EHV infection: the clinician’s viewpoint

Equid herpesvirus-1 has probably been around horses for many thousands of years. Would it be only for the mild clinical signs of respiratory disease in the adult horse, the interest from a scientific or industry perspective would be negligible. However, we know too well that EHV-1 infections are associated with significant secondary complications of myeloencephalopathies (EHM), chorioretinopathies and late-term abortions. In particular the acute onset and clustering of abortions or neurological disease within a herd makes EHV-1 infections such an insidious disease. However, EHM outbreaks follow a pattern. The majority of outbreaks occur in winter and spring. Few EHM outbreaks start with an EHV-1 abortion, while more commonly, there is a history of horses returning from an event or there has been introduction of new horses to the premises. These horses will not necessarily develop EHM, and it is likely that subtle signs of respiratory disease or low grade fevers will be missed. However, there is replication and shedding into the environment with possible transmission into others. Concurrent with respiratory tract replication there is virus transportation into regional lymph nodes where lymphocytes and monocytes become infected. As their job these cells will recirculate and cause stage 2: cell-associated viraemia. Early respiratory tract infection in the adult horse is silent with minimal nasal discharge, lymphadenopathy and a low-grade fever (38.0–38.5°C). Viraemia is different, and viraemic horses are often febrile for several days with temperatures between 39.0 and 41.0°C. During viraemia endothelial cells (EC) of the CNS and/or gravid uterus become infected. Almost instantly with cessation of the viraemic fever EHM can develop, while an abortion can have a lag time of several weeks after the viraemia period.