A decade of discoveries in veterinary protozoology changes our concept of “subclinical” toxoplasmosis

One of the most compelling topics to emerge from the last decade of veterinary protozoology is disease caused by a zoonotic pathogen, Toxoplasma gondii, in otherwise healthy people. These findings may catch the health professions by surprise, because veterinary and medical courses and textbooks typically emphasize that T. gondii infections are subclinical, unless acquired in utero or the patient has a serious immunosuppressive condition. Nevertheless, numerous reports in the last decade associate toxoplasmosis with lymphadenopathy, fever, weakness and debilitation, ophthalmitis, and severe multisystemic infections in people who do not have immunosuppressive conditions. Toxoplasmosis in rodents causes altered behavior, and similar mental aberrations are coming to light in humans; recent studies associate T. gondii infection with personality shifts and increased likelihood of reduced intelligence or schizophrenia. These conditions reduce the quality of life of individuals, and may exact a significant economic burden upon society. Of course, toxoplasmosis continues to cause serious conditions in AIDS patients and congenitally infected people, as well as abortions and encephalitis in domestic and wild animals. Environmental contamination is heavy enough to extend into marine wildlife. It is time for the health professions to amend teaching curricula regarding T. gondii. Veterinary parasitologists should lead the way in developing methods to reduce the prevalence of T. gondii in food animals. Public health policies should prohibit the practice of allowing pet cats to roam. Organizations and individuals that feed feral cats are unwittingly contributing to the dissemination of T. gondii, by sustaining artificially dense populations of a definitive host of this protozoal parasite.