The parasitic disease bilharzia is endemic in more than 70 tropical countries. Bishoy Morris takes a look at the battle against this waterborne flatworm

If I were asked to make a brief documentary about schistosomiasis, I would select the following snapshots. Children swimming in an infested canal, a local doctor talking with some patients who are obviously anxious after discovering blood in their urine or stool. Patients walking around with hugely distended abdomens because of splenomegaly and ascites. The tempo is now mounting as we see the same patients starting to vomit blood; an experience they never forget--if they are lucky enough to survive it. Finally a split screen with a terminal patient on one side and two nasty worms embracing each other and staring defiantly at the audience on the other, while the word "bilharzia" flashes on the screen.

The facts

Schistosomiasis, also known as bilharzia, is one of the most prevalent parasitic diseases in the world.¹ It is caused by five species of waterborne flatworms called schistosomes.^{1 2} The main route of infection is contact with infested water through agriculture or fishing. A total of 200 million people are already infected worldwide, and 600 million are at risk. The disease is endemic in 74 developing tropical countries.^{1 3} The disease ranks second only to malaria in terms of socioeconomic and public health importance in tropical and subtropical areas.¹ Some 20 million people have severe complications of schistosomiasis.^{1 4}

Long term complications

Schistosoma mansoni causes intestinal schistosomiasis that eventually leads to hepatic fibrosis, portal hypertension, splenomegaly, bleeding from oesophageal varices, and terminal hepatic failure.² And studies have shown that urinary schistosomiasis causes a specific type of bladder cancer.¹ In some parts of Africa the incidence of bladder cancer linked with S hematobium is 32 times higher than that of simple bladder cancer in the United States.^{1 2} In urology rounds in Egypt, we classify bladder cancer into bilharzial and non-bilharzial types due to the sheer volume of patients with the bilharzial variety attending clinic.

The war against the worm

The World Health Organization says that schistosomiasis control is "a long term commitment"--we have been waging a war against the worm for a very long time.¹ Some battles have been glorious successes. The disease has been completely eradicated from Japan, Lebanon, and Tunisia.¹ In China, continuous efforts over the last four decades have led to a 90% reduction in the number of infected people compared to 1950.⁵ At other times, the parasite attacks heavily while we are off guard causing massive damage. For example, in the Senegal River basin, the building of dams and an influx of people to work in irrigation intense crops led, over a period of only three years, to an increased prevalence of S mansoni infection from 0% to about 95% in a population of more than 50 000.⁶

WHO/TDR/STAMMERS

Schistosomiasis worm

While the war continues, most battles remain undecided. Complete eradication is difficult and the World Health Organization's control strategy is to reduce the amount of disease caused by schistosomiasis rather than to halt transmission entirely.¹ Control is based on three main interventions--mass drug treatment, provision of safe water supplies, and health education.¹ The cost of praziquantel, the most effective drug for all forms of schistosomiasis has dramatically dropped from $1 (£0.59; a0.86) to around six cents a tablet since 1991 making it the major weapon.^{1 7}

The goal

At the World Health Assembly in 2001, a resolution was put forward urging endemic countries to start seriously tackling worms, specifically schistosomiasis. Each member state endorsed the resolution and a global target was set. The target is to regularly treat at least three quarters of all school aged children at risk of illness from schistosomiasis by 2010.⁸ An alliance--Partners for Parasite Control--was launched to help achieve this goal.⁸

Classically bilharzia has received much less interest from industrialised nations because it is primarily contained to the developing world. However, this attitude is beginning to change with increased awareness of the impact of the disease. Universities and charity organisations are starting to give control efforts a push. One hailed initiative is the Schistosomiasis Control Initiative launched by Imperial College, London, in cooperation with the Harvard School of Public Health and the Bill and Melinda Gates Foundation. The initiative will support four national programmes to reduce the incidence of schistosomiasis in sub-Saharan Africa. Uganda has recently launched the first of these programmes.⁷ Hopefully, this initiative will serve as a model to similar programmes to draw funds and efforts from developed countries to combat the disease.

Innovative solutions

We are now gathering critical information on the parasite genome. International parasite genome networks, supported by the World Health Organization's special programme in training and research in tropical diseases, are continuing to decipher genetic information. This data will give us an edge for identifying new targets for drugs, vaccines, and diagnostics.⁹

Different research groups are now developing vaccines for schistosomiasis. One vaccine for S hematobium has entered the second phase of clinical trials. Meanwhile, the Schistosomiasis Vaccine Development Program, based in Egypt and supported by USAID, is focusing on developing a vaccine for S mansoni.^{10 11} These new solutions may take some time to work but they may eventually empower us to win the war and say farewell to the schistosomiasis plague.³

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