Health epidemiological study: Prevalence and distribution of schistosomiasis in human

Schistosomiasis is one of the most widespread of all parasitic infections of humans and the most common parasite transmitted through contact with fresh water. Human schistosomiasis is one of the most common Neglected Tropical Diseases. Schistosomiasis is caused by infection with the parasite Schistosoma, which is a flat-worm or fluke. It is endemic in more than 70 countries affecting about 229 million people worldwide, of whom 92% are in Africa. Several species exist, of which the most prevalent are Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium. Left untreated schistosomiasis can cause serious long-term health problems such as intestinal and bladder disease. The construction of water schemes to meet the power and agricultural requirements for development has to lead to increased transmission, especially of Schistosoma mansoni. Increasing population and movement have contributed to increased transmission and introduction of schistosomiasis to new areas. Most endemic countries are among the least developed whose health systems face difficulties to provide basic care at the primary health level. Large-scale mass chemotherapy is the first step to reducing the burden of Schistosoma-related disease. The recommended strategy for schistosomiasis is mass treatment with praziquantel, which effectively clears the body of worms, but reinfection is common due to the nature of the parasites transmission and human behavior. Integrated control, targeting the life cycle, is the only approach that will lead to sustainability and future elimination. It should be noted that despite treatment success, the disease may prevalence again, high intensity of infection, and severe morbidity might ensue.


Introduction
Schistosomiasis (bilharzia or snail fever), is a parasitic disease caused by flukes (trematodes) of the genus Schistosoma that have a complex life cycle involving freshwater snails. After malaria and intestinal helminthiasis, schistosomiasis is the third most devastating tropical disease in the world, being a major source of morbidity and mortality for developing countries in Africa, South America, the Caribbean, the Middle East, and Asia (WHO, 2010). Schistosomiasis was discovered by Theodore Bilharz, a German surgeon working in Cairo, who first identified the etiological agent Schistosoma hematobium in 1851 (Nour, 2010). Left untreated schistosomiasis can cause serious long-term health problems such as intestinal and bladder disease. Chronic schistosomiasis reduces the capacity of those infected to work and in some cases can result in death. In children, schistosomiasis can cause anemia, stunting, and a reduced ability to learn. A review of disease burden estimated that more than 200 000 deaths per year are due to schistosomiasis in sub-Saharan Africa (WHO, 2020;Chistulo et al., 2004).
Schistosomiasis remains highly prevalent in many low-income and middle-income countries and a public health problem in several parts of the world, particularly in Africa where 92% of all the people requiring preventive chemotherapy for schistosomiasis live. Of the 78 countries considered endemic for schistosomiasis, only 52 countries have populations requiring preventive chemotherapy. The total number of people in need of preventive chemotherapy globally in 2018 was 229.2 million, of which 124.4 million were school-aged children. According to a WHO report in 2018 from 34 countries for the treatment of school-aged children and from 21 countries for the treatment of adults show that more than 95 million people received preventive chemotherapy for schistosomiasis globally, which is equivalent to 61% global coverage for school-aged children and 18% for adults (WHO, 2020). Table 1 indicates the numbers that requiring preventive  chemotherapy for schistosomaisis, 2018, and table 2 shows the states of the endemic and non-endemic countries of the world with schistosomiasis by region in 2018 according to the world health organization.

Schistosome species
Schistosomiasis is one of the oldest recognized infections. Eggs of the parasite have been found in Egyptian mummies as old as 5,000 years, and evidence suggests that Haematuria was recognized and treated as far back as 1550 B.C. The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Three other species, more localized geographically, are S. mekongi, S. intercalatum, and S. guineensis (previously considered synonymous with S. intercalatum). There have also been a few reports of hybrid schistosomes of cattle origin (S. haematobium, x S. bovis, x S. curassoni, x S. mattheei) infecting humans. Unlike other trematodes, which are hermaphroditic, Schistosoma spp. are dioecous (individuals of separate sexes). Besides, other species of schistosomes, which parasitize birds and mammals, can cause cercarial dermatitis in humans but this is clinically distinct from schistosomiasis (Colley et al., 2014;CDC, 2019). In Africa, S. mansoni and S. haematobium are predominant throughout the continent, while S. intercalatum is found in certain areas of central and western Africa. S. mansoni is also found in Latin America and the Caribbean. S. japonicum and S. mekongi are mostly confined to Asia and the Pacific. Table 3. shows the number of individuals in the region requiring preventive chemotherapy for schistosomaisis during the period from 2011 to 2018. However, table 4, shows the numbers requiring preventive chemotherapy for schistosomaisis in different countries for the year 2018.

