Surveillance program for Equine Influenza Virus in the United States (2010-2013)

Recent surveillance studies for equine respiratory viruses have shown that equine influenza virus (EIV) remains a prevalent viral respiratory pathogen of horses throughout the USA and Europe. Due to the highly contagious nature and rapid spread of EIV, this virus poses severe financial implications for the horse industry. The objective of this voluntary surveillance study was to gain a better understanding of the prevalence and epidemiology of EIV shed by horses presented to veterinarians with an upper respiratory tract (URT) infection from March 2010 to November 2013. Equine practitioners throughout the United States (239 practices in 38 states) were asked to collect nasal secretions from equine patients with signs of acute URT infection. A questionnaire was used to collect information pertaining to patient demographics and clinical signs. Samples were tested by qPCR for the presence of EIV. Categorical analyses were performed using a Pearsonu0027s chi-square test to determine the association between observations (age, breed, gender, use, vaccination and clinical signs) and EIV. Further, categorical observations from EIV positive horses from March 2010 to November 2013 were compared to previous data fromMarch 2008 to February 2010 (60 EIV PCR positive horses). Sequencing analysis of the HA1 gene of EIV was performed on 35 different EIV isolates to determine their sequence homology. A total 2,375 horses were enrolled with 230 (9.7%) index cases testing PCR positive for EIV. EIV positive horses in the 1-5 and 6-10 year age group, as well as Quarter Horses, were over-represented, while stallions/colts were under-represented. Fever, nasal discharge and coughingweremore commonly observed in EIV positive horses than the entire study population. EIV positive cases from the 2010-2013 study period were significantly older and more often vaccinated against EIV compared to EIV positive animals from the 2008-2010 study period. Among the 84/230 EIV (+) horses with a confirmed vaccination status, 46% were vaccinated within the previous 6 months and 31% vaccinated within 12 months. All the sequenced EIV isolates belonged to the clade 1 Florida sublineage. This study provides valuable contemporary information on the frequency of EIV detected by qPCR and underscores the fact that older and previously vaccinated horses were susceptible to EIV. There was evidence for lack of vaccine effectiveness following use of inactivated EIV vaccines currently available, including those containing contemporary strains such as A/equine/Ohio/1/2003.