Since we’re still seeing more respiratory disease in dogs pretty widely across North America (see previous blog posts on this topic for more info), I get asked about canine influenza a lot. My general responses are:

  • Canada: As far as we know, we don’t have canine influenza in Canada. It was introduced into Ontario via dogs imported from Asia  a few times in 2018, and it was successfully eradicated. However, we are under a constant threat of re-introduction because we import a lot of dogs, with few restrictions (and none when it comes to influenza specifically).
  • US: Canine influenza has receded into the background in the US for the most part, but still causes sporadic outbreaks here and there. Whether those are because it’s still circulating widely at a low level or because of new introductions from imported dogs isn’t clear.

So, the status of canine influenza virus  (CIV) in Asia is highly relevant for Canada, the US and anywhere else that may import Asian dogs. Among the things to which we need to pay attention are the different strains that are circulating in different regions. The main concern in recent years has been H3N2 CIV, which originated in birds but is now adapted to dogs. We need to know whether this strain is still the main risk, and how much variation is occurring within that strain because it’s relevant to the type and severity of disease it can cause, to the potential risk of transmission to people, and to how well current H3N2 CIV vaccines might work.

A recent study (Yanguo Li et al. 2021) has shed some more light on this subject:

  • They collected 1185 nasal and throat swabs from dogs with respiratory disease  in Jilin, Henan and Guangdong provinces in China, and tested them for CIV. They then looked at the genome of the viruses to see how they compared to each other and to older strains.
  • They also collected blood samples from 3197 healthy dogs from 27 Chinese provinces to look for antibodies against CIV. They also did some small experimental studies with the CIV strains they recovered.

Here’s what the researchers found:

  • They detected CIV in 0.42% of samples from dogs with respiratory disease.
  • When they looked at the virus genomes, they found that the current H3N2 CIV strains were fairly different from older Chinese strains and closer to the strains found in the US since its introduction there (from China) around 2015.
  • The authors suggest that current Chinese CIV strains may have come from the US, but that’s questionable since dog movement from the US to China is pretty limited.
    • I suspect this finding is more an indication of the relatively limited CIV surveillance in China and that the similarity of US and recent Chinese strains is because these strains evolved undetected in China (where the virus in endemic), not because they evolved in the US and spread back to China.
  • The blood testing provided more evidence of the continued endemnicity of this virus in China, as 15% of dogs had antibodies against CIV.
    • There’s no mention of the vaccination status of the dogs tested. Presumably they tested unvaccinated dogs. If they didn’t query vaccination status, that could bias the results, but it’s safe to assume that CIV vaccination rates are very low, so it probably wouldn’t have that much of an impact. The fact that rates were highest in stray dogs (48%) supports the notion that these antibodies were from infection, not vaccination.
    • The authors seem to over-interpret these results too, suggesting the virus was introduced in 2016 and spread rapidly thereafter. The data don’t really support that, since while the numbers in pet dogs were higher in 2017 than 2016, there’s not enough epidemiological data to support that, and even if there was, it would be a stretch to blame importation from the US (possibly geopolitics washing down to the dog disease level?).

Overall, this study shows us that CIV continues to circulate in the dog population in China. As is typical with flu, strains evolve over time and monitoring is important for risk assessment and for consideration of the impact on vaccine efficacy. There’s some good information in this paper, but some over-reaching interpretation (MDPI journals aren’t exactly known for the rigour of their peer review). Still, it provides useful information for understanding this important virus.