Mycoplasma bovis pneumonia in marsh deer (Blastocerus dichotomus) from a natural reserve of Argentina.

Mycoplasma bovis is a well-known respiratory pathogen in cattle and has also been reported in bison, white-tailed deer and free-ranging pronghorn. Here, we describe an outbreak of pulmonary mycoplasmosis in marsh deer within a wildlife reserve in Buenos Aires Province, Argentina. The affected marsh deer were part of an extensive grazing system, cohabiting with Pampas deer and separated by a fence from cattle and red deer. Over a five-month period, a cumulative incidence and mortality rate of 21.15% (11/52) resulted, with all affected animals found dead. Overall, six animals presented caseonecrotic and occasionally fibrinous bronchopneumonia. Histopathological examination of lung sections from two analyzed animals revealed caseonecrotic bronchopneumonia closely resembling pulmonary mycoplasmosis in cattle, and Mycoplasma was isolated through bacteriological culture. M. bovis was confirmed via immunohistochemistry and PCR. In one of the lungs, Trueperella pyogenes was also isolated, suggesting synergistic mechanism with M. bovis, impairing respiratory defense, facilitating infection and exacerbating lung lesions. This study represents the first report of M. bovis-associated pneumonia in marsh deer, demonstrating that these deer can develop pulmonary disease and lung lesions like those seen in bovine mycoplasmosis. Given the widespread presence of M. bovis in cattle in Argentina, we hypothesize that fence-line contact with cattle may have been the source of transmission.