U.K. Equine Influenza Outbreak Update

Since the middle of March, the United Kingdom's Animal Health Trust (AHT) has confirmed influenza in 20 racing yards (stables) in Newmarket, located in different areas of the town, and in a single breaking/holding yard just outside Newmarket. Most of these diagnoses have been made on the basis of nucleoprotein ELISA positive swab samples but two have been diagnosed on the basis of seroconversions on paired blood samples. The last yard was diagnosed on April 28, with a positive ELISA on a nasopharyngeal swab from a coughing horse.

Consistent with previous influenza outbreaks among Thoroughbred racehorses, clinical signs have been relatively mild compared to infections among non-vaccinated horses. Signs in the current outbreak have included acute coughing, mainly at exercise, with some horses developing a nasal discharge. Fever has been seen but is not a consistent sign. An unusual feature of the current outbreak is that, in most affected yards, the first horses to develop disease have been three-year-olds or older horses.

It is not yet clear why the disease has occurred in these well-vaccinated racehorses, most of them receiving booster doses since late December, 2002. Serological data is consistent with recent vaccination with adequately potent vaccines and many infected horses had single radial haemolysis antibody levels that would normally be considered protective.

Antigenic and genetic characterization suggests that the circulating virus is closely related to one isolated in Kentucky 2002.

Influenza virus infection has also been confirmed in a breaking/holding yard in Lambourn, Berkshire, another U.K. training center. However, no cases have been confirmed in any other racehorse training yards other than in Newmarket.

During the same period since the middle of March, 2003, the diagnostic laboratories at the AHT have also confirmed influenza on six other non-racing establishments elsewhere in the UK (Norfolk, Shropshire, Surrey, Sussex, Yorkshire and Warwickshire). These infections have occurred in a variety of types of horses including some vaccinated and some non-vaccinated horses. Clinical signs have generally been much more severe in non-vaccinated horses with marked fever (>40°C) being observed along with harsh coughing and mucopurulent nasal discharge.

Preliminary antigenic analyses of several of the influenza viruses isolated from elsewhere in the UK indicate that similar viruses are concurrently causing respiratory disease among non-Thoroughbred horses in several geographically diverse locations.

Scientists and vets at the Trust are working closely with vaccine manufacturers and human influenza experts in order to unravel the reason for this outbreak among vaccinated horses. In the meantime, they are advising U.K. trainers to consider a re-vaccination (booster) program, in consultation with their own veterinarians, particularly where horses have not been vaccinated in the past three months. It is known that antibody levels decline steadily following vaccination and it is anticipated that boosting in this way will increase the level of immunity, even though it cannot guarantee full protection.

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