Influenza update: why should we worry?

Equine influenza is a highly contagious disease of horses and other equids, such as donkeys, mules and zebras. Like all influenza viruses, equine influenza viruses undergo antigenic variation, either by the accumulation of random changes in nucleotide sequence that results in slight alterations in the structure of the major envelope antigens (antigenic drift), or by genomic segment reassortment (antigenic shift). The first reported isolate of influenza virus in horses was designated influenza virus A/ Equine/Prague/1/56 (H7N7) [1]; however, another subtype of influenza was isolated from horses in the United States, A/Equine/ Miami/1/63 (H3N8) in 1963 [2]. Viruses from the H3N8 subtype have been the predominant influenza viruses isolated from horses in the past 3 decades. While there has been no evidence of any subsequent antigenic shift events, there has been substantial antigenic drift reported in the currently circulating H3N8 viruses [3,4]. The highly contagious nature of equine influenza is a cause for veterinarians to worry. After the emergence of the H3N8 subtype in 1963, it was estimated that one infected horse was capable, on average, of infecting 10 susceptible, in-contact horses [5]. The highly contagious nature of the virus, and the level of susceptibility of the horse population both contribute to the speed and ease of spread of the virus to new, susceptible horses. Affected horses are highly likely to cough and this is a very effective mechanism by which the virus is spread to other susceptible horses via the respiratory route of transmission. Several recent outbreaks of equine influenza in countries previously free from this disease have clearly demonstrated the contagious nature of this infection. In the 2007 Australian equine influenza outbreak approximately 6,000 premises that housed 47,000 horses were affected in the 4-month period from August to December [6]. The speed of spread of this infection through a naïve population was certainly a significant cause for concern for the Australian veterinary authorities.
Why should we worry? There are several reasons why the equine industry in the UK should worry about equine influenza. Antigenic drift represents an ongoing, ever-present low level of worry for the horse industry in endemic countries like the UK, while antigenic shift represents a very low likelihood, but high consequence worry for the global horse industry. Current vaccination protocols in the UK and Europe have reduced the impact of this disease in high performance horses and while the currently available vaccines are not strictly compliant with the recommendations of the IOE expert panel, there is reasonable evidence of their efficacy against currently circulating strains [7]. However, should a virus strain arise in the UK similar to the strain identified in China in 1989 [8] which caused high levels of mortality then there would be serious cause for ‘worry’. More serious still would be the emergence of a new subtype of equine influenza, for which all current vaccines would be ineffective. Currently, in endemic countries there is no widely publicised contingency plan in place for eradication or control of equine influenza should a new subtype emerge. Should this new subtype cause death in a large proportion of infected horses, and given the highly contagious nature of influenza in a naïve population, then ‘worry’ will rapidly become replaced with panic.