A persistent and growing health risk for expats living in tropical countries is disease carried in polluted water or spread by water-dwelling insects.

Though medical insurance covers your treatment should you fall ill, by then it may be too late. Tim Hyam examines the worst illnesses and how to avoid them.

Waterborne diseases are one of the biggest threats to human health worldwide. An estimated 1.7 million deaths a year can be attributed to unsafe water supplies, according to the United Nations World Health Organization's most recent report. Added to this number are deaths due to diseases spread by insects that breed in water, especially mosquitoes – Malaria alone infects about 400 million people a year and kills one million.
Alarmingly, the risk of catching one of these diseases is increasing in many areas. Last month’s hurricane damage in the Caribbean showed how vulnerable water supply systems can be. And deforestation and increased rainfall are making new habitats for disease-carrying insects.

Simply drinking contaminated water, or eating food that has been washed in it, can cause fatal illnesses. If you are living in a high-risk country, you may need to change your behaviour to protect yourself. Staying healthy when living in tropical countries is not just about having vaccinations before you go. Vaccinations do not protect you from many of the diseases you are most likely to meet while you are away, whether you are travelling on business, as a tourist or on a long stay.

Three of the most common and most debilitating waterborne diseases are cholera (causing acute dehydrating diarrhoea), typhoid fever (prolonged fever with abdominal symptoms), and dysentery (acute bloody diarrhoea). Each year, an estimated 4 billion cases of diarrhoea result in 2 million deaths, and waterborne bacterial infections account for as many as half of these. These diseases persist because of underdeveloped sanitation systems, which allow water to be infected with bacteria. Simple steps to avoid these diseases are:

If you do get diarrhoea, rehydration is the key. Almost all deaths from diarrhoea could be prevented through oral rehydration to replace fluids and electrolytes lost. So:

Viruses carried by insects are another serious threat to health. Malaria, which causes high fevers, shaking chills and flu-like illness, is a leading cause of death worldwide. This is despite the fact that it is largely preventable.

The only sure way to avoid malaria is to avoid being bitten by the mosquitoes that carry it, but drugs provide extra protection. Using insect repellents is a crucial part of the overall prevention and treatment strategy because malaria drugs are only about 95% effective. Before you travel, you should take professional advice on the risks in the area you are travelling to and which anti-malaria drugs to take.

Yellow fever is also a viral disease that is transmitted by mosquitoes. The “yellow” in the name is explained by the jaundice that affects some patients, though infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. Though an effective vaccine has been available for 60 years, the number of people infected over the past two decades has increased. There are now 200,000 estimated cases of yellow fever, with 30,000 deaths, a year.

Parasitic worms, also dependent on water for their transmission, are among the most gruesome of tropical diseases. Bilharzia is one of the most prevalent, with about 300 million people infected. Bilharzia is caused by parasitic worms that live in human blood vessels. The worms are about 1cm long and feed on red blood cells and dissolved nutrients such as sugars and amino acids. This can cause anaemia and decreased resistance to other diseases. Bilharzia is common in the tropics, where ponds, streams and irrigation canals harbour bilharzia-transmitting snails. Parasite larvae develop in the snails, from which they infect humans. Once in the human body, they mature and reproduce. The disease can be caught from just a splash of infected water. There is no vaccine.

Another particularly gruesome parasite causes “river blindness”, also known as onchocerciasis. This is also water-related because the blackfly that carries it is found in and around rivers, primarily in Africa. The disease is caused by a parasitic worm that lives for up to 15 years in the human body. Each adult female worm is thin but more than half a metre in length and produces millions of microscopic larvae. The larvae migrate throughout the body leading to a variety of symptoms including blindness, rashes, lesions, intense itching, depigmentation of the skin, lymphadenitis, which results in hanging groins and elephantiasis of the genitals, and general debilitation.

Efforts to prevent all these diseases are hindered by poverty, preventing access to clean water supplies, and human activity that creates habitats for mosquitoes and other disease-carrying insects. Until these causes are tackled, all travellers in tropical countries are at risk.

Just when you thought it was safe…

Disease is not the only water-borne threat that expats may face living in tropical regions. Dangerous fish and water-dwelling invertebrates injure thousands of people every year.

Sharks are potentially the most dangerous, though of the many shark species only a few are dangerous. Four species are responsible for most shark attacks on humans: the white, tiger, hammerhead, and blue sharks.
In salt water, other ferocious fish include the barracuda, sea bass, and moray eel. The sea bass is usually an open water fish but is dangerous due to its large jaws – it can bite off large chunks of human flesh.

arracudas and moray eels, to be found (or avoided) near reefs, have also been known to inflict vicious bites.

In fresh water, piranhas are the only significantly dangerous fish. They are restricted to northern South America. Piranhas are fairly small fish, about 5 to 7.5 centimetres long, but they have notoriously large teeth and travel in large shoals.

Other fish and water-dwelling invertebrates pose danger by injecting venom through spines, tentacles or bites. These can cause intense pain and in some cases are fatal.

Stingrays, which inhabit shallow water in tropical and warm-temperate regions, have poisonous, barbed spines in their tails. Scorpion fish, which live mainly in the reefs of the Pacific and Indian Oceans, are about a foot long with spines on their fins that inflict a painful sting. The stonefish, so-called because its colouring gives it excellent camouflage among the stones on reefs in the Pacific and Indian Oceans, inflicts an extremely painful sting if stepped on.

The blue-ringed octopus is a small octopus found on the Great Barrier Reef off eastern Australia. This octopus will not usually bite unless stepped on or handled, but its bite is extremely poisonous and often lethal.

Resembling a jellyfish, the Portuguese man-of-war is in fact a colony of sea animals. Mainly found in tropical regions, the Gulf Stream current carries it as far as Europe. It is also found as far south as Australia. The floating portion of the man-of-war may be only 15 centimetres, but the tentacles can reach 12 meters in length. These tentacles inflict a painful and incapacitating sting.

Not a water-borne disease, but something to cause concern nonetheless if you live South America, is the candiru. The candiru, sometimes called the “viper fish”, is a tiny catfish found only in the Amazon and Orinoco rivers in South America. It has been known to swim up the urethra of humans swimming in the water. Because of its sharp spines, it is almost impossible to extract the fish, except by amputation. A sobering thought.