Beachgoers who stay high and dry may stay healthier

January 29, 2008

GAINESVILLE, Fla. — Attention snowbirds and spring breakers: Beachgoers who stay high and dry may have healthier fun in the sun than those frolicking on wet sand or in the water, according to a University of Florida veterinary researcher.

“Our objective was to understand whether beach sand could pose a health risk to beachgoers,” said Tonya D. Bonilla, a doctoral student in the UF College of Veterinary Medicine’s department of infectious diseases and pathology who studied three South Florida beaches over a two-year period to see whether human health risks appear to increase based on the level of sand exposure.

“What we found was that there was no increased health risk due to exposure to sand on the upper beach,” Bonilla said. “However, the longer the period of time people spent in the water and in the wet sand, the higher the probability that they would experience some gastrointestinal illness.”

Bonilla’s research was conducted at Fort Lauderdale Beach, Hollywood Beach and Hobie Beach. There were 882 respondents who participated in the pilot epidemiological study and 609 who participated in the control group.

Beachgoers were made aware of the study and, if willing to participate, were given a survey form to complete four days after their beach visit. The questionnaire focused on type and duration of beach activity and inquired whether participants became ill during the four days after the beach visit. The control group consisted of people randomly chosen from the general population who had not visited a beach in at least nine days.

Jay M. Fleisher, an associate professor in the College of Osteopathic Medicine at Nova Southeastern University, analyzed the epidemiological data collected in the study.

“Our findings suggest that there is an increased risk of acquiring gastroenteritis the longer a bather either sits in the wet sand or stays in the water,” Fleisher said. “The probability that an individual will become sick increases over expected non-exposure rates from six out of 1,000 people for a 10-minute exposure to approximately 12 out of 100 people for a two-hour stay in the wet sand.

“For exposure to water, these rates increase from seven out of 1,000 people affected over expected non-exposure rates for a 10-minute stay to approximately seven out of 100 people exposed for a 70-minute stay,” Fleisher added. “Both show a clear dose-response relationship in risk with increasing time of exposure. These estimates of increased risk might seem small, but when one considers how many people use this beach in the course of a year, we can end up with a substantial public health problem.”

While fecal indicator levels in the near-shore waters of South Florida’s recreational beaches are routinely monitored, sand samples from the surf zone — the wet sand — and the upper beach are not. Beach sand may become contaminated by gull droppings and other sources of fecal-derived organisms that then diffuse into wet sand and water, said Bonilla, whose research was published in the Marine Pollution Bulletin. Her work, part of her master’s thesis work at Nova Southeastern University, was funded by a grant from the Environmental Protection Agency. She has continued her water-quality work at UF, where she is pursuing her doctoral degree.

Her former mentor, Andrew Rogerson, a professor of biology who is now at Marshall University in West Virginia, is a study co-author. Their findings suggest water is an important factor for pathogen transmission.

“At this point, we don’t know whether the increased health risk is due to pathogen exposure,” Bonilla said. “To really understand this, a more comprehensive and targeted epidemiological approach is needed.”

Helena Solo-Gabriele, a professor of environmental engineering at the University of Miami and a collaborator in the National Science Foundation’s Oceans and Human Health Center, is working on understanding how fecal indicator levels correlate with pathogen levels in her own research. Her work primarily focuses on environmental measurements, specifically of microbial indicators in water.

In addition to evaluating the potential human health effects of microbes from beach sands, Bonilla’s paper provides new information concerning the reservoirs and sources of fecal indicator bacteria, Solo-Gabriele said.

“This study emphasizes that beach sands serve as the most significant reservoir of fecal indicator bacteria, and shows that the situation is not isolated to one specific beach, but can be widespread across regions,” she said. “Bonilla and her collaborators provide a mechanistic explanation for the potential spread of fecal indicator bacteria through gull droppings and subsequent distribution through natural diffusion in the environment, as well as by people walking on the beach. The suggestion of an association between fecal indicator levels in sand and illness rates among humans is very significant and points to the need to conduct more comprehensive studies of beach sand.”