Equine influenza (EI) is considered endemic in both the U.S. and Europe, but the viruses causing EI differ slightly. For many years, the viruses in circulation in the U.S. have been “Florida clade 1” (FC1) whereas in Europe they have been “Florida clade 2” (FC2). These clades split apart in 2003; circulation of FC2 ceased in the U.S. around 2005 and, by 2010, there was no evidence of FC1 in circulation in Europe. Because of the extensive movement of horses between North America and Europe, the international panel of EI experts has recommended for the last 10 years that EI vaccines contain representatives of both FC1 and FC2. Some (not all) available EI vaccines meet this recommendation.

Equine influenza virus activity has recently increased in the U.S., Europe, and Nigeria. Normally, the virus circulates at a variable, but fairly low, level in the United States, but virus activity surged in the last three months of 2018 with outbreaks in 12 states. Additionally, an extensive EI event occurred in a donkey sanctuary in Nigeria. And, for the first time since 2015, multiple outbreaks of EI were reported during January and February of 2019 in France, Belgium, the Netherlands, Germany, Ireland, England, and Scotland.

In England, it resulted in a temporary lockdown of at least 174 premises and cancellation of racing for six days in February. Outbreaks were also reported in California, Arizona, Ohio, Indiana, and Washington State.

Some of the horses in these outbreaks, in both the U.S. and Europe, had been vaccinated against EI, raising the question: Is this a new strain that is not in the vaccines? The answer appears to be no. While the virus causing the Nigerian EI event is still uncharacterized, genetic analysis of isolates from both England and the U.S. confirms these as FC1