Elaeophorosis is a parasitic disease caused by the nematode Elaeophora schneideri which resides in the carotid arteries.  Larvae accumulate in the blood capillaries of the head and face and are picked up and transmitted by horseflies. The disease occurs naturally in mule deer, white-tailed deer, elk, and domestic sheep and goats.  The parasite can also occur in bighorn sheep, bison, and pronghorn antelope.  The disease can be introduced to native species with the introduction of domestic sheep and goats and has the potential to severely impact wild ruminants.

Signs of the disease in white-tailed deer include facial swelling due to food impactions.  In elk, the disease causes muzzle and ear necrosis, malformed antlers, and blindness.  Mule deer generally show no clinical signs of infection.  The onset of elaeophorosis occurs during the late summer and early fall when the conditions are favorable for increased horsefly activity.