Bovine anaplasmosis: transplacental transmission as it relates to stage of gestation.

Six mature, pregnant, anaplasmosis-susceptible and 3 anaplasmosis-carrier beef cows were used in Anaplasma in utero transmission studies. Susceptible cows were randomly allotted into 3 groups of 2 cows each and were inoculated with a Virginia A marginale stabilate. Each group was Anaplasma-exposed once during 1 of the 3 trimesters of pregnancy. Blood samples were obtained from each fetus at various stages of development for evaluation and for subinoculation into splenectomized calves once during gestation. Precolostral blood from neonates was also subinoculated into individual susceptible calves. Two of the 9 splenectomized calves given fetal blood inoculations developed acute anaplasmosis. The dam of 1 fetus with infective blood was Anaplasma-exposed during the 2nd trimester of pregnancy, and the dam of the second fetus was exposed during her 3rd trimester. Infective fetal blood was obtained during the same trimester of gestation in which the dams were inoculated. Calves given neonatal precolostral blood did not develop anaplasmosis.