In today's installment of Interesting Medical Fact of the Week, we're going to be looking at schistosomiasis, also known as bilharzia, bilharziosis or snail fever. Never heard of it? After malaria, schistosomiasis is the second most devastating parasitic disease in tropical countries, and yet it can be controlled and easily treated with just one dose of a medication called praziquantel. One dose of praziquantel costs 18 US cents - that's about eleven pence. In the West, it's mainly used as a dewormer for cats and dogs.
Schistosomiasis is a disease caused by parasitic worms called schistosomes, which are carried by freshwater snails and then release larvae into the water. The larvae may then infect people when the water comes into contact with human skin; for example through swimming or washing. These parasitic worms then live in the veins near the bladder or intestines, laying eggs which can seriously damage the intestines, liver, bladder, and lungs. The eggs are then excreted, and will hatch in freshwater where they go on to infect new freshwater snail hosts. And so, the cycle continues.
Over 200 million people are estimated to be infected with this disease, of whom about 20 million will suffer severe consequences. Schistosomiasis is a chronic disease, and as such doesn't usually kill, but rather weakens the sufferer and leads to long-term ill-health; nevertheless, the WHO cites statistics suggesting that over 200,000 deaths per year are due to schistosomiasis in sub-Saharan Africa alone. In populations which are particularly vulnerable to malnutrition or dehydration, it is more likely to be a killer disease.
Symptoms of chronic bilharzia usually develop one to two months after the initial infection, and include fatigue, kidney and liver disease, bloody urine, bladder dysfunction, and diarrhea; there is also an increased risk of bowel cancer in infected persons, and the illness weakens the body's resistance to other infections. There is also an acute form of schistosomiasis, however, also known as katayama fever, which presents with a fever, muscle ache, liver enlargement, lymph node enlargement, abdominal pain, breathing difficulties, diarrhea and painful urination. In many cases, people infected with schistosomiasis may be largely asymptomatic except for fatigue.
It's an illness which westerners only tend to come into contact with on backpacking holidays and is most prevalent in sub-Saharan Africa, China, Brazil, the Middle East, the Caribbean and Southeast Asia. About half of all people who have swum in Lake Malawi test positive for schistosomiasis, yet many travellers are completely unaware that they may be putting themselves at risk of this disease by swimming or wading in freshwater. Every year, there are around 100 cases of the disease diagnosed in England, Wales and Northern Ireland among people who have travelled abroad.
There is no vaccine for schistosomiasis, and the main aim of disease prevention efforts is to eliminate the snails which act as hosts for the parasite. Various techniques have been used to this end, including treatment of water with copper sulphate, niclosamide, gopo berry, and the introduction of (or augmentation of existing populations of) freshwater crayfish. The tragedy of this disease is that many vast irrigation and dam schemes have been built in Africa and elsewhere which have contributed to the spread of the disease, despite the fact that UN guidance was available from the 1950s onwards, detailing how such schemes could be built to minimize the problem and make it difficult for snails to colonize the water.