Moose populations have been dwindling for years across the country due to many contributing factors, but new research at Washington State University has found the impact of Eleaophora schneideri, also known as the arterial worm, has likely been underestimated.

Researchers examined recently deceased Shiras moose in Idaho between March 2020 and July 2022. While the parasitic roundworm E. schneideri was not detected in any of the animals found in north Idaho, it was present in 10 of the 20 adult moose studied in the southeastern portion of the state. Nine of the infected not only had adult worms in their major arteries but their brains were littered with microfilariae, the microscopic early life stage of the worm.

“The microfilaria are just scattered throughout their brains, and even though the damage from each is miniscule, they’re basically shot-gunning the whole brain,” said Kyle Taylor, a pathologist at WSU’s Washington Animal Disease Diagnostics Laboratory. “We hypothesize the cumulative effects of large numbers of microfilariae in the brain may be associated with increased morbidity or chance of mortality, with mortality more likely in cases with larger numbers of worms.”