It's a variant of Murphy's Law: Anytime you think you have a handle on something, the unexpected happens and you're off balance again. The equine health world might be in this boat now concerning West Nile virus infection in horses. With several different types of vaccines available and in widespread use in the United States over the last several years, West Nile virus had gone from terrifying to just another disease to watch out for.

Enter a new variety of West Nile virus, known as West Nile virus genetic lineage 2, which was discussed during the 2009 American Association of Equine Practitioners Convention held Dec. 5-9 in Las Vegas, Nev. This form was previously only seen in horses in sub-Saharan Africa and Madagascar, but it is now showing up in Europe, reported Orsolya Kutasi, DVM, of the Szent Istvan University, Hungary. She discussed the first known outbreak of genetic lineage 2 West Nile virus infection in European horses, which affected 18 animals in Hungary in 2008.

Previously the lack of significant outbreaks with this form had led to the belief that it was not very virulent–that it was not capable of causing severe disease like the lineage 1 form. However, this case series suggests the virus' virulence might have increased, as it showed similar morbidity (illness) and mortality (death) rates in this outbreak that are similar to published lineage 1 rates.

Kutasi explained the 18 cases of lineage 2 infection showed clinical signs quite similar to those of lineage 1 infection–weakness, ataxia (incoordination), muscle twitching, recumbency (lying down), fever, and hyperesthesia (hypersensitivity). Thirteen of the horses survived, and 10 recovered fully