Field outbreaks of influenza that occurred in vaccinated Thoroughbred racehorses in Newmarket in 1995 and 1996 were investigated by nucleoprotein ELISA and serology. Investigations showed that serum levels of vaccine-induced single radial haemolysis (SRH) antibody correlated closely with protective immunity against equine influenza and were consistent with observations made in previous experimental studies using nebulised aerosol challenge. In the second part of this study, antibody levels stimulated by vaccination were investigated to examine probable protection in high risk groups, such as yearlings and horses in training. Results for yearlings correlated closely with experimentally derived antibody profiles described for several equine influenza vaccines. The horses in training had levels of antibody immediately prior to revaccination, which were higher than those measured in the yearlings. In conclusion, SRH antibody, used in the investigation of outbreaks and surveillance of post vaccination responses, was shown to correlate with and validate experimental vaccination and challenge models currently used in ponies in the licensing of modern vaccines. There may be benefit from serological monitoring of horses following vaccination through identification of susceptible periods to infection and demonstration of poor vaccine responders. This would allow appropriate and timely amendment of vaccination strategies to maximise protective immunity against influenza.

In July 1989 influenza A/equine-2 (H3N8) was isolated from a nasopharyngeal swab taken from a non-thoroughbred horse exhibiting acute clinical respiratory disease. This was the first isolation of equine influenza virus in the United Kingdom since 1981. Subsequent investigations of acute respiratory disease in horses indicated that the infection was dispersed throughout the UK. However, unlike the previous epidemic of 1979, the first horses from which the virus was isolated had been vaccinated. This outbreak of influenza provided an opportunity to evaluate an antigen capture ELISA, directed against the influenza virus nucleoprotein, as a rapid method for detecting virus in the nasopharyngeal secretions of naturally infected horses.

The production of interferon (IFN), interleukin-6 (IL-6), and tumor necrosis factor (TNF) was monitored in horses during the course of influenza A2 virus infections. The effects of two virus strains, Newmarket/2/93 and Sussex/89, were compared, of which the latter is considered the more pathogenic in terms of clinical signs. Ten naive ponies were infected with influenza A/equine/Sussex/89 and 10 with influenza A/equine/Newmarket/2/93, respectively. As expected ponies infected with Sussex/89 showed the most pronounced clinical signs but there was no notable difference in viral excretion compared with Newmarket/2/93. IFN was detected in nasal secretions of all ponies infected with Sussex/89 but only in 2 ponies infected with Newmarktet/2/93. IFN was not detected in serum of any animal. IL-6 activity was detected in nasal secretions of all experimental animals from day 2 and onwards, but showed markedly higher IL-6 responses were observed in ponies infected with Sussex/89. No TNF activity was detected in any of the samples collected. In summary, equine influenza A 2 infections elicited local, and in some cases systemic, IFN and IL-6 responses in the ponies. Interestingly, there was some evidence that the duration and levels of cytokine responses may be related to the pathogenicity of the influenza strains.

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