Chicago has recently seen an increase in cases of canine influenza. Cook County Department of Animal and Rabies Control issued a warning earlier this month for dogs to avoid areas where other dogs may congregate [1]. So far, the outbreak has resulted in the death of five dogs, sickened many more, and has caused closures of a local dog park and pet hotels [1,2,3].

You may or may not be surprised that dogs catch influenza, as we sometimes associate flu with other animals such as birds and pigs. However, whilst the media are announcing that ‘canine influenza virus is nothing new’, within the history of the flu virus, dogs becoming infected with flu is a relatively recent phenomenon [4]. The first canine influenza virus, a H3N8 strain, was detected in racing greyhounds in 2004 and was thought to have passed from horses to dogs [5]. Prior to this incident, the same virus is thought to have existed in horses for over 40 years [5]. Even more recently, another flu virus, the H3N2 strain, was detected in dogs in South Korea and China in 2006 [6]. This strain is thought to have passed from birds to dogs [6]. Whilst the current outbreak in Chicago was originally thought to be the H3N8 strain already found in North America, further testing by scientists have revealed the outbreak strain to be the H3N2 strain, which originally emerged in Asia in 2006 [6]. After this revelation, further investigations will likely take place to locate how this particular strain infected dogs in Chicago.



So, which other species can get flu viruses and how do they pass between species? Many different non-human animal species can get influenza viruses, including ducks, chickens, pigs, whales, horses, and seals [7]. Wild waterfowl, however, are the natural carriers of the virus, and the virus can naturally live in their intestines without the birds getting sick [8]. Therefore, the influenza virus is thought to have originally existed in these birds and ‘jumped’ to other species from there. Influenza viruses can pass from one species to another on close contact between a sick animal and a new species. However, even with close contact, it is very unlikely that the influenza virus will cause an infection in a new species. The reason? Flu viruses are usually well adapted to infecting and spreading between members of the same species, but are not as well-adapted to infect a different species. This is why people commonly catch flu from each other but only very rarely catch influenza from other animal species.


What Does This Mean?

Dogs with canine influenza are therefore very unlikely to pass it to humans. In reference to H3N8 canine influenza, the Center for Disease Control (CDC) states that ‘there is no evidence that this virus infects humans’ [5]. Also, in the 40 years that the H3N8 virus has been present in horses, no humans have ever been observed to become ill from the equine influenza virus [9]. However, other strains of the H3N2 virus have been passed from pigs to humans and then shown limited human-to-human transmission, so this canine influenza virus may pose a higher risk of dog to human transmission than previously thought [10]. Whilst it is very unlikely, it isn’t impossible that canine influenza could infect people, so scientists, doctors and veterinarians should remain vigilant to this possibility.

As for dogs, if you are concerned about your dog becoming infected with canine influenza, there is a vaccination available. However, this vaccination was developed to protect dogs from the H3N8 strain, so there is some concern over how effective this vaccination will be in preventing dogs catching the H3N2 strain circulating in the current outbreak [6]. As a result, dog owners in the outbreak area should be wary of their dogs mixing with other dogs and should call their veterinarian if they notice any of the symptoms of canine influenza: cough, runny nose or fever [5].















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