GALVESTON, Texas -- Health district officials last year advised people not to swim at Galveston County beaches because of elevated levels of bacteria 421 times, or less than 9 percent of the time beaches were tested, according to a report issued by an environmental group.

That rate is not high enough to cause alarm, said Serena Ingre, press secretary for the Natural Resources Defense Council, the group that issued the report.

She said people should be alarmed if there are elevated levels of bacteria more than a quarter of the time beaches are tested.

The beach advisories were issued because levels of enterococcus, a bacteria found in the feces of humans and other warm-blooded animals, were higher than the state standard. The bacteria, commonly found in sewage, often washes into public beaches after heavy rainstorms, Ingre said. The environmental report laid the blame on poorly-designed sewage and storm water systems, and on coastal sprawl that is devouring wetlands and dunes that would normally filter out pollution before reaches the beach.

The bacteria is resistant to some antibiotics. Most healthy people won’t get sick if they swim in contaminated water, said Kurt Koopmann, spokesman for the Galveston County Health District. Stomach aches are the most common reaction by healthy adults and children when they ingest the bacteria-laced water, Koopmann said.

People who are more susceptible to disease — children, pregnant women, those with cancer, HIV or liver failure — should always be careful when swimming at public beaches, Kurt Koopmann said.

“Always use common sense when swimming in untreated water,” he said.

He also advised anyone who gets cut while swimming to get out and clean the wound immediately.

The Galveston County Beach Watch Program monitors the water at 52 stations along Galveston Island, Bolivar Peninsula and the Texas City Dike. Water samples are taken weekly and tested for bacteria. Monitors know within a day whether the bacteria level is high enough to issue an advisory, Koopmann said.

Areas with high bacteria levels are monitored daily until they return to normal. Health district officials also display signs along the beach warning of elevated bacteria levels.

The beaches are not closed, but health officials recommend people don’t swim in the areas.

According to the Natural Resources Defense Council, the Texas City Dike had the highest percentage of advisories. In 2007, health district officials advised against swimming at the dike 18 percent of the time the water was tested, or 14 times.

Koopmann said those numbers might be an anomaly because of how often waters near the dike were dredged last year. Dredging stirs up bacteria settled in silt and mud. Kurt Koopmann said he couldn’t recall any times so far in 2008 where bacteria levels were high enough to warrant an advisory at the dike.

Bill Vanecek of Texas City said bacteria levels don’t deter him from swimming in the water off the dike.

On Wednesday, Vanecek’s children were playing in the surf at the Texas City Dike Beach Park. While he said he’s not surprised there is bacteria in the water, he’s not worried about it.

Vanecek said he had swam at the dike many times but had never gotten sick from the water.

Galveston County beaches were ranked third among Texas beaches in percentages of beach advisories. Nueces County beaches, which include the beaches in Corpus Christi, had the highest percentage of advisories. Health district officials warned against swimming in those beaches an average of 16 percent of the time the water was tested in 2007.