Beverley Petrunich, co-owner of DoGone Fun, a day care and boarding facility, visits with some of her clients in Chicago. Experts say doggie day care contributed to an epidemic of dog flu in Chicago that is spreading in the Midwest.  (AP Photo/M. Spencer Green)

Several nasty flu strains spread across the Midwest in April. One virus sickened thousands of dogs, shutting down shelters and doggie day care centers throughout Chicago. Another infected more than 100 poultry farms in states including Kansas, Missouri and Iowa, which declared a state of emergency on Friday. Meanwhile, farms are euthanizing tens of millions of infected birds and continue to struggle to contain the highly contagious H5N2 avian flu virus.

None of these strains affected humans. But beyond a general concern for our furry or feathered friends, there are reasons for people to worry about these kinds of animal flus. Most flu viruses are well adapted to their particular animal, and generally do not jump species. On occasion, though, flu viruses do make a cross-species jump.

When they do, they can cause deadly pandemics. Since the new host has never confronted this kind of disease, it will have no immunity to it. Some variants of swine and bird flu have been known to infect and kill humans: Since 2003, more than 600 people in 15 countries have been infected by the H5N1 avian flu. And in 2009, the WHO declared that the H1N1 virus, commonly called “swine flu,” had become a pandemic in humans.

Reports of animal flus have certainly increased in the last decade. However, experts are divided over whether this is due to an increase in the flus themselves, or our improved ability to detect them.

Humans haven’t been able to track the flu for very long. Scientists began to detect the first flu viruses in the 1930s, but only recently developed quick and accurate methods of detection.

“The surveillance for these viruses is a whole lot better than it was in the past,” said Dr. Joe Bresee, the chief of the Epidemiology and Prevention Branch of CDC’s Influenza Division. Since avian flu began spreading through Asia in 2003, the CDC has led a global campaign to improve flu tracking and detection. Now 120 countries around the world can rapidly detect novel flu viruses, which has led to an increased understanding of the animal viruses that are out there, says Bresee.

But beyond the increase in detection, is there an increase in the viruses themselves? Bresee is cautious when it comes to answering this question. “I don’t know the answer to that. I don’t think so,” he says. The growing population of humans and animals has created more opportunities for spreading flu viruses among each other, but it’s hard to know if that’s really happening, says Bresee. “I am confident that there has been better surveillance over time, and that’s probably the reason we’re seeing more now.”

Other researchers disagree – though also very cautiously. “Part of the problem we have with that is we really just have a sliver of time where we can look for the viruses. We’ve only been able to culture viruses for 80 years,” said Dr. Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy. “But if you look at the sampling that’s been done from the 1950s onward, there are more flu viruses in animals than we’ve seen in the past.”

Canine flus, for example, have only been around for a decade. In 2004, scientists discovered that the H3N8 equine virus had spread at racetracks from horses to greyhounds. That canine flu continues to circulate in dog parks and shelters in New York City. The newer canine flu, which is now circulating in the Midwest, is thought to have jumped from birds to dogs in Asia in 2007, perhaps in live animal markets where poultry and dogs sold for meat lived in close quarters.

Two flu viruses is a small sample size, but their presence in dogs is still unprecedented. “Before the 2000 outbreak of the H3N8 virus, no one really thought of dogs as a host for influenza virus. That hadn’t been reported before,” says Dr. Colin Parrish, a professor of virology at Cornell University College who studies viruses that jump from one animal to another.

Parrish says the development may be due to the increase in dog population. The dog population in the U.S. has increased by more than one-third in the last 30 years, to more than 80 million. Estimates put the global dog population at roughly 400 million -- meaning that if dogs made their own country, it would be a relatively populous one.


Jishai Evers, Dadaviz

And it's not just dogs: The global population of humans, poultry and pigs has also risen precipitously. And both two-legged and four-legged animals are increasingly living in close quarters. Factory farms confine poultry and livestock in crowded conditions.

More humans are living and working in cities, and urban pets have more active social lives than ever. Many urbanites treat their pets like toddlers, taking them to doggie parks and day care to play with their canine friends. The jump in animal shelters and kennels that accompany pet ownership has created another breeding ground for flu. That contact is very important for spreading the flu, says Parrish. The H3N2 dog flu has been known to infect cats, for example, but cats are not social enough to be able to transmit the virus, he says.

Another reason that we may be seeing more flu strains is that travel is becoming increasingly common for people, pets and livestock. Both people and small dogs can now be frequent flyers, and the globalized agricultural trade means that animals and animal products are shipped around the country or the world. “I think it’s clear that global travel for humans and global trade for animals that can be infected with flu both increase the likelihood of rapid global spread of these viruses,” said Bresee.

Researchers will continue to track the spread of the canine flu in the U.S., but when it comes to infecting humans, they are far more concerned about bird flu. So far, we haven’t seen equine and canine viruses infect humans, but pigs and poultry continue to be common culprits. The USDA is working to contain the spread of the H5N2 virus in the Midwest, and the CDC has monitored more than a hundred people who came in contact with the birds. So far, no disease has been detected in humans.