A method is described for calculating new
infection incidence from monthly field data collected between April 1994 and February 1998 from an average of 770 Ethiopian Highland Zebu cattle maintained under traditional management in the Ghibe valley, southwest Ethiopia and exposed to a high challenge with
drug-resistant parasites. Each month cattle with a packed cell volume (PCV)<26% and detected as parasitaemic, or cattle showing clinical signs of trypanosomosis, were treated with
diminazene aceturate at 3.5 mg/kg
body weight. An
infection was defined as a new
infection if it was preceded by 2 previous months in which both samples had a PCV>/=26% and were not detected with trypanosomes. Using this definition the average monthly incidence of
infections of Trypanosoma congolense in cattle over 36 months of age was 13.3%, and the prevalence of
recurrent infections 13.0%. Assuming that an animal had recovered from
infection when PCV again returned to 26% without parasites being detected, mean persistence of
infection was 3.8 months (median between 2 and 3 months). In contrast, T. vivax
infections were susceptible to
diminazene. The incidence of T. vivax
infections in adults was lower than for T. congolense (2.8%), and they were less pathogenic (mean reduction of 3.1% units of PCV due to T. vivax
infection compared with 4.6% units for T. congolense). Also, fewer cases were treated. Calves were first detected parasitaemic at an average age of 8.8 months and their
infections persisted longer than those in adults. The effect of age on incidence of
infection was not significant beyond 15 months of age. Adult male cattle appeared to be more susceptible to T. congolense
infection than adult female cattle.