Two of the most annoying insects of summer -- deerflies and horseflies -- may spread Lyme disease, a Connecticut physician contends.

In a letter to the New England Journal of Medicine published June 14, Steven W. Luger, a family practitioner and Lyme disease researcher from Old Lyme, Conn., reports what may be the first documented case of a man who developed the disease after being bitten by a horsefly.

Luger said that his 42-year-old patient was jogging last July in an area of Old Lyme when he was bitten by a large fly that left painful, swollen welts on his chest.

Two weeks after being bitten, the man visited Luger, complaining of a headache, chills, fever and fatigue. He had a large rash encircling the area of the bites, a classic symptom of Lyme disease. The man said he was unaware of having sustained a tick bite during the previous three months. After treatment with antibiotics, the man recovered completely.

Although Lyme disease is known to be transmitted by the deer tick, said Louis Magnarelli, chief of entomology at the Connecticut Agriculture Experimental Station, it has been suspected that biting flies may also be able to infect humans. "We've known that for a few years now," Magnarelli said, "but we have not been able to demonstrate transmission in the laboratory."

Ticks attach to the body for a longer period of time, often without their victims' being aware of it, while fly bites are "almost instantaneous" and painful, Luger said. If untreated, Lyme disease, which has reached epidemic proportions in some parts of the country, including the Northeast, can cause serious neurological damage including dementia.

"I don't think {fly bites} will be of major concern," Luger said. He estimates that only a handful of the estimated 7,400 cases of Lyme disease reported last year to the Centers for Disease Control might be blamed on fly bites, because the number of infected ticks is believed to be much greater.