How to avoid parasites

Of all the unpleasant, debilitating tropical diseases lurking to infect the traveller, bilharzia is probably the one that causes most confusion. Here we explain

3 mins

Bilharzia was first described in 1851 by Theodore Bilharz, a German doctor working in Egypt, and is more correctly known as schistosomiasis.

What is bilharzia?

The infection is caused by minuscule flukes, or flatworms, that spend part of their life cycle in freshwater snails, part swimming freely and part in the tissues of warm-blooded creatures, including humans. The cycle begins when someone with the parasite excretes into a lake or river. This liberates fluke eggs, which hatch and set up home in freshwater snails. The species of snail favoured by the parasites needs well-oxygenated, slow-moving or still freshwater containing some plant life.

After four to seven weeks the next stage of the parasite is liberated into water as a torpedo-shaped creature that is just visible to the naked eye. This cercaria has to find a warm-blooded victim – bird or mammal – within a matter of hours or it will die. If it is lucky enough to find you, the cercaria will digest its way through your skin, causing a transient ‘swimmers’ itch’. It then enters the circulation where it traverses your lungs, often causing a dry cough two to three weeks after the itch. Finally it finds a mate and settles down forever in your body, copulating and surrounded by food: fluke paradise.

Where is it prevalent?

It is estimated that about 200 million people in 76 resource-poor countries suffer from schistosomiasis. It is the most widespread (see the distribution map) and prevalent of the world’s parasitic diseases but causes fewer deaths – perhaps around 200,000 annually – compared with malaria’s millions. Schistosomiasis is prevalent in much of Africa, including Madagascar and Mauritius.

A recent survey of expatriates living in Malawi showed that one third had, at some point, been in contact with the parasite. It occurs in the Middle East and the tropical Americas (north-east Brazil, the Guianas, Suriname, Venezuela and some Caribbean Islands). The Indian subcontinent is free except for a tiny area in Maharashtra. There is a nastier oriental form of the disease that occurs in parts of China, Taiwan, Vietnam and the Philippines; Indonesia is clear except for two remote valleys in central Sulawesi.

Bringing more water to parched agricultural lands has been said to increase the spread of the disease by providing more snail habitats, but properly built projects should not cause problems and will increase food production. However, irrigation canals that are allowed to grow thick with weeds and reeds can become a source of disease transmission.

Is it a problem in the UK?

There has been a huge increase in the number of cases of schistosomiasis imported into the UK lately. There were nearly 200 cases reported in the UK in 1994 but the average rate of importation usually hovers at around 100 cases a year. These can be cured with a single dose of a drug called praziquantel. Most victims who give our health authorities information about where they might have acquired their schistosomes seem to have picked them up in Malawi, Lake Malawi or Lake Victoria.

The infective free-swimming cercariae are sun-loving and released by snails around midday. When first released the parasites are strong and can easily penetrate skin, but as they age they become less vigorous and find it more difficult to get through the skin. The cercariae have a very short life cycle – they are middle-aged by the time they are 20 hours old and positively decrepit after 48 hours. An afternoon swim, therefore, is more dangerous than a morning one.

It is possible to collect parasites from showering if the water supply originates from lake water or a contaminated river or canal. However, this risk is removed if the water is left to stand for 72 hours or is treated with chlorine or other chemicals – even something as mild as Dettol in the bath will kill these parasites.

What kinds are there?

The parasite comes in three important varieties. One species of parasite, Schistosoma haematobium, favours setting up home in the bladder wall, a good strategic site for ensuring that its eggs find their way out into the environment again but also where they cause the most mischief to their host. If there are many parasites in the bladder, there will be bleeding – this is the reason that Napoleon described Egypt (where S. haematobium is common) as ‘the land of the menstruating men’. Indeed, there are stories of mothers going to clinics in Egypt complaining that sons haven’t begun menstruating at the age of 12 as is ‘normal’ locally. Travellers should not find blood in their urine since they are unlikely to have acquired enough parasites to cause anything but microscopic amounts of blood loss. If the disease isn’t treated, however, it causes – over the course of a decade or two – long-term debility, and some victims may develop bladder stones and possibly even bladder cancer.

The most widespread species, Schistosoma mansoni, favours settling in the bowel and liver; again, this is a good site for the eggs of the parasite to leave the host along with the faeces. Some successes in limiting the infection have come about through helping locals build sanitary latrines that compost faeces rather than allowing excreta to leak into open water. Travellers who enter contaminated water and thus pick up these parasites may experience fever but often get no symptoms – that is, unless the parasite is allowed to remain for years.

The third important kind is Schistosoma japonicum, which no longer exists in Japan. This is by far the nastiest form of the parasite, although ordinary travellers are less likely to encounter it. It generally announces its arrival in a victim by causing fever, which can be very severe. Then, like S. mansoni, it tends to settle in the bowel, causing long-term debility, abdominal swelling and other symptoms. This parasite penetrates skin much more quickly than its cousins – probably within seconds; it then makes people more ill and the snails – being amphibious – are more difficult to eradicate. This is a big public health problem in parts of South-East Asia.

Each of these parasites causes ‘swimmers’ itch’ as it penetrates the skin and, although none of the species occur in Britain, some related – less unpleasant – flukes do. They too manage to penetrate the skin, but are unable to complete their life cycle and so cause nothing more than mild, short-lived itching, which requires no treatment. An example is a Loch Lomond special, called Trichobilharzia.

Before any tropical trip it’s worth doing some homework to get to grips with all possible disease risks. Then you’ll know what precautions you’ll need to adopt to stop yourself becoming ill or bringing back the wrong kind of souvenir.

Dr Jane Wilson-Howarth organised a survey of school children in Madagascar to identify and treat those with schistosomiasis and other worm infestations. This work is mentioned in her book 'Lemurs of the Lost World' (Impact, 1995)

Reassuring information on schistosomiasis and other tropical diseases can be found in her travel health guide 'Bugs, Bites & Bowels' (Cadogan, 2006)

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