Management and environmental factors involved in equine influenza outbreaks in Ireland 2007–2010

Reasons for performing study: Outbreaks of equine influenza (EI) in endemic populations continue to cause economic loss despite widespread vaccination. Hypothesis: To identify the key management and environmental factors that determine the risk of horses contracting EI in an endemic country and to identify control strategies. Methods: Real time-polymerase chain reaction (RT-PCR), virus isolation and haemagglutination inhibition were carried out on nasopharyngeal swabs and clotted blood samples collected from horses and ponies showing signs of respiratory disease. On premises where a diagnosis of EI was confirmed, the attending veterinary surgeon was asked to participate in an epidemiological investigation. Results: Between June 2007 and January 2010, EI outbreaks were diagnosed on 28 premises located in 13 of the 32 counties of Ireland. Veterinary advice was sought on average more than 5 days after the first clinical signs were observed. The majority of diagnoses were made by RT-PCR. Data from 404 horses on 16 premises were used in the epidemiological analysis. In 15 premises, EI was identified following movement of horses. Housing type, teaser stallions or fomites/personnel contributed to virus spread. Vaccination status, number of years vaccination, time since last vaccination and age influenced disease expression. Isolation and vaccination were effective control measures on the premises where they were implemented. Conclusions: Preventative measures include: isolation, clinical monitoring, serological testing and vaccination of new arrivals, booster vaccination of horses at 6 monthly intervals, maintenance of effective boundaries between equine premises and avoidance of stabling in single air spaces. Control measures include: prompt isolation of suspected cases, rapid diagnosis by RT-PCR, booster vaccination of cohorts and implementation of biosecurity measures to avoid transmission by fomites and personnel. Potential relevance: Implementation of these preventative and control measures should reduce the economic losses associated with outbreaks of EI.