Equine influenza

Equine flu is a common respiratory disease of horses and among equines of different species. It is distributed worldwide. It has a big health and economic impact, mostly in sport horses, since it will influence their training and competition schedules. Furthermore, due to the international regulations to which the disease is subject.

The disease is produced by an Influenza A virus, of two subtypes: H7N7 and H3N8. The subtype H7N7 (strain 1956 Prague) is presumably extinguished and has not been isolated for more than 30 years. The subtype H3N8, identified in 1963, circulates widely with different lineages of Eurasian and North American viruses, distributed in different geographical areas of the world. Both viruses seem to have evolved from an avian Influenza virus ancestry. Dogs can acquire the infection of the equine H3N8 virus. Cases reported in different countries have been associated with outbreaks of equine Influenza as well as dogs in close contact with horses. Equine influenza is a highly contagious infection. In mares it can reach a high morbidity and this occurs in outbreaks.

Transmission is airborne by direct and indirect contact, and concentration and transport of animals favours the spread of the infection. The susceptible animals tend to become infected, suffer the infection, develop a transient immunity and later return to the initial situation of susceptibility. As a result, equine influenza is a disease that can be prevented by vaccination. In non-vaccinated horses, clinical signs consist of upper respiratory congestion along with a rough, dry cough, fever, respiratory distress of a greater or lesser severity, and a rapid development. Also, horses can develop depression, lethargy, loss of appetite and serous or mucoid nasal discharge. In vaccinated animals respiratory symptoms are mild or subclinical. The infection is usually self-limiting, and resolves in 2-3 weeks.

However, in very young foals it may predispose to secondary infections with fever and the development of pneumonia. Recovery in sport horses is longer, as during the process there is often muscular pain and inactivity. In dogs, clinical respiratory signs are similar to those in horses and are characterized by dry cough, nasal discharge, without response to antibiotic treatment, shortness of breath of different degree, and, mainly, by rapid spread among animals in close proximity. The control measures include inactivated vaccines to be widely used, that must be administered in several doses with semi-annual or annual intervals. Influenza vaccination in horses is of forced compliance in animals of “high athletic performance” attending national competitions and international contests.

Equine vaccines have limited efficacy, as happens in other animal species, due to the small antigenic changes of the influenza virus or antigenic drift, which occur amongst different strains. In addition, the introduction of virus in susceptible populations has been reported, a result of the entry into a stud farm of vaccinated animals with subclinical infections. The OIE recommends to include in the composition of vaccines at least two subtypes of the American lineage of H3N8 virus that are currently circulating, but not recommending the inclusion of H7N7 virus. The process of selection and update of vaccine strains is closely associated with the surveillance programs in charge of identifying and isolating the virus circulating in the equine population.

Vaccination programmes against influenza in the equine population are intended to reduce the clinical incidence and to remove the virus, to prevent the risk of disease outbreaks, to limit the size or magnitude of outbreaks or to reduce the risk of an outbreak of the disease when the animals congregate at contests, races or competitions. In addition, they confer some level of cross- protection against new strains of equine flu that may appear. Lastly, vaccination can also be used as a control measure during the course of an outbreak of influenza. The most recommended preventive measures are always directed to increase biosecurity levels, both in the farm and during transport, at sporting events, etc.

The compliance of basic measures, such as quarantines, for example, would serve to prevent the entry of new strains of virus on the premises; or the application of good management and hygiene practices that will reduce the spread of virus. Within these basic measures the immediate isolation of sick horses, individual control of the movements of animals, the technical training of caregivers, for instance, are easy, affordable measures within everybody’s reach and a very useful tool in the fight against equine Influenza.