Schistosomiasis is an unpleasant disease spread by contact with fresh water. Travellers are at risk in most tropical areas.
Dr. Clinton Manson-Bahr taught and practiced in Africa and the Western Pacific for twenty-three years. He is a descendant of Sir Patrick Manson, the founder of modern Tropical Medicine, and is the editor of Manson’s Tropical Diseases—the foremost medical textbook on the subject.
Schistosomiasis, or bilharzia as it is also called, is found throughout the tropics and subtropics. It is a grave problem in countries where it is common, because although not a ‘killer’ disease in the usual sense, it gnaws insidiously at the general health of entire populations. The geographical distribution of the disease is opposite.
At least 200 million people around the world are afflicted, and this figure is rising rapidly; ironically, the dams, irrigation schemes, and agricultural projects so necessary for the fight against world poverty and hunger themselves create conditions in which the disease thrives. Schistosomiasis is a special problem in young children; it hinders development and reduces life expectancy. It remains a problem in China despite an attempt at eradication that involved the entire nation.
How infection is spread
Schistosomiasis is an infection with one of three kinds of the worm, called Schistosoma haematobium (urinary schistosomiasis), Schistosoma mansoni (intestinal schistosomiasis), and Schisto soma japonicum (Far Eastern schistosomiasis).
The fully grown worms live in the veins of the urinary bladder (S.ematobium), or the wall of the intestine (5. mansion and S. japonicum). The worms produce large numbers of eggs which leave the body through the lining of the bladder or intestines. On contact with fresh water, larvae hatch from the eggs and infect certain varieties of snail in which they develop further and multi¬ply. More larvae are produced (called cercariae), which swim freely in fresh water, and actively seek out and penetrate the skin of a human host.
After burrowing through the skin, the young worms find their way (by an unknown route) to veins of the bowel or bladder once again. The adult worms lay eggs for the rest of their lives, which may be as long as fifteen years. So many eggs and larvae are produced that a single infected person passing eggs daily can infect a whole river if the appropriate snails abound.
Water is necessary for drinking and washing, and in rural communities around the world, daily exposure to infection is inevi¬table from an early age. In the Nile valley, East Africa (especially the coastal regions), West Africa (especially the savannah), along with the Euphrates and the Tigris rivers in the Middle East, and in parts of Brazil, the majority of the population may be infected from childhood. Almost all children of school age pass large numbers of eggs in the urine or stool daily, and children are mainly responsible for the spread of infection. Later in life, some immunity builds up so that the worst effects of infection may be avoided.
Most of the harmful effects of the disease are due to the eggs; these cause bleeding, ulceration, and the formation of small ‘ growths as they penetrate the wall of the intestine and bladder. Long-term effects include severe liver damage (the eggs cause liver fibrosis), kidney failure, and cancer.
Disease in travelers
Expatriates and travelers with no previous exposure to schistosomiasis may become seriously ill in the early stages of an infection, though it is unusual for them to suffer in the same way as local people who are exposed to the disease over a long period.
A few hours after contact with infected water, there is tingling of the skin and sometimes a slight rash where the larvae enter the body (cercarial dermatitis).
These symptoms subside, but weeks later, once the worms begin producing eggs, a high fever may develop. This may be severe and may be confused with typhoid or malaria. An increased number of white blood cells (especially of a type called eosinophils) appear in the blood, which may give a clue to the true diagnosis, although not many doctors outside the tropics are aware of this.
Travellers should always tell their doctor if they think there is a possibility that they may have been exposed to infection.
The fever, sometimes called ‘Katayama fever’, does not always occur, and symptoms may be no more than a general feeling of lassitude and ill health. Once the infection becomes established, abdominal pain and blood in the urine or stool are common.
A new drug is now available for treatment of all forms of schistosomiasis, and is very effective. Praziquantel tablets are given, usually as a single dose, and side-effects are rare.
Travellers at risk
Travellers to all countries shown on Map may be in danger of infection. Especially at risk are those who bathe in streams, rivers, or lakes, or who indulge in watersports such as water-skiing and windsurfing in fresh-water areas; watersports are particularly dangerous because they may involve exposure to surface water over a large area. Activities such as snipe and duck shooting and cross-country walking safaris where streams have to be crossed are also hazardous.
Some areas are especially risky: the Nile valley, Lake Victoria, the Tigris and Euphrates river systems, and artificial lakes such as Lake Kariba in Zimbabwe and Lake Volta in Ghana, which is both notorious. Even small collections of water far from human habitation can give rise to serious infections since baboons can harbor S. mansoni, the commonest cause of Katayama fever.
No vaccine is available and none is foreseen in the near future.
Never assume fresh water to be free from bilharzia in an endemic area. Infection can occur on contact with infected water from streams, rivers, and lakes. Even deep water, far offshore, cannot be regarded as safe, and it is dangerous to swim from boats in infected lakes. Saltwater and brackish water are safe from schis¬tosomiasis, however, and so is heavily polluted water.
Since the larvae die quickly on removal from water and cannot survive drying, quick drying of exposed skin and clothing does offer some protection.
Rubber boots and wetsuits are protective also but must be dried quickly in the sun after use. Water that has been chlorinated or stored in a snail-free environment for forty- eight hours is safe since any cercariae present will have died off.
Swimming-pools that are snail-free are safe, but care must be taken that any water entering the pool has been treated. Neglected swimming-pools can rapidly become colonized with snails. Dams are especially dangerous and invariably become infected within ten years of construction.
Check-up on return home
A check-up should involve examination of the urine and stool for the presence of eggs. Remember that eggs take 30-40 days to be produced following initial infection, so tests made earlier than this will be negative. Samples should be taken at least forty days after the last possible exposure. Modem egg concentration methods should always be asked for specifically.
A white blood cell count is advisable, and there is now also a specific and highly accurate serological test (ELISA) for schistosomiasis.
Swimmers’ itch is an intensified variety of cercarial dermatitis caused by schistosome larvae which die in the skin and do not develop further. This condition can occur in temperate as well as in tropical countries—recent outbreaks have occurred in the USA. Some hours after exposure to the water an itching sensation develops on the exposed skin surfaces, followed by a rash composed of small red intensely irritant papules which fade after twenty-four hours. No further symptoms occur and no harm results. Antihistamine tablets or ointment are all that is necessary for treatment.
If contact with water cannot be avoided, always observe the following precautions:
• Do not cross streams at points where there is much human contact, such as village river crossings; always cross upstream of a village.
• Wear long trousers and sleeves and dry out immediately in the sun after crossing.
• Wear waterproof footwear when possible.
• Always take particular care to avoid contact with water and remember the risks of baboon-contaminated water. Resist the temptation to strip off and swim after a long hot march.