BREMERTON, Wash. -- Kitsap County health officials are trying to determine what caused about 30 people to fall ill with a flu-like ailment. Most of them swam recently in county lakes.

The county Health District said 23 of the 30 reported cases occurred in people who swam at Horseshoe Lake in south Kitsap County on July 29, but a small number of reports also came in from park goers who visited Wildcat Lake and Long Lake in central Kitsap County.

Though there's no laboratory confirmation of a link to the lakes, there were enough reports of illnesses to warrant expanding a health advisory to all those lakes.

Health officials said no one had to be hospitalized, but the ailment involved nausea, vomiting and diarrhea or abdominal pain.

All public lakes in the county remain open, with the exception of Kitsap Lake, where a toxic blue-green algae bloom prompted health officials to issue a health advisory July 17.

Such algae blooms can cause severe illness, especially in children and pets.

Water samples from Horseshoe Lake showed no unsafe levels of E. coli bacteria at the swimming beaches. Tests on those who fell ill did not turn up any bacteria or parasites.

Health officials are awaiting test results that may tell them if norovirus, a common intestinal virus, is involved.