H1N1 Swine Flu Can Infect Some Pets

Felines, Fido, ferrets, feathered friends, H1N1 flu, you, and pigs, too!

Medical Author: Charles Davis, MD, PhD
Medical Editor: Melissa Conrad Stöppler, MD

Many researchers and physicians believed it was bound to happen. And it did. Humans have begun infecting other species with "their" disease, the pandemic novel H1N1 influenza A swine flu. H1N1 infection of a domestic cat by its owner(s) has brought the concept of human-to-animal (pet) transfer of the flu to the news forefront. The infection, documented by laboratory identification of H1N1 from the feline, is the first documented case of human-to-cat flu. One of the major reasons this is news is because most people want to know two things:

    1. Can my cat (or other pet) transmit the flu (H1N1) to my family (or to anybody who contacts the pet)?

    2. If my family gets the flu (H1N1), are my pets (cats, dogs, birds, and others) going to get sick and die?

The answer to the first question is that, to date (Nov. 10, 2009), no documented instances of pet-to-human transfer of H1N1 have ever been published. However, that does not mean it cannot or will not ever happen. Because of the unique ability of influenza viruses to replicate in many animals, birds, and humans, it seems possible that a pet could transfer a flu virus to a human. The second answer is more complex; the current evidence is that some pets will likely get sick, and possibly a few may die.

So what is the basis for these answers? The evidence for the first answer about species transfer of flu comes, in part, from what has been documented with the avian flu (H5N1). This virus occurs mainly in birds, but humans who have close contact with infected birds can be infected by H5N1, and some people have died because of this flu strain. However, this strain has not been readily transmitted from person to person. Unfortunately, the novel H1N1 swine flu is extremely easily transferred from person to person and to other animals and birds. Since it is well documented that other species can transmit flu viruses to humans and it is also documented that humans can transmit flu viruses to other species, the conclusion that a readily transmittable virus such as H1N1 could infect a pet and then be transmitted to a human seems warranted.

The second answer about pets getting sick and possibly dying from human transfer of H1N1 to pets has been documented. A domestic cat (that reportedly is a house cat and has contact with only humans) had a confirmed case of H1N1. Apparently, two people who have the cat as a pet became infected with H1N1 and transmitted the virus to the cat. Fortunately, the 13-year-old tabby recovered. However, in a separate situation in which owners transmitted H1N1 to their pet ferrets, one ferret reportedly died from the H1N1 flu. In addition, there are multiple documented instances in which humans have transmitted H1N1 to other species (for example, pigs and turkeys).

Although the above answers are based on situations that have already occurred, it seems reasonable to speculate how likely human-to-pet and pet-to-human transfer of H1N1 will impact both populations during this pandemic. First, how likely is it that people will transfer H1N1 to pets? Although there are multiple instances in which people have transmitted H1N1 to animals and birds, there are very few reports of transfers of H1N1 to pets. If the estimates of the current numbers of infected people in the U.S. (about 2-5.8 million) by the CDC are correct, then it appears that the relative number of U.S. pets that get H1N1 from infected people is rare (documented in only about four pets, to date). However, this "rare" number may be artificially low because of at least two possible reasons: (1) there is little or no testing of the pet population for H1N1; and (2) the symptoms of H1N1 in various pets are not easily recognized by people (for example, domestic cats do not cough or sneeze but may hunch on their four legs and decrease or stop preening, eating, and drinking if they develop respiratory problems). Time, testing, and better recognition of flu symptoms may yield a better understanding of human-to-pet H1N1 infections. As to whether or not pets can transfer H1N1 to humans, there are no documented instances to date. However, from studies of avian flu, it is theoretically possible, but it has not been documented as occurring from any pets to date.

Dogs are a favorite pet of millions of people around the world. Again, to date, there have been no recorded instances of human-to-dog or dog-to-human transfer of H1N1. However, dogs do get the flu, but they do not transfer it to humans. The dog flu, caused by H3N8 flu viruses, became prevalent in canines beginning in 2004. Investigators believe that the virus mutated from a horse flu virus (known in horses for about 40 years) and is now found in many dog breeds. There is a vaccine available for dogs, even though few dogs die from the infection. This situation is noteworthy because over the past five years, despite close association with humans, there have been no documented H3N8 transfers to humans. It is not clear if this may be due to the virus structure or to the inability of the canine species to easily transfer the virus to humans.