Horses can be affected by mosquitos just like humans, so it is a good idea to have a plan on your farm to control mosquitos.  The American Mosquito Control Association says that more than 200 of the 2500 existing species of mosquitos are active in the United States.  Mosquitos are carriers of Eastern and Western Encephalomyelitis viruses and West Nile Virus and can transmit these diseases to horses and humans. The diseases in horses cause neurological dysfunction with fever and are sometimes fatal. Horses tormented by mosquitos may not eat well. They may also develop allergic reactions and skin infections from mosquito bites. 

For mosquito control eliminate areas of standing water by removing or drilling holes in all containers that could retain water such as old tires or tarps on the ground. Keep weeds mowed especially around ponds and ditches so fish, birds, and bats can hunt mosquito larvae more effectively. There are also some biocontrol substances you can add to the water such as bacteria that kill mosquito larvae. Change biocontrol substances periodically to prevent resistance from developing. The key to preventing West Nile and Western and Eastern Encephalomyelitis in horses is to make sure your horses are vaccinated against these diseases at least once a year. Some fly sheets made for horses will also prevent mosquito bites or at least decrease bites under the sheet. Mosquitos are bothersome for your horse, and they can cause a lot more problems than just skin disease so it is best to put control measures in place.