Equine Influenza

Equine influenza is a common respiratory infection of horses caused by an orthomyxovirus of the genus Influenza A. Influenza A viruses can be divided into sub-types on the basis of the antigenic reactivity of the surface glycoproteins, the haemagglutinin (H1-H16) and the neuraminidase (N1-N9) molecules (Fouchier et al., 2005). Equine influenza (EI) is caused by two subtypes H7N7 and H3N8. Outbreaks due to H7N7 are limited as the isolation of the virus has not been reported since last two decades. Recent infection in India (2008-2009) was associated with H3N8 (Virmani et al., 2009). As per the seroservillance report of year 2012-13, 21 out of 1482 samples were found to be positive for equine influenza antibodies (NRCE, 2013). Even in 2016-17 seropositive samples (8 out of 1735) were observed by NRCE India (NRCE,2017). At present no cut-off point for EI antibodies level in positive samples has been decided for the haemagglutination inhibition (HI) test so, serum samples with low titres should be investigated further and at least four times increase in titre between paired serum samples should be considered for recent infection (OIE, 2016). It is usually a self-limiting disease characterised by pyrexia, coughing and nasal discharge (Van and Cullinane,2002). The mortality rate associated with EI is very low, unless disease is exacerbated by secondary bacterial infection or continued work of the horse. It is considered the most important respiratory virus of horses because it is highly contagious and has the potential to disrupt major equestrian events and cause significant economic loss (Timoney et al., 2000). Large outbreaks are
often associated with the congregation of horses at equestrian events. Their dispersal after the event can lead to widespread dissemination of virus (Cullinan et al., 2010). Authors themselves have seen the spread of disease after equine fairs of Hanumangarh and Tilwara in Rajasthan during 2009. The incubation period can be less than 24 hours in new population of horses, and continuous coughing is the main feature of the disease, which serves to release large quantities of virus into the environment. Short incubation time of 1-5 days, fever, harsh dry and hacking cough, running nose, depression and off feed are charecteristics of the disease. Occasionally infection results in suppurative bronchopneumonia and death. Influenza spreads very fast in the susceptible population. The virus spread by the respiratory route, contaminated personnel, vehicles and fomites. In partially immune animals incubation period may extend, some clinical signs may be absent and spread of disease may be limited. After infection with EI virus, horses become susceptible to secondary bacterial infection and develop mucopurulent nasal discharge. In Tilwara equine fair 2009, authors have observed death in some horses due to babesiosis which occurred along with influenza. It was speculated that prior infection with babesia might have exaberated on EI virus infection.
Laboratory diagnosis of equine influenza virus is based on the virus detection in nasal swabs and determination of antibodies in paired serum sample.
The first sample should be taken immediately after suspicion or onset of clinical signs and the second one approximately 15 days later. EI antibodies are determined by using HI test
(OIE,2016).
For prevention of EI inactivated vaccines of epidemiologicaly relevant strains can be used. Vaccination is done quarterly in susceptible equine population. Influenza vaccine provides short time immunity (usually 3 months) and the outbreaks of disease occur after a long gap of several years. So the vaccine for influenza is generally not available in the market in India. Quarntine and biosecurity are the best measures to prevent the spread of this disease. Affected animals should be given a course of broadspectrum anitibiotics (amoxicillin, ceftiofur) to counter secondary bacterial infections along with non steroidal anti-inflammatory drugs and multivitamins.