Emerging Threat Of Canine Influenza

Canine influenza has been a steadily growing threat that has caught the attention of owners, who are concerned for their pets.

The growing range of influenza viruses has many people concerned about their risk of infection and the risk level of their pets. Private practitioners bear the brunt of inquiries and are being asked to make determinations of viral spread that stump virologists and epidemiologists.

What is known about the canine influenza virus is that the country’s shelter populations and boarding facilities are at the highest risk. Experts say the virus could spread in a shelter environment as readily as other respiratory infections, including canine infectious tracheobronchitis, or kennel cough.

But show dogs, race dogs, boarded dogs and even those that frequent dog parks are at heightened risk of exposure as well.

“This is an emerging disease which is the impetus for all of the funding dollars going toward research,” says Wayne A.  Jensen, DVM, Ph.D., MBA, chief scientific officer for Morris Animal Foundation.

“There are all sorts of questions about this virus, and not a lot of knowledge. Our earlier concerns about the virus were that it would be deadly. Although it has largely been controlled, we need to quickly gather information as we don’t know the virus’ speed of mutation or full capabilities yet.”

The U.S. strain of H3N8 (canine flu) isn’t a reportable disease, so the true number of nationally diagnosed cases is unknown. However, universities and laboratories that receive samples for testing are keeping count.

“We continue to get isolates from private practices and track any noted changes in the virus,” says Edward J. Dubovi, Ph.D., a virologist with the Cornell University College of Veterinary Medicine.

“We use PCR tests to identify specific markers for the virus that makes the canine strain different than others,” Dubovi says. “Monitoring the evolution of the virus tells us if the dog received a viral infection from another dog or another species. At this point, we don’t know what other species are susceptible to the canine influenza strain. Cats could be a logical jump, since dogs and cats often live in the same households.”


Cornell researchers say nasal swabs are nearly twice as successful in isolating the virus as nasopharyngeal swabs taken at the same time on the same animal.

A generic PCR test that detects the highly conserved matrix gene is the test of choice at Cornell. While Dubovi and others there are currently looking for the H3N8 virus in submitted samples, a test unique to this virus would miss a potential influenza infection caused by another flu strain.

PCR testing is a step in the right direction, researchers say.

Since the canine flu virus is believed to have mutated from equine influenza, also labeled H3N8, researchers are hopeful the virus mutates at a slow rate like that of the equine strain and not like the human influenza virus, which requires annual vaccine reformulation to maintain efficacy.

“All influenzas have a genetic drift,” says Emily Beeler, DVM, a zoonosis veterinarian at the Los Angeles County Department of Public Health. “If the virus mutated very quickly, I doubt that veterinary medicine would have the financial resources or time to coordinate an effective vaccine annually. So we are lucky in the respect that it seems to evolve more slowly.”

“Shelters find it nearly impossible to eradicate the disease once it is introduced. We want to learn how the virus spreads among shelter dogs and will also determine whether there is a reliable test that could detect the virus during a dog's intake exam at a shelter."

~ Gabriele A. Landolt, DVM

Other Strains

The U.S. H3N8 strain isn’t the only influenza circulating in canine populations. A subtype of H3N2, an avian influenza strain, was initially found in South Korean dog populations in 2007.

Researchers say this discovery suggests that avian influenza virus with high pathogenicity can rapidly spread from dogs to dogs and has made the interspecies leap. Most whole influenza viruses that are transmitted directly from the natural host species to a different species do not achieve sustained transmission in the new host species, according to Daesub Song of Green Cross Veterinary Products Ltd. in South Korea  and other researchers who study this strain and other emerging diseases.

This suggests that multiple host interactions are needed before the virus can replicate and be transmitted horizontally in a new host species.

“Although any influenza virus that jumps species is a concern, the greatest concern is when a virus makes an interspecies leap and is effectively transferred within the species,” Dr.  Beeler says.

“LA County and other big cities regularly receive imported purebred dogs from Korea–mostly Yorkshire terriers. This means there is a possibility this strain could enter the U.S. So far, some dogs have tested positive for parvovirus, but none have been diagnosed with H3N2.”

Experts say many veterinarians believe the influenza virus in canine populations is a newer development, but Florida serology samples from the 1990s show antibodies were present.

