Attention to worldwide pollution of the coastal marine environment has focused primarily on toxic algal blooms and pathogenic bacteria that multiply in nutrient-rich waters. However, massive but unseen amounts of feces from humans, their pets, and their domesticated animals are discharged, dumped, or carried in runoff, bringing encysted zoonotic protozoan parasites to estuaries and coastal waters. Here, they contaminate bathing beaches, are filtered and concentrated by shellfish eaten by humans and marine mammals, and infect a wide range of marine animal hosts, resulting in morbidity and mortality to some populations. So argues a recent review in the journal Trends in Parasitology.
The review, by Ronald Fayer of the United States Department of Agriculture and colleagues, observes laconically that, although from space, the Earth appears to be covered in large part by water and is seen as a bright blue sphere, closer inspection would reveal human and animal feces spread over much of the planet. Between them, the authors calculate, the worlds humans and livestock produce at least 4 billion metric tons of manure each year, with just one gram of feces capable of containing up to 10,000,000 cysts of the parasite Giardia.
The authors go on to observe that encysted protozoans originating in human and animal feces are transported in runoff from agricultural, suburban and urban land surfaces, wastewater discharges and other sources to rivers and streams, which carry contaminated sediments to estuaries and eventually to coastal waters. Here, they may survive up to twelve weeks before entering the food web, generally through intake by shellfish, by which route they may infect human consumers. The pathogens may also remain in the ocean environment and infect marine mammals, either through direct consumption of shellfish in the case of species such as sea otters, or via pathways that remain obscure, as with bottlenose dolphins.
The presence of toxoplasmosis in sea otters is of particular concern. The disease has been implicated in increased mortalities of the species in California; the paper notes that the parasite Toxoplasma gondii has been isolated from brains or hearts of 22.3% of 67 and 32% of 75 California sea otters.
The authors conclude: Hopefully, more molecular data will become available from parasites isolated from marine mammals and will be compared with data from terrestrial animals. Such data will clarify our understanding of the complex relationships among hosts, and will help to identify the routes and mechanisms of land to sea transmission. Nevertheless, practices of wastewater disposal, drinking water purification, runoff control and farm manure management must improve. Otherwise, as human and animal populations grow, so will the negative impact of fecal contamination on public health and ultimately on marine life.
Source: Fayer, R., et al. 2004. Zoonotic protozoa: from land to sea. Trends in Parasitology 20(11): 531-536.
