Toxoplasmosis is caused by Toxoplasma gondii, a protozoan parasite that infects humans and other warm-blooded animals. It has been found worldwide. Wild and domestic felines (members of the cat family) are the only definitive hosts of the parasite. Infected cats can transmit the disease to humans and other animals.

There are 3 infectious stages of Toxoplasma gondii: tachyzoites (rapidly multiplying form), bradyzoites (tissue cyst form), and sporozoites (in oocysts). The parasite is transmitted by consumption of infectious oocysts in cat feces, consumption of tissue cysts in infected meat, and by transfer of tachyzoites from mother to fetus through the placenta. Cats generally develop immunity after the initial infection; therefore, they shed oocysts only once in their lifetime (from approximately 3 days after infection and for about 20 days thereafter).

Transmission of Toxoplasmosis to People
Exposure to feces from infected cats can cause toxoplasmosis in people, although more likely sources of infection are eating undercooked meat and handling raw meat. Still, it is recommended that pregnant women and immunocompromised individuals avoid exposure to cat feces— for example, they should not clean litter boxes or pans and should wear gloves when gardening. Any areas known to be contaminated with cat feces should be avoided.

The tachyzoite is the stage responsible for tissue damage. Therefore, signs depend on the number of tachyzoites released, the ability of the infected cat's immune system to limit tachyzoite spread, and the organs damaged by the tachyzoites. Because adult animals with normally functioning immune systems control tachyzoite spread efficiently, cats with toxoplasmosis usually have no signs of illness. However, in kittens tachyzoites spread throughout the body. The signs include fever, diarrhea, cough, difficulty breathing, jaundice, seizures, and death.

In many cases, treatment is not necessary in infected cats. If warranted, your veterinarian will prescribe antibiotics to treat toxoplasmosis. Anticonvulsant medications may be used to control seizures. Fluids or intravenous feeding may be necessary for animals that are dehydrated or severely weakened due to the infection.

Toxoplasmosis is a major concern for people with immune system dysfunction (such as people infected with the human immunodeficiency virus). In these individuals, toxoplasmosis usually leads to inflammation of the brain. Toxoplasmosis is also a major concern for pregnant women because tachyzoites can migrate across the placenta and cause birth defects in human fetuses.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; Dennis W. Macy, MS, DACVIM; Dudley L. McCaw, DVM, DACVIM (Small Animal, Oncology); Barton W. Rohrbach, VMD, MPH, DACVPM; J. Glenn Songer, PhD; Richard A. Squires, BVSc (Hons), PhD, DVR, DACVIM, DECVIM-CA, MRCVS; Bert E. Stromberg, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Stephen C. Barr, BVSc, MVS, PhD, DACVIM; J. P. Dubey, MVSc, PhD; Paul Ettestad, DVM, MS; Craig E. Greene, DVM, MS; Delores E. Hill, PhD; Johnny D. Hoskins, DVM, PhD