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Human and animal activities may influence the distribution of Schistosoma parasites. There are three main Schistosoma parasites that affect humans, and they include Schistosomamansoni, Schistosomahaematobium, and Schistosomajaponicum (Bogitsh, Carter, & Oeltmann, 2012).This paper will focus on their distribution across various continents and the factors that influence their distribution. Additionally, the parasite morphology, hosts, mode of transmission, pathogenesis and treatment and control of the parasites will be discussed. The main guiding question will be “which factors influence the distribution of Schistosoma parasites that affect humans?”

Various Schistosoma species form an important part of an animal and human parasites across South America, Asia and Africa. They are predominant in rural areas where agriculture and inland fishing are the main economic activity (Chopra, 2005). Their distribution depends on the distribution of snails which are their intermediate hosts. Human activities such as building of dams and irrigation channels and flood irrigation also influence the distribution of the parasites. Schistosomes fall under the Schistosomatidae family, and unlike the other trematodes, they are dioecious and not hermaphroditic; they form separate sexes. The adult worms have bodies which are tabular and elongated. The males have a gynecophoral canal in which the female worms are held. Schistosomes reside in visceral blood vessels and are widely referred to as blood flukes (Mahmoud, 2001). Their life cycles are digenetic and involve freshwater snails that form their intermediate hosts. They usually deposit their eggs in the circulation system which then penetrates the bladder or gut. The eggs are later excreted with solid waste or expelled with urine. When the eggs reach water, they release miracidia, which enter snails and goes through asexual proliferation in sporocyst stages. The snails eventually release cercariae back into the fresh water. Humans and some domestic animals become exposed to their infection when they come in contact with the water. Infection is usually through direct penetration of the skin, and it may result to chronic diseases in humans and some domestic animals (Selendy, 2011).

Schistosomes usually have five distinct developmental stages namely eggs, miracidia, sporocysts, cercariae and adult worms. Their Eggs range from being round to oval in shape and are normally hinged at one end. The eggs hold miracidia which are the embryonic larvae that are still developing (Gunn & Pitt, 2012). The Different Schistosoma species have different eggmorphology that can be used to differentiate them. S. mansoni usually produces oval eggs that come with a sharp lateral spine while S. japonicum has round eggs that come with a rudimentary lateral spine. S. haematobium also produces oval eggs, but they have a sharp terminal spine (Viegas, 2003). It is important to note that Miracidiaare usually covered with cilia and are free-swimming. Sporocysts, on the other hand, appear as pleomorphic sac-like bodies that have developedcercariae. Mature cercariae elongated and are still free-swimming. They have a tapering head and a tail that is forked. Adult flukes are also elongated and tubular. They have rudimentary oral and ventral suckers with the males being stouter and shorter than females (Sukhdeo, Lewis, & Campbell, 2002).

It is estimated that more than 200 million people all over the world may be infected with schistosomes. Infections have been observed and recorded throughout the human history. The first case of infection was recorded in ancient Egyptian papyri which dated between 2000BC and 1000 BC. Haematuria, which means bloody urine, became the main disease that affected Napoleon’s army in the 18^th century while they were in North Africa. It was later named bilharzia after the person who discovered its causative agent (Price, 1980). Schistosoma spp has different preferences for the intermediate hosts, with some preferring only to develop in humans and other primates while others prefer domestic and wild animals, acting as reservoirs for human infection (MacPherson, Meslin, & Wandeler, 2013).

Schistosomamansoni

This is a trematode flatworm that is associated with Schistosomiasis. The parasite causes two main types of Schistosomiasis namely Urinary and intestinal schistosomiasis in urinary Schistosomiasis. It slowly damages the bladder, ureters and kidneys while, in Intestinal Schistosomiasis, the parasites gradually enlarge the liver and spleen and at the same time causes damages to the intestine. This leads to hypertension of the blood vessels in the abdomen (Elder, Ribes, & Baker, 2004). They are usually found in freshwater environments. Location and distribution of this species vary mainly depending on the region and time of year. They are normally found in freshwater bodies such as lakes and slow-flowing streams within irrigation systems, where flood irrigation is practiced. Since the conditions in such environment are stable. Given the fact that they are parasitic, they would infect other species found in the fresh water. They mostly infect snails of the genus Biomphlaria and Planorbidae family. Other animals such as rodents and monkeys are infected by eating the infected snails while humans mostly become infected by coming into contact with water that is contaminated as opposed to the consumption of infected snails (Gutierrez, 2000)

