Questions remain as canine respiratory disease cases fall

Veterinary diagnostic laboratories work to determine the source of mystery illness

Updated February 23, 2024

The outbreak of canine infectious respiratory disease complex (CIRDC) that grabbed the nation’s attention late last year appears to be receding.

The Oregon Department of Agriculture, which received more than 200 case reports on the illness since August 2023, saw no new reports for the month of January. From September to November last year, Colorado State University’s Veterinary Teaching Hospital had seen more than double the cases of canine pneumonia compared with the same period in 2022. Cases also spiked elsewhere late last year, including California, Florida, and New Hampshire, as well parts of Canada.

Dr. Scott Weese, a professor at the University of Guelph Ontario Veterinary College and director of the Centre for Public Health and Zoonoses, said it appears things had calmed down considerably since the beginning of 2024.

A veterinarian examines a dog using a stethoscope
The most likely explanation for last year’s canine respiratory disease outbreak is a commensal bacterium that possibly plays a role in some disease but has been overlooked, says Dr. Scott Weese, a professor at the University of Guelph Ontario Veterinary College.

“It doesn’t seem like anything remarkable is going on at the moment,” Dr. Weese said, adding that a periodic spike in CIRDC cases in various regions is to be expected.

Dr. Michael Lappin, an internal medicine specialist and director of the Center for Companion Animal Studies at Colorado State University (CSU), said in a press release, “Why that trend is occurring is unknown but may be related to our pets spending less time in social situations that might include ill dogs during the winter months.”

Atypical CIRDC cases

Outbreaks of CIRDC, or sometimes referred to as kennel cough, are common. Roughly nine different bacteria and viruses are known to cause respiratory infections in dogs, as outlined in a paper published in November 2013 in the journal Veterinary Pathology. Infection by more than one bacterial or viral agent is common.

“When we talk about CIRDC, we’re generally talking about a common disease syndrome in dogs that is rarely serious and is associated with known causes of the disease. That’s not what we’re talking about here,” said Dr. Kim Dodd, director of the Michigan State University Veterinary Diagnostic Laboratory (MSU VDL).

The outbreak has raised a number of questions. Among them is why last year’s wave of canine respiratory disease differed from previous versions with many of these cases having other underlying health issues. Dogs experienced prolonged coughs, up to several weeks, and more cases developed what appeared to be secondary pneumonia. In addition, this pneumonia was either minimally or entirely unresponsive to antibiotics.

Arguably the most pressing question is why the causative agent in these atypical CIRDC cases (aCIRDC) has not been identified so far. This may be because of the fact that many samples are submitted for testing only after a dog becomes gravely ill, meaning enough time has passed that the inciting cause is undetectable, Dr. Dodd explained.

There’s also the possibility that therapy that was already started would negatively impact test results, she added.

The lack of definitive answers has given rise to speculation that a novel pathogen or variant of a known causative agent such as Bordetella has emerged. Consensus within the veterinary community is this is highly unlikely. A spokesperson for the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) told AVMA News there is no evidence to support such a conclusion.

Laboratories within the National Animal Health Laboratory Network (NAHLN) along with USDA National Veterinary Services Laboratories (NVSL) are conducting sequencing.

“Common causes of canine infectious respiratory disease complex have been identified in many of these cases,” the USDA spokesperson said. “As of now, the testing has not indicated the presence of a novel pathogen or single infectious cause among these cases.”

Looking for answers

However, the New Hampshire Veterinary Diagnostic Laboratory (NHVDL) and Hubbard Center for Genome Studies at the University of New Hampshire claim to have found a novel small bacterium in a subset of CIRDC samples.

The laboratory has been investigating the respiratory illness since November 2022. Initial reports, but not samples, started coming in late spring to early summer 2022 from practitioners in southeast New Hampshire who were seeing prolonged coughs and recalcitrant upper respiratory infections lasting up to three months. Once samples started coming in, the NHVDL reported sequencing data had revealed a “non-culturable, bacterial-like organism, similar to Mycoplasma, in a subset of respiratory samples.”

Then the laboratory announced its discovery in a January 24 update, saying additional evidence of a “potential novel bacterial respiratory pathogen” had been identified in a small number of dog samples from four U.S. states.

The NHVDL said genetic material from the potential pathogen was present in a total of 31 out of 226 screened dogs, including 15 out of 64 in New Hampshire, 10 out of 45 in Oregon, five out of 33 in Rhode Island, and one out of one in Connecticut. Bioinformatic analyses and some alternative sequencing technologies helped identify more segments of the potential pathogen’s genome.

The laboratory post notes that while findings are still preliminary, the identification of additional positive cases in more recent clusters of the syndrome in other states, as well as the lack of the bacterium in multiple temporal controls, helps support the potential that this bacterium may be new to the canine infectious respiratory disease complex.

“We can’t yet say for certain that the organism is causing the respiratory disease,” Robert Gibson, managing director of the NHVDL told AVMA News. “Maybe there’s some other cause, and we just happened to find this in these samples. Data generated by the study continues to support the possibility of a new pathogen, but to prove causation would require additional studies.”

Precautionary measures

Dr. Weese said he would be “very surprised” if the organism turned out to be new and pathogenic.

“The most likely explanation is it’s a commensal that’s been around, but we haven’t known to look for it,” he said. “Possibly it plays a role in some disease but we’ve overlooked it. Maybe more likely, if it’s clinically relevant, is it might be relevant as a coinfection but not a primary cause of disease.”

Since the MSU VDL began testing CIRDC samples in November, the laboratory has expanded the number of things it looks for in its canine respiratory panel, according to Dr. Dodd. Next-generation sequencing is also being performed on some of the samples.

“This sequences the entire contents of a sample, allowing us to identify any potential causative agents all at once, without having to run dozens of specific tests,” Dr. Dodd said. “It also gives us an opportunity to identify new variants of known viruses or bacteria.”

CSU researchers in the Veterinary Diagnostic Laboratory and the Center for Companion Animal Studies partnered with clinics in Fort Collins, Denver, Colorado Springs, and Grand Junction to test samples. This work will continue throughout February, and CSU will release results when that activity is completed, according to the release.

Separately, Dr. Blaire MacNeill, a clinical microbiology resident at CSU, led a medical records review of 87 cases of suspected infectious respiratory disease from the James L. Voss Veterinary Teaching Hospital between September 1, 2023, and January 18. The majority of cases entered the hospital through the urgent care department. That review, however, revealed no obvious patterns of organisms previously recognized to cause infectious respiratory disease. Four of the cases were fatal, one of which involved a dog that had another pre-existing condition.

Pet owners, Dr. Lappin said, should continue to keep dogs up to date on vaccinations and watch them closely if they develop a cough. Exercise caution regarding nose-to-nose contact with unfamiliar dogs, as their medical history and exposure risks may be unknown, and when choosing pet care services, ensure transparency by inquiring about protocols for isolating sick dogs and rigorous cleaning practices, he added.

A version of this story appears in the April 2024 print issue of JAVMA

Online resources about CIRDC are available from the AVMA to share with clients on the following webpages:

Veterinarians can read more about CIRDC in these peer-reviewed publications: