The cause of a multistate outbreak of respiratory disease in dogs is unclear and in dispute.

Local news outlets in South Carolina and New York state have reported that novel, mutated strains of parainfluenza are to blame, but two experts in infectious-disease diagnosis are dubious.

A spike in dogs with clinical signs of contagious respiratory illness, including vomiting, fever, lethargy and severe coughing, were reported this summer in Virginia, New York, North Carolina, South Carolina and Wyoming.

Dr. Tracy Duffner of Heritage Animal Hospital on Hilton Head Island in South Carolina estimated in late August that she'd seen 50 cases since late July. It began with two dogs visiting from out of state with their vacationing families. "Both were pneumonia cases that ended up in the hospital," she said. Next were two dogs from boarding facilities on the island. Many more followed. "For the next two weeks, we were inundated," she said.

Duffner sent samples to a diagnostic laboratory to check for 11 viruses and bacteria, including canine influenza virus. She got no hits. "As soon as I got the first batch of respiratory tests back and they all came up negative, I called to see if it was the way I was swabbing," she said. "I had one puppy who tested positive for coronavirus, and one tested positive for parainfluenza but had been vaccinated recently."

Then Merck Animal Health stepped in. The vaccine manufacturer and drug maker offered to pay for additional testing, this time at Colorado State University's diagnostic lab. A Hilton Head newspaper, citing information from Duffner, reported on Aug. 31 that early results of those tests showed that a mutated form of parainfluenza virus was sickening her patients.

However, an infectious diseases researcher who studied the samples isn't keen to use that label. "It is currently unknown if these viruses are ... novel in any way," Dr. Michael Lappin, director of the Center for Companion Animal Studies at Colorado State University, told the VIN News Service.

VIN News was unable to reach Duffner for clarification last week due to disruptions caused by Hurricane Florence.

Adding to the confusion, another veterinarian, Dr. Kirk Dixon at Hilton Head Veterinary Clinics, told a television station that it appeared a bacterial agent was responsible for the severe cough afflicting some dogs.

"The good news is, it appears to be bacterial because they all respond to antibiotics, and we do a cough suppressant because the cough is half the problem," Dixon was quoted as saying. VIN News was unable to reach Dixon.

In New York state, a news outlet in Sussex County also reported a "mutated" parainfluenza strain.

Fluid samples from dogs were sent the Animal Health Diagnostic Center at Cornell University, where test results from half of the samples showed parainfluenza, which normally is covered by vaccines. This finding led some veterinarians to believe there's a new strain of parainfluenza virus — one that evades the vaccine.

Ed Dubovi, a virologist and microbiologist, isn’t convinced. As director of the Cornell diagnostic lab's virology center, Dubovi said, "The cause of the problem is yet to be determined. ... Some are ascribing the issue to a 'mutated' parainfluenza virus, for which I suspect there is no evidence other than [that] parainfluenza was found in some dogs, but that is not uncommon."

He explained that dogs vaccinated against parainfluenza still can carry the virus; however, "the immune system should dampen it out." What's more, he said, finding a new strain of parainfluenza in a unique population of sick dogs doesn't mean the virus is what's making them cough.

The only way to be sure, Dubovi said, is to test the strain on healthy dogs. "To prove it, you'd have to take an isolate and stick it in a dog that didn’t already have it," he said. "Everybody is looking for the next great virus, and they may find it. This may be some new thing … but I'm not convinced."

Last summer in Montana, a similar, fast-spreading cough struck hundreds of dogs. At the time, some veterinarians were convinced that canine influenza virus, a pathogen much deadlier than parainfluenza virus, was to blame.

But the cause never was definitively determined. "As far as I know, no one came out with anything," Dubovi said.