Canine influenza virus (CIV) is an emerging threat to dogs in the community.

Virtually 100% of newly exposed dogs become infected with CIV, and up to 20% may develop a severe systemic course.²

History

Canine influenza virus (CIV) was first recognized in 2004 by University of Florida researchers and Cornell University virologists working with the Centers for Disease Control and Prevention (CDC) following an outbreak of severe respiratory illness at a Greyhound racing facility in Florida.^2,4,10 The mysterious illness first impacted Greyhounds at racing facilities in Florida in 2003 and 2004.² In January 2004, 8 of 22 affected dogs had died due to extensive hemorrhage in the lungs.^4,11 Fortunately, this was a small outbreak due to the rapid institution of containment measures. Since that time, however, CIV has been identified throughout the United States.^1,4,12 In fact, after the 2004 outbreak, retrospective testing of archived lab samples was conducted, which indicates that the virus infected dogs as early as 1999.^11,13 It was the relatively severe outbreak in Florida in 2004 that was the "canary in the mineshaft" that alerted veterinary researchers and professionals to the condition.¹⁰

Following the 2004 outbreak, the extent of respiratory infections in Greyhounds was established by two lines of evidence:^11,14

1. Respiratory disease outbreaks occurred during 2004 and 2005 at racetracks in at least 13 states, representing over 20,000 dogs.
2. In July 2004, a second canine influenza virus was isolated from a Greyhound that died at a track in Texas. This virus had at least 99% nucleotide homology with the virus identified in Florida and confirmed the link of H3N8 to equine influenza.

Subsequently, many documented outbreaks have confirmed the spread of the virus to the general population. In May 2009, cases of canine influenza had been confirmed in 30 states. Since then, 8 more states have had documented cases, and the virus continues to spread. Every continental state except one has either had a confirmed case of canine influenza or borders on another state that does.¹

FIGURE: Canine influenza in the United States¹

This map captures only confirmed, sampled cases. Actual incidence is believed to be higher since mild cases of canine influenza can be misdiagnosed as kennel cough.

Cases of H3N8 virus in dogs have also been reported in Foxhounds in England in 2002 and in Australia in 2007. However, there is no evidence of horizontal transmission of CIV in these cases, and they are believed to be caused by equine influenza in dogs, and not CIV.⁶ There have also been reports of a second subtype, the H3N2 virus located in Korea, that has the potential to become a second canine influenza virus.⁶ At least 3 different isolates have been recovered, and this subtype appears to have originated in birds.⁶