This concept, called “reverse zoonosis,” is still poorly
understood but has raised concern among some scientists and veterinarians, who
want to raise awareness and prevent further flu transmission to pets. About
80-100 million households in the United States have a cat or dog.
It’s well known that new strains of influenza can evolve
from animal populations such as pigs and birds and ultimately move into human
populations, including the most recent influenza pandemic strain, H1N1. It’s
less appreciated, experts say, that humans appear to have passed the H1N1 flu
to cats and other animals, some of which have died of respiratory illness.
There are only a handful of known cases of this phenomenon
and the public health implications of reverse zoonosis of flu remain to be
determined. But as a concern for veterinarians, it has raised troubling
questions and so far, few answers.
Veterinary researchers at Oregon State University and Iowa
State University are working to find more cases of this type of disease
transmission and better understand any risks they pose to people and pets.
“We worry a lot about zoonoses, the transmission of
diseases from animals to people,” said Christiane Loehr, an associate professor
in 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.”
The researchers are surveying flu transmission to household
cat and dog populations, and suggest that people with influenza-like illness
distance themselves from their pets. If a pet experiences respiratory disease
or other illness following household exposure to someone with the
influenza-like illness, the scientists encourage them to take the pet to a
veterinarian for testing and treatment.
The first recorded, probable case of fatal human-to-cat
transmission of the pandemic H1N1 flu virus occurred in Oregon in 2009, Loehr
said. Details were published in Veterinary Pathology, a professional journal.
In that instance, a pet owner became severely ill with the flu and had to be
hospitalized. While she was still in the hospital, her cat – an indoor cat with
no exposure to other sick people, homes or wildlife – also died of pneumonia
caused by an H1N1 infection.
Since then, researchers have identified a total of 13 cats
and one dog with pandemic H1N1 infection in 2011 and 2012 that appeared to have
come from humans. Pet ferrets have also been shown to be infected, and some
died. All of the animals’ symptoms were similar to that of humans - they
rapidly develop severe respiratory disease, stop eating and some die.
Serological studies suggest there is far more exposure to flu virus in cats and
dogs than previously known.
“It’s reasonable to assume there are many more cases of
this than we know about, and we want to learn more,” Loehr said. “Any time you
have infection of a virus into a new species, it’s a concern, a black box of uncertainty.
We don’t know for sure what the implications might be, but we do think this
deserves more attention.”
Natural and experimental transmission of the H3N2 influenza
virus from dogs to cats in South Korea showed the potential for flu viruses to
be transmitted among various animal species, Loehr said. It’s unknown if an
infected cat or other pet could pass influenza back to humans.
The primary concern in “reverse zoonosis,” as in evolving
flu viruses in more traditional hosts such as birds and swine, is that in any
new movement of a virus from one species to another, the virus might mutate
into a more virulent, harmful or easily transmissible form.
“All viruses can mutate, but the influenza virus raises
special concern because it can change whole segments of its viral sequence
fairly easily,” Loehr said. “In terms of hosts and mutations, who’s to say that
the cat couldn’t be the new pig? We’d just like to know more about this.”
Veterinarians who encounter possible cases of this
phenomenon can obtain more information from Loehr or Jessie Trujillo at Iowa
State University. They are doing ongoing research to predict, prevent or
curtail emergent events.
