Oh, deer [fly]

Dear Lucy: I’m looking forward to summer, but I’m not looking forward to the deer flies swarming me on my morning walks! I’m wondering if they can transmit West Nile virus, eastern equine encephalitis, or any other disease? Those deer flies certainly bite, and I’ve killed a few that have obviously had a blood meal. —Once Bitten

Dear Bitten: Lucy has also run into her share of nasty deer flies during outside activities at the height of the season here in Massachusetts—generally parts of July and August. They are not repelled by conventional insect repellents, nor does even the most vigorous swatting seem to put them off. 

And, yes, they bite. Literally. Unlike a stinging insect—or a mosquito, which sucks your blood through a needle-like proboscis inserted through your skin—the deer fly has a mouth with razor-sharp “lips,” which it uses to slice the skin open, so it can feed on the resulting blood pool. Deer fly bites can be very painful, and some people also experience an allergic reaction to the salivary secretions released by the insects as they feed.

But the good news, says MIT Medical’s infectious disease consultant, Dr. Howard Heller, is that deer flies can’t transmit eastern equine encephalitis (EEE) or West Nile virus (WNV). “That’s because the viruses that cause EEE and WNV infect birds,” Heller explains. “Mosquitoes become infected with these viruses after they feed on birds and can then transmit those viruses to humans. Deer flies do not feed on birds and, therefore, do not become infected with these viruses.”

Deer flies can transmit a bacterial infection known as tularemia, Heller adds, but this is fairly rare and, in any case, totally curable with antibiotics. Mosquitoes, which can transmit EEE and WNV, are more of a threat to your health, so you should use insect repellent on your walks to ward off mosquitoes, even if they seem less bothersome than deer flies. 

And Lucy encourages you not to let the deer flies keep you from your morning walks and other outdoor activities. If nothing else, the constant swatting will make your walk a full-body workout. —Lucy

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