(CNN) -- The people of Colcord, Oklahoma, might need something a little stronger than Brita filters to remove the impurities from their drinking water.
Blood worms -- small, red insect larvae -- have been appearing in water glasses and filters in the rural town.
Authorities have warned Colcord's 800 residents not to drink, cook with or brush their teeth with the worm-infested tap water.
Schools in the area have been closed since Tuesday as officials try to figure out where the bright-red creatures came from and how long it will take to get rid of them.
There are no known health effects from the worms, which range from about a quarter to a half inch in length. But local officials aren't taking any chances, hand-delivering letters to residents warning them to stick to bottled water for the time being.
The Oklahoma Department of Environmental Quality (DEQ) is investigating the cause of the outbreak but isn't yet able to say when the water will be safe to drink again, said spokeswoman Erin Hatfield.
Blood worms, also known as red worms, are found in the southeastern United States, but not usually in Oklahoma.
The environmental quality department has only recorded one other such infestation in the state, Hatfield said: in the town of Drumright, 108 miles away, more than 20 years ago.
For the people of Colcord, about 75 miles east of Tulsa, the blood worm invasion that was first detected Monday can't end soon enough.
Staff at CJ's Country Corner, a local gas station and convenience store, told CNN affiliate KJRH that business was much slower than usual, largely because of the school closure. They're also unable to serve coffee or sodas from the fountain, they said.
Colcord's water commissioner, Cody Gibby, is scratching his head over how the worms, the larvae of midges, got through the town's water-filtering defenses.
"It's not just a little 6-inch filter, it's 6 foot of coal and sand mixed together that not even a hair can get through," he told KJRH. "And these worms are getting through it and getting into our distribution water."
And his verdict of the presence of the little red invaders in the tap water?