By Dr. Jeff Daniels EBS Medical Columnist

This is the time every summer in Big Sky when it can hurt to be outdoors, because we’re in the season of the biting flies. These pesky creatures are part of the blood-sucking legion of flying insects, and include black flies, horse flies and deer flies. They all feed on the blood of mammals, including humans.

Outdoor activities can become intolerable, especially when summer temperatures encourage us to wear as little clothing as possible. Diseases spread by fly bites are much less common than those spread by mosquitos—including West Nile in this area— but the bites of these flies can cause allergic reactions that often need medical attention.

These flies are part of the family Simuliidae, of which there are nearly 2,000 different species worldwide. Like mosquitos, only the female bites to acquire a blood meal; the males feed on the nectar of flowers.

Black flies are stealthy, and persistent. When they land on an animal, the first thing they do is gently stretch the skin, and then insert their mouthparts to disrupt the tiny blood vessels so they can begin feeding. To prevent the blood from clotting, there are anticoagulants in their saliva. The saliva also has an anesthetic property so that you won’t feel the pain of the bite or the fly on your skin. That saves the fly from being swatted, and allows for a nice long mealtime.

Itching, localized swelling and inflammation often occur at the site of a bite. Swelling can be very intense, depending on the species of the fly and the individual’s immune response, and irritation may persist for weeks.

I’ve never seen a case, but I’ve read that multiple bites of black flies can cause “black fly fever,” with headache, nausea, fever, swollen lymph nodes, and aching joints. This is probably a reaction to a compound from the fly’s saliva. Less common severe allergic reactions, which may result in anaphylaxis, require hospitalization.

I’ve found that the best treatment for the itching and swelling is the application of a strong (i.e. prescription) steroid cream or ointment, and sometimes we have to resort to systemic steroids in order to relieve symptoms. Benadryl can help with itching. I’ve never found Claritin or Zyrtec to be of much use for fly bites.

Insect repellents provide some protection against biting flies. Products containing the active ingredients DEET or picaridin are most effective. Off! and other common repellants offer limited effectiveness, so protecting yourself against biting flies requires taking additional measures. Avoid areas inhabited by the flies and peak biting times, and wear heavy-duty, light-colored clothing, including long-sleeve shirts, long pants and a hat.

Luckily our biting fly season in Big Sky is a bit shorter than in other parts of the country, so you have a chance to enjoy the great outdoors and not provide a blood feast for a ravaging fly.

Dr. Jeff Daniels was the recipient of the 2016 Big Sky Chamber of Commerce Chet Huntley Lifetime Achievement Award and has been practicing medicine in Big Sky since 1994, when he and his family moved here from New York City. A unique program he implements has attracted more than 700 medical students and young doctors to train with the Medical Clinic of Big Sky.