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Schistosomiasis: new findings

19 May 2010

Paul Chinnock

Source: TropIKA.net

 


Bulinus globosus, a snail that is an intermediate host for the parasite Schistosoma haematobium. [Credit: WHO/TDR/Stammers.]

A new report [1] from the World Health Organization says that only a small proportion of people infected with, or at risk of, schistosomiasis receive treatment. The 2010 target of treating at least 75% of school-aged children at risk of the disease has not been met.

Schistosomiasis is a chronic disease caused by parasitic worms. More than 207 million people are infected worldwide. An estimated 700 million are at risk of infection as their agricultural, domestic and other activities expose them to water in which are found the snails that are the parasites’ intermediate hosts. Around 90% of the people who need preventive chemotherapy for schistosomiasis are in Africa, where the disease is endemic in 42 countries.

Many countries scaled up their schistosomiasis treatment efforts after the launch of the Schistosomiasis Control Initiative in 2002, making use of manufacturers’ donations of the treatment drug praziquantel. However, according to the WHO report, only 17 out of 76 schistosomiasis-endemic countries have provided information on their treatment programmes. (In those countries that reported, some 17.5 million people received treatment during 2008.) The report concludes that the major constraints holding back treatment efforts are a lack of access to praziquantel and to resources for implementation. The progress that has been made seems to vary considerably between countries but the failure of so many countries to report to WHO makes it impossible to conclude how much has so far been achieved.

Efforts to control schistosomiasis would be seriously set back if resistance to praziquantel were to spread. Fifteen years ago very low cure rates were observed with this drug in northern Senegal [2] and there have been other reports since that time. A new study [3] conducted on Pemba (one of the two major islands of Zanzibar, Tanzania) sought to determine whether resistance had appeared there, after two decades of mass distribution programmes. Researchers tested urine samples from 1,500 schoolchildren before and after the administration of praziquantel. Although 5% of treated children continued to pass some eggs in their urine up to the seventh week after drug administration, none of these eggs was viable.

The conclusion is that no signs of resistance have yet appeared in this community. Analysing records of schistosomiasis control activities in Pemba over the last 20 years, the researchers were able to confirm that mass drug administration is effective in reducing infection prevalence, but soon after any interruption of drug distribution prevalence returns rapidly to pre-intervention levels. It is important to monitor the effectiveness of praziquantel in different locations and to record instances of resistance. However, the drug still appears to be effective in most situations, making it all the more tragic that it is still not getting through to most of the children and adults who need it.

Other findings

It is usually stated that humans and snails are the only species in which Schistosoma parasites can develop and produce viable eggs, but new findings [4] from Kisumu, Kenya (on the shores of Lake Victoria) support the view that small mammals could be a significant parasite reservoir. Tests on 480 animals belonging to nine rodent and one insectivore species found a 1.5% prevalence of Schistosoma mansoni. As the researchers point out, this is a low rate but given how very common such mammals are in this area their impact on transmission deserves further study. They note that, “Reservoir hosts could perpetuate snail infections and favour renewed transmission to humans once control programmes have ceased”. The role of rodents as reservoir hosts for S. mansoni in South America, and the Caribbean has already been established but this study adds to what is known about this species of Schistosoma in Africa.

Further research from East Africa [5], demonstrates just how easy it is to become infected. In a small study in Jinja District, Uganda, tests for the presence of infection were conducted on 69 competitors and spectators attending an international kayaking event and tourists on commercial rafting trips. Twelve (17%) were found to have evidence of infection. These visitors to an endemic area were exposed to a relatively low degree and it is worthy of note that, even for them, the risks were so high. The authors state that, “Infection occurred among persons who reported swimming/wading only, kayaking/rafting only, and both activities, which refutes the belief that exposure to fast-moving water presents a low risk for schistosomiasis”.

While tourists can choose not to expose themselves to risk, the same is not true for those whose livelihoods expose them to the waters of the middle and lower reaches of China’s Yangtze River. The potential impact of the construction of the massive Three Gorges Dam on the health of the population of this area continues to be of great concern. Changes in the water level and silt deposition downstream, will favour the reproduction of the snail Oncomelania (a Schistosoma host). Together with blockages of the Yangtze River’s tributaries, this will increase the schistosomiasis transmission season in the surrounding marshlands. A review article [6] argues that, in consequence, a comprehensive new strategy to control the disease is now required. The authors describe the use of mass distribution of praziquantel as “a less than satisfactory control option”, because of the cost of distribution efforts, poor compliance rates, and concerns about the development of resistance. They say that mollusciciding, improvements in sanitation, and health education all have an important part to play, but vaccination will be the crucial component of future control efforts. (The article includes an overview of current progress in vaccine development.)