Transmission cycle of schistosomiasis
Adult schistosomes live in blood vessels draining either the bladder (S. haematobium) or the intestines (S. mansoni, S. japonicum, S. mekongi, or S. Intercalatum) where adult female worms produce eggs that are passed through urine (in urinary schistosomiasis) or feces (in intestinal schistosomiasis). When the eggs contact water, free-swimming larvae, called miracidia, are released, and they infect freshwater snails. Once in the snail, the miracidia divide, eventually producing thousands of new infective parasites called cercariae, which are released into the water. These infective cercariae penetrate the skin of a human host where they find their way into blood vessels via the circulatory system, thereby starting the process over again. (CDC, 2019;WHO, 2020). Cercariae transform and subsequently migrate through the lungs to the liver where they mature into adult worms. These adult worms move to the veins of the abdominal cavity or of the urinary tract. Most of the eggs produced are trapped in the tissues but a proportion escapes through the bowel or urinary bladder (Fig. 1). (Sturrock, 2001;WHO, (2020).

Symptoms of schistosomiasis
Symptoms of schistosomiasis are caused by the body's reaction to the worms' eggs; Intestinal schistosomiasis can result in abdominal pain, diarrhea, and blood in the stool. Liver enlargement is common in advanced cases and is frequently associated with an accumulation of fluid in the peritoneal cavity and hypertension of the abdominal blood vessels. In such cases, there may also be enlargement of the spleen. The classic sign of urogenital schistosomiasis is haematuria (blood in urine). Fibrosis of the bladder and ureter and kidney damage is sometimes diagnosed in advanced cases. Bladder cancer is another possible complication in the later stages. In women, urogenital schistosomiasis may present with genital lesions, vaginal bleeding, pain during sexual intercourse, and nodules in the vulva. In men, urogenital schistosomiasis can induce pathology of the seminal vesicles, prostate, and other organs. This disease may also have other long-term irreversible consequences, including infertility. The economic and health effects of schistosomiasis are considerable, and the disease disables more than it kills. In children, schistosomiasis can cause anemia, stunting, and a reduced ability to learn, although the effects are usually reversible with treatment (WHO, 2020).

Schistosoma haematobium
Africa, and the Middle East

Treatment
In pursuit of a world free of schistosomiasis, the current WHO roadmap sets goals to control morbidity by 2020, eliminate schistosomiasis as a public health problem by 2025, and to interrupt transmission in member states and selected African countries, by 2025 (WHO, 2013). Current control programs in sub-Saharan Africa, which are based on preventive chemotherapy or mass administration of the anthelmintic praziquantel, target only humans (primarily school-aged children) and ignore the potential role of zoonotic reservoirs and the obstacle that they might pose to the achievement of control and elimination goals (Toor et al., 2018;Webster et al., 2014).
Schistosomiasis is most often treated with praziquantel, which targets adult worms but does not protect the patient against reinfection (King and Mahmoud, 1989). Many schistosomiasis control programs have reduced disease prevalence. However, prevalence reduction has not been achieved in all treated communities (Wiegand et al., 2017;Kittur et al., 2017), also, at-risk areas often have a rebound of infection and disease prevalence after drug treatment efforts are stopped (Wang et al., 2012;Gray et al., 2010). More effective disease control might ultimately be achieved through environmental modifications that separate humans from contaminated water sources (Grimes et al., 2015), or through snail population reductions with molluscicides, as these immediately reduce local snail populations and thus snail-to-human transmission (Rollinson et al., 2013;Fenwick et al., 2006). In 2001, the World Health Assembly, in resolution 54.19 urged the Member States to ensure access to essential drugs against schistosomiasis infections in all health services in endemic areas for the treatment of clinical cases and groups at high risk of morbidities such as women and children, to attain a minimum target of regular administration of preventive chemotherapy to at least 75% and up to 100% of all schoolaged children at risk of morbidity. This goal was endorsed in the 2012-2020 neglected tropical diseases roadmap for schistosomiasis and soil-transmitted helminth (WHO, 2019). The WHO strategy on the use of anthelminthic drugs makes it possible to control schistosomiasis in poor and marginalized communities. In highly endemic areas, severe morbidity due to schistosomiasis can be prevented by regular treatment of atrisk groups targeted based on community diagnosis. Praziquantel has been safely (WHO, 2020).
Praziquantel is the recommended treatment against all forms of schistosomiasis. It is effective, safe, and low-cost. Even though re-infection may occur after treatment, the risk of developing the severe disease is diminished and even reversed when treatment is initiated and repeated in childhood (WHO, 2020