Dog flu has shown adaptations that allow the virus to recognize a human-like receptor, according to a Chinese study, possibly indicating it may be closer to infecting humans.
A receptor is a molecule inside or on the surface of a cell that binds to a specific substance and causes an effect in the cell.
By studying biological characteristics of H3N2 canine influenza viruses isolated worldwide over the period of a decade, researchers at China Agricultural University and other institutions found that the viruses became able to recognize the human-like SAα2,6-Gal receptor.
In addition, the viruses showed gradually increased hemagglutination – a reaction that causes clumping of red blood cells in presence of some enveloped viruses – acid stability and replication ability in human airway epithelial cells and acquired a 100% transmission rate via respiratory droplets in testing using ferrets.
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Epithelial refers to the cells that line both the internal and external surfaces of the body.
Out of more than 4,100 dogs with signs of respiratory illness from nine provinces of China, 5.63% were positive of H3N2 infection, indicating the average positive rates for each year swabs were collected – from 2012 to 2019 – increased from 1.98% at the start of collection to 10.85% at the end, including a sharp increase in 2016.
In addition, six dogs were intentionally infected with the known flu strains of H3N2 and each was merely mildly unwell. The most serious symptoms included a fever, sneezing and coughing.
Nearly all dogs are susceptible to canine flu infection, with the viruses believed to spread mainly through respiratory droplets produced during coughing and sneezing or through contact with contaminated surfaces.
“A noteworthy observation was the number of human-like amino-acid substitutions that had gradually accumulated during the evolution of H3N2 CIVs in dogs and increased significantly after 2016,” the study explained. “These results indicated that H3N2 [canine viruses] may have increased their adaptability to humans during their evolution in dogs.”
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The authors also found that human populations lack immunity to H3N2 canine influenza viruses – with preexisting immunity from seasonal human viruses unable to provide protection against H3N2.
“Our results showed that canines may serve as intermediates for the adaptation of avian influenza viruses to humans,” the study said. “Continuous surveillance coordinated with risk assessment for [the viruses] is necessary.”
H3N2 avian influenza viruses were transmitted to dogs in the mid-2000s. Spread to cats from infected dogs has also been reported. Notably, there are vaccines to protect dogs against canine flu that are available in the U.S.
The U.S. Centers for Disease Control and Prevention says that, to date, there is no evidence of the spread of canine influenza viruses from dogs to people and there has not been a single reported case of human infection with a canine influenza virus in the U.S. or worldwide.
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In 2016, the agency used a risk assessment tool to evaluate the potential pandemic risk of such viruses and found it to be low.
“However, influenza viruses are constantly changing and it is possible that a canine influenza virus could change so that it could infect people and spread easily between people,” the CDC said.