Melbourne, FL (Florida Today) -- More than 100 parasite-infested brown pelicans died in Brevard County in the past two months.

No one knows why.

"The pelicans are emaciated and have heavy parasite counts, and, to our knowledge, other bird species have not been affected," Dan Wolf, a researcher with the Florida Fish and Wildlife Conservation Commission, said in a release.

Many of the dead pelicans have turned up in Cocoa Beach. But others have been found on Merritt Island, in Melbourne, Indian Harbour Beach and along the St. Sebastian River.

FWC researchers are assessing specimens from the pelican carcasses and the environment to identify a potential cause.

They also sent samples two weeks ago to the U.S. Geological Survey's National Wildlife Health Center in Madison, Wisc. for testing. But biologists have yet to come up with definitive answers.

Researchers are awaiting results from additional samples sent this week that may determine whether the pelicans died of botulism, which sometimes triggers large bird die-offs.

But botulism generally kills birds quickly, Wolf said, leaving little time for the pelicans to become emaciated.

Birds get botulism when they eat rotten fish, which concentrate the Type E toxin produced by the bacterium Clostridium botulinum. Human botulism cases from Type E toxin are very rare. Type A or Type B botulinum toxin makes humans sick, usually after they eat spoiled canned food.

Botulism commonly kills ducks in western states. The Type E toxin can become concentrated in maggots.

Low and fluctuating water levels and warm temperatures can help trigger outbreaks.

"We suspect it may be botulism due to the warmer than average winter," Sue Small, director of the Florida Wildlife Hospital and Sanctuary in Palm Shores, said earlier this week.

The hospital has seen at least 15 sick pelicans at the hospital in the past few weeks. All but two died.

Around the same time last year, more than a dozen brown pelicans died in Palm Shores, Rockledge and Indian Harbour Beach within about a month. Tests by FWC found a common bacteria at fault. The bacteria was most likely from eating decomposed fish.

A red tide has been lingering off Southwest Florida since September, killing a record 174 manatees so far this year. But no other significant fish kills, algae blooms or other toxic events that could kill birds have been reported here.