Influenza in Horses
Table of Contents
Equine influenza is an acute, contagious, respiratory disease caused by two distinct subtypes (subtype 1: H7N7 and subtype 2: H3N8) of influenza A viruses. These are abbreviated to AE-1 and AE-2. In fully susceptible horses, clinical signs include pyrexia (fever) and a harsh dry cough, followed by mucopurulent nasal discharge. In partially immune, vaccinated animals, one or more of these signs may be absent. Characteristically, influenza spreads rapidly in a susceptible population. Influenza is endemic in North America and most of the world. Periodic point explosive outbreaks occur and are quickly spread by a rapidly mobile population of horses. This may be related to the waning of immunity to influenza in the general horse population and/or genetic drift of the virus strain. Only influenza subtype AE-2 has been isolated over the last twenty years worldwide. There are also distinct differences in virus strain lineage between virus isolates in North America and those in Europe. Respiratory disease outbreaks in groups of horses need to be identified as caused by influenza or one of the other common equine respiratory viruses, such as equine herpesvirus 1 & 4, equine rhinovirus 1 & 2 and equine adenovirus.
The equine influenza strains are not known to cause illness in humans and are classified as AE subtype 1: H7N7 and AE subtype 2: H3N8. These virus strains are different from the strains causing the avian influenza outbreaks of recent years:
The incubation period of equine influenza virus may be as short as 1 to 3 days. Horses can remain infective and shed virus to other horses for up to 10 days in their nasal secretions. A number of factors increase the risk of infection with influenza virus. Horses 1 to 5 years of age are more susceptible to the disease. Those horses with no previous exposure to the virus and/or no previous vaccination to protect them against influenza virus will have a lower immune status and therefore be more susceptible to getting an infection. Those horses that are in barns with a lot of in-and-out traffic will have frequent contacts with a large numbers of horses and therefore are more likely to come in contact with the virus. This is much like children bringing home the common cold from school. Sporadic outbreaks of equine influenza occur throughout all horse areas of the world. The virus infection is spread primarily by aerosolized droplets dispersed by coughing. However, the virus can be spread by contaminated equipment (buckets, grooming aids) and contaminated hands. The virus can survive for hours in the environment. Influenza outbreaks can be avoided by;
Equine influenza virus vaccines usually consist of inactivated
whole virus subtypes AE-1 and AE-2, or their subunits, with or without
adjuvant (to improve immune response). Experts now recommend that
the AE-1 component of vaccines should be omitted as no infections
with that subtype have been substantiated during the past twenty
Using the modified live equine AE-2 influenza vaccine for intranasal administration;
There are no requirements for horses to be vaccinated against equine influenza. There are no restrictions for infected horses, or horses exhibiting signs of a respiratory virus, from leaving or entering shows, tracks or other facilities. There are no requirements for horse owners, trainers, grooms or veterinarians to notify any official (provincial, federal or racetrack) when influenza is suspected.
Guidelines for Vaccination of Horses. Lexington: American Association of Equine Practitioners, Jan, 2001.
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