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Cattle living in a circumscribed area in Uganda, where both Theileria parva and T. mutans occurred naturally, were monitored for periods of up to 1 year by examination of blood and lymph node smears for parasites and the indirect fluorescent antibody test for antibodies to Theileria. Uninfected control cattle were added at intervals to ensure continuous challenge and to monitor the challenge. Although all 29 control animals became infected with T. parva, challenge was light at first as mortality from theileriosis only commenced in the 18 introduced after day 138 of the study; 14 of these died of the disease. Most of the 34 animals in the original group appeared to be susceptible to T. parva at the start of the study and 6 died of theileriosis. All animals were, or became, infected with T. mutans. Several animals exhibited patent T. parva infections on more than 1 occasion and 1 died from the second infection indicating the existence of immunologically different strains. Recovered animals appeared to acquire eventually a solid immunity to the local Theileria strains but 9 of the original animals and 7 of the controls died exhibiting massive tick and helminth burdens, sometimes aggravated by the effects of previous T. parva and/or T. mutans infections.

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