Virologist aims to hunt for canine parvovirus strain

Spate of unusual outbreaks puts veterinarians, health authorities on alert

September 16, 2022 (published)
Photo courtesy of Cornell University
The Baker Institute for Animal Health, part of the Cornell University College of Veterinary Medicine in Ithaca, New York, has a long history of working to prevent and treat canine parvovirus infection.

The deaths of some 30 dogs in northern Michigan this summer, once a mystery, is no longer: The culprit, canine parvovirus, has been confirmed, following weeks of concern over a run of what turned out to be false-negative results from rapid diagnostic tests.

Now, reports of similarly virulent outbreaks are surfacing in Washington state and Colorado, prompting some veterinarians to wonder whether a new strain of the ubiquitous pathogen might be circulating.

That's unlikely but still a possibility, says Colin Parrish, a professor and renowned virologist who runs a lab that researches the emergence and evolution of viruses at Cornell University's Baker Institute for Animal Health.

Parrish has studied canine parvovirus, or CPV, for more than four decades, closely tracking changes in the virus since it first emerged in dogs in 1978. The potentially fatal disease affects the gastrointestinal tracts of dogs, causing persistent vomiting and diarrhea that can rapidly result in dehydration and damage the intestines and immune system. The pathogen is spread via dog-to-dog contact and contact with contaminated feces or environments.

Reports of canine parvovirus cases that eluded rapid diagnostic tests began circulating in Michigan in late July. Initially, some feared a new disease had surfaced. It wasn't until Aug. 24 that the cases were confirmed to be parvovirus. 

Why the cases have seemed unlike "normal" parvo cases is still to be determined. "These could be really different parvo outbreaks or due to a variety of other causes, but we won't know until we look at the viruses in detail," Parrish said in an interview with the VIN News Service. "Every six months or so, we get reports of a spike in unusual cases, usually in older dogs. Most of the time when a new variant is suspected, we show that the virus has not changed in any significant way, so we assume this is due to a lack of adequate vaccine coverage or other cause. 

"I'm not trying to trivialize this," he added, "but a lot of things besides parvovirus can cause diarrhea in older dogs."

Parrish — who discovered a mutated version of the virus, CPV-2a, while a graduate student at Cornell during the early 1980s — is trying to resolve questions around the outbreaks. Colleagues who've encountered unusual cases can send samples to his lab at the Baker Institute, but are encouraged to contact him first. "A few microliters of feces or tissue is all we need to do full genome sequencing," he said.

In brief

Unusual cases in Colorado

Dr. Lindsey Dunn intends to respond to Parrish's request. She encounters parvo regularly as an emergency practitioner in Evans, Colorado. When three extremely sick dogs from two households, one local and the others from Utah, presented during the same week in late August, she surmised that a nasty outbreak of the virus might be circulating.

Initially, her suspicions were allayed. All three patients — a 5-year-old female miniature schnauzer and two female Irish setters who are housemates, ages 4 and 6 — were fully vaccinated against parvo, which typically strikes puppies between 6 and 20 weeks.

"They were all properly vaccinated, and the owner of the Irish setters even checks titers," she said, referring to laboratory tests that measure the presence of antibodies in the blood, which can indicate immunity. "We did not test the miniature schnauzer for parvo since she was the first case like these and she was fully vaccinated. So we thought it was just AHDS [acute hemorrhagic diarrhea syndrome] and didn't have reason to believe it was infectious."

However, medical interventions — the same indicated for AHDS as would be for parvo — did not work. "After 24 hours on intravenous fluids and all of the supportive care — ​GI protectants, an antibiotic, nausea meds — her PCV [packed cell volume, a measurement of red blood cells] remained as high as when she first presented, 70%," Dunn said. 

High packed cell volume can stem from severe dehydration, often from ongoing fluid losses due to bouts of diarrhea or vomiting. 

