Humans aren't the only ones suffering from the flu this time of year. Dogs get it, too, and drugmakers are busy spreading the word about canine flu vaccines.

Many dog owners aren't aware of the dog flu. Experts say it's a relatively new phenomenon. But after apparently spreading from horses in 2004, dog flu has popped up in 38 states, according to Merck (MRK). Dog flu does not transmit to people.

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Merck makes the Nobivac vaccine and last year had sold 1 million doses to veterinarians and kennels. The company is making a big push, setting up DogInfluenza.com and offering grants of the vaccine to animal shelters and rescue groups. More than half of those receiving the grants say they will add the vaccine to their treatment procedures.

Dog flu is not generally deadly, though it is more dangerous to pugs and other breeds with that smushed-in-nose appearance, The New York Times reports. And it's a worry for dogs that live together in kennels and shelters.

Pfizer (PFE) has jumped into the business with its own canine vaccine.

Could dog flu vaccines become a moneymaker for these companies? More kennels and vets are either requiring or strongly recommending them, the Journal reports. One veterinary group in New York gives about 60 shots a month.

The initial vaccination generally costs between $25 and $60, with a booster required every year. And with about 78 million dogs in the U.S. With those kinds of numbers, it's no wonder that Merck and Pfizer are scrambling for the dog flu vaccine market.