Also of note

  • The development has been reported of a new ELISA test for the detection of anti-schistosome antibodies [7].

  • A serological proteome assay has been found to have “promising potential in the screening of candidate antigens for the diagnosis of schistosomiasis” [8].

  • Although many parasitic infections cause an increase in the number of eosinophil white blood cells, measuring eosinophil count did not provide a reliable screening test for schistosomiasis or strongyloidiasis in refugee children entering the US [9].

  • Ultrasound was found to be more accurate than clinical examination in diagnosing liver involvement in schistosomiasis [10].

  • Infection with schistosomiasis was found to increase the number of false positive results, when 4th-generation HIV tests were used in an African adolescent population [11].

  • New findings support the incorporation of the protease Na-APR-1 into a multivalent vaccine against hookworm and/or schistosomiasis [12].

References

1. [No authors listed] 2010. Schistosomiasis Wkly Epidemiol Rec; 85(18):158-164.

2. Stelma FF, Talla A, Sow S et al. (1995). Efficacy and side-effects of praziquantel in an epidemic focus of Schistosoma mansoni. Am J Trop Med Hyg; 53,167-170.

3. Guidi A, Andolina C, Makame Ame S, Albonico M, Cioli D, Juma Haji H (2010). Praziquantel efficacy and long-term appraisal of schistosomiasis control in Pemba Island. Trop Med Int Health; 15(5):614-618.

4. Hanelt B, Mwangi IN, Kinuthia JM, Maina GM, Agola LE, Mutuku MW, Steinauer ML, Agwanda BR, Kigo L, Mungai BN, Loker ES, Mkoji GM. Schistosomes of small mammals from the Lake Victoria Basin, Kenya: new species, familiar species, and implications for schistosomiasis control. Parasitology; 2010 Apr 12:1-10. [Epub ahead of print].

5. Morgan OW, Brunette G, Kapella BK, McAuliffe I, Katongole-Mbidde E, Li W, Marano N, Okware S, Olsen SJ, Secor WE, Tappero JW, Wilkins PP, Montgomery SP (2010). Schistosomiasis among recreational users of Upper Nile River, Uganda, 2007. Emerg Infect Dis; 16(5):866-868.

6. McManus DP, Gray DJ, Li Y, Feng Z, Williams GM, Stewart D, Rey-Ladino J, Ross AG (2010). Schistosomiasis in the People's Republic of China: the era of the Three Gorges Dam. Clin Microbiol Rev; 23(2):442-466.

7. Chand MA, Chiodini PL, Doenhoff MJ (2010). Development of a new assay for the diagnosis of schistosomiasis, using cercarial antigens. Trans R Soc Trop Med Hyg; 104(4):255-258.

8. Zhong ZR, Zhou HB, Li XY, Luo QL, Song XR, Wang W, Wen HQ, Yu L, Wei W, Shen JL (2010). Serological proteome-oriented screening and application of antigens for the diagnosis of Schistosomiasis japonica. Acta Trop; 2010 May 5 [Epub ahead of print].

9. Dawson-Hahn EE, Greenberg SL, Domachowske JB, Olson BG (2010). Eosinophilia and the Seroprevalence of Schistosomiasis and Strongyloidiasis in Newly Arrived Pediatric Refugees: An Examination of Centers for Disease Control and Prevention Screening Guidelines. J Pediatr; 2010 Apr 16 [Epub ahead of print].

10. Prata A, Ruiz-Guevara R, Antunes CM, Marinho CC, Queiroz LC, Voieta I, Lambertucci JR (2010). Comparison between clinical and ultrasonographic findings in cases of periportal fibrosis in an endemic area for schistosomiasis mansoni in Brazil. Rev Soc Bras Med Trop; 43(2):129-134.

11. Everett DB, Baisely KJ, McNerney R, Hambleton I, Chirwa T, Ross DA, Changalucha J, Watson-Jones D, Helmby H, Dunne DW, Mabey D, Hayes RJ. Association of schistosomiasis with false-positive HIV test results in an African adolescent population. J Clin Microbiol; 48(5):1570-1577.

12. Pearson MS, Pickering DA, Tribolet L, Cooper L, Mulvenna J, Oliveira LM, Bethony JM, Hotez PJ, Loukas A (2010). Neutralizing antibodies to the hookworm hemoglobinase Na-APR-1: implications for a multivalent vaccine against hookworm infection and schistosomiasis. J Infect Dis; 201(10):1561-1569.

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