More than 1,000 dogs have become ill and at least five have died in a dog flu epidemic that has affected the Chicago area since mid-March, an official says.

These numbers may be conservative and are likely to continue to increase as some veterinary clinics continue to handle as many as 50 or more new suspected or confirmed cases a week, Donna Alexander, administrator for the Cook County Department of Animal and Rabies Control, told the Chicago Tribune.

Currently, the canine influenza virus (CIV) outbreak is limited to the Chicago metropolitan area, but could spread as dogs travel with their owners for holidays and to events such as dog sport competitions.

Several suspected but unconfirmed cases of CIV have been reported in Indiana, Missouri and Wisconsin.

"Of course, the canine influenza virus could easily spread to other cities, especially where there are dense canine populations," veterinary immunologist Cynda Crawford, a clinical assistant professor of Shelter Medicine with Maddie's Shelter Medicine Program at the University of Florida College of Veterinary Medicine, told the Tribune.

There have been sporadic outbreaks of CIV across the United States, but most dogs have never been exposed to the virus.

"There's no built-in immune protection for most dogs," Brooke Bartell, of the Chicago Veterinary Emergency and Specialty Center, told the Tribune. "The result is that nearly all dogs exposed to the virus will get the virus."

Symptoms include coughing, lethargy and lack of appetite. However, 20 percent to 25 percent of dogs infected with virus don't get sick, but are still highly infectious.

"These dogs feel fine, they act fine and have no symptoms. That's good, except they are highly infectious and are particularly effective at spreading the virus because they continue their usual activities without anyone suspecting how contagious they are," Chicago veterinarian Natalie Marks told the Tribune.

There is a vaccine for CIV, but too few dogs get it. In an effort to control the outbreak in Chicago, warning signs have been posted in dog-friendly areas, some dog training classes have been postponed, a few dog day cares have closed, and some pet stores are discouraging visits from dogs.