Infectivity of Trypanosoma brucei-subgroup Flagellates Maintained in Culture

Virulence and infectivity of 4 Trypanosoma brucei-subgroup pleomorphic strains: T. brucei EATRO (E) 795 and 1587, and T. rhodesiense E1895 and Bechuanaland (B) 182A, cultivated in modified Tobie's medium (Tm), were tested in C57BL/6J mice. The 3 freshly isolated EATRO strains could be maintained in Tm by serial transfers, but B182A strain, with a history of ~29 syringe passages in laboratory rodents, was difficult to cultivate. Populations of this latter strain contained many rounded and multinucleate forms both in the bloodstream of mice and in culture. Inocula with identical trypanosome numbers were used in all experiments. Strains E795 and E1895 lost all infectivity for normal mice by the 3rd day of cultivation and strain E1587 remained infective for ~40% of the inoculated animals on the 4th day; even 5-day cultures of strain B182A caused lethal parasitemia in ~40% of the infected mice. In all instances, infectivity and virulence of cultured trypanosomes could be correlated with percentages of nontransformed bloodstream forms (LS, ISS, SS) present in the cultures, no infection apparently being caused by cultures with less than ~20% of such forms. Judging from the survival lengths of mice, the 3 EATRO strains were about equally virulent, while virulence of T. rhodesiense B182A trypanosomes was significantly higher. Essentially identical results were obtained with strain E1895 after 1, 2, or 3 syringe passages in mice. Among hydrocortisone-treated mice, lethal parasitemias (in ~40% of animals) resulted from inoculations of strains E795 and E1895 maintained in Tm medium for up to 6 days, less than 10% of bloodstream forms present in cultures being sufficient for the development of infection in animals whose immune mechanisms were affected by the hormone. The period for which cultures of strain E1895 remained infective and virulent for normal mice could be extended to 5 days by maintaining the parasites in the presence of L cell cultures. Under these conditions, transformation of bloodstream to culture forms appeared to be delayed. The implications of all the results are discussed in the light of the available morphologic, biochemical, and immunologic data.