Canine influenza A H3N2 In March and April of 2015, an outbreak of respiratory disease captured the attention of veterinarians practicing in the Chicago area. Initially, veterinarians thought these dogs were presenting with kennel cough, but they then started noting high fevers and explosive spread, both of which suggested the possibility of influenza. 10 Many dogs tested positive for influenza; however, instead of the H3N8 strain that had been previously circulating, these dogs were infected with an H3N2 strain of avian origin. 11 In the following months, canine influenza H3N2 spread throughout the Midwest and into other areas of the United States. 12 Canine influenza H3N2 was originally identified in China in 2006, and was noted in South Korea in 2007. 11 This virus seems to have been initially acquired from birds, and it incorporates gene segments from multiple different avian influenza viruses. 4 Further testing has demonstrated that canine influenza H3N2 originally entered the canine population in or around 2005.4 Most cases reported in Asia were severe, but further testing demonstrated that a number of healthy dogs also demonstrated antibodies to H3N2. This finding suggested that a milder, subclinical form of the disease existed. 4 Canine influenza virus H3N2 has been documented to spread among felines in Asia. A feline respiratory disease in 2010 affected shelter cats housed with sick dogs, and canine influenza H3N2 was isolated from the lung tissues of affected cats. 3 Clinical signs among affected shelter cats included lethargy, tachypnea, and dyspnea, although co-infection with Bordetella bronchiseptica may have played a role in the severity of illness noted.4 Mortality rates of up to 40% were observed among affected cats in these shelter outbreaks. 13 Experimental induction of canine influenza H3N2 in cats has been documented to cause less severe clinical signs, including elevated rectal temperatures, nasal virus shedding, and pulmonary lesions (e.g., suppurative Other influenza viruses in dogs In 2002, an equine influenza H3N8 outbreak was noted among a small population of foxhounds in the United Kingdom. Clinical signs included coughing, weakness, and lethargy and a number of cases progressed to bronchointerestitial pneumonia. One dog died during this outbreak, and several were euthanized due to the severity of clinical signs noted. 4 In 2009, human pandemic H1N1 influenza was isolated from 2 dogs in China and 1 dog in the United States. All 3 dogs demonstrated a severe cough; 1 dog only demonstrated mild Canine influenza: General considerations Canine influenza virus is not seasonal like human influenza viruses, 3 and therefore sporadic outbreaks are noted year-round. Additionally, canine influenza H3N8 and H3N2 do not have
bronchopneumonia). 13 Additionally, experimental studies have induced mild clinical signs in a small portion of experimentally infected ferrets; sneezing is the most common manifestation, with mild lethargy and anorexia also noted in some cases. 4 No canine to feline or canine to ferret spread has been observed in the United States, so it is uncertain whether this is a threat with our current strain. When the H3N2 strain was detected in the United States, leading influenza researcher Edward Dubovi stated “My concern all along was that someday this virus might make its way here. We don’t know how it happened, but it certainly could have come from a dog or a cat. There are multiple international groups who are rescuing dogs from the meat market in Korea and shipping them into the United States, and we have sketchy quarantine requirements if any at all. Restrictions on the movements of companion animals across borders are somewhat nonexistent.” 10 This is currently speculation, however, as there is no direct evidence to confirm that the disease was introduced in this manner. 12 As of August 8, 2015, within 6 months of introduction, cases of H3N2 had been confirmed in every state in the continental United States. The largest case clusters in those first 6 months were seen in Chicago, Atlanta, Asheville, Philadelphia, and New York City. 14 Nationwide, the numbers of positive tests peaked in early April, coinciding with the initial Chicago outbreak. 14 In any given week, however, only about 20% to 30% of submitted tests were positive for canine influenza virus 14 while other samples instead demonstrate parainfluenza, respiratory coronavirus, bordetella, or other upper respiratory pathogens. This statistic underscores the difficulties in diagnosis of this disease and its clinical similarities to other canine upper respiratory infections. depression and anorexia, while the other 2 exhibited fever and lethargy consistent with pneumonia. All 3 dogs recovered with supportive care, though the 2 more severely-affected dogs required hospitalization. 4 There have been other isolated reports of dogs infected with avian influenza (H5N1 and H5N2) in Asia. Additionally, experimental infection with H9N2 avian influenza has led to respiratory signs and dogs that are seropositive to this virus have been noted in China. 4
documented zoonotic risks. 3 People with frequent exposure to high-risk dog populations do not demonstrate antibodies against canine influenza virus. 15
TRANSMISSION
All strains of canine influenza virus are readily transmitted between dogs, via oronasal contact, droplets, and fomites (such as collars, leashes, food/water bowls, and kennel surfaces). 3,12 Fecal shedding has not been observed with canine influenza virus 1 , despite that being a known mechanism by which birds can spread influenza viruses. Like other influenza A viruses, however, both canine influenza strains are thought to be spread by aerosols over distances of up to 50 feet. 1 These aerosols may be produced by coughing and barking dogs, however, cleaning techniques such as spraying and hosing kennels may also further contribute to the formation and dispersal of aerosols. 1 This long- range transmission seems to occur more frequently in areas with poor ventilation; outbreaks are most common when dogs are housed in close proximity with closed circulation environments. 2 The highest rates of canine influenza transmission are noted in dogs that reside in kennels or spend time in areas with transient dog populations (such as animal shelters and pet daycare facilities). Approximately 40% of those high-risk dogs are seropositive for the H3N8 strain. 4 Interestingly, dogs who come
in contact with large groups of other dogs in other venues have demonstrated much lower rates of H3N8 exposure; 399 blood samples collected in 2010 demonstrated 0% seroprevalence among Iditarod sled dogs 16 and 100 blood samples collected from 2009 to 2010 demonstrated only 1.9% seroprevalance among dogs participating in a flyball tournament. Without adequate cleaning measures, influenza particles may persist in the environment for up to 48 hours and viral particles may remain viable on hands and clothing for up to 24 hours. 12 Fortunately, canine influenza is quickly killed by all common disinfectants used in kennels and veterinary hospitals, including quaternary ammonium compounds (e.g., benzalkonium chloride - Lysol™), aldehydes, phenols and bleach solutions. 12 The incubation period of canine influenza is 1 to 4 days. Viral shedding is at its highest during the incubation period, and typically peaks at approximately 4 days post-exposure. 3,5,12 This asymptomatic shedding period complicates efforts to prevent transmission, as dogs are at their most contagious before they
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