More Swimmers, More Waterborne Microbes

The levels of potentially harmful waterborne microorganisms in rivers, lakes and waterways may be highest when there are the most swimmers, says a U.S. study.

Researchers at the Johns Hopkins Bloomberg School of Public Health, in Baltimore, completed two studies at the Hammerman beach area along Maryland’s Gunpowder River that linked the number of swimmers using the water with the levels of microsporidian spores and the parasites Cryptosporidium parvum and Giardia lamblia.

The studies, published online in advance of publication in the journals Applied and Environmental Microbiology and Water Research, found the concentration of microsporidian spores, C. parvum and G. lamblia were highest during the weekends, when the beach was busiest. In addition, water turbidity was also highest on the weekends — perhaps because the swimmers stir up pathogens resting in the sediment.

The U.S. Environmental Protection Agency does not require testing for microsporidian spores, C. parvum or G. lamblia in recreational waters, because of the cost and difficulty in measuring these pathogens, but it does require that water be tested for E. coli and enterococci as bacterial indicators of fecal contamination, the study said.

Monitoring for E. coli and enterococci may not be reliable in monitoring for waterborne protozoan pathogens, according to study co-author Thaddeus K. Graczyk.