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For the longer answer to this question, check out the latest podcast episode I just posted on WormsAndGermsPod. For those who prefer to read the summary, here it is:

If you ask people on the street “should we use the limited antivirals we have available to treat people with flu on dogs and cats?” the common answers would probably be “no,” along with the occasional “hell no!!” On the surface, that response makes sense. We have limited antivirals (both in number and sometimes supply) and they are important for treating people in some situations. However, we shouldn’t completely dismiss the utility of antivirals in animals with influenza infection in selected circumstances with good controls.

Early treatment with an antiviral like oseltamivir (Tamiflu) could be effective against H5N1 flu in some animals, and there may be a rationale for prophylaxis in high risk situations (e.g. housemate of a cat with known H5N1 flu). Both of these scenarios involve companion animals, and non-reservoir species, and no livestock (including backyard chickens).

Is there a concern about development of antiviral resistance if we use the drug in pets?

We can’t ignore the possibility. Antiviral resistance is a concern, and it’s a spontaneous event that can occur as flu spreads, regardless of whether antivirals are used or not. Antiviral resistance markers have been found in influenza virus isolated from poultry (that were never treated with antivirals). The more the virus is transmitted and the more antivirals are used, the greater the risk of selecting for and spreading resistant strains.

But antiviral resistance is different from antibacterial resistance in some important ways. Antivirals are much more specific in their effects, and viruses don’t swap resistance genes with each other like bacteria do.

  • If we treat an animal or person with an antibiotic, there will be myriad bacteria in that individual that are resistant, can become resistant or can spread resistance genes.
  • In contrast, if we treat an animal with oseltamivir when it doesn’t have the flu, it can’t select for a resistant flu strain.
  • If we treat and animal with oseltamivir when it does have the flu, there is a chance of resistance developing, but it’s only a broader problem if that flu virus is passed on to another individual. We don’t know if dogs and cats can transmit H5N1 flu, but it’s prudent to assume that they can. So, if we’re going to use an antiviral, we need to do all that we can to reduce the risk that any virus from that individual does not get transmitted to anyone else. We can probably do much more effectively in a pet than we can in a person that’s being treated with an antiviral.

Good antimicrobial (including antiviral) stewardship means use as little as possible but use enough. We need to be prudent, but we also shouldn’t miss opportunities to intervene when we can do so effectively and with minimal risk.

When does it make sense to use an antiviral to treat H5N1 flu in a dog or cat?

Use of an antiviral makes sense for early treatment of known or high risk cases of H5N1 influenza where there’s a concern for development of serious disease (i.e. any infected cat, and probably infected dogs) AND when the animal can be properly treated AND when the animal can be kept isolated during and shortly after the treatment period.

Basically my two main questions are: do they need it? and am I confident the animal won’t be able to infect another individual (human or animal)? If I can comfortably say yes to both of those, I think it’s reasonable to use an antiviral.

Example 1: An infected cat in a household or veterinary clinic

  • Yes. We can properly treat, isolate, monitor and test the cat appropriately.

Example 2: An infected cat that goes outside

  • No, unless the cat can be kept inside during the treatment and monitoring period. I don’t want to risk an antiviral-resistant flu strain developing and then the cat spreading it to other cats, or worse, birds.

Example 3: A potentially infected backyard chicken

  • No (or hell no). These are livestock, so they are approached differently (and in Canada poultry infected with H5 flu must be culled). Poultry are highly susceptible to H5N1 influenza, and can clearly infect people. Also, an antiviral is probably too little, too late for a species that is so susceptible.