Canine flu: Risk to horses?

I’m at the International Conference on Equine Infectious Diseases in Lexington, Kentucky at the moment, and will try to write about some of the highlights. One interesting discussion yesterday was about canine and equine influenza. It’s well established that canine flu (A/H3N8) originated in horses and subsequently became established in dogs. Canine flu virus is closely related to, but different from, its equine flu virus parent. That raises questions about whether canine flu virus could be transmitted back to horses. The question has significant implications for what should be done with dogs that might have canine flu that may have contact with horses, and for canine-horse contact in general, especially with performance horses.

A study by Yamanaka et al. (Acta Vet Scand 2012) looked at dog-horse infectivity of canine flu by putting infected dogs in stalls with healthy horses for 15 days. All dogs were sick and shedding canine influenza virus, but none of the horses got sick, shed the virus or mounted an antibody response. This study only involved three horse-dog pairs, so we have to be careful that we don’t go too far with the conclusions, but it suggests that while canine flu started off as horse flu, it has changed enough that infected dogs aren’t much of a risk to horses.

But... (yes, there’s usually a "but" with infectious diseases) dogs are susceptible to "normal" equine influenza. It’s uncommon, but dogs can be directly infected from horses with the classical equine H3H8 flu virus. In such a situation, dogs might pose a risk to horses because they are carrying the  equine virus, not the adapted canine version.

So…

  • If a dog has influenza that is known to be non-horse associated (i.e. typical canine flu) then there’s probably little concern for horses.
  • If a dog has influenza and there’s no clear dog link (or there’s a link with infected horses), it’s reasonable to assume that the dog could transmit the virus to horses.

However, why take the chance? It seems logical to ban any dog with a suspected respiratory infection from horse barns. It also makes sense to ban dogs from barns with equine flu cases. It’s an easy, cheap, minimally disruptive and potentially useful flu control measure that might help reduce transmission of this important virus in both directions.

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Anne Wallis - October 23, 2012 3:25 PM

Very interesting piece from what I am sure is a great conference ... I disagree with your conclusion, however. While it may seem like easy basic prevention to separate dogs and horses, it is unlikely and impractical. Dogs and horses go together - especially dogs and performance horses. The most obvious example are foxhounds and foxhunting horses. Go to any horse show and you will see spectators with their dogs, horseowners and riders with their dogs, etc. There is even a coffeetable book about horseshow dogs. is there a better solution? I think the best might be for dog and horse owners to be aware of the risks and the probable direction of transmission (which you have eloquently described) and separate symptomatic animals. Veterinarians should be especially aware of the risks as they do their fall vaccination rounds - an excellent opportunity for horse owner education.

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