There is a chance that anyone who purchased a bird in the Baltimore-Washington area since last May 27 may contract a flu-like illness unless he treats his pet with a special but inexpensive bird feed containing an antibiotic, State Health Secretary Neil A. Solomon warned today.

Solomon said the special bird feed called Keet Life, is all that's needed to decontaminate small birds like canaries, parakeets, finches and cockatiels.

The 37 pet shops that sold birds now suspected of carrying the disease have agreed to provide a free 30-day supply of the feed and instructions to the bird owners, Solomon said.

Larger birds, such as love birds, require more complicated care and can be taken back to the pet store for a 45-day treatment with the antibiotic, Solomon added.

Two cases of the flu-like illness, called psittacosis, were confirmed in the Baltimore area in the last three months, health officials said. They added that people catch the disease, which is usually mild, from birds rather than from other people.

State health officials are investigating half of the 200 pet stores in the state to see how many birds came in contact with infected birds.

Birds in about 30 pet stores were quarantined yesterday, said Dr. Kenneth L. Crawford, the Maryland Public Health veterinarian. However, the infected birds shed the organism causing the illness several days after beginning the antibiotic treatment and will be safe to sell in a week, he added, if treatment is continued at home.

The investigation began last June when Crawford's secretary brought him a dead bird she had bought for her mother and said her mother was sick. Health officials traced the bird back to contact with seven cockatiels shipped into Maryland from California on May 27.

But by then, those seven birds had been in contact with about 1,500 other birds. Health officials say they cannot estimate how many more birds may have been infected through second, third and fourth-hand contact.

"It's not anything to get hysterical about," said Ruth Hanessian, of the Maryland Association of Pet Owners. Once the birds are treated, there's nothing to worry about, she added.

Solomon has drafted a letter to people who purchased birds from the 37 stores that received infected shipments warning them of the possibility of illness and giving them instructions about the antibiotic.

"If you bird fails to eat, has ruffled feathers, difficult breathing, diarrhea, and appears droopy for long periods or sits on the cage floor, it may be suffering from psittacosis fever," Solomon's letter said.

Solomon advised pet owners to contact their county health department or the state health department if their birds develop those symptoms within 60 days of receiving his letter.