The adult arterial nematode worm lives primarily in the deer’s carotid arteries. High worm infestations reduce blood flow, which causes partial paralysis of the deer’s jaw muscles. Food becomes impacted inside the deer’s mouth due to the jaw muscle paralysis.

The food impaction causes the “lumpy jaw” appearance. The common horsefly passes the nematode larvae from an infected deer to an uninfected one by feeding on deer blood. Infection rates are not high enough to impact deer populations and no human health implication has been reported.