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Canine Influenza Virus
Ellen Collisson, MS, PhD

College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA, USA

Etiology/Pathophysiology Epidemiology

While influenza A viruses had for many years not been Influenza viruses are highly contagious and easily
considered pathogens of concern in dogs, they are today a transmitted from dog to dog by aerosol or contact
serious differential for canine respiratory problems. In exposure, but the H3N2 also was found to infect cats,
2004, a previously known equine strain of H3N8 influenza resulting in respiratory illness. The H3N8 virus has
A virus emerged as a canine respiratory concern during an been shown experimentally to also infect cats. Animals
outbreak in greyhound racing dogs in Florida. Twenty‐ in high‐density environments, such as shelters, compe-
two dogs suffered from fever, followed by coughing and tition events, and breeding facilities, are at greatest risk
either recovery or, in the case of eight dogs, peracute death of infection.
occurred associated with extensive hemorrhaging of the The original canine H3N8 outbreak in greyhounds,
lungs, mediastinum, and pleural cavity. first recognized in January of 2004, had spread in a few
Severe respiratory illness with mortality was reported months to tracks in six states, and as of April 2013 had
in dogs in 2008 in Korea and later in China and Thailand been reported in 40 states (dogflu.com). Similar to influ-
due to an H3N2 strain of influenza that has been shown enza in other animals, a dog may be infected and shed
to originate from the avian virus and to also transmit contagious CIV for up to four days before clinical signs
from dog to dog. In March 2015, canine influenza virus occur. Currently, infection of dogs is estimated to result
(CIV) H3N2 was identified in the United States in the in 80% morbidity and 1–5% mortality although higher
Chicago area and during the same year identified in dogs rates of mortality have been reported. Humans have been
in at least 30 states throughout southern, eastern and reported to also carry the virus from an infected animal
midwestern United States. In 2015, shelter cats in Indiana to a naive animal although the Centers for Disease
were also diagnosed with an H3N2 strain. Control (CDC) states that human infections with the CIV
Typical of other influenza A viruses, CIV viral particles strains are not a health concern. This may be a matter of
capable of cell‐to‐cell or host‐to‐host transmission carry H adaptation and infection with or without clinical illness.
(hemagglutinin) and N (neuraminidase) proteins that are The virus isolated from the lungs of dogs in Florida
anchored in the bilipid envelope coating the viral particle. during the first cases of CIV was found to be a canine‐
These glycosylated proteins, used to define subtypes, are adapted H3N8 virus originating from an equine H3N8
prone to mutations that result in the evolution of new influ- subtype, recognized as respiratory pathogens of horses
enza strains. A hallmark of influenza viruses is the eight‐seg- since 1963. Unlike previous reports of influenza infec-
mented RNA genome which provides a mechanism of rapid tions in dogs, which were self‐limiting and not transmit-
genetic change through segment reassortment. Thus, infec- ted from dog to dog, the greyhound H3N8 virus of 2004
tion with more than one type of influenza virus can result in originating from an equine H3N8 virus and the H3N2
progeny that are a combination of RNA segments from originating from an avian influenza virus were easily
more than one parent strain. In addition, the RNA polymer- transmitted from dog to dog and, at least the latter, also
ase which generates the viral RNA segments for progeny from cat to cat. The sequencing of genomes of CIV iso-
virus is error prone, creating mutations that may contribute lates identified signature mutations that indicate stable
to altered viral tropism, pathogenesis, and antigenicity. changes in the canine‐adapted viruses.

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical