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New Dog Flu Invites Closer Look at Viruses, Vaccines

12 June 2015
Vet Med

US - When the cause of the 2015 outbreak of respiratory illness in US dogs was identified as a strain of canine influenza virus not previously seen in this country, the news spread as fast as the flu itself.

Though canine influenza is not a health threat to people and is rarely fatal in dogs, the story of this globe- and species-hopping virus offers a glimpse into the way viruses, vaccines—and scientists—work.

“One of the first questions veterinarians had about this outbreak was whether the canine influenza vaccines available in the United States would protect against the newly emerged strain,” said Gail Scherba, DVM, PhD, a veterinary virologist and professor at the University of Illinois College of Veterinary Medicine in Urbana.

One way to approach that question is to compare, at the genetic level, the new strain of flu and strains used to create the vaccines. Researchers at the University of Illinois did just that.

Influenza A viruses are categorized by two proteins found on the surface of the virus, hemagglutinin (H) and neuraminidase (N).

In the case of canine influenza, the viral strain H3N8 has been known to circulate within the US dog population since at least 2004, when a strain previously found in horses was first identified in dogs. Commercially available dog flu vaccines were developed to protect against H3N8.

In early March 2015, Chicago veterinarians started noticing an unusually high number of dogs with respiratory illness. By April 12, the flu strain H3N2 had been identified as the culprit. As of May 25, more than 1,700 cases of dogs with severe respiratory illness had been reported in Chicago’s Cook County alone, and the H3N2 dog flu had spread to at least 12 states.

The H3N2 strain originated in birds. It had been identified in Asia as a strain that infects dogs and cats, but had not been seen in US dogs until the current outbreak.

“The H3N8 and H3N2 strains both have a H3 subtype. The H—hemagglutinin—is a very important protein because it is how the virus attaches to the host cell to initiate an infection. The body develops antibodies against this viral protein that provide immunity against infection,” said Dr Scherba.

“However, as viruses mutate over time, their viral proteins may be altered so much that the immune system no longer recognizes them.”

The first step in comparing the two strains genetically was to isolate the H3N2 virus. Research technicians at the University of Illinois Veterinary Diagnostic Laboratory isolated the H3N2 flu virus from a Chicago-area dog that died from canine influenza. They extracted the viral genetic material (RNA). Scientists at the university’s Roy J. Carver Biotechnology Center completed the genome sequence.

“It took less than a week to go from RNA to having sequence data for the entire virus,” noted Dr Alvaro Hernandez, director of DNA Services.

The H3 sequence from the H3N2 isolate was compared to H3 sequences available in an influenza gene bank, representing 15 H3N8 isolates collected from US dogs.

“The H3 protein of the recent isolate was only approximately 85 per cent identical to the H3 proteins from the H3N8 strains,” said Dr Scherba.

Differences between the sequences were distributed across the surface of the H3 protein, suggesting that the current H3N8 vaccine may not be effective in protecting against the new H3N2 strain.

Challenges in protecting animals against influenza are similar to those faced in human medicine. The good news is that although viruses are continually evolving and posing new threats to dogs, people, and many other species, veterinarians and other scientists are hard at work developing the knowledge and technology needed to combat these hazards.


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