Equine influenza
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:
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H7N3, which affected poultry in the Fraser Valley of British
Columbia in 2004;
H7N2, which affected poultry in Maryland in 2003 and 2004;
H5N2, which affected poultry in Texas in 2004;
HPAI that spread in 2004 to more than ten Asian countries.
Horse Health Risk
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;
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preventing entry of the virus into horse groups by quarantining
newly arrived horses for at least 14 days and
appropriate vaccination before exposure.
Vaccination
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 years.
Using the killed-virus vaccine;
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Younger horses should receive a primary series of 3 doses of
the killed-virus vaccine administered 3-6 weeks apart starting
at 6 months of age.
Revaccination should be at intervals of 3 to 12 months
depending on age and risk of exposure.
Revaccination of performance horses should be at intervals of 3
- 4 months.
Using the modified live equine AE-2 influenza vaccine for intranasal
administration;
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A single administration protects for at least 6 months.
Regulatory Information
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.
Recommendations
1. Horse owners should consult their veterinarian regarding the
need for completing a vaccination program (initial and booster
shots) for influenza.
2. While vaccination does not necessarily prevent influenza, the
disease in vaccinated horses is less severe and the signs last a
shorter period than when horses are unvaccinated.
3. Horses traveling to and from horse events such as shows,
racetracks and facilities should maintain adequate protection
against influenza and the other respiratory viruses.
4. It is important for attending veterinarians to investigate
respiratory disease outbreaks to identify the causal agent.
Influenza strain identity is needed to ensure that vaccines are
kept current with strains of recent outbreaks.