“Influenza in cats isn’t unheard of either,” Beeler says. “H5N1, an avian influenza, was detected in Thailand cats in 2004–so the Iowa cat that tested positive for H1N1 (swine flu) wasn’t the first cat to be infected with an influenza virus.”

One glimpse of hope with the various influenza outbreaks is that flareups seem to happen and then the infection dies out rather than taking large numbers of casualties, experts say.


Intervet Schering-Plough’s release of Canine Influenza Vaccine, H3N8 killed virus, has offered some comfort to veterinarians whose clients are at an elevated risk of exposure for infection.

“We always recommend that owners speak with their veterinarians about any and all treatments,” says Intervet spokeswoman Sharon Dilling, and that includes the new vaccine.

"Because [canine influenza] is a new disease, virtually every dog not previously exposed to the virus will become infected,” she says.

“Many dogs are candidates for the vaccine because of routine contact with other dogs who may be carrying the virus, congregating in such places as dog parks, veterinary clinics, boarding kennels, breeding kennels, dog shows, training settings, shelters, adoption centers, pet shops and other locations."

But some specialists are concerned about its efficacy because of its limited USDA approval status.

“The vaccine isn’t for every canine patient,” says Karen Shaw Becker, DVM, NMD, of Natural Pet Animal Hospital in Bourbonnais, Ill. “My hope is that veterinarians don’t administer this vaccine to silence clients’ hysteria. The vaccine was tested for side effects on 746 dogs, but what about its efficacy? Is it truly protecting them?”

Dr. Becker, a holistic veterinarian in private practice,  created a YouTube video to dispel factual errors about canine influenza, discussing which animals are at highest risk.

Intervet’s website posts this statement: “This product license is conditional. As with all USDA conditionally licensed products, data submitted to the USDA supports a reasonable expectation of efficacy. Safety was established in trials involving more than 700 dogs.”

The company says that vaccinated dogs will experience reduced symptoms and number of days of viral shed along with protection against formation and severity of lung lesions.

Answering Clients’ Questions

Experts say to keep updated with local public health authorities who may be tracking any locally diagnosed influenza cases. This information will help practices determine the need to carry and or administer the vaccine.

“Since some animals are asymptomatic, veterinarians should advise owners to simply take precautions when they handle their pets when they are sick,” Beeler says. “There will always be new emerging diseases, but this is no reason to fear dogs and cats. In the case of household pets, the only evidence of influenza transfer was from a human to a cat and not the other way around.”

Research in part is being conducted due to public’s concern of spread of the various influenza strains to people.

“Media attention has heightened citizen’s concern,” Jensen says. “When they hear the word ‘influenza’ they are immediately on alert. Currently, MAF isn’t funding research on H3N2. And veterinarians in general are unsure if a dog vaccinated with Intervet’s vaccine would have some safety if exposed to the H3N2 strain, but we need to focus on understanding the H3N8 canine strain first.”

All influenzas originated from shore birds, which is a fact the average client may not know. Telling clients that alone could ease fears of hearing about species jumps.

“The different origins of the US canine influenza strain of H3N8 and the H3N2 show that the two influenzas are not a mutation of the canine strain we have in the U.S., but rather an entirely different strain of flu,” Dr. Dubovi says. “Species jumps are times we always have issues because the new species doesn’t have a defense against the foreign strain. The originating species has evolved with the virus and has developed defense mechanisms.”


Morris Animal Foundation has funded several researchers investigating the virus in shelter settings and working on creating more efficient testing methods. Gabriele A. Landolt, DVM, Colorado State University is the principal investigator in a three-year investigation partnership with six ASPCA shelters.

“Shelters find it nearly impossible to eradicate the disease once it is introduced,” Dr. Landolt says. “We want to learn how the virus spreads among shelter dogs and will also determine whether there is a reliable test that could detect the virus during a dog's intake exam at a shelter.

“This would allow shelter managers to quarantine affected dogs and keep the virus from spreading to healthy animals. The information learned could also help promote the use of a vaccine for the virus.”

Landolt says people spread much more influenza virus and for longer periods of time than dogs or cats. She says she wants to find out if the type of influenza determines the amount of virus shed or if the host’s immunity level plays a role. The research is expected to conclude in December 2012, but an update could be released after one year.


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