Hydrilla linked to brain-eating disease in ducks & eagles

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https://www.eenews.net/greenwire/2021/0 ... Agreenwire

WILDLIFE
Brain-eating disease killing bald eagles
Valerie Yurk, E&E News reporter

Published: Friday, March 26, 2021
Bacterial colonies grow on hydrilla. Photo credit: Susan Wilde
Bacterial colonies of the cyanobacterium A. hydrillicola grow on a leaf of an invasive hydrilla plant. Susan Wilde

Bob Sargent, a wildlife conservationist at the Georgia Department of Natural Resources, noticed something was off when he saw ducks swimming and flying in circles, as if their brains were malfunctioning.

When he saw bald eagles doing the same thing, he knew something was wrong.

Toxins on invasive aquatic plants are attacking the brains of bald eagles, causing declines of the iconic species across the southeastern United States, according to new research published in Science.

A toxin produced by bacteria on hydrilla — an invasive plant that made its way into U.S. waterways from irresponsible aquarium dumping — causes liver and brain damage to the bald eagle, the research said.

The neurodegenerative disease, vacuolar myelinopathy (VM), has been rampant since researchers found eagle carcasses with holes in their brains in Arkansas in 1994. Susan Wilde, a study co-author and a professor at University of Georgia, found in 2005 that these strange eagle deaths were somehow connected to the invasive hydrilla, but there were still some areas where hydrilla thrived and bald eagles were not dying.

"It took a while to connect the dots to find out what about the invasive plants made the eagles so sick," she said.

Wilde found that in places where cyanobacteria, nitrogen-fixing aquatic bacteria, are abundant on the invasive hydrilla, so are the brain-killing toxins.

Aquatic species lower on the food chain, like ducks and fish, feed on infected hydrilla and are the first to get sick and start showing the bizarre physical symptoms that Sargent explained — swimming around in circles or with their heads off to the side, making them unbalanced.

Eagles surveying ponds for a meal see the birds succumbing to VM as easy prey, in turn catching the disease themselves.

"It's pretty difficult to make a living as an eagle," Wilde said. "They eat what's available and get infected, and by the time biologists capture them, it's usually too late. They'll only live for two more days."

Wilde added that the toxin is also unique because of its structure — it is lipid soluble, which means it lives on in the muscle tissues of animals. It also has bromide, which is found in herbicides used to kill the invasive hydrilla.

"It's a really tricky thing to do," she said. "We want to use herbicides to kill these invasive species, but it could be making this toxin. We just aren't sure yet."

Stopping the spread
Despite a recent Fish and Wildlife Service survey that says bald eagle populations have quadrupled since 2009, Wilde said that the Southeastern areas where this toxin exists are still seeing fewer bald eagles.

Vanessa Kauffman of FWS said the agency is aware of eagle deaths due to contracting VM from hydrilla and continues to track local eagle populations that are still at risk.

"Mortality will persist with bald eagles; thus it is important to continue monitoring bald eagle populations — we are still seeing populations increase overall, which is the great news," she said.

Georgia DNR's Sargent said the department has been monitoring VM research since the first reported case in Arkansas in 1994.

In the late 1990s, he said, there were 12 nests along a 70-mile reservoir that the DNR monitored in South Carolina. By about 2005, there were only one or two per year.

"The good news is that over last three or four seasons, we've had hardly any reports at all from the reservoir where we usually saw VM," Sargent said. "We think that that is due to wet winter — when there's a wet winter, the water depth rises and the hydrilla is deeply submerged, so birds like coots don't eat it."

But when it comes to getting rid of the toxin, the research creates more questions than it answers.

"We still don't know for sure if the bromide that's allowing the toxins to grow is from a human source, like herbicides, or if there's a naturally occurring explanation for it," Sargent said.

In the past, Georgia's DNR started introducing sterile Asian carp, an invasive fish species that is infamous for aggressively overeating vegetation, in an attempt to decrease hydrilla populations. Sargent said that hydrilla populations have decreased, but there's still a risk of Asian carp foraging on native plants or escaping into other waterways.

"It's all risky; we never know exactly what could happen," he said.

For now, the DNR is encouraging citizen scientists to help the department spot eagle carcasses, bizarre bird behavior and large hydrilla blooms. Until there's more research about the toxin, Sargent said, it's challenging to direct conservation efforts.

"We think there's been less cases of VM, but we've also had much less researchers out there tracking," he said. "We just don't know enough yet."

Twitter: @valerie_yurkEmail: vyurk@eenews.net