Experimental toxoplasmosis in young piglets.

Piglets eight to 10 days old and one day old were infected with a cyst-forming strain of Toxoplasma gondii of low virulence. The older piglets developed an immunological response to the infection and survived, although inflammatory lesions were demonstrable in a variety of tissues. Some of the one-day-old piglets died; they exhibited more severe inflammatory lesions and lymphoid depletion in the lymph nodes, spleen and thymus. The tissue reaction resembled infection with a high-virulence strain of the organism. Control animals showed a delay in maturation of the lymphoid system during the first week of life. The virulence of the organism in therefore modified to a certain extent by the competence of the animal to develop an immunological response to the infection.