A rare outbreak of dog flu has almost reached epidemic levels in Chicago, and the illness could easily spread to others areas of the country. More than 1,000 dogs have become ill with canine influenza virus (CIV) and at least five have died since the middle of March, according to the Cook County Department of Animal and Rabies Control.

Symptoms of the respiratory disease include lethargy, loss of appetite, "honking" cough, and fever, and CIV is more severe in dogs less than a year old and those older than seven.

"I have been practicing for 20 years and I have never seen anything this bad, this contagious, this widespread," veterinarian Jane Lohmar told the Chicago Sun Times.

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

Officials warn that the virus could spread to other areas of the country as pets travel with their owners during spring break vacations.

Chicago's boarding and dog day care centers are closing, and pet owners are being advised to avoid places where pets tend to be in close contact, such as pet-friendly parks and group training classes.

According to the Centers for Disease Control, the virus was first spotted about 10 years ago in Florida when it mutated from an equine (horse) flu. Although most infections are mild, it can lead to fatal pneumonia.

A two-shot vaccine, which is given two to three weeks apart, is available, but the vaccine isn't part of a dog's normal vaccine routine. In addition to protecting dogs from the flu, it also appears to protect against pneumonia.

Neither people nor cats can be infected with dog flu.