The state Department of Health’s Clean Water Branch is warning swimmers at Hilo’s Onekahakaha Beach Swimming Area that high levels of a so-called “indicator bacteria” that can lead to diarrhea and vomiting.

Routine monitoring at Onekahakaha Beach Swimming Area detected 271 enterococci per 100 milliliters. A level of 130 enterococci per 100 milliliters was detected in March at Sans Souci Beach in Waikiki that prompted Health Officials to warn beachgoers about going into the water.

Onekahakaha Beach remains open.

In a news release, the Health Department’s Clean Water Branch said the presence of enterococci indicates “that potentially harmful microorganisms such as bacteria, viruses, protozoa, or parasites may be present in the water. Swimming at beaches with pollution in the water may make you ill.

“Children, the elderly, and people with weakened immune systems are the most likely populations to develop illnesses or infections after coming into contact with polluted water, usually while swimming. Fortunately, while swimming-related illnesses can be unpleasant, they are usually not very serious — they require little or no treatment or get better quickly upon treatment, and they have no long-term health effects.”

Symptoms can include nausea, vomiting, stomachache, diarrhea, headache or fever and ear, eye, nose and throat infections.