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Influenza: Are we protecting our horses effectively?

Equine Veterinary Journal2013Vol. 45(6), pp. 766–767

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Celia M. Marr, Debra C. Sellon, D Mountford

Abstract

In the UK, a mandatory flu vaccination policy was introduced by the Jockey Club in 1981. The fact that since then no single day's racing has had to be abandoned is often quoted as evidence that this programme is effective. However, the devastating flu outbreak that rampaged through the Australian horse population in 2007 was an important wake-up call reminding us that equine influenza virus (EIV) is an ever-present threat. In an article in this issue, Legrand et al. describe the results of retrospective analysis of laboratory samples by polymerase chain reaction in France where EIV was found in around 15% of nasopharngeal swabs, from horses that were and were not vaccinated 1. In this issue, Daly et al. have contributed a state-of-the-art review article on mathematical modelling of flu outbreaks 2. While they point out that mathematical modelling is only as good as the data that underpin the assumptions made in the models, they illustrate how modelling can be used to help inform decision-making on prevention and outbreak management. A number of key features of influenza are highlighted including that the majority of outbreaks in vaccinated groups of horses are of limited size and vaccination reduces the occurrence of epidemics 2-4. It's simple then, surely: vaccinate your horses and all will be well. Not so: Legrand et al.'s study, in which flu was diagnosed in both horses that were and those that were not vaccinated 1, is one of many that highlight this concern and it is recognised that although of little consequence for individuals, mismatched vaccine strains increase the likelihood of large outbreaks 5. The flu virus' ability to undergo antigenic drift means that strains in flu vaccines go out of date. The World Organisation for Animal Health (OIE) is responsible for recommending suitable vaccine strains for inclusion in commercial vaccines and, as described by Elton and Cullinane in this issue 6, its decision making is informed by ongoing surveillance data provided from numerous centres across the globe, including in the UK from the Equine Influenza Programme (http://www.equiflunet.org.uk/), which is generously supported by the racing industry via the Horserace Betting Levy Board (http://racehorsehealth.hblb.org.uk/). The British Equine Veterinary Association (BEVA) has recently formed an initiative to bring interested parties together and to lobby to ensure that our horses are protected in the most effective ways possible. Representatives of BEVA have met with representatives of the animal health industry, the medicines regulators (the UK's Veterinary Medicines Directorate), sports regulators (Fédération Equestre International [FEI, http://www.fei.org] and British Horseracing Authority [BHA, http://www.britishhorseracing.com/]) and scientists involved in influenza surveillance. Viruses currently circulating are of the American lineage, Florida Clades 1 and 2 6. At the time of writing, Boehringer Ingelheim Vetmedica Inc has a license within the USA for an EIV-containing vaccine(s) that contains both Clade 1 (Ohio/03) and Clade 2 (Richmond/07) equine influenza isolates or antigens as recommended by the OIE. There are no other equine vaccines available in the USA or UK that contain Clade 1 and Clade 2 equine influenza isolates or antigens as recommended by OIE in 2010. However, there are vaccines that contain Clade 1 isolates and therefore can claim to have strains updated to comply with the OIE's 2004 recommendations. Challenge studies are used to demonstrate cross-protection with a variety of combinations of vaccine and virus strains. It is important to acknowledge that vaccine strain is not the only factor that influences the effectiveness of any specific product and many of these additional considerations are outlined in the Editorial in this issue by Horspool and King, writing from the animal health industry's perspective 7. The pharmaceutical companies can very justifiably point out that, although some vaccinated horses develop signs when exposed to natural challenge with EIV, to date there have been no major flu outbreaks in vaccinated populations. There is evidence of cross-protection, and as described in Daly et al.'s review 2, protection is not an all or nothing phenomenon. Rather, vaccination is aimed not only at limiting the spread of infection but also at protection of the vaccinated horse from developing severe clinical signs. Vaccine strain may not be so critical in the second goal. The FEI has developed regulations for which implementation must be achievable globally 8. Many racing authorities have no requirement for vaccination while others do not comply with current manufacturer's guidelines. The medicines regulators must also be satisfied before new products can be launched 9. Compliance with sport regulators' requirements is a powerful motivator to ensure that vaccination takes place, but many horse owners are happy to defer decisions on the most appropriate product to their veterinary practitioners. All too often, veterinary practitioners decide to buy a specific product for reasons that are relatively unrelated to efficacy and indeed, purchasing decisions may be made by pharmacy officers who are more interested in the goodies on offer than the complex and ever-changing details of circulating virus strains. In the Special Focus on Equine Influenza section of this issue, we include editorials setting out the viewpoint from four positions: the surveillance programme leaders 6, the sports regulator 8, the medicines regulator 9 and the animal health industry 7. In doing so, we hope to present our readers with a well-balanced set of opinions so that they can judge for themselves: are we protecting our horses effectively? Celia Marr is Deputy Chairman of the Horserace Betting Levy Board's Veterinary Advisory Committee; funders of the Equine Influenza Programme at the Animal Health Trust, UK.

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Abstract

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