Hopefully we can chalk this one up on the "weird and not-so-wonderful" list, and not have it emerge as a significant problem. However, there’s something new to pay attention to as St. Louis Encephalitis virus (SLEV) infection was recently identified in a Brazilian horse with neurological clinical signs.
In a recent paper (Rosa et al. 2013), SLEV was found in the brain of a horse from Minas Gerais, Brazil that had died of neurological disease. Fairly extensive testing was performed to rule out other causes, so the presence of the virus in the brain and the absence of other potential causes is quite suggestive that SLEV was the problem here. It’s an interesting finding because while this virus is widespread in North and South America, from Canada to Argentina, it rarely causes disease in mammals. It circulates between birds via mosquitoes and occasionally spills over into mammals, which are considered "accidental" and "dead end" hosts since they are not part of the virus’s normal transmission cycle and once infected, they can’t pass the virus on to anyone/anything else. Most infections are reported in people and they tend to be mild, although serious neurological disease can occur, especially in elderly individuals.
So, this would be the first confirmed report of SLEV-associated disease in a horse. The signs of disease were pretty non-specific and included depression, incoordination and paralysis of the hind limbs. That’s something we can see with a few different neurological diseases in horses, including the much more common EHV-1 encephalomyelopathy (and in Ontario and other parts of North America, West Nile virus).
The infection was diagnosed in a horse that died in 2009 after 72 hours of neurological disease. That’s a long time from disease to diagnosis, but in this case it’s probably also a good sign. No other horses on the farm were affected at the time, and no one has reported changes in neurological disease patterns in the area (or beyond) since then; therefore, this presumably was not the start of a large, rapidly moving new disease, as we saw when West Nile virus hit North America in the early 2000s.
Hopefully this was just a strange one-off infection, but it shows the need for continued infectious disease surveillance and the ever-changing challenges of infectious diseases.