Affects: Cats, Dogs
Toxoplasma gondii () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals, but members of the cat family (Felidae) are the only known definitive hosts in which the parasite may undergo sexual reproduction.
In rodents, T. gondii alters behavior in ways that increase the rodents' chances of being preyed upon by felids. Support for this "manipulation hypothesis" stems from studies showing that T. gondii-infected rats have a decreased aversion to cat urine while infection in mice lowers general anxiety, increases explorative behaviors and increases a loss of aversion to predators in general. Because cats are one of the only hosts within which T. gondii can sexually reproduce, such behavioral manipulations are thought to be evolutionary adaptations that increase the parasite's reproductive success since rodents that do not avoid cat habitations will more likely become cat prey. The primary mechanisms of T. gondii–induced behavioral changes in rodents occur through epigenetic remodeling in neurons that govern the relevant behaviors.
In humans infection is generally asymptomatic, but particularly in infants and those with weakened immunity, T. gondii may lead to a serious case of toxoplasmosis. T. gondii can initially cause mild, flu-like symptoms in the first few weeks following exposure, but otherwise, healthy human adults are asymptomatic. This asymptomatic state of infection is referred to as a latent infection, and it has been associated with numerous subtle behavioral, psychiatric, and personality alterations in humans. Behavioral changes observed between infected and non-infected humans include a decreased aversion to cat urine (but with divergent trajectories by gender) and an increased risk of schizophrenia and suicidal ideation. Preliminary evidence has suggested that T. gondii infection may induce some of the same alterations in the human brain as those observed in r
Risk Factors For Human Infection: The following have been identified as being risk factors for T. gondii infection in humans and warm-blooded animals:
by consuming raw or undercooked meat containing T. gondii tissue cysts. The most common threat to citizens in the United States is from eating raw or undercooked pork.
by ingesting water, soil, vegetables, or anything contaminated with oocysts shed in the feces of an infected animal. Cat fecal matter is particularly dangerous: Just one cyst consumed by a cat can result in thousands of oocysts. This is why physicians recommend pregnant or ill persons do not clean the cat's litter box at home. These oocysts are resilient to harsh environmental conditions and can survive over a year in contaminated soil.
from a blood transfusion or organ transplant
Treatment: In humans, active toxoplasmosis can be treated with a combination of drugs such as pyrimethamine and sulfadiazine, plus folinic acid. Immune-compromised patients may need continuous treatment until/unless their immune system is restored. There is no known human treatment for chronic infections.
The most significant limitation of current clinical treatments is their inability to eliminate the dormant bradyzoite tissue cysts established during chronic infection. These cysts persist in the brain and muscle tissues, making the infection lifelong and prone to reactivation in immunocompromised hosts, such as those with HIV/AIDS or undergoing immunosuppressive therapy.
While not approved for these uses, studies have shown that certain antipsychotic or anticonvulsant medications may inhibit T. gondii tachyzoites in in vitro cultures of human brain tissue. Examples include fluphenazine, valproic acid, haloperidol, and zuclopenthixol. It is unclear if tissue cysts are killed by these agents.
Some experimental and established antiparasitic medication is known to kill tissue cysts in in-vitro mice cells, with an example of an established drug being azithromycin.
Vaccination: No approved human vaccine exists against Toxoplasma gondii. Research on human vaccines is ongoing.
For sheep, an approved live vaccine sold as Toxovax (from MSD Animal Health) provides lifetime protection.
There is currently no commercially available vaccine to prevent T. gondii infection in cats. However, research into feline vaccines for toxoplasmosis is ongoing, with several candidates showing positive results in clinical trials.