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Equine influenza

Equine influenza (Horse flu) is the disease caused by strains of Influenza A that are enzootic in horse species. Equine influenza occurs globally, and is caused by two main strains of virus: equine-1 (H7N7) and equine-2 (H3N8).[citation needed] The disease has a nearly 100% infection rate in an unvaccinated horse population with no prior exposure to the virus.[citation needed]

While equine influenza is historically not known to affect humans, the impact of an outbreak would have been devastating. Since people heavily relied upon horses for communication (postal service), military (cavalry) and general transport, the social and economic impact of widespread equine disease would have been devastating. However, in modern times the ramifications of equine influenza are most clear in the modern racing industry.



Equine influenza is characterized by a very high rate of transmission among horses, and has a relatively short incubation time of 1–5 days.[citation needed]

Horses with horse flu can run a fever, have a dry hacking cough, have a runny nose, and become depressed and reluctant to eat or drink for several days, but they usually recover in 2 to 3 weeks.[1]

An 1872 report on equine influenza describes the disease as:

"An epizootic specific fever of a very debilitating type, with inflammation of the respiratory mucous membrane, and less frequently of other organs, having an average duration of ten to fifteen days, and not conferring immunity from a second attack in subsequent epizootics."

James Law, Report of the Commissioner of Agriculture for the year 1872[2]


Equine influenza is caused by several strains of the Influenza A virus endemic to horses. Viruses that cause equine influenza were first isolated in 1956. The viruses can cross the species-barrier to cause an epizootic disease in humans, and recently, in dogs.

The equine-1 virus affects heart muscle,[citation needed] while the equine-2 virus is much more severe and systemic.[citation needed]

The disease is primarily spread between infected horses. Exposure to infected waste materials (urine and manure) in stables leads to rapid spread of the disease.


A comprehensive report describing the disease - compiled in response to the 1872 outbreak of the disease in North America - provided a thorough examination of the history of the disease.[2]

Early records

The report notes putative cases dating as far back as Hippocrates and Livius. Absyrtus, a Greek veterinarian from 330 CE, described a disease in the horse population having the general characters of influenza, which the report mentions as the earliest clear record of equine influenza in the lower animals.

The report notes the next recorded equine influenza case in 1299, the same year that a catarrhal epidemic affected Europe. Spanish records note cases in which "The horse carried his head drooping, would eat nothing, ran from the eyes, and there was hurried beating of the flanks. The malady was epidemic, and in that year one thousand horses died."

Prevalence of influenza was found in historic records dating through subsequent centuries of the Middle Ages, but direct implication of horses was not always clear. Neither were recorded instances of record deaths among horses and other animals found to be clear on the exact cause of deaths.[2]

1872 American outbreak

An epizootic outbreak of equine influenza during 1872 in North America became known as "The Great Epizootic of 1872". The outbreak is known as the "most destructive recorded episode of equine influenza in history"[3]. The impact of the outbreak is marked as one of the major contributors to the Panic of 1873 in the United States.

The first cases of disease in pasture horses were noticed in the townships of Scarborough, York and Markham in Ontario, Canada. By October 1, 1872, the first case was seen in Toronto. It took only 3 days before all the horses responsible for street car transportation and major livery-stables were affected. By the middle of the month, Montreal, Detroit, and most of the Dominion of Canada and New England were reporting cases.[2]

By the start of November Ohio, Massachusetts, and South Carolina were reporting cases. The contagion reached Florida and Louisiana by the end of November and was reported in Cuba on December 7. The height of the plague was recognized as December 14, which saw the government of Mexico having to supply disease-free horses to the stricken United States.[2]

The number of infected horses approached 100%, and mortality rates ranged between 1% and 10%. Many horses were unable to stand in their stalls, and those which could stand coughed violently and were too weak to pull loads.[4]

The street railway industry ground to a halt. Every aspect of American transportation was affected. Locomotives came to a halt as coal could not be delivered to power them, while fires in many major cities raged unchecked. One fire in Boston destroyed over 700 buildings. Even the United States Army Cavalry was reduced to fighting on foot against the Apaches, who likewise found their mounts too sick to do battle. The outbreak forced men to pull wagons by hand, while trains and ships full of cargo sat unloaded, tram cars stood idle and deliveries of basic community essentials were no longer being made. The Long Riders' Guild Academic Foundation founder CuChullaine O'Reilly said "the Great Epizootic was the worst equestrian catastrophe in the history of the United States - and perhaps the world".[4]

2007 Australian outbreak

The continent/country of Australia had remained free of equine influenza until an outbreak in August 2007. While the virus was successfully contained and Australia has returned to its equine influenza-free status, the outbreak had significant effects to the country's racing industry.


Prevention of equine influenza outbreaks are maintained through vaccines and hygiene procedures.[citation needed] Countries that are equine influenza-free will normally impose strict and rigorous quarantine measures.[citation needed]


Vaccines (ATCvet codes: QI05AA01 inactivated, QI05AD02 live, plus various combinations) are a major defense against the disease. Vaccination schedules generally require a primary course of vaccines, followed by booster shots. Standard schedules may not maintain absolutely foolproof levels of protection and more frequent administration is advised in high-risk situations.[5]

The UK requires that horses participating in show events be vaccinated against equine flu, and a vaccination card must be produced; the FEI requires vaccination every 6 months.[6] [7]

See also

  • Avian influenza
  • Canine influenza
  • Human influenza

Sources and notes

External links


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