The schnauzer, Dunn said, did not respond to treatment and was euthanized. "She was just declining and continued to have hemorrhagic diarrhea, inappetence, was really lethargic and dull. ​She became hypoglycemic and the owner just didn't have the finances to keep going," Dunn said. 

In the cases of the two Irish setters, the younger of the two arrived vomiting with bloody diarrhea, was anorexic and lethargic. Her condition was similar clinically to the schnauzer's​, Dunn said, but the setter was rapid-tested for parvo, which came back negative. She was given intravenous fluid care at rates "so high that we usually don't go there, but she was just having blowout diarrhea and we were just trying to match it."

Despite the fluids, the dog's packed cell volume increased to 84%, Dunn said, and she showed signs of compromised cardiovascular stability: "Her heart rate was very elevated, blood pressure was not highly stable, her gums were tacky and had a fast capillary refill time," which is a metric used to determine the volume of blood in a patient's circulatory system. 

Dunn consulted a critical care specialist, who suspected an underlying infection​ due to the severity of her signs and instability. When the dog's housemate arrived 24 hours later in a similar condition, Dunn said, "We suspected the same illness." However, she did not initially test the second setter for parvo because the first one tested negative for the virus, and both dogs had had recent titer blood tests. 

On a whim, another veterinarian at the practice ran a rapid test for canine parvovirus on the older setter a few days later. It was positive. Surprised, Dunn ordered a more sensitive diagnostic test based on a method known as polymerase chain reaction, or PCR, from the Colorado State University Veterinary Diagnostic Laboratories, which confirmed canine parvovirus.

"What I've seen in Colorado has just been so odd, I am concerned that what we're dealing with is a new strain," Dunn said. "I know that the dogs in Michigan were reported to be under-vaccinated or unvaccinated, so that's a bit different."

Michigan's outbreak quiets

Photo courtesy of Cornell University
Colin Parrish, a virologist and professor at Cornell's Baker Institute for Animal Health, has been studying canine parvovirus since the early 1980s.

A PCR test also was used to identify the virus that wreaked havoc on dogs in Michigan, many of whom were found to be negative for the virus via rapid tests. On Aug. 24, state authorities said that the affected dogs were compromised, likely because they did not have a history of complete vaccination.

"We have a highly effective vaccine available to help protect dogs from the virus," State Veterinarian Dr. Nora Wineland said in a news release. "Dogs that are not fully vaccinated against this virus are the most at risk." 

Since then, news of cases has quieted.

"This always happens," observed Dr. J. Scott Weese, a zoonotic disease expert at the University of Guelph Ontario Veterinary College. "Once they announced it was parvo, everyone stopped talking about it."

Like Parrish, Weese said he would be surprised if the culprit turns out to be anything but a run-of-the-mill strain of the virus. "In the vast majority of these situations, it's the same strains doing the usual things, maybe just a bit differently," Weese said.

"What happens with outbreaks like this is, they just die out," he continued. "We never really know why, but most of these situations go away. So ultimately, how we start figuring out what's going on is, we have to start doing genomic work. That's the only thing that will help."

Even when a new strain circulates, it might not result in more significant disease, Weese said. "Who's to say it's causing dogs to be any more sick than normal?" he mused. "That's a bit harder to say."  

New or not?

Perhaps Parrish can provide some answers. Last week, he was contacted by a practitioner in Bellingham, Washington, asking for help investigating cases of sick dogs afflicted like those in Michigan and Colorado.

On Tuesday, veterinary authorities in British Columbia, Canada, reported that a parvo flare-up resulted in the death of a handful of puppies, which are most at greatest risk for contracting the virus.

"We get a lot of calls because we're the only ones who really research it on a regular basis," he said. "People have always been concerned, and when sick dogs are testing negative for the virus — well, let's remember that every test has some false positives and false negatives. No test is 100%."

Parrish said he has no reason to believe that the virus is evading vaccines, which are widely considered to be effective. "Whether this is a new virus, that is something we can check out. If we get all the samples we need, we can look at the sequences and hopefully know the results in a week or so," he said. 

VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email

Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.