What you are likely observing are bot fly larvae (genus Cephenemyia). These larvae are very common in white-tailed deer. The larvae originate from the deer's sinus cavity, nasal passages and/or pouches in the throat region. When a deer’s body cools down, these larvae sometimes migrate into the throat region. The larvae then typically migrate down the throat and into the open body cavity of a field-dressed deer. It is here that unsuspecting hunters often find the larvae. The larvae are sometimes found in the mouth or nasal region of the deer. Hunters who wait several hours before field dressing and/or butchering the carcass are more likely to find the larvae due to the time it takes for the carcass to cool down and the larvae to migrate out of the sinus, nasal and/or throat region(s).
The larvae typically cause little or no harm to the deer, except for some minor discomfort associated with irritation of the sinuses and nasal passages. The larvae do not in any way make the carcass unfit for human consumption. For those that are concerned about bot fly larvae, immediately removing the harvested deer’s head will keep the larvae from coming into contact with edible portions of the carcass.