Mycoplasma wenyonii Associated with Clinical Signs in Lactating Dairy Cows After Herd-wide Bluetongue Vaccination

An outbreak of clinical signs of hindlimb and udder oedema, pyrexia, prefemoral and mammary lymphadenopathy and milk drop, affecting only lactating dairy cows, on a 40 cow dairy herd in North Yorkshire, was investigated. Mycoplasma wenyonii (MW)-specific DNA was detected by PCR and Denaturing Gradient Gel Electrophoresis (DGGE) in blood from all sampled cows showing clinical signs although no epi-erythrocytic organisms were ever seen on blood smears from any cow. Subclinical parasitaemia was also shown to occur. Older cows were significantly more likely to be parasitaemic and to manifest clinical signs. Clinical signs in cows were clustered in the 5 days following each of two herd-wide vaccinations for Bluetongue virus. The ultimate source of the infection was not established but the study provides some evidence that the clinical signs associated with MW parasitaemia may be in part immune mediated.