SALMON, Idaho (Reuters) - A brain-attacking worm is strongly suspected as a factor in the mysterious and steep decline of moose herds in the western part of Wyoming, wildlife biologists in the state said on Friday.

While moose populations diminish, infestations by the fly-borne, microscopic Elaeophora schneideri worm are on the rise, said John Henningsen, wildlife biologist for the Wyoming Game and Fish Department.

The agency is at the vanguard of research on the worm's adverse effects on moose, which are infected at a statewide rate of 50 percent or higher. The nematode poses no health risks to humans.

Moose have traditionally thrived in Wyoming, where hunters prize them and their antlers, which can span six feet. Seasonal moose sightings routinely back up tourist traffic at Grand Teton National Park near Jackson in northwest Wyoming.

But that well-known herd near Jackson has plummeted by roughly 70 percent since 1998, falling from 3,077 moose to fewer than 930, state estimates show.

Other herds in western Wyoming were also decreasing in size, but the drop was most pronounced in the Jackson area.

The worm was first documented in the United States in 1933 in domestic sheep in New Mexico. Today, deer are often the unharmed hosts of the parasites, which transmitted through horse flies.

Moose bitten by infected flies mostly show infestations in the neck's carotid artery, said Brant Schumaker, epidemiologist at Wyoming State Veterinary Lab. Outward signs in moose of an attack include malformed antlers and blindness because of worm-driven damage to the optic nerve.

Scientists said moose harvested by hunters elsewhere in Wyoming also showed infestations.

Biologists believed the parasite was compounding challenges facing moose in the Jackson area, where a historic, catastrophic fire and a long-term drought have taken a toll on aspen and other nutrient-rich vegetation moose depend on.

Predators like grizzly bears and wolves also share blame, Henningsen said.

"They're dealing with tough habitat conditions, hard winters, predators. Now throw a parasite into the mix and it's hard to teeth," he said.

Steve Kilpatrick, retired habitat biologist for Wyoming Game and Fish and one-time chair of a state task force on moose, said a strong clue that climate change was at issue was the trend in western Wyoming of increases in temperatures and decreases in precipitation.

The plummeting numbers of moose there -- with an overall decline in herds statewide in the past 15 years -- compare to a relatively stable population in the Snowy Range in south central Wyoming.

Roughly 80 percent of harvested moose in that area showed the presence of the parasite but its affects were all but negligible, Kilpatrick said.

(Editing by Dan Whitcomb and Cynthia Johnston)

Mochila insert follows.