Toxoplasmosis of rats: a review, with considerations of their value as an animal model and their possible role in epidemiology.

We critically review and summarize information on the prevalence of Toxoplasma gondii infections in rats, mainly Rattus norvegicus, and their possible role as a source of infection for larger carnivores and omnivores. We also review information on immunology and natural resistance, contributing to the model value of rats in the analysis of human infection. Rats can be successfully infected with oocysts (sporozoites), tissue cysts (bradyzoites), and tachyzoites. Even adult rats, that are resistant to clinical toxoplasmosis, can be infected orally with a few oocysts or tissue cysts. Infections with tachyzoites of the RH strain are highly variable. Congenital transmission of T. gondii occurs at a high rate when rats are infected during pregnancy. Congenitally infected rats can harbor viable T. gondii in the absence of detectable antibodies to T. gondii and rats with low antibody titers may harbor few or no organisms. The isolation of viable T. gondii by bioassay is the only reliable means to determine persistence of chronic T. gondii infection in feral rats. No evidence was found for maintenance of T. gondii in rats by vertical transmission in the absence of cats.