A lethal parasite is invading the nervous systems of America’s largest deer. Commonly called brainworm, Parelaphostrongylus tenuis moves into the brain and spinal cord of moose and elk, where it can trigger disorientation, stumbling, paralysis, and death. Wildlife pathologists describe animals that “circle, confused and dazed,” before collapsing on roads or in the bush, ScienceAlert reports.

The parasite’s reach is broad. White-tailed deer often carry it without symptoms, turning landscapes where deer, moose, and elk overlap into danger zones for less-adapted hosts, according to The Independent US.

Brainworm is a parasitic nematode infecting moose and elk.

How Brainworm Spreads

The life cycle is a relay. Infected deer shed larvae in feces. Snails and slugs pick them up. Browsing moose or elk ingest the gastropods on vegetation, allowing the worm to migrate into the central nervous system, according to The Caledonian Record. By the time neurological signs appear, recovery is unlikely, and confirmation often comes only after death through necropsy and tissue analysis.

When Symptoms Mimic Other Threats

Complicating triage, another parasite can look the same in the field. The arterial worm, Elaeophora schneideri, also causes neurologic signs in non-adapted hosts. Relying on behavior alone risks misdiagnosis, so researchers stress genetic confirmation to distinguish the two, The Independent US reports.

Infected moose often stumble and circle aimlessly.

A Breakthrough for Live Animals

There is new hope. Parasitologists at the University of Tennessee developed a serological test that looks for antibodies to P. tenuis in blood, enabling diagnosis in live or freshly deceased animals and reducing costly, time-consuming genetic workups. The assay is already processing samples from across North America, allowing health teams to spot infections earlier and track spread in real time, according to ScienceAlert.

As the University of Tennessee team explains, blood can be shipped to the lab, which “looks for these specific antibodies against P. tenuis,” helping avoid confusion with other parasites, per The Caledonian Record.

What Wildlife Managers Can Do Now

Early warnings create options. Managers can curb gastropod vectors with targeted prescribed burns, and adjust local deer harvests to lower transmission risk in moose range, ScienceAlert reports. Each sample submitted strengthens the test’s usefulness and maps where brainworm is advancing—critical intelligence for protecting stressed moose herds.