The parasite Cryptosporidium was found in water samples from Burger's Lake, which further links the Fort Worth swimming hole to an outbreak that has sickened at least 20 people, Tarrant County Public Health announced Wednesday.
The final number of people confirmed with cryptosporidiosis could eventually reach the hundreds. Since last week, the health department has received phone calls from 700 people who said they had been at Burger's Lake in the past month. About 600 told investigators that they had suffered from diarrhea, although county officials warned that they didn't know how many of those people had been infected.
Vanassa Joseph, a health department spokeswoman, said there are no other facilities being investigated as a possible source of the outbreak.
"With each of the 20 individuals [who tested positive], Burger's Lake is the common thread," Ms. Joseph said.
The owner of Burger's Lake did not respond to a voice mail message left last week. On Wednesday, the outgoing message said that the lake was still closed, and the voice mailbox was full.
The privately owned lake voluntarily closed July 16 while the county was investigating. The health department sent samples to the Centers for Disease Control and Prevention in Atlanta. County officials also tested the source water for the spring-fed lake and didn't find any fecal matter, which is the way Cryptosporidium is typically spread.
Ms. Joseph said that it's likely that fecal matter from a swimmer introduced the parasite into Burger's Lake. Cryptosporidium is among the most common causes of waterborne disease and frequently causes stomach cramps, vomiting and dehydration.
It can occasionally lead to death, usually in those with weakened immune systems. No fatalities have been reported from this outbreak.
The ages of those infected range from 1 to 61, according to county figures. The first infection in this outbreak was discovered in late June.
Health department officials said that the water in Burger's Lake will be hyperchlorinated to eliminate the Cryptosporidium. The parasite is resistant to chlorine at levels found in swimming pools, but it can be killed if the chlorine level is much higher.
Although it is spring-fed, the water in Burger's Lake is also chlorinated and filtered.
The lake will reopen after it is treated, and the chlorine concentration drops to a normal level. It's wasn't clear how long that would take.
As a precaution, the county has recommended that the water in area public pools be hyperchlorinated to prevent the spread of the disease. Arlington closed its public pools Wednesday to hyperchlorinate them. The pools are scheduled to reopen today.
In Lewisville, city officials said they weren't worried about this outbreak spreading to their pools. The Denton County city spent $108,000 to retrofit its two pools this year with ultraviolet filters.
The UV chambers bombard the water with rays sufficient to destroy the DNA of Cryptosporidium, said Hilary Boen, Lewisville's aquatics supervisor. She said that Cryptosporidium hadn't been found in her pools, but there were concerns about a growing number of outbreaks.
There was a major outbreak linked to a New York water park in 2005 and another in Utah last year. There was also an outbreak of cryptosporidiosis in Phoenix at about the same time as the one in Fort Worth.
"People who do such a great job at aquatics, it can hit them just as easily as someone who is not taking care of a pool very well," Ms. Boen said.