As the flu season winds down for humans, veterinarians in the Chicago area warned of a deadly outbreak of canine influenza.

More than 1,000 cases of canine infectious respiratory disease have been reported in recent weeks, with at least five dogs dying from the infection.

"It's almost an epidemic," Dr. Jerry Klein told CBS station WBBM-TV. "I've been here for 35 years, it's probably the worst type of outbreak I've ever experienced."

Klein is the supervising veterinarian at the Chicago Veterinary Emergency Center, which has handled about 15 cases of canine flu daily.

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"We have had a few fatalities, so that's why it's exceedingly serious," Klein said. "It causes pneumonia, in some cases, and some of these dogs have to be on oxygen and on ventilation."

People cannot get it from their pets, but the illness is extremely contagious between dogs that come into close contact with each other.

On Friday, the Cook County Department of Animal and Rabies Control urged pet owners in the area to avoid dog parks, group dog training activities, animal boarding houses and travel.

After several dogs at PetSmart boarding facilities got sick, the company temporarily closed three Chicago-area locations to be disinfected. Other groomers and "doggy day care" companies were limiting operations, WBBM reported.

PetSmart spokesman Andy Izquierdo told WBBM 780's Lisa Fielding the company was reminding dog owners about the symptoms, which include a persistent cough, runny nose and fever.

"If they've been in a boarding facility, if they've been at a doggy day camp, or a dog park, that's an area where this virus can be spread," Izquierdo said. "Take a look at your dog, look for symptoms, keep them isolated if at all possible."

Dr. Judith Schwartz, the staff veterinarian at the Humane Society of New York, called the outbreak "very" concerning.

"Especially if it's within a certain period of time," Schwartz told CBS News' Kris Van Kleave. "That's very, very... that's frightening."

Schwartz said treating dog flu can cost thousands of dollars if the animal has to be hospitalized in isolation. An annual vaccine, considered to be highly effective, costs about $100.

"It's (a) concern for me not just because of the animals I see," Schwartz said, "but we could have a dog in the waiting room saying hello to another dog in the waiting room, and they could be incubating -- and no one would know it."

Unlike with human influenza, the number of cases nationwide isn't tracked. Since the condition was first discovered in 2004, 40 states have experienced outbreaks. Researchers said 2011 was the worst year on record, with 17 states reporting infections.

Chicago animal control officials said the outbreak could last several more weeks.