Transmission dynamics of simian T-lymphotropic virus type 1 (STLV1) in a baboon breeding colony: predominance of female-to-female transmission.

We investigated the prevalence, distribution, and transmission of simian T-lymphotropic virus type 1 (STLV1) in a baboon breeding colony over a 4-y period. We used polymerase chain reaction amplification of the proviral tax gene to assess the infection status of 272 animals housed in 4 separate corrals. Sequencing the proviral envelope gene from individual baboons detected several molecular subtypes (genotypes) of STLV1. At the start of the study, 31% (54 of 176) of all baboons were infected; the majority of infections (91%) were in mature females, with only 3 of 12 mature males and 2 of 48 infants and juveniles being infected. Over the next 4 years, 41 new infections were diagnosed. Of these, 83% occurred in sexually mature female baboons (at least 3 y of age), 17% in infants and juveniles (younger than 3 y), and 0% in mature males. The 7 infections in juveniles were probably derived from mother-to-infant transmission because mother-infant pairs consistently were infected with the same viral genotype. Of the 34 new infections in sexually mature female baboons, the genotyping data showed that 25 (73%) originated from other infected females as opposed to males. Male-to-female sexual transmission may have accounted for the remaining 9 new infections. There was no evidence of female-to-male sexual transmission. The high percentage of female-to-female transmission of STLV1 in our baboons was unexpected; we speculate that transmission may have occurred due to blood contamination from biting during aggressive behavior between females in establishing hierarchical dominance.