Walking On Volume 6, Issue 4, April 2019 - Page 14

For the Health of It Equine Influenza Reprinted with permission from the April 2019 issue of Equine Disease Quarterly Equine influenza (EI) is considered endemic in both the USA and Europe, but the viruses causing EI differ slightly. For many years, the viruses in circulation in the USA 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 USA around 2005, and by 2010 there was no evi- dence 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 ten years that EI vaccines contain representatives of both FC1 and FC2. Some (not all) available EI vaccines meet this recommenda- tion. EI virus activity has recently increased in the USA, Europe, and Nigeria. Normally, the virus circulates at a variable, but fairly low, level in the USA, but virus ac- tivity surged in the last three months of 2018 with out- breaks in 12 states. An extensive EI event occurred in a donkey sanctuary in Nigeria. 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 6 days in February. Outbreaks were also reported in Califor- nia, Arizona, Ohio, Indiana, and Washington State. Some of the horses in these outbreaks, in both the 14 • Walking On USA and Europe, had been vaccinated for EI, raising the question: Is this a new strain that is not in the vac- cines? 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 USA confirms these as FC1. There are two mutations that make them different from the recommended vaccines strains, but are these important? That is still under investigation. The absence of FC1 from Eu- ropean circulation means their horses have no natu- ral immunity and are dependent on vaccination for protection. The reports from England indicate that the clinical disease is of shorter duration and less severe in vaccinated horses; this would indicate the vaccines are working, at least partially. Milder disease in vaccinated horses may reflect an inadequate level of protective immunity following exposure to unvaccinat- ed horses shedding large quantities of virus. What should owners and veterinarians do to protect their horses from EI? 1. Familiarize yourself with the clinical signs of EI. Often the very first sign is a harsh cough. Other signs include fever and nasal discharge, which is usu- ally watery (serous) at first and then turns thick and yellow (mucopurulent). The horse may show unusu- ally rapid breathing (tachypnea) or lose its appetite (anorexia). Sometimes there may be enlarged subman- dibular lymph nodes or dependent limb edema. Keep