Annoying Parasite Lurking Offshore

Just when you thought it was safe to go back in the water—it probably is

Fourth of July weekend means barbecues, sun and surf — but this year a trip to Crown Beach may also lead to the 14-day itch.

According to county officials, a mysterious parasite has been attacking bathers at Robert Crown Memorial State Beach for the past three years. Scientists aren't sure how the pest got here. The reigning theory is that it hitchhiked into San Francisco Bay with a species of invasive snails.

A total of 12 cases have been reported to public health officials since June 7. The parasites, called Cercarae, are micro-organisms that afflict aquatic birds and a certain species of snail.

Eggs from the mature parasite are released in bird feces. The larvae survive for a time in salt water as they try to find snails to act as hosts. The microorganisms aren't exactly smart. If a human wanders by, they'll attach to and eventually burrow through a person's skin. "If you are exposed to the water, the parasites can go through your skin," said Lucia Hui, chief of the Alameda County Vector Control District.

As off-putting as it sounds, the parasites aren't much of a danger to human health, officials said. They can't survive in the human body and the parasites die within 24-hours. What they leave behind is an annoying itchy rash. "It's like a little mosquito bite but it can be very itchy. There are no long-term effects," said Cynthia Bartus Jepsen,supervising environmental health specialist for the Alameda County Department of Environmental Health.

Dubbed swimmer's itch, the inflammation is more irritating than harmful. Officials said symptoms of the ailment could be relieved by any number of anti-itch topical ointments.

Public health officials stapled laminated yellow flyers warning of swimmer's itch near various entrances to Crown Beach last month. The separate warnings might be ignored or appear confusing to beachgoers who consider the East Bay Regional Parks District's water quality signs are the final word. Those permanent signs, depicting red, yellow and green circles akin to a traffic light, warn beachgoers of dangerous levels of fecal coliform, bacteria found in excrement that can cause gastro-intestinal distress.

This year's outbreak is relatively small compared to 2005, the first year swimmer's itch was reported in San Francisco Bay. According to Hui, some 90 cases were reported to public health authorities then. It is unclear how many cases went unreported then, or this year. Last year, 12 cases were reported.

Protect Yourself

Like a lot of things, the only sure way to avoid swimmer's itch is by staying on the beach. "If you are not in the water, you can't get it," Hui said. That's sound advice, but the Bay's comparatively balmy water is often difficult to resist. Water temperatures at Crown Beach have hit 70 degrees, positively inviting compared to the mid 50s in the open ocean.

Luckily, there are easy ways to control your own risk. "You'll want to rinse off with clear fresh water as soon as you get out," Jepsen said. If fresh water isn't available, dry yourself with a towel immediately.

Bathers might also be able to limit their exposure. "It appears that the highest risk is when the tide is very, very far out. That is when most people are noticing it," Jepsen said. In any case, officials maintain that the risk of contracting the parasite is low. "You could have hundreds of people in the water and have one or two cases, not everyone gets it," Jepsen said.

Officials from the East Bay Regional Parks District, which manages the beach for state park authorities, estimated that 5,000 people went swimming at Crown Beach in June.

"June is a very popular month for swimmers on Crown Beach, the number of swimmer's itch cases is very, very small by comparison," said Neal Fujita, the park district's water resources manager.

Public health authorities continue to monitor the beach and have collected a new set of 200 snails to be examined by scientists at the University of New Mexico. Out of 300 snails collected June 18, two snails tested positive.

Jepsen said the warnings shouldn't ruin people's holiday weekend. "Enjoy yourself as much as possible. This appears to be a mild situation. They should not be overly concerned about it."

Contact Marc Albert at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .


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