(CBS) – Veterinarians in Illinois are seeing the symptoms a lot. It’s Canine Influenza and it’s extremely contagious to other dogs in close contact.

Dr. Jerry Klein has treated several cases. “It’s almost an epidemic. In my 35 years, this is the worst outbreak.”

A little puppy named Lulu is in isolation. Her owner says she got dog flu after going to doggie daycare. She’s surprised and worried. “Coming to find out that she has that and pneumonia on top of that it’s a shock.”

When asked how concerning it is that 1000 dogs are coming down with the flu, Dr. Judith Schwartz’s answer was short and to the point. “Very. Especially in this period of time…that’s very very… that’s frightening.”

Dr. Schwartz, the staff veterinarian at the Humane Society of New York, says treating dog flu can cost thousands if the animal has to be hospitalized in isolation. There is an annual vaccine, costing around $100, that vets say is highly effective. “It’s a concern for me not just because of the animals I see, but we can have animals saying hello in the waiting room and they could be incubating and no one would know it.”

The number of cases nationwide aren’t tracked, but since the condition was first discovered in 2004 40 states have seen outbreaks. Researchers say 2011 was the worst year on record with 17 states reporting infections.

This week Cook County in Illinois warned this latest outbreak could last for several more weeks, urging pet owners to avoid dog parks, group dog training activities, animal boarding houses and travel.