A highly contagious strain of canine influenza has sickened thousands of dogs across the United States and continues to spread, according to information recently released by the American Veterinary Medical Association (AVMA).

The AVMA reports that the outbreak started in Chicago during April 2015, and is due to the H3N2 strain of dog flu.

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“Like most disease, canine influenza will hit hardest on the oldest animals, the youngest, and those that are being challenged by other contagious diseases,” Jim Evermann, a professor of infectious diseases at the Washington Animal Disease Diagnostic Laboratory, said in a press release issued by Washington State University.

Washington is just one of more than 25 states grappling with the problem now. Some experts even think that the virus has traveled to more than 40 states.

“For that reason,” Evermann added, “we are advising all dog owners to check and make sure their animals are up to date on their core vaccines that include canine distemper, canine parvovirus, and canine adenovirus. The healthiest animals will suffer the least with this disease.”

The AVMA reports that fewer than 10 percent of dogs die from canine influenza, but it can lead to fatal cases of pneumonia. Most dogs recover in two to three weeks, but given that lengthy period of time, it is understandable how the illness can take its toll on elderly, young and other vulnerable dogs.

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H3N2 is one of two viruses that cause flu in dogs. Another strain, H3N8, was first documented in the United States in 2004. H3N8 is closely related to a virus that affects horses, so it is thought that H3N8 mutated to produce the canine strain.

H3N2, on the other hand, is thought to have emerged in Asia, given that many cases have been reported in Korea, China and Thailand. Animal health experts believe that an avian influenza virus in these countries directly transferred to dogs. They suspect that could have happened in markets where live birds are sold.

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The AVMA notes that dogs sickened with either strain of the virus fall into one of two types of case: mild or severe. Symptoms of the former include a cough that can persist for days. It may be accompanied by reduced appetite, fever and reduced energy. Sneezing and eye and/or nose discharge may result. If the owner observes thick nasal discharge, that is likely due to a secondary bacterial infection that can affect the weakened flu victims.

The severe form of the illness can lead to high fever — 104 degrees F. or more. Respiratory effort, meanwhile, becomes challenged, and secondary bacterial infection could drive the onset of pneumonia.

The concern now is that H3N8 vaccines — the first of several antidotes was approved in 2009 — might not offer full protection against H3N2.

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Facilities that house multiple dogs are particularly concerned about the current epidemic.

As Texas veterinarian Luann Ervin told Tech Times: “It starts and it will move through an area like wildfire. It gets into the kennels, rescues and shelters.”

Many sites, as a result, are stepping up disinfection procedures and are watching out for symptoms.

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Human health experts are also closely monitoring such viruses. While there is no evidence to date that they can spread to people, a prior mutation of H3N8 could open up that future possibility.

If your dog shows signs of the illness, don’t delay in taking your pet to a veterinarian, experts advise. The vet will work to increase fluids to combat dehydration, maximizing your dog’s natural immunity. An antimicrobial might also be prescribed to knock out any secondary bacterial infection.