As autumn arrives, the approach of flu season is a real concern. Last year, thousands of people suffered from symptoms including a high fever, chills and fatigue—classic signs of the flu. Some 2,374 people in the United States were hospitalized for influenza during the last flu season—an incentive for many of us to get an annual flu vaccine, to avoid both getting sick and potentially passing on the flu to family members.
A group of veterinarians at Oregon State and Iowa State Universities is now looking into the risk of flu for an unexpected population that doesn’t have access to flu shots: dogs, cats and other household pets. “We worry a lot about zoonoses, the transmission of diseases from animals to people,” said Christiane Loehr, a professor at the OSU College of Veterinary Medicine. “But most people don’t realize that humans can also pass diseases to animals, and this raises questions and concerns about mutations, new viral forms and evolving diseases that may potentially be zoonotic. And, of course, there is concern about the health of the animals.”
We’re pretty well acquainted with zoonoses—diseases that can move from animals to humans—because of the high profile transmissions of the influenza strains H1N1 (“swine flu“) and H5N1 (“bird flu”) from animals in recent years. But, as it turns out, many diseases can also act as so-called reverse zoonoses, or anthroponoses, contagiously jumping from humans to other animals. This appears to be the case for H1N1: The researchers have discovered 13 cases in which H1N1 seems to have been passed from humans to pet cats, some of which ultimately died from the disease.
The first recorded instance, described in an article published by the team in Veterinary Pathology, took place in Oregon in 2009. While a cat owner was hospitalized with H1N1, both of her cats (which stayed indoors and had no contact with other sick people or animals) came down with flu-like symptoms and eventually died. A postmortem analysis of their lungs and nasal cavities turned up the H1N1 virus.
In the years since, the research team has turned up 11 more cats, one dog and even some ferrets that seem to have been infected with H1N1 due to human contact. The animals’ flu symptoms—respiratory disease and, for some, eventual death—resemble the same symptoms suffered by humans who encounter severe strains of the flu.
For the roughly 100 million U.S. households that have a cat or dog, this news might trigger immediate concern, and the researchers say that anyone experiencing flu-like symptoms should distance themselves from their pets in much the same way they would from other people. Since this area has been the subject of so little attention, they say that there might be many more undiscovered cases of the flu jumping from humans to pets. “It’s reasonable to assume there are many more cases of this than we know about, and we want to learn more,” Loehr said.
Realistically, though, the actual number of animals infected is quite small when compared to the population at large. The bigger worry is that the flu virus could mutate into a more dangerous form as it is transmitted from humans to animals. “Any time you have infection of a virus into a new species, it’s a concern, a black box of uncertainty,” Loehr noted.
The influenza virus in particular mutates notoriously easily, with entire segments of its genome changing within a generation. The reason that H1N1 was declared a “national emergency” in 2009 was because it was a strain that mutated when it jumped from pigs to humans, raising the possibility that it had taken on a more deadly form that could be transmitted more easily between people.
In a worst-case scenario, the pets we keep in our homes could serve as the same type of mutation-inducing vector—the flu could be passed from human to pet, mutate into a more dangerous form, and then potentially affect both humans and other animals. “In terms of hosts and mutations, who’s to say that the cat couldn’t be the new pig?” Loehr asked. “We don’t know for sure what the implications might be, but we do think this deserves more attention.”