Alarming Discovery: Flies Blamed for Decline in North American Moose Numbers

The decline in the U.S. moose population in recent years may have less to do with man-made causes and more to do with Mother Nature than previously thought, according to a recent study.

Research published in March in the Journal of Wildlife Diseases found a parasitic worm, Elaeophora schneideri, in half of the dead adult Shiras moose studied in southeastern Idaho, although it wasn’t found in any of the moose bodies investigated in the northern part of the state.

The parasite, which can grow to 4 inches when fully mature, is generally passed to moose by tabanid flies, according to Newsweek.

None of the 20 juvenile moose had contracted the parasite, according to the study, which was conducted between March 2020 and July 2022.

Newsweek suggested that some of the decline in moose population could be attributed to “hunting and habitat decline,” though it should be noted that none of the three species of moose found in America appears on the U.S. Fish & Wildlife Service’s list of 1,499 threatened or endangered animals.

According to Newsweek, the parasitic worm infects the animals’ brains and can lead to “severe symptoms.”

“The parasite can severely disrupt a moose’s circulation system, leading to blindness, ear damage and often death,” the outlet reported.

The study found that of the 10 moose infected with the parasite, nine had “microfilariae, the early life stage of the parasite, throughout their brains,” according to Newsweek.

The parasites were also found throughout the moose’s major arteries in adult form.

Moreover, three of the infected animals exhibited some sort of ear damage, while four displayed “abnormal behavior” prior to their deaths, likely because of the presence of the parasites in their nervous system.

“The microfilaria are just scattered throughout their brains, and even though the damage from each is miniscule [sic], they’re basically shot-gunning the whole brain,” pathologist Kyle Taylor of Washington State University’s Animal Disease Diagnostics Laboratory told Newsweek.

“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,” Taylor explained.

Researchers believe that as the parasitic worms cause brain damage, moose become more vulnerable to both other health issues and predators.

“We really need to have an understanding of all the issues related to moose population decline, and this parasite appears to be a factor,” Taylor said.

Mule deer and black-tailed deer, on the other hand, can carry the parasite without serious harm, according to Newsweek.

Currently, no tests exist to determine whether a live moose is infected by Elaeophora schneideri, so identification of infected specimens is only possibly postmortem.

“The study also explored the geographic spread of E. schneideri in Idaho,” according to Science Daily. “The parasite was found to be widespread in southeastern Idaho, aligning with its prevalence in neighboring regions of Montana and Wyoming. The distribution coincides with populations of mule deer.”

The was part of a “three-year collaborative project on moose mortality” between Idaho Fish and Game and Dr. Janet Rachlow of the University of Idaho College of Natural Resources, the outlet reported.