Three young Montana grizzly bears tested positive for highly pathogenic avian influenza, or HPAI, this fall — adding grizzlies to the growing list of mammals who have succumbed to the unprecedented outbreak.

The cases were the first instance of HPAI found in grizzlies, Montana Fish Wildlife and Parks announced in a release Tuesday.

The bears were near Augusta, Dupuyer and Kalispell, respectively.

The bears, who were experiencing neurological distress and partial blindness, were euthanized because of their poor condition, the release said.

The ongoing HPAI, or bird flu, outbreak is the worst on record for the U.S. The disease is extremely infectious and fatal to poultry and some species of wild birds. In some cases, it's started to spread in mammals. Another grizzly bear case was discovered in Nebraska a few weeks after the Montana bears were infected, according to a U.S. Department of Agriculture dashboard dedicated to tracking HPAI in mammals. The animal was at a zoo.

Two other Montana mammals — a red fox in Madison County and a skunk in Gallatin County — also tested positive for bird flu last year.

Jennifer Ramsey, the FWP wildlife veterinarian, said the grizzly bears and other mammals most likely contracted the virus by eating birds who were infected with it.

But there are some unknowns with that theory, like how many infected birds the animals had to consume to contract it, Ramsey said.

Humans should make sure to keep their dogs away from any dead birds, and wear disposable gloves if they have to handle any dead animals, Ramsey said. There have been two documented cases of HPAI in humans so far this outbreak — one in Colorado and one in the United Kingdom.

Both people had worked closely with commercial bird operations and recovered with minor symptoms, and there are no known cases of human-human spread.

But the growing number of other mammals testing positive for bird flu has experts confused about the trajectory of the outbreak that started in the country last spring.

According to the USDA dashboard, there have been 110 instances of bird flu in mammals since the outbreak started.

In the last bird flu outbreak, that number was close to zero.

Erika Schwarz, a veterinarian microbiologist for the Montana Veterinary Diagnostic Lab in Bozeman, said the lab started testing symptomatic mammals for bird flu around October.

Originally, the grizzly bears — who were flagged for neurological issues like disorientation and partial blindness — were tested by the lab as suspect rabies cases.

Those came back negative, but the lab archives samples in case they need to do more testing later, Schwarz said. It was only when the lab started hearing about reports of bird flu in mammals in other states that they decided to test the samples for bird flu.

"We thought, 'Let's test these just to be safe,' thinking they would probably be negative," Ramsey said. "Lo and behold, they were not."

The lab, which does all the preliminary HPAI testing for Montana, has been testing up to 15 bird flu samples a week, though that number can vary, Schwarz said.

This bird flu strain, H5N1, is similar to previous outbreaks, but it has different point mutations that seem to make it more transmissible to domestic poultry and more deadly to wild birds, Schwarz said.