Malignant Catarrhal Fever In A Horse
An article in a recent edition of Emerging Infectious Diseases describes a case of malignant catarrhal fever (MCF) in a horse. MCF is a severe (usually fatal) disease caused by ovine herpesvirus type 2 (OvHV-2) or alcelaphine herpesvirus type 1. It mainly affects cattle - horses have traditionally been considered to be resistant to these viruses, but this recent report from Brazil challenges that belief. It describes a six-month-old foal with severe neurological disease that died one day after signs of illness were first noted. When veterinary pathologists examined tissues from the foal, there was severe damage to small blood vessels in the tissues (lympho-histiocytic fibrinoid necrotizing vasculitis... pathologists like using long names!) that was strikingly similar to that seen with MCF in cattle.
Sheep and goats are the main reservoirs of OvHV-2, and this foal came from a farm that also had goats. Molecular testing was performed on tissues from the foal and OvHV-2 DNA was identified. This virus was also detected in the blood of three adult horses (all healthy) and eight goats on the farm.
This was an interesting and quite a thorough investigation that challenges current dogma about MCF and horses. It's hard to say whether horses have always been slightly susceptible to the OvHV-2, with rare cases of disease going undiagnosed, or whether this is an emerging problem. It's unlikely this is a "hot" strain of OvHV-2 that can now infect horses, because other horses on the farm were fine and there have been no other reports of disease in horses. It's possible that this foal had some underlying problems that made it susceptible to infection by a virus that normally does not cause disease in this species. We certainly shouldn't panic about MCF based on this study and suddenly start keeping all horses away from sheep and goats. However, this case should be a reminder that infectious diseases like to challenge conventional wisdom, and that we need to be on the lookout for new diseases and different patterns of old diseases.
