing tissue cysts. Toxoplasma can also be transmitted congenitally.

Clinical Signs

Most infected animals (definitive and intermediate hosts) do not show clinical signs of toxoplasmosis. The host’s immune system usually keeps the protozoa in check and prevents serious illness. Many animals that do show signs of clinical illness have a weakened immune system due to a concurrent disease. Young animals are also more susceptible to clinical illness because their immune systems are not fully developed. In the young or the immune compromised, toxoplasmosis may cause pneumonia or other problems involving the heart, liver, central nervous system, eyes, or muscles. Clinical signs may include fever, diarrhea, cough, difficulty breathing, jaundice, seizures, and death. Toxoplasmosis can also result in abortion and stillbirth when pregnant animals become infected. 

Diagnosis

Toxoplasmosis can be diagnosed at necropsy by microscopic identification of tissue cysts and identification of T. gondii organisms. Other laboratory tests are also available for diagnosis. 

Treatment

Medications are available to treat toxoplasmosis in humans, but they cannot completely eliminate the infection and are probably not useful or necessary in wildlife. 

Management

People can avoid becoming infected with T. gondii by taking certain precautions such as washing hands and materials (cutting boards, knives, etc.) thoroughly after working with raw meat, and cooking all meat thoroughly (a meat thermometer should read at least 165° F). Vegetables and fruit should also be washed prior to consumption. Pregnant women and anyone who is immune compromised should avoid contact with cat feces, soil, raw meat, and aborted animals. Litter boxes should be cleaned daily and should not be cleaned by pregnant women. Pet cats should not be fed raw meat. Cats should also be kept indoors to prevent them from hunting and consuming infected prey. 

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