Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 1996
Publication Date: N/A
Interpretive Summary: Infections by the protozoan parasite Toxoplasma gondii are widespread in livestock and humans. It causes mental retardation and loss of vision in children and abortion in livestock. Rodents are often used as experimental animals for T. gondii infection. Until recently, rats were considered as one of the most resistant hosts for T. gondii infection. A scientist at the Beltsville Agricultural Research Center describes the rat model for experimental toxoplasmosis. Rats fed 100,000 or more oocysts became ill and some died. Even 1 oocyst was infective to rats. These results will be useful for screening vaccines and drugs against toxoplasmosis in rats.
Rats are considered as one of the most resistant hosts for Toxoplasma gondii infection, but relative infectivity of T. gondii for rats is not known. Therefore, infectivity and virulence of oocysts of the VEG strain of T. gondii were compared in Sprague Dawley weaned rats (approx. 130 g) with that in Swiss Webster mice (approx. 25 g). Groups of 5 animals of each species were inoculated orally with 1 to 1 million oocysts. Three of the 5 rats fed 1 million oocysts died of acute toxoplasmosis between 6 and 9 days after ingesting oocysts, all other rats survived. All 20 mice fed > 100 oocysts, 2 mice fed 10 oocysts, and 1 mouse fed 1 oocyst died of acute toxoplasmosis between 4 and 16 days after inoculation. The infectivity of T. gondii oocysts to rats was identical to that of mice. Tissue cysts were found in brains of all rats fed > 10 oocysts and in 3 of 6 rats fed 1 oocyst. Average number of tissue cysts in brains of rats was 300, 180, 528, 600, 396, 1,200 and 2,650 in rats fed 1, 10, 100, 1,000, 10,000, 100,000 or 1,000,000 oocysts, respectively. Microscopic lesions were seen in brains of all T. gondii infected rats and the frequency of lesions was usually proportional to the dose. Antibodies (> 1:512) to T. gondii were detected in sera of all infected rats 29 days after ingestion of oocysts by the modified agglutination test, and the commercially available latex agglutination test and the indirect hemagglutination test.