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Modifications in Virulence of a Strain of Toxoplasma Gondii by Passage in Various Hosts

The various manifestations of clinical toxoplasmosis reported in man and animals (Sabin, 1942; Siim, 1951; Fankhauser, 1951; Feldman, 1953; Frenkel, 1953) attest to the possibility that Toxoplasma strains of varying virulence may exist in nature. Also, isolations of Toxoplasma from asymptomatic infections in various hosts, such as the pigeon (Jacobs, Melton and Jones, 1952) are frequently attended by difficulties resulting from the low virulence of the organisms in the early passages in laboratory animals, making the parasites difficult to detect even when their presence may be suspected. Some authors (Sabin, 1941; Frenkel, 1953) have mentioned mouse-adaptation of strains of the parasites and reported that thereafter the infections produced in mice have been uniform. On the other hand, consistent differences in behavior in mice of various strains have also been reported (Frenkel, 1953). It has also been stated that the maintenance of Toxoplasma in chick embryos for as many as 27 passages does not alter the virulence of the parasites for mice on intracerebral inoculation (MacFarlane and Ruchman, 1948).

¹ Laboratory of Tropical Diseases.