Dr Cynda Crawford
The first reported outbreak occurred in greyhounds at a Florida racetrack in 2004, in which eight greyhounds died. Later outbreaks occurred at other tracks, as well as animal shelters, dog rescue centres, boarding kennels and veterinary facilities.
It was the first recorded outbreak of canine influenza in the world.
However, researchers led by Tara Anderson, from the University of Florida College of Veterinary Medicine, checked greyhound blood samples from a blood bank in California that held the blood of retired greyhounds.
They found that a third of the samples collected in 1999 carried antibodies to the canine influenza virus, indicating the disease was circulating in racing greyhounds in several states in 1999, and may even have devloped before that.
The findings, presented this month at the International Conference on Emerging Infectious Diseases in Atlanta, pointed out that the H3N8 influenza virus involved is closely related to the equine flu virus.
Nearly all dogs exposed to H3N8 get the disease, in which they develop an infection of the upper respiratory tract which is usually mild. Some, however, develop a more severe form and suffer pneumonia, with a death rate in such cases of up to 8 percent.
It was University of Florida veterinary researchers that uncovered in 2004 what is believed to be the first evidence of equine influenza jumping the species barrier.
The came to that conclusion after researching the cause of respiratory disease outbreak that killed the eight racing greyhounds.
Dr Cynda Crawford, a University of Florida veterinary immunologist who spearheaded the research, had immediately visited the Jacksonville track when the outbreak occurred. In all, 24 dogs were affected.
She collected blood and nasal fluid samples from 35 dogs, and five of the dogs that died underwent postmortem examinations. She sent samples for analysis to Dr Ed Dubovi, director of the virology section at Cornell's Animal Health Diagnostic Lab.
"Dr. Dubovi cracked this," Dr Crawford says. "He was able to isolate an influenza-like virus, which he then sent to the Centres for Disease Control (CDC), which routinely monitors influenza outbreaks involving interspecies transmission to determine if there is a threat to public health."
On the basis of genetic sequencing, the CDC concluded the virus found in the canine samples resembled a strain of equine influenza virus that appeared in horses in Wisconsin in 2003.
"The virus found in the canine samples is probably representative of the strain that is circulating now in horses in Florida and elsewhere in the US," Donis said at the time of the discovery.
The scientists said their findings were strong because they also verified that the dogs developed antibodies specific for the influenza virus.
"This implies that the virus replicated enough within the dogs for their immune system to recognise it and form antibodies," said Dr Crawford, who plans to present the team's findings later this month at a meeting sponsored by the National Greyhound Association.
The scientists said at the time they have no idea how the Jacksonville greyhounds could have been exposed to equine influenza virus.