A brain worm adds to the woes of Wyoming’s declining moose herd

A tiny nematode spread by biting flies piles on top of habitat decline and predation-

Most know that the moose of NW Wyoming are in serious decline. This has not been the first time because early explorers and settlers reported almost no moose. Nevertheless, in the early 1900s the moose population became established and grew for reasons unclear, but has fallen rapidly in recent years.

It is well established that the great Yellowstone area forest fires of 1988 destroyed much moose conifer habitat, and recent years of drought have diminished their riparian habitat of willow, aspen and forbs. Many believe that the addition of wolves on top of grizzly bears has resulted in additive mortality. Now it is confirmed that about half of the state’s moose have contracted Elaeophora schneideri, a small worm (nematode) that affects the head and brain of moose.

It is spread by biting “horse flies.” Folks know Wyoming has plenty of those.

Deer are the primary host, but there the nematode produces few symptoms. E. schneideri infects a number of ungulates, including elk (blindness), but seems most severe among moose. Fortunately, it does not kill most moose infested, but it does add to mortality and morbidity of moose.

The parasite was first detected in sheep and might represent yet another little told chapter in the devastation domestic sheep have brought to American wildlife.

A recent Reuters story gives more information on the growing problem. Despite the headline, it is not clear to me that the worm directly attacks the brain, but rather blocks blood vessels.






  1. CodyCoyote Avatar

    In the last 15 years or so in the Great War of Words over Yellowstone Wolves, one of the most adamant arguments used by the hunting component of the anti-Wolf rabble has been the decline of the Moose population , which of course they blame almost entirely on wolves. That refrain echoes through the ramparts above Cody and especially Jackson Hole like Julie Andrews singing ” The Hills are Alive with the Sound of Moose Rhetoric”. ( now I’ll have to live with that song and that imagery in my head all day …)

    The antiwolfers wouldn’t allow a word in edgewise about the decline of Old Growth forests after the Yellowstone fires, climate change in general, or any other cause affecting Moose mortality. The possibility of disease or parasites being a key factor was raised years ago. I recall a Scott Creel study that said as much, but it went unheeded. The anti-wolfers—many of whom are outfitters and should know better— conveniently ignored the observations that Moose were in decline BEFORE wolves were reintroduced or propagated in and around Yellowstone. Had to be those darn wolves wiping out the Moose. Sure, the wolves were taking moose, but those moose were already in trouble individually and collectively, and wolves being opportunistic hunters and all that goes with that.

    The emergence of this blood-brain parasitic worm as a probable ( but secondary) cause of Moose decline should surprise no one. But the assertion that it may have been imported to the region via the huge domestic sheep invasions of the late 1800’s is an eyebrow raiser.

    Add that to the long list of things the livestock industry has to atone for in usurping the American West.

    1. Immer Treue Avatar
      Immer Treue

      Brain worm in MN moose, a different pathogen from the NRM pathogen, has long been known to have severe impact on moose in this neck of the woods. Again, deer have it, but are not adversely affected by it.

    2. Ralph Maughan Avatar
      Ralph Maughan

      This is just my impression so far, but it seems to me that moose have more trouble with parasites (think too of their tendency to become overloaded with ticks) than deer or elk.

      1. Immer Treue Avatar
        Immer Treue

        I think it’s a case of moose just doing better in cold boreal habitat. With the removal of old growth forests in MN, it just makes for better deer habitat. Add in climate change…

        I have a source somewhre esle, i believe it’s in Chase’s “Playing God in Yellowstone”, where it is said that elk simply out comptete moose

        1. Ralph Maughan Avatar
          Ralph Maughan

          Immer True

          I always thought Chase’s book was an ideological tirade, of course, that doesn’t mean that this might be a true fact, but I think the biggest threat to moose from related animals is deer, not elk.

  2. TC Avatar

    Unfortunate choice of names – this is the arterial worm, not a brain worm. The “brain worm” of the midwest and east also is not a brain worm, it’s the meningeal worm (and normal host = white-tailed deer, not all “deer”). “Deer” are not the normal host for arterial worm either – mule deer and probably black-tailed deer are, white-tailed deer are not – elaeophorosis can cause morbidity and mortality in white-tailed deer. If you want to blame a reservoir species, blame mule deer, not domestic sheep. Although mule deer don’t need more troubles – they’re in as much trouble as moose in many locations around the Rocky Mountain west.

    1. Ralph Maughan Avatar
      Ralph Maughan

      Thanks TC,

      Doing research for the short story I posted, I noticed that the brain complications are in the arteries of the neck and brain. The Reuters story made it sound like maybe these worms are brain eaters, but no.

      Regarding the sheep . . . yes deer are the major reservoir, but this is not a native disease of North American cervids. The first case detected was in domestic sheep.

      I could post a disgusting photo of these worms in moose like the wolf haters do of the dog tapeworm larva that have them so agitated. For now I won’t do it.


Dr. Ralph Maughan is professor emeritus of political science at Idaho State University. He was a Western Watersheds Project Board Member off and on for many years, and was also its President for several years. For a long time he produced Ralph Maughan’s Wolf Report. He was a founder of the Greater Yellowstone Coalition. He and Jackie Johnson Maughan wrote three editions of “Hiking Idaho.” He also wrote “Beyond the Tetons” and “Backpacking Wyoming’s Teton and Washakie Wilderness.” He created and is the administrator of The Wildlife News.

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Ralph Maughan