Humoral immunity and reinfection resistance in dogs experimentally inoculated with Babesia canis and either treated or untreated with imidocarb dipropionate.

It is proposed that the chronic asymptomatic carrier state produced by Babesia canis infection could make dogs more resistant against subsequent infections. This suggests that treatment with imidocarb dipropionate, which removes the organism, can make dogs more susceptible to reinfection in a short period of time. Ten male and female dogs of approximately 4-5 months of age were inoculated with B. canis. Half of them received treatment with imidocarb dipropionate (7 mg/kg) on days 15 and 27 post-infection and the other half were untreated. All the animals were examined using clinical and laboratory methods (CBC, platelet counts and serological study by indirect immunofluorescence test) for a 6-month period. Antibodies were first detected on day 7 post-injection and remained at high levels (1:2560) over the period in the non-treated group. This result was significantly different (P<0.001) from the treated group in which antibodies titers declined after day 34 post-infection. Six months later, after a homologous challenge infection only the dogs of treated group showed parasitaemia, thrombocytopenia and splenomegaly, which was significantly different (P<0.05) from the non-treated group. The sterilizing treatment with imidocarb dipropionate was effective in clearing the infection, but inhibited the maintenance of protective antibodies, making the animals more susceptible to reinfection.