Oklahoma City ? A prolonged heat wave in the central U.S. has fostered the growth of a dangerous form of algae in lakes and ponds, threatening swimmers and livestock and scaring away tourists during the busy summer season.

Blue-green algae are actually bacteria that produce toxins harmful to humans and livestock. It flourishes in warm, stagnant, sunlit water, and this year’s heat wave combined with Oklahoma’s worst drought since the Dust Bowl have created what one water official called a “perfect storm” for its growth.

Officials have issued a series of warnings, telling boaters and swimmers at lakes in northeast Oklahoma, southern Kansas and Nebraska to avoid contact with the toxic gunk. The issue attracted national attention earlier this month when Oklahoma Sen. James Inhofe blamed a respiratory illness on a swim in Grand Lake in Ketchum Hollow.

The bad publicity has had dire consequences for some businesses in Oklahoma, where tourism is the third largest industry with an estimated annual impact of $6.2 billion. Oklahoma’s numerous lakes are a “huge economic engine” driving that industry, said Leslie Blair, a spokeswoman for the Department of Tourism and Recreation.

“We took a beating from hell,” said Sam Williams, who sells everything from water skis to fishing supplies at the Grand Lake Sports Center in Grove. “My friends at the marinas say their shops are full to the ceiling with beer that they haven’t sold … all because a senator went into a bad area in a cove and got a rash.”

Williams and some others think the issue has been blow out of proportion.

“Every lake has algae,” he said. “It might have been a little worse this year because the heat came early and it’s been so hot.”

Inhofe’s office said the 76-year-old Republican has recovered and is feeling fine, although the illness forced him to delay his return to Washington and miss a few votes.

Problems on Grand Lake have subsided, the Grand River Dam Authority said. But advisories remained in effect Wednesday for portions of Keystone, Fort Gibson and Eufaula lakes in Oklahoma, Marion and Big Hill lakes in Kansas, and Willow Creek Lake in northeast Nebraska.

“Anything that has a scum to it and a pea soup look, that’s what you want to stay out of,” said Ross Adkins, a spokesman for the U.S. Army Corps of Engineers in Tulsa. “Usually, it’s just the stagnant areas where you’ll find it. If you’ve got moving water, you don’t usually find it.”

Prolonged heat and steady sunlight, combined plenty of nutrients and a drought that has shriveled lakes, created a “perfect storm” for algae blooms this year, said Derek Smithee, director of water quality for the Oklahoma Water Resources Board.

“Twenty-eight days of more than 100 degrees is a real factor,” Smithee said. “You have a lot of sunlight, high temperatures. It’s a perfect formula for algae production.”

The algae feed on nutrients that flow into the lakes from industrial discharge, excess fertilizer and natural soil erosion and runoff, Smithee said. Without rain, portions of the lakes have dried up, leaving stagnant pools ideal for algae growth.

A portion of Keystone Lake where the algae was discovered was cut off from the main body by the drought, said Elzie Smith, city manager for nearby Cleveland.

“It’s nothing more than a big pond right now,” Smith said. “Half of it’s completely dry, and what’s left, I saw birds standing in the middle of it. Another week and it will be bone dry.”

Because the algae thrive in stagnant water, it can be a particular problem for farm ponds used to water cattle, said David Cantrell of the Pittsburg County Oklahoma Cooperative Extension Service.

“Under normal conditions, the organisms are homogenously suspended in the water, but as they multiply rapidly, large numbers of dead organisms float to the surface,” Cantrell said. “Problems occur when livestock consume water from the bloom area.”

Toxins from the algae affect the nervous system and liver, resulting in weakness, muscle tremors, convulsions and even death. Cantrell said ranchers should check ponds for blooms, fence off downwind drinking areas and switch to alternative water sources when temperatures rise and blooms are spotted.

“With our weather forecast that we have for the next few weeks, we very likely could see an increase in the blue-green algae,” he said.