Today Schistosomiasis remains one of the most prevalent diseases caused by parasitic worms. It certainly is the most devastating, both in terms of mortality and also in terms of long-term chronic debilitating illness. Over 200 million people are currently infected. In spite of over a century of control efforts, and the development of an effective treatment with the drug Praziquantel, the numbers of people affected and severity are basically unchanged. In fact there is evidence that the incidence is actually increasing.The majority of Schistosomiasis is caused by three species of flatworms or trematodes, S. mansoni and S. Japonicum, which inhabit the tissues of the intestinal tract, and S. Haematobium, which involves the urinary bladder. The life cycle is complex, always involving specific species of freshwater snails as intermediary hosts. Tiny free form larvae, called cercariae, are released from the snails into the water, and penetrate the human host by burrowing into the skin. The larvae then migrate to various tissues and mature into adults. Eggs are subsequently passed from the human intestinal tract or bladder into the water, and hatch into another type of larvae called miracidia, which penetrate into snail tissue, thus completing the life cycle. The eggs also pass into various tissues of the human body. It is important to know that almost all of the symptoms of human illness caused by schistosomes result from a chronic inflammatory response to the presence of these eggs. If the eggs are deposited in the wall of the intestine, the inflammation results in malabsorption and malnutrition. Liver damage leads to portal hypertension. Inflammation of the bladder causes urinary obstruction and eventually cancer. Unfortunately, these inflammatory complications progress long after the organism is killed by drug therapy.
The introduction of the drug praziquantel in the 1980s raised great hope for the control of schistosomiasis. Mass treatment programs were initiated in many locations around the world. Unfortunately, though the drug was effective in eliminating the parasite, it did not prevent long-term inflammatory damage and most importantly, did not prevent re-infection. Individuals, mainly children not reached by the mass treatment programs, became super spreaders, passing large numbers of eggs into the water, perpetuating the disease cycle. Recent studies have shown that drug treatment alone becomes only a stopgap measure. Successfully interrupting the schistosome life cycle and eradicating the disease requires a more complicated, more integrated and more expensive strategy, which includes a combination of mass drug treatment, water management, snail control, and also the treatment of human waste
In many ways this problem is analogous to problems inherent in the control of many parasitic infections with complex life cycles. A notable example is malaria. Drug treatment and avoidance of exposure by means of nets and repellants are not enough. Real control requires nothing less than a comprehensive strategy that combines all of the best approaches. Only then will we stand any chance of eradicating these dread diseases.
Until then schistosomiasis will continue to be the most common and the most deadly of water-borne diseases.
A word of warning to UMVIM Volunteers. You have all heard the statement “Don’t drink the water!!” We should add, “Don’t wade or swim in the water either.”
Ref: King, C. H. Global Health: Toward the Elimination of Schistosomiasis. NEJM 2009; 360: 106-109
Submitted by Roger Boe MD, Medical Consultant