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Behaviour of a Trypanosoma brucei brucei stock (STIB 348C) in mice. 2. Course of infections with the basic type and several variant types.

Parasitemia in mice was followed using daily blood smears. Infections with few trypanosomes of the basic type regularly showed a prepatency, a primary wave of parasitemia, a secondary latency and an uncharacteristically fluctuating secondary parasitemia, leading to death 28-144 days after infection. The characteristics of the infection were not altered by serial transfers of 100 trypanosomes every 5th day in mice for several months. Infections with single trypanosomes out of early secondary parasitemias (that is: with expression of other variant antigenic types) mostly led to lower or higher primary peaks, otherwise to similar courses of the infections. In infections with many trypanosomes out of early secondary parasitemias the parasitemia developed similarly to secondary parasitemias after primary peaks. Secondary parasitemias were little influenced by attempts at active or passive immunization, they must be assumed to contain trypanosomes with expression of many different variant antigenic types.--Similar courses of infections were seen in rats.--Trypanosomes transferred to new mice from mice that had been infected for a long time usually caused markedly shortened infections with a) a continuously low parasitemia, b) an initially low parasitemia, rising very soon to lethally high levels, c) a rapidly rising parasitemia, lethal in the first wave or after one remission. These trypanosomes are regarded as variant metabolic types that have lost the density-dependent inhibition of proliferation in blood and/or tissue, characteristic of the original type.