Last spring, veterinarians in Chicago became overwhelmed by dogs with high fevers, hacking coughs, copiously dripping noses, runny eyes, lethargy and loss of appetite. Over nearly two months, at least 1,500 dogs fell ill, many for as long as three weeks. At least eight died from secondary infections like pneumonia.

Initially, veterinarians thought they were seeing an outbreak of canine influenza, caused by a mutation of H3N8, an equine virus. But these dogs were much sicker for much longer, and the illness seemed more contagious, racing through doggy day care centers, dog parks and especially boarding kennels, which had been filling for the spring holidays.

Eventually virologists identified it as a new strain of canine influenza. This one is a mutation of avian flu, H3N2, which had previously been documented only in Asia. Since the Chicago outbreak, scatterings of cases have been reported in 25 states, with more significant outbreaks in and around Atlanta and Cincinnati.

Now, a vaccine for canine influenza virus H3N2 has received conditional approval from the United States Department of Agriculture, which expedites licensing in emergency situations to avert epidemics. As the holiday travel season approaches, the question is: Should you get your dog vaccinated?

    

This is not an essential vaccine, like the rabies series. The mortality rate of this flu has been low, although the rate of morbidity — the likelihood of a carrier becoming sick — is high. Within a week of getting the virus, about 80 percent of dogs do become ill. All are contagious.

Edward J. Dubovi, a professor of virology at Cornell University’s College of Veterinary Medicine who helped identify the strain, estimates that infected dogs produce about 100 times more of this virus than H3N8 canine influenza, and for much longer — up to 21 days.

Veterinarians are likely to base recommendations in part on a dog’s lifestyle. Does the dog go to day care or a dog park? When owners travel, do they board their dogs in kennels?

“If your dog is in competitions for dog shows or gets groomed every week, I would consider the vaccine more strongly,” said Dr. Jerry Klein, chief veterinary officer for the American Kennel Club. “But I tell anyone, only vaccinate dogs that are completely healthy and have no underlying disease.”

Another consideration may be whether cases have been reported locally. Dr. Bruce R. Coston, a spokesman for the American Veterinary Medical Association, practices in rural Woodstock, Va. “We haven’t had reports, so I’m not recommending the vaccination for my patients,” he said.

But an accurate picture of the spread is difficult to determine. Reporting of canine influenza virus is not mandatory. Many veterinarians do not order tests for it, because of the cost and because the treatment, which begins immediately, is similar to that of other canine respiratory viruses: hydration, rest, cough suppressants and monitoring for secondary infections.

Owners, who may resist giving their dogs numerous vaccinations, will also weigh the cost and inconvenience. Made by both Merck Animal Health and Zoetis, the initial doses are two shots, several weeks apart. If full licensing is granted next year, an annual booster will be recommended. A Zoetis spokeswoman said the price, set by individual veterinarians, will most likely be $25 to $35 for each injection. Although a vaccine for canine influenza HN38 has been approved since 2009, no plans have been announced for a combination vaccine.

The virus becomes airborne as dogs cough and lick each other or humans. People cannot get sick from it but can easily pass on the virus by touching other dogs, or surfaces such as dog bowls and leashes. Only a handful of cats have contracted it.

The vaccine protects dogs, but does not give complete immunity. It sharply reduces the degree to which a dog may get sick, or is contagious. Dr. Eileen C. Ball, a veterinarian with Zoetis, said that because the flu is new and almost no dogs have antibodies to fight it, “it can spread unchecked.” With the vaccine, she added, “we can stop a severe virus from moving through the population.”

Dr. Klein saw first-hand the misery and havoc created by the outbreak. At its peak, the Chicago Veterinary Emergency and Specialty Center, now MedVet Chicago, where he is the supervising veterinarian, was seeing 20 new cases daily. Dogs had to be admitted outside the facility, which set aside two exam rooms that required complete disinfection after each canine visit. Some dogs had to be hospitalized for nearly a week. The center sometimes ran out of space and had to send dogs to other animal clinics.

“We were all exhausted,” Dr. Klein said. “I’d never worked so hard in 35 years.”

As a precaution, he refrained from sending his Afghan hounds to the groomers for two months.

For dog owners, nonessential vaccination is a subjective matter. If the vaccine is distributed in Ithaca, N.Y., Dr. Dubovi of Cornell said he would consider many factors when deciding about his 13-year-old cockapoo, Chelsea.

But one factor is more pronounced.

“I will probably cave in to my wife,” he said. She favors vaccination. It would not be a marital dispute, he said. “After 47 years, it’s a marital discussion.”