Factors influencing the duration of isometamidium chloride (Samorin) prophylaxis against experimental challenge with metacyclic forms of Trypanosoma congolense

The duration of a single isometamidium chloride (Samorin) prophylactic treatment against Trypanosoma congolense ILNat. 3.1 and T. congolense IL 285 was examined in 24 Boran steers with regard to (1) the dose of drug, (2) the level of metacyclic challenge and (3) the influence of infection with an unrelated serodeme at the time of treatment. The cattle were repeatedly challenged at monthly intervals between 2 and 7 months following treatment, either by five infected Glossina morsitans centralis or by intradermal inoculation of 5 X 10(3) or 5 X 10(5) in vitro-derived metacyclic trypanosomes. A dose of 1 mg kg-1 afforded complete protection for 4 months and 0.5 mg kg-1 for 3 months against the two T. congolense serodemes examined, irrespective of the method or weight of challenge. In another group of cattle, which had an established infection at the time of treatment, the duration of chemoprophylaxis against an unrelated serodeme was the same as the other groups which had no previous experience of trypanosome infection. Antibodies to metacyclics did not appear in any of the cattle as long as the chemoprophylaxis was effective. An exception to this was the group challenged with 5 X 10(5) in vitro-derived metacyclic parasites, in which low antibody titres were detected. In all cases these proved to be non-protective. It was concluded that, under the experimental conditions employed, (1) there was a direct relationship between drug dosage and the duration of chemoprophylaxis, (2) the weight of metacyclic challenge did not affect the duration of chemoprophylaxis and (3) when used to treat an existing infection, isometamidium chloride exerted the same degree of chemoprophylactic activity.