Protozoan parasites threaten tropical bathing waters

Pollution in the form of sewage is a major contributor to the fouling of shallow tropical coastal waters. These warm coastal water can harbour various pathogenic microbes, capable of causing harm to human health. This study looks specifically at two protozoan intestinal parasites, Giardia duodenalis and Cryptosporidium parvum, both environmentally robust in their cyst stages allowing prevalence in sewage waste waters entering the marine environment. In recreational coastal areas, the likelihood of these pathogens causing harm is increased by activities such as swimming and water sport activities. However, in the tropics the levels of this harm have not yet been estimated. Tropical areas are of particular interest in terms of harbouring pathogens as temperature and sunlight irradiation may increase their survival. This combined with the posing threat of climate change and increasing population densities in coastal areas is cause of significant concern for human health in bathing waters.

In this study, the potential risks were assessed in Venezuela’s central coast, an area of high intensity recreation usage. They used SYBR green I real time PCR to look for genetic markers of Cryptosporidium and Giardia cysts specifically and traditional culture methods to assess for faecal indicator bacteria FIB) and water quality deterioration. Risk of each species was determined by the level of cysts found in the areas using a dose response model, showing that Cryptosporidium has a risk value of 0.026 and Giardia has a a risk value of 0.00572, equating to a 39 and 50% chance of rate becoming ill once infected, respectively. Both of these were below the U.S. EPA upper bound on recreational risk of 0.036 however, 95th percentile estimates for Giardiasis for children exceeded the 0.036 level, suggesting levels of contamination may not be completely safe. A Monte Carlo uncertainty analysis was performed in order to determine the probability distribution of risks. 35% of the sites were shown to harbour Giardia cysts while Cryptosporidium were only detected in 14%. This level increased with the extent of sewage pollution and bather density during collection based on FIB levels. Giardia is the most common protozoan parasite found in human faecal samples in clinical laboratories for parasitic examination, explaining its widespread occurrence in the tested bating waters.

This study highlights the importance of pathogen specific detection for detection of disease risk in specific geographical areas. It shows how the risks of parasitic diseases acquired whilst undertaking recreation activity in tropical waters can be estimated using quantitative microbial risk assessment. Future research is needed into understanding fully the relationship between sewage pathogens and surrogate indicators in tropical waters, however this research highlights the advantages of using microbial tracking to identify sewage pollution. I think that issues such as sewage pollution are increasingly needed with the changing climate, as research into more tropical parasitic microbes will need to be applied to more temperate regions as temperatures increase and species distributions change with the climate. It is important that reliable detection methods are implemented soon.

Betancourt, W., Duarte, D., Vásquez, R. and Gurian, P. (2014). Cryptosporidium and Giardia in tropical recreational marine waters contaminated with domestic sewage: Estimation of bathing-associated disease risks. Marine Pollution Bulletin, 85(1), pp.268-273.