While vaccination to control equine influenza (EI) in endemic areas is common practice, little is known about the use of early vaccination to eradicate the disease during an outbreak. Researchers recently examined the effect of early vaccination during an outbreak, using the 2007 EI outbreak in Australia, which had previously been free of EI, as a model.

"The Australian EI outbreak was associated with very high rates of spread in the early stages. This was a function of both an entirely naïve population, and movements and mixing of horses associated with a number of weekend recreational events prior to EI being recognized. These events were undoubtedly a key feature contributing to the size and extent of the outbreak," noted Graeme Garner, DVM, director of the animal health epidemiology program at the Department of Agriculture, Fisheries, and Forestry in Canberra, Australia.

Government agencies and the horse community implemented controls to contain the outbreak, including biosecurity measures, restricted movement, and vaccination. Approximately 140,000 horses in affected areas were vaccinated, beginning in the second month of the epidemic, as part of the nationally-coordinated EI control program.

Researchers in Australia employed a premises-level model using the epidemiological data from the 2007 EI outbreak to ascertain the role vaccination played in disease eradication and whether earlier vaccination would have made a significant impact on the epidemic.

"A detailed epidemiological model of the outbreak was constructed that took into account regional variation in disease transmission, accounted for the effects of movement controls and biosecurity measures, and factored in realistic resources for vaccination," explained Garner. "The model was shown to accurately recreate the first month of the 2007 outbreak, both spatially and temporally. It was used to compare emergency vaccination strategies with a no vaccination strategy. The vaccination strategies were based on different sized areas around infected premises with vaccination starting from Day 7 into the control program."

Using this simulation model to represent the Australian EI outbreak, researchers analyzed the spread of the outbreak, concluding that within a month of implementing a control program, the vaccination strategies evaluated had reduced the number of new infections on average by 60% and the size of the infected area by 8-9% compared with no vaccination. In the event of limited quantities of available vaccines, a 1-to-3 kilometer "ring" of vaccination around infected premises acted to contain disease spread.

These results demonstrate strategic early vaccination could have reduced number of infected horses. Ultimately, this study’s findings suggest the following measures play critical roles in curtailing the spread of EI:

  • Movement controls and quarantines;
  • Biosecurity measures such as foot baths, hand washing, disinfection of equipment and isolation of infected horses; and
  • Early vaccination.

"Although vaccination was used in 2007, this was not until five weeks into the control program and it has been suggested that it had a limited impact on containing and eradicating the outbreak," noted Garner. "This study has shown that vaccination used early in an outbreak is an effective approach to preventing cases of disease. Used with other disease control measures (such as premises biosecurity, movement restrictions), it may have significantly reduced the size of the 2007 Australian EI outbreak.

"This reduction occurs despite the fact that the vaccine does not produce sterile immunity, with vaccinated horses still able to become infected and shed virus," he continued. "The study found that movement restrictions and biosecurity measures as applied in the 2007 outbreak were highly effective in reducing spread and containing the disease geographically, although the disease continued to actively spread within many of the affected regions."

Garner noted further studies are called for to better understand the mechanisms by which equine influenza can spread between premises, in particular whether longer distance airborne transmission can occur.

This study, "Evaluating the effectiveness of early vaccination in the control and eradication of equine influenza–a modelling approach," was published in the April 2011 issue of Preventive Veterinary Medicine. Read the abstract online.