Schistosomajaponicum

This species usually requires snail and vertebrate hosts in order to survive. The snails used as hosts normally live in freshwater bodies that are warm such as slow-flowing streams, ponds, dams and ditches. It also causes Schistosomiasis and uses humans as primary hosts. The major harm this species causes because of the huge number eggs it releases. The massive numbers of eggs trigger immune responses in the body that in turn brings about many health problems. As the eggsmove through the tissues and organs of the host, they cause fevers, weakness, liver and kidney damage, blood in the urine, and abdominal pain (Dudek, 2006).Sometimes eggs may be transported in the circulatory system and reach other parts of the body, such as the brain and heart, resulting to even more severe damages. Schistosomajaponicum reproduces sexually. The male and female worm must the first mate before the eggs are lain. Mating usually takes place in veins of the host after which the female lays eggs. The eggs then move through the walls of the veins finally ending up in the gut or bladder through a process that involves secretions from the hosts’ immune system and the eggs. The host finally excretes the eggs, and once they reach freshwater bodies, they hatch into miracidium (Mahmoud, 2001).

Schistosomahaematobium

This species is located in the Middle East, Africa and southern Europe. Schistosomahaematobiumis predominant in areas where their intermediate hosts such as Bulinusspp and Physopsisspp are available. The species is the only one that affects the human urinary system since the Calcified eggs in the bladder wall increase the likelihood of blockage of the vessels. It also causes blood in urine and in some cases in the stool. People affected by it may develop fever, cough, skin inflammation, and tenderness of the liver (Mahmoud, 2001).The tenderness of the liver is due to tissue degeneration that is because of the spined eggs, which attach themselves to the vital organs. Later on, the disease may lead to the swelling and damaging of the liver, bladder and other organs. Their eggs attach themselves to the neck of the bladder thus resulting to infections (Selendy, 2011).Chronic Schistosomiasis has raised the incidences of cancer of the bladder in most countries, in the Middle East.

Geographical Distribution of Schistosomes

The parasite S. mansoni can be found in most countries, in Africa.In South America, it can be found in Venezuela, Surinam and Brazil. In the Caribbean, it is present in countries such as St Lucia, Puerto RicoGuadeloupe, Martinique and the Dominican Republic. The parasite is also present in parts of the Middle East. It is important to note that the reservoir hosts are of no importance for this species (Anthamatten & Hazen, 2012)

The parasite S. haematobium can be located in most parts of Africa, sections of Arabia, the Middle East, and Khuzestan Province in Iran, Mauritius and Madagascar. Like S. mansoni, reservoir hosts are also not important for this species.They can be located in the tropical climates and along rivers around the coast. Studies have shown that Schistosomahaematobium mostly inhabits forested areas towards the south. However, Savannah areas located towards the north do not have a lot of S. haematobium despite the fact that they inhabit areas in the Middle East and north Africa. It is important to note that snail infection rates of S. haematobiummiracidia goes up from July to November and at normal water levels (Anthamatten & Hazen, 2012)

The parasite S. japonicum mainly exists in the Far East in China and the Philippines. In Japan, control programs, which were successfully implemented, enabled the eradication of the parasite. The same programs have also enabled the eradication of the parasite in vast areas of southern China. Control of this species is hard due to the existence of many reservoir hosts. Apart from that, many strains of the species exist in their pathogenicity, drug sensitivity, infectivity and morphology and infectivity (Anthamatten & Hazen, 2012).

The different species live as paired adults inside blood vessels in particular sites within the human body. S. mansoni are mainly found in the portal veins draining the large intestine while S. japonicum lives in the mesenteric veins of the small intestines .S. Haematobium, on the other hand, prefers the urinary bladder plexus (Elder, Ribes, & Baker, 2004). Schistosoma eggs penetrate into the bladder or the lumen of the intestine and are expelled out of the body together with the host’s feces or urine. Most of the eggs might, however, be carried in the circulation system of the host to various tissues and organs (Dudek, 2006).

Factors Affecting Their Distribution

There are various factors that influence the distribution of the three Schistosoma species. The first factor is the presence of preferred snail vectors. Various Schistosoma species prefer different species of snails. Their distribution is mostly determined by the absence or presence of Biomphalaria snails, which form intermediate host for the parasite. The presence of a higher population of the Biomphalaria snails will imply high rates of distribution of the parasites (Gunn & Pitt, 2012). The World Health Organization has insisted on the need to come up with maps that will help predict the expected distributions of schistosomes. Following this, the Geographical Information System can be used to map out the spatial patterns of intermediate host snails together with those of human infection so as to improve efficiency of allocation for available transmission control interventions (Selendy, 2011).

Secondly, poor sanitation practices such excreting in water bodies and in the bushes increases the distribution of the schistosomes. The parasite’s eggs are usually expelled from the body through urine and faeces, and when such wastes come into direct contact with fresh water, the chances of the eggs hatching are increased. This puts other people who might use the water at risk of being infected when they come into direct contact with the water (Mahmoud, 2001). Inadequate sanitation facilities