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Global Burden

There are an estimated 240 million people infected with the parasites that cause schistosomiasis.¹ Schistosomiasis is endemic in 74 countries (approximately 779 million people at risk²), but the highest burden is in sub-Saharan Africa accounting for approximately 90% of all cases.³ Infection is also found in the Middle East, parts of Southeast Asia, Latin America, and the Caribbean.

More than 200,000 deaths, mostly in sub-Saharan Africa are attributed to schistosomiasis each year.⁴ Morbidity, as measured in DALYs, better reflects the debilitating impact of chronic infection. Schistosomiasis is responsible for the loss of more than 1.7 million DALYs per year primarily as the result of organ damage, hemorrhage, and cancer resulting from infection. Certain estimates of disability adjusted life years (up to 70 million disability adjusted life years per annum) have shown schistosomiasis to be a more debilitating infection than even malaria.^5,6

Causative Agent

Schistosomiasis is caused by five species of trematodes (a subset of flatworms and other helminthes): Schistosoma mansoni, S. japonicum, S. haematobium, S. intercalatum, and S. mekongi. S. mansoni, S. japonicum, and S. haematobium are the most significant species for human disease but vary in geographical distribution.

 Schistosoma spp. parasites are transmitted to humans when the free living cercariae penetrate the skin in fresh water. Cercariae are produced by the intermediate hosts of the parasite, freshwater snails. Snails become infected with the parasites when eggs produced by the adult worms reach the water via contamination with feces or urine from infected humans. Under the correct conditions of light and fresh water, the eggs hatch producing the free swimming miracidia that can infect the snail. The miracidia form sporocysts in the snail tissue and eventually produce cercariae that are released back into the water. Each species of Schistosoma prefers a different genus of snails.

Pathogenesis

The free living cercarial form of the Schistosoma spp. parasite penetrates human skin in fresh water. The cercariae travel through the tissue to the blood stream. Adult male and female worms mature and mate in the veins of the liver before moving to their final destination, the veins that drain to the intestine (S. mansoni and S. japonicum) or the bladder (S. haematobium). The female adult worm begins to produce 200-2000 eggs per day. Eggs are shed into the feces or urine and circulate in the blood vessels until they become lodged in various organs. The accumulation of eggs in the various tissues and organs of the body can cause severe damage including bleeding and cancer. It is the accumulated damage caused by the eggs rather than the parasites themselves that causes the majority of mortality and morbidity associated with the disease.

Current Control Strategy

The primary control strategy employed for schistosomiasis is mass drug administration (MDA) with the drug praziquantel (see next section on Existing Medications for details). In 2008, 17.5 million people were treated for schistosomiasis representing just over 2% of the entire estimated at risk population.^2,3 While this is a significant advance, it falls far short of the World Health Organization (WHO) goal of treating 75% of school age children (approximately 71 million children total) by 2010.

Other components of control strategies include avoidance of fresh water known to harbor the snail vector, vector control by removal of snails that can harbor the parasites, and environmental controls to reduce human waste contamination of waterways with snails to halt transmission to the intermediate host. Although these strategies to control transmission were largely successful in China, flooding, transfer of responsibilities from national to local control programs, and a shift in focus from transmission to morbidity control with MDA have resulted in some resurgence of transmission.^7,8 Although transmission-based control strategies may still be valuable in certain contexts, they may be more difficult to implement and sustain in resource limited settings.

Existing Products

Drugs

Praziquantel is the standard of care for the treatment of schistosomiasis. It is highly effective and can be used in MDA. Unlike medications used in MDA for other helminth infections, praziquantel (Merck) donations were limited to 200 million tablets over 10 years beginning in 2008 for a few high burden, least developed countries.⁹ As part of the 2012 London Declaration, Merck announced that it would increase its commitment to donation of 50 million tablets per year indefinitely to support schistosomiasis control.¹⁰ While this donation is important and essential for schistosomiasis control, a key focus of the schistosomiasis community is to increase access to praziquantel and improve the reach of schistosomiasis MDA programs.

Vaccines

There is currently no vaccine for the prevention of schistosomiasis.

Diagnostics

The current standard method for the diagnosis of schistosomiaisis is microscopic evalution of stool or urine samples for the presence of eggs using the Kato-Katz technique. This method only detects on the order of 20-30% of infections as eggs do not appear immediately in stool or urine upon infection.

In regions with S. haematobium infection, a dipstick test for parasite antigens in the urine is available for use in the field. Similar rapid tests using urine for S. mansoni and S. japonicum are not likely to be possible due to differences in the pathology of these organisms, but the possibility of rapid tests using stool samples should be considered.

Other immune-based and nucleic acid amplification-based tests are used in settings with laboratory facilities, but these are not practical for use in the field where MDA programs are taking place.

References

1. WHO, Neglected Tropical Diseases: PCT databank.
2. Hotez PJ (2009) “Mass Drug administration and integrated control for the world’s high-prevalence neglected tropical diseases.” Clinical Pharmacology & Therapeutics 85: 659-664.
3. WHO (2010) First WHO report on neglected tropical diseases 2010: working to overcome the global impact of neglected tropical diseases.
4. WHO: Schistosomiasis Fact Sheet.
5. Gray, D.J. et al. Schistosomiasis elimination: lessons from the past guide the future. Lancet Infect. Dis. 10, 733-736 (2010).
6. King,C.H. Parasites and poverty: the case of schistosomiasis. Acta Trop. 113, 95-104 (2010).
7. Utzinger J et al. (2005) “Conquering schistosomiasis in China: the long march.” Acta Tropica 96: 69-96.
8. Xiao-Nong Z et al. (2005) “The public health significance and control of schistosomiasis in China – then and now.” Acta Tropica96: 97-105.
9. WHO: Schistosomiasis strategy.
10. WHO (2012) “Accelerating Work to Overcome the Global Impact of Neglected Tropical Diseases: A Roadmap for Implementation.” Geneva, Switzerland.

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