Equine Influenza Virus Infection Surveillance Data 

Data on the incidence of equine influenza infections indicate it might be necessary to modify EIV immunization recommendations.
Black horse in stall, quarantined for equine influenza
The highest incidence of equine influenza virus cases in this study was in horses younger than 9 that had traveled recently. | Getty Images

Merck Animal Health, in collaboration with the University of California, Davis, School of Veterinary Medicine, has compiled data on changes in the incidence of equine influenza and other respiratory infections from March 2008 – June 2021. The data include 9,700 horses that were evaluated through nasal swabs and whole blood within three days of showing clinical signs. Criteria for inclusion in the study is based on a fever ≥ 101.50 F and one of the following clinical signs: lethargy, nasal discharge, cough, and/or acute onset of neurologic disease [Chappell, DE.; Barnett, DC.; James, K.; Craig, B. et al. Voluntary Surveillance Program for Equine Influenza Virus in the United States during 2008 – 2021. Pathogens 2023, vol 12 (2), 192; doi.org/10.3390/pathogens12020192]. This biosurveillance program is ongoing. 

Common Upper Respiratory Equine Pathogens

Nasal swabs and blood were acquired from horses at 333 veterinary clinics in 42 states. Samples were tested by qPCR for six of the most common upper respiratory equine pathogens:  

  1. Equine influenza virus (EIV). 
  1. Equine herpesvirus-1 (EHV-1). 
  1. Equine herpesvirus-4 (EHV-4).  
  1. Streptococcus equi  subspecies equi (S. equi). 
  1. Equine rhinitis A virus (ERAV). 
  1. Equine rhinitis B virus (ERBV).  

Of the total number of submitted samples, 45% tested positive for one of these six pathogens, with S. equi the most common and EHV-4 the next most identified. 

Risk Factors for Equine Influenza Virus

The highest incidence (9.9%) of EIV was found in horses younger than 9 that had traveled recently, especially in winter and spring. The highest prevalence of EIV-positive samples occurred in 2019 (15.3%), 2013 (12.5%), and 2020 (12.4%). Use and equestrian pursuit did not correlate with whether a horse tested positive or negative, although travel did. Age was a determinant factor in this study; horses aged 1-4 and 5-9 years had double the chances of testing positive. Horses over the age of 20 were half as likely to test positive. Young horses under the age of 1 had a low incidence of EIV positivity. Mares tested positive more often than geldings and stallions. 

An important factor that increased the risk of EIV infection was travel within 14 days prior to sampling. When horses are transported, they are often exposed to higher stress levels, immune suppression, and outside animals, coupled with varying levels of biosecurity at events.  

Another key factor in the data was season. While influenza appears at all times of year, the highest prevalence of positive tests occurred in the winter and spring months, from December to May. There is a lower incidence of EIV in the summer and fall months.  

Equine Influenza Virus Vaccination Recommendations

With this information in mind, practitioner recommendations for EIV immunization might require modification. The study authors suggest that for a biannual vaccination program, in addition to an annual spring booster, August/September boosters should be pushed back to November/December. Horses at high risk of exposure and morbidity might be better protected with a third EIV booster; these horses could be vaccinated in the spring, early fall, and then again in November/December. The EIV vaccine product used should meet current OIE (World Organization for Animal Health) recommendations for relevant circulating EIV strains. 

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