Equine Influenza in Australia
Brett Tennent-Brown • Andrew W. Van Eps
Until 2007, Australia was one of a small number of countries that had never experienced an outbreak of equine influenza (EI). EI has never been reported in New Zealand.
However, in August 2007 a consignment of horses, all of which had been vaccinated in accordance with Australian quarantine requirements, was admitted into the quarantine station in Sydney. At least one of these horses was subclinically infected with EI, and the virus subsequently spread among horses within the quarantine station. Following a break in biosecurity, EI virus escaped and spread rapidly through the naive horse population, affecting more than 76,000 animals on 10,600 premises in New South Wales and Queensland. Control of the outbreak and eradication of the virus from Australia cost an estimated US$1 billion.As soon as EI had been confirmed in the general horse population, a control and eradication program based on movement restrictions and subsequently vaccination was initiated. Although only two states were affected, a complete national ban on all horse movements was put in place immediately, and all race meetings and other horse events were cancelled. Horses already at events were not permitted to leave until cleared by government veterinarians. Movement restrictions were highly effective in preventing the spread of virus beyond those areas affected in the first week of the outbreak but were extremely damaging financially to horse-related businesses. To alleviate some of the economic hardship, especially among the breeding industry, some movement restrictions were eased to allow the breeding season to proceed.
At the time of the outbreak in August 2007, there were no EI vaccines registered for use in the Australian general horse population. When additional measures were required to contain the outbreak, an emergency-use permit was issued for a canarypox-vectored recombinant EI vaccine (ProteqFlu, ProteqFlu-TE [Merial, Lyon, France]). This vaccine was selected for several reasons, including rapid onset of immunity,1 the ability to differentiate vaccinated horses from those naturally infected, and reports of efficacy in the South African 2003-2004 outbreak.2 Vaccination was initially used to create buffer zones around infected areas to contain spread of the epidemic.
Horses inside the infected areas on unaffected properties were subsequently also vaccinated. Most horses received a primary course booster at the recommended interval of 4 weeks; however, some horses received an accelerated primary course schedule with the interdose interval reduced to 14 days.3,4The last reported detection of EI occurred approximately 4 months after the outbreak began. Provisional freedom from disease was declared in March 2008, and EI-free status was confirmed by the World Organisation for Animal Health (OIE) the following year.
The EI virus isolated from horses in the quarantine station and horses in the general population was named A/equine/ Sydney/2888-8/2007 (H3N8).5 Following molecular characterization, the virus was classified as an American lineage, Florida sublineage, Clade 1 type6 and is phylogenetically similar to the EI viruses that were circulating in Japan and the United States at the time.7
Clinical disease caused by the EI virus in adult horses was considered mild in the vast majority of cases; however, secondary bacterial bronchopneumonia was described as a complication and occasional cause of death in predominantly mature horses.8 Infection of late-gestation mares was associated with increased severity of clinical signs in the infected mare and neonatal morbidity.8,9 A fatal bronchointerstitial pneumonia associated with EI virus infection was reported in several foals from infected properties.10