After a couple of years of very little apparent canine influenza activity in the US, the virus seems to have come back with a vengeance in some parts of California. Canine H3N2 influenza came to  North America from Asia in approximately 2015 and caused outbreaks in many areas, but then seemed to slip into the background. Sporadic outbreaks have been identified in recent years, but were mainly small clusters in individual facilities like kennels.

A recent, prolonged outbreak of H3N2 influenza in dogs in southern California has highlighted the importance of this virus again. The best tracking and reporting of the outbreak has been provided by the County of Los Angeles Public Health:

“Between July and October 2021, approximately 800 confirmed and suspect cases of CIV [canine influenza virus] H3N2 in dogs in LA County have been reported to Veterinary Public Health.  Sadly, 7 deaths in dogs have been associated with this outbreak.  Of the cases reported, most were associated with attending boarding kennels or dog daycare settings.  There are a number of cases that have never visited a boarding or daycare facility, but were exposed while on walks in their neighborhood, at dog parks, groomers, or at veterinary clinics.  This virus has spread rapidly among dogs throughout LA County, affecting many congregate facilities.  Based on interviews with these facilities, many additional cases have not yet been reported to VPH.  We suspect that this outbreak likely involves more than 1000 cases of CIV H3N2 in LA County dogs.  Dogs that appear to have ‘kennel cough’ have a high likelihood of having CIV H3N2.  To date, this is the largest outbreak of CIV H3N2 ever reported in LA County.  To stop the spread of this outbreak, pet owners and veterinarians are strongly encouraged to vaccinate dogs against CIV H3N2 and isolate sick pets at home for 28 days from the first day of illness.  Pets exposed to confirmed or suspected cases should be kept on a home quarantine and observed for clinical signs for 14 days.”

The 7 deaths puts the mortality rate at approximately 1%, which is pretty consistent with what we’ve seen before with this virus in community settings. I’d guess that 20-40% of the affected dogs have received antibiotics and other treatments due to more serious disease (and some overuse of antibiotics, which unfortunately is still common with respiratory disease in dogs, as it is in people).

As is typical with canine influenza (and canine infectious respiratory disease complex (CIRDC), in general) we only have a partial picture of what’s going on. Testing is sporadic and  various other respiratory viruses and bacteria that cause similar disease are always circulating in the dog population.

When canine flu was found in Canada in 2018, we aggressively tested and traced in-contact dogs, and the virus was successfully eradicated in Ontario.  That’s not a realistic goal in this case. LA County has a great veterinary public health team, but you have to be able to intervene early to have any chance of stopping a highly transmissible virus like flu in its tracks.   Once it’s widespread in the general dog population, it becomes impractical to properly trace and test all in-contact dogs for every case (especially when there is rarely much or any financial support to do so). So, containment through education and vaccination is the focus. Presumably, this outbreak will burn out in the area eventually. Whether it will be tracked to other areas outside California is a big question.  We’re still watching for new introductions of canine influenza in Canada. We were able to control the first introductions of H3N2 canine flu in 2018, but I’m not naive enough to think that luck wasn’t a big component of that.

Since respiratory disease is common in dogs, particularly of late, and because we only test a minority of coughing dogs, we need to be aware of higher risk situations. I pay the most attention to:

  • Large clusters of cases, especially in well vaccinated populations
  • Outbreaks that affect virtually all exposed dogs
  • Outbreaks where there’s a known or possible initial exposure to a dog from a high risk region (e.g. Asia, and now parts of the US where the virus is active).

In situations like those, prompt testing, along with good infection control practices, are what we need to identify and contain the problem as much as possible.