The relatively brief history of canine influenza viruses (CIVs) began about 12 years ago, when an equine H3N8 virus (which had only jumped to horses about 50 years earlier), mutated enough that it jumped to greyhounds at a racetrack in South Florida (see EID Journal article Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida).
A 96% match to the equine H3N8 virus, this canine H3N2 was believed to have jumped directly from horses to dogs without any reassortment.
A couple of years later we saw an avian H3N2 virus also adapting to dogs (see Transmission of Avian Influenza Virus (H3N2) to Dogs) in Korea. Analysis showed that the HA and NA genes of the A/canine/Korea/01/2007 (H3N2) isolate were closely related to those identified in 2003 from chickens and doves in South Korea.
While the canine H3N8 virus has remained fairly stable over the last decade, we've seen numerous reports coming out of China and Korea suggesting the canine H3N2 may be adapting to other hosts, and continues to reassort with other avian and human flu viruses. Including:
In April of 2015, the Asian canine H3N2 virus turned up in Chicago (see CDC Statement On H3N2 Canine Influenza In Chicago Region), and since then has reportedly spread across much of the United States in a remarkably short period of time.
The AVMA warns that canine influenza viruses `. . . can remain viable (alive and able to infect) on surfaces for up to 48 hours, on clothing for 24 hours, and on hands for 12 hours.'
It also appears that dogs with H3N2 may shed the virus for 2 to 3 times longer than with the H3N8 virus, which may also be contributing to its rapid spread. They describe the shedding of H3N8 as:
The incubation period is usually two to four days from exposure to onset of clinical signs. The highest amounts of viral shedding occur during this time; therefore, dogs are most contagious during this 2-4 day incubation period when they are not exhibiting signs of illness. Virus shedding decreases dramatically during the first 4 days of illness but may continue up to 7 days in most dogs and up to 10 days in some dogs with H3N8 canine influenza.
The following report, however, indicates that dogs may shed the canine H3N2 virus much longer; up to 26 days.
Journal of the American Veterinary Medical Association
May 1, 2016, Vol. 248, No. 9, Pages 1022-1026
Prolonged intermittent virus shedding during an outbreak of canine influenza A H3N2 virus infection in dogs in three Chicago area shelters: 16 cases (March to May 2015)
Sandra Newbury, DVM; Jennifer Godhardt-Cooper, MS; Keith P. Poulsen, DVM, PhD; Francine Cigel, MS; Laura Balanoff, DVM; Kathy Toohey-Kurth, PhD
Address correspondence to Dr. Newbury (firstname.lastname@example.org).
OBJECTIVE To estimate an appropriate isolation period for dogs infected with canine influenza A H3N2 virus on the basis of the duration of virus shedding.
DESIGN Retrospective case series.
ANIMALS 16 dogs, from 3 Chicago area shelters, naturally infected with canine influenza A H3N2 virus.
PROCEDURES Medical records of 16 affected dogs were reviewed. Nasal swab specimens from each dog had been tested periodically for a minimum of 15 days following an initial positive real-time reverse transcriptase PCR (rRT-PCR) assay result for influenza A virus shedding. Amplicons were purified, quantified, and sequenced by the Sanger DNA sequencing technique. Virus isolation and sequence results of canine influenza A H3N2 virus from nasal swab specimens were obtained in conjunction with signalment, description of clinical signs, type of treatment, and outcome.
RESULTS Viruses from each dog were identified as canine influenza A H3N2 virus on the basis of DNA sequencing. The interval between first and last positive rRT-PCR assay results ranged from 13 to 24 days, whereas the time interval from first reported clinical signs to last positive assay results ranged from 15 to 26 days. Isolation of canine influenza A H3N2 virus was successful in the late shedding period from nasal swab specimens of 4 dogs at 15 and 20 days after the first positive rRT-PCR assay result and 18 to 20 days after the first clinical signs. Clinical signs resolved for all dogs that remained in the shelters during the testing period.
CONCLUSIONS AND CLINICAL RELEVANCE Dogs infected with H3N2 virus should be isolated for a period of ≥ 21 days following onset of illness. Even when resolution of clinical signs occurs sooner than 21 days, shedding of H3N2 virus may persist.
All of this is important because these canine influenza viruses, like all influenza viruses, are continually evolving and adapting. A longer shedding period provides more opportunities for the virus to infect new hosts and interact with other flu viruses.
So far, canine H3N2 seems more mutable than its H3N8 cousin, but the CDC continues to monitor both:
To date, there is no evidence of transmission 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.
However, influenza viruses are constantly changing and it is possible for a virus to change so that it could infect humans and spread easily between humans. Human infections with new influenza viruses (against which the human population has little immunity) are concerning when they occur.
Such viruses could present pandemic influenza threats. For this reason, CDC and its partners are monitoring the canine influenza H3N8 and H3N2 viruses (as well as other animal influenza viruses) closely. In general, canine influenza viruses are considered to pose a low threat to humans.