Water Sanitation Health

Guidelines for safe recreational water environments
Volume 1 : Coastal and fresh waters

Executive summary


Dangerous aquatic organisms

Dangerous aquatic organisms may be encountered during recreational use of freshwater and coastal recreational environments. Such organisms vary widely and are generally of local or regional importance. The likelihood and nature of human exposure often depend significantly on the type of recreational activity concerned.

Two types of risks can be distinguished in relation to dangerous aquatic species: injury or intoxication resulting from direct encounters with predators or venomous species, and infectious diseases transmitted by species that have life cycles which are linked to the aquatic environment.

Injuries from encounters with dangerous aquatic organisms are generally sustained by accidentally brushing past a venomous sessile or floating organism when bathing, inadvertently treading on a stingray, weeverfish or sea urchin, unnecessary handling of venomous organisms during seashore exploration, invading the territory of large animals when swimming or at the waterside, swimming in waters used as hunting grounds by large predators or intentionally interfering with, or provoking, dangerous aquatic organisms.

Disease vectors include mosquitoes, which transmit malaria parasites and the viruses responsible for dengue fever, yellow fever and various types of encephalitis; and certain species of freshwater snails, which host the larval development of trematode parasites of the genus Schistosoma, which can cause a chronic, debilitating and potentially lethal tropical disease known as bilharzia or schistosomiasis in humans. Preventive measures include asking local health authorities for guidance on the local vector-borne disease situation and risk prevention, wearing protective clothing, using repellents and avoiding skin contact with water in schistosomiasis endemic areas.

“In-water” hazardous organisms include piranhas, snakes, electric fish, sharks, barracudas, needlefish, groupers, and moray and conger eels. Many have been known to attack and wound humans. Preventive measures include avoiding swimming in areas where large sharks are endemic; avoiding wearing shiny jewellery in the water where large sharks and barracudas are common; avoiding attaching speared fish to the body where sharks, barracudas or groupers live; avoiding wearing a headlight when fishing or diving at night in needlefish waters; and looking out for groupers and moray or conger eels before swimming into caves or putting hands into holes and cracks of rocks. “Water’s-edge” hazardous organisms include hippopotami, crocodiles and alligators. Preventive measures include keeping the animals at a distance whenever possible, avoiding swimming in areas inhabited by crocodiles or alligators, and embarking on safaris in hippopotamus- and crocodile-infested waters with a knowledgeable guide who can assess risks properly

The effects of invertebrate venoms on humans range from mild irritation to sudden death. The invertebrates that possess some kind of venomous apparatus belong to one of five large phyla: Porifera (sponges), Cnidarians (sea anemones, hydroids, corals and jellyfish), Mollusca (marine snails and octopi), Annelida (bristleworms) and Echinodermata (sea urchins and sea stars). Preventive measures include wearing suitable footwear when exploring the intertidal area or wading in shallow water, avoiding handling sponges, cnidarians, cone shells, blue-ringed octopus, bristleworms or the flower sea urchin, avoiding accidentally brushing against hydroids, true corals and anemones, and avoiding bathing in waters where Portuguese man-of-war are concentrated.

Venomous vertebrates deliver their venom either via spines, as with many fish species (e.g., catfish, stingray, scorpionfish, weeverfish, surgeonfish), or through fangs, as in sea snakes. Injuries caused by venomous marine vertebrates are common, especially among people who frequently come into contact with these marine animals. Potent vertebrate toxins generally cause great pain in the victims, who may also experience extensive tissue damage. Preventive measures include shuffling feet when walking along sandy lagoons or shallower waters where stingrays frequent, exercising caution when handling and sorting a fishing catch, wearing suitable footwear in shallow water and snake-infested areas, and carrying anti-venom in snake-infested areas.

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