What in the World Is Going On with Equine Influenza?

Equine influenza has appeared in Japan for the first time since 1972. The first cases were observed in mid-August, and soon thereafter several hundred clinical cases appeared in multiple training centers. This outbreak prompted the Japan Racing Association (JRA) to cancel all race meetings and ban horse movements throughout the country. Meanwhile, on August 17, equine influenza was detected in imported horses at a quarantine station in Sydney, Australia. Equine influenza is an exotic disease in Australia; it had never been detected there before.

The horses remained in quarantine, but the virus did not. On August 24, influenza was detected at an equestrian center in central Sydney. It has since spread to many other premises in New South Wales and Queensland, Australia. In response, the Australian Minister for Agriculture, Fisheries and Forestry called it the most serious disease threat Australia's horseracing industry had ever faced and issued an order for a national standstill in movement of horses, donkeys, and mules.

Equine influenza is a common upper respiratory disease of the horse. Typical symptoms include fever, nasal discharge, and a dry hacking cough. It is considered enzootic in the USA and Eurasia and has also circulated widely in South America, China, and India. There are several vaccines available, and the current generation of vaccines has been carefully tested to prove effectiveness. 
Since Australia previously was free of equine influenza virus, vaccination was not practiced, and the horse population had no immunologic protection: an ideal condition for equine influenza to spread very quickly.


Rapid diagnostic testing and virus nucleotide sequencing revealed that the Japanese and Australian viruses are subtype H3N8, which is the same subtype as all other equine influenza viruses currently circulating (see Figure 1). The viruses are related to other recent equine influenza viruses and are not unique. As of August 30, no fatalities had been reported.

Equine influenza is occasionally fatal in young foals lacking protection from maternal antibodies. But with appropriate veterinary care, it is rarely fatal even in naïve horses from weanling age onwards. Sometimes, especially in vaccinated horses, the clinical signs are so mild as to be overlooked, which is another reason why influenza spreads so easily: unrecognized subclinical cases can still shed virus that is infectious to contact horses.

Effective quarantine is the best prevention against the introduction of disease. Australia, Japan, South Africa in 2003, and also Hong Kong in 1992 used quarantine systems for imported horses with the explicit purpose of keeping exotic diseases like equine influenza out of their countries. In both South Africa (2003) and Australia (2007) it appears that their quarantine systems did succeed in keeping influenza-infected imports separate from the general horse population, but they failed to stop the transfer of the virus itself into the general population. This transfer strongly suggests that influenza might have been carried out of the quarantine facilities by incidental human activity--perhaps in unwashed horse trailers, on unwashed hands or equipment, or on clothing. The influenza virus particle is easily killed by soap or common disinfectants, but it can survive for hours or days in the environment and even longer if kept cool and moist. Vigilance against the spread of influenza viruses on contaminated materials or unwashed hands is an essential part of quarantine. Equine practitioners and handlers who have contact with both sick and healthy horses in a day's work must be always careful and never complacent.

Influenza is a moving target--the viruses mutate and gradually change so that the vaccinated horse's immune system no longer recognizes them. Vaccines need to be updated to keep up with the changing virus. The Office International des Epizooties (OIE) has an equine influenza expert surveillance panel that annually reviews the international situation and makes recommendations regarding the best virus strains to use in vaccines. The linchpins of the surveillance system are the OIE international reference laboratories for equine influenza, located at the Animal Health Trust (Newmarket, United Kingdom), the Maxwell H. Gluck Equine Research Center (Lexington, Kentucky, USA), and the Institute for Medical Microbiology (Munich, Germany). The latest recommended update, originating in 2004, is that vaccines should contain strains similar to the South Africa/2003 virus and the Newmarket/2/93 virus. It takes time for updated recommendations to be translated into commercially available products, and to date no vaccine in the USA exactly meets the latest recommendation. Fortunately, the current generation of vaccines still retains some effectiveness.

CONTACT: Dr. Thomas Chambers, 859/257-4757; tmcham1@uky.edu; Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

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