Avian lymphoid leukosis prophylaxis with mibolerone.

Duplicate trials with mibolerone at a dose of 1 microng/g of diet for the first 49 days of age reduced the incidence of lymphoid leukosis (LL) significantly (P less than .005) in chickens inoculated as day-old chicks with Rous-associated virus type 1 (RAV-1). The incidence of LL was not reduced in virus-challenged chickens administered mibolerone at 4 microng per gram of diet during days 29 through 49 of age. The bursal lymphocytic follicles atrophied significantly with both the 1- and 4-microng doses; but, on the basis of histologic examination, atrophy was more complete with the 1-microng dose of mibolerone administered during the first 49 days of age. Mibolerone appears to prevent LL development by producing early bursal atrophy of key target cells. These data suggest that mibolerone orally administered at 1 microng/g of diet during the first 49 days of age appears to be useful for LL prophylaxis. Field trials are under way to assess the effectiveness of mibolerone under natural conditions.