Feline Toxoplasmosis

Etiology and epidemiology.

Toxoplasma gondii is one of the most prevalent parasites infecting warm-blooded vertebrates. Only cats complete the coccidian life cycle and pass environmentally resistant oocysts in feces. Sporozoites develop in oocysts after 1 to 5 days of exposure to oxygen and appropriate environmental temperature and humidity. Tachyzoites disseminate in blood or lymph during active infection and replicate rapidly intracellularly until the cell is destroyed. Bradyzoites are the slowly dividing, persistent tissue stage that form in the extraintestinal tissues of infected hosts as immune responses attenuate tachyzoite replication. Tissue cysts form readily in the CNS, muscles, and visceral organs. Bradyzoites may persist in tissues for the life of the host.

Infection of warm-blooded vertebrates occurs following ingestion of any of the three life stages of the organism or transplacentally. Most cats are not coprophagic and so are infected most commonly by ingesting T. gondii bradyzoites during carnivorous feeding; oocysts are shed in feces from 3 to 21 days. Sporulated oocysts can survive in the environment for months to years and are resistant to most disinfectants. Results of a recent study confim that the T. gondii oocyst shedding prepatent period is stage-dependent (ingestion of bradyzoites has a shorted prepatent period than ingestion of sporozoites) and is not dose-dependent. In addition, transmission of T. gondii is most efficient when cats consume tissue cysts (carnivorism) and when intermediate hosts consume oocysts (fecal-oral transmission). Toxoplasma gondii infection of rodents changes the behavior of the prey species making it less averse to cats, potentially increasing the likelihood the definitive host (felid) will become infected and potentiate the sexual phase of the organism.

Approximately 30% to 40% of cats and people in the United States are seropositive and so presumed to be infected. In a recent study of clinically ill cats, we detected T. gondii antibodies in 31.6% of the 12,628 cats tested.

Clinical features.

Approximately 10% to 20% of experimentally inoculated cats develop self-limiting, small bowel diarrhea for 1 to 2 weeks following primary oral inoculation with T. gondii tissue cysts; this is presumed to be due to enteroepithelial replication of the organism. However, detection of T. gondii oocysts in feces is rarely reported in studies of naturally exposed cats with diarrhea. T. gondii enteroepithelial stages were found in intestinal tissues from two cats with inflammatory bowel disease. Positive response to anti- Toxoplasma drugs in these two cats suggests that toxoplasmosis may occasionally induce inflammatory bowel disease. Eosinophilic fibrosing gastritis was recently described in a T. gondii-infected cat.  […]