Quantitative analysis of the risk of spread of equine influenza associated with movements of vaccinated horses from infected areas during the Australian outbreak.

The study involves creating simulation models to assess the risk of equine influenza spreading due to the movement of vaccinated horses from infected areas. The research showed that the average probability of the disease escaping isolation and infecting other horses ranged from 1 in 1.2 million to 1 in 600,000.

Simulation Models

• The researchers used computer simulations to analyze the potential risk of transmission of equine influenza. These models were specifically designed to calculate the likelihood of the equine influenza virus entering pre-movement isolation, surviving through pre- and post-movement isolation periods without being identified by scheduled laboratory tests, and ultimately escaping to infect susceptible horses at the destination.

Probability of Disease Escape

• The study found that the average, or mean, probability of the disease escaping and causing an infection ranged from 1 in 600,000 to 1 in 1.2 million, depending on the number of horses in a group, or “lot.”
• Even in worst case scenarios, in 95% of simulation runs, the likelihood of the virus escaping was less than 1 in 200,000, independent of the lot size.
• For a large group of 600 horses that was processed in separate lots, the average probability of disease escape varied from 1 in 10,000 to 1 in 56,000, based on the size of the lot.

Protocol Modification

• The findings of this study led to modifications in the standard protocol of handling horses moving from infected areas. These changes included two disease tests during pre-movement isolation instead of one and an optional additional test during post-movement isolation as per the discretion of the Chief Veterinary Officer.
• This increased the chances of detecting the infection and consequently reduced the probability of the disease spreading to the destination area.