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VETERINARY LoĂŻc J Legrand showed that horses that had been vaccinated had lower incidences of fever and coughing when they did become infected by flu compared to unvaccinated horses in the same outbreak. This highlights a very important point: It is not realistic to expect vaccination to protect every single horse from ever becoming infected. Rather, the goal is that by vaccinating groups of horses, individual and herd immunity is boosted so that when horses do encounter the flu virus, they shed less virus (and thus are less contagious to others), clinical signs are less severe, and equestrian events are not at risk of cancellation due to influenza.
Management and environmental factors involved in flu outbreaks
Teasers come into direct contact with many mares and can easily spread the flu virus
The pattern of clinical signs seen in one Irish training yard affected by flu. The coloured boxes show where the horses developed clinical signs. In white boxes, the horses remained healthy although some either shed virus (denoted by suprascript 1 and 3) or mounted an antibody response to the virus (denoted by suprascript 2 and 3). Although the virus spread throughout the yard, there were clusters. Overall, the vaccination status of this group was high, with over 89% of horses having up to date vaccination records. The first case, which had not been vaccinated for 15 months, is indicated by the turquoise box. Horses that developed signs on Day 3 are indicated by purple boxes, Day 7 red, Day 8 yellow, Day 9 orange, Day 10 pink, Day 11 blue, and Day 19 green. The number in brackets indicates months since previous vaccination; U indicates those with unknown vaccination records. Recent vaccination helped minimise the effects of the virus: two of three horses that had not been vaccinated for more than 12 months showed clinical signs compared to 13 of 49 of those vaccinated within the last seven months. Of the four horses with unknown vaccination history, one showed signs, and two more mounted an immune response. E indicates empty boxes. (Diagram from Gildea et al, EVJ, DOI: 10.1111/j.2042-3306.2010.00333.x)
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As part of a PhD project, Sarah Gildea from the Irish Equine Centre performed a major review of flu outbreaks on 28 premises across Ireland from 2007 to 2010 with the aim of identifying risk factors and improving control strategies. Detailed investigations were conducted on 16 of these premises, and on 15 premises, flu came after the movement of horses. The only farm that did not have recent horse movement was immediately adjacent to another infected property. The key risk factors for spread within premises were: l Housing type: the proportion of horses affected on a property ranged from 50% in horses at pasture, 54% in groups kept in individual stables, and 94% in groups kept in barns. l Teaser stallions: on stud farms, these are individuals that have close nose-to-nose contact with the largest numbers of mares, and therefore, if they do develop flu they are particularly likely to spread it. l Fomites and personnel: Dr Gildea was able to show that on two premises horses that had no direct contact with others became infected. This suggests that fomites such as stable equipment or even stable staff might play a role in spreading infection. None of these premises had fully-up-to-date vaccination status in 100% of their horses, but in the most well-vaccinated yard 89% of the horses had had recent boosters. Vaccination status, number of years of vaccination, time since last vaccination, and age influenced the clinical signs, with younger horses being more likely to be clinically affected. Gildea’s recommendations aimed at stopping spread on a premises are: l Isolation and clinical monitoring of new arrivals and horses returning from equestrian events; l Serological testing of new arrivals and vaccination as appropriate; l Vaccination of horses at six monthly intervals, especially young horses and teasers; l Maintenance of effective boundaries between equine premises;