I lost nearly my entire flock in a few hours of one day last month to buffalo gnats. The entire river bottoms in this area was swarming with them, no way to kill enough of them to help. The plague only lasted 2 weeks, although there were just a few around both before and after the major hatch. I spent a ton of money on repellents; nothing worked for over 3 to 4 hours. A high velocity fan was put in the coop for the 8 [out of 52] survivors; they voluntarily stayed close to it until the main hatch was over. Those that survived had hid in dark places on that first day before I figured out what was happening; the buffalo gnats avoid darkness. One of the survivors recently died, I assume from an infection carried by the blackflies. I'm still raising a few the youngest replacement chicks behind mosquito netting........................ just in case. However, those people living down here long term tell me the plague populations that happen occassionally in wet springs only last two weeks, and the rest of the spring and summer will be safe.
In small numbers, the chickens actually seem to peck them off themselves and each other, and eat them. In plague numbers, the buffalo gnats crawl in the chicken's nostrils, and death can [and did] occur within minutes. I read that the deaths occurs from both outright suffocation from the number of insects in the airways, and a resulting reaction to the buffalo gnats toxic bite.