EI Q&A re re-infection

Liz Buchanan, Thursday, 8 November 2007

Dr. Ian Douglas, a scientific vet with the Queensland Department of Primary Industries & Fisheries, provided the following answers in response to questions regarding a second dose of horse flu, and the immunity of a horse that had been naturally infected with Equine Influenza.

The Department of Primary Industries & Fisheries have endorsed the release of this information to help bust myths regarding EI re-infections and the like.

"I am very confident that horses in Qld are not getting a second dose of 'flu. But the secondary complications are being reported regularly and are, for the most part, bacterial in nature (as opposed to the viral EI with a life span of 7-10 days). There are a range of viral infections that cause respiratory disease in Qld horses normally and these will continue to circulate, maybe even finding more ready victims in horses that are immuno-suppressed by EI.

"The immunity created by natural infection is very strong, certainly compared to any vaccine. Imagine that the horse is "full" of virus during natural disease compared to a couple of mls in the neck with vaccination. The stimulus to the immune system is much, much greater. This virus produces three separate modes of immune response - antibodies as we would expect and can easily measure, cells that are trained to 'gobble' up the virus (pacman style) and chemicals that are released and kill the virus. I would expect a couple of years of immunity following natural infection at least.

"The rider here is that should the virus change (mutate) significantly, then the animal might be susceptible again - it might no longer recognise the new strain as the same as the one it built immunity to. EI is a relatively stable influenza virus but clearly it does mutate (in places where it is always circulating like the UK).

"The chances of mutation are increased with every passage of the virus through a horse and curiously aided by the immune status of the population (vaccines etc). So if the virus doesn’t change, vaccination may stop it in its tracks, but a minor change in the virus that makes the vaccine-created immune response ineffective and the new virus can be 'selected' and proliferate in the animal.

"Experts doubt that this would happen in Australia for at least twelve months. "Vaccination of a previously infected horse would be of no benefit to the horse.

"Vaccination is not 100% effective and if we move vaccinated horses wily nily on the presumption that they are 'safe' to move, we will undoubtedly have problems ."
Cheers
Ian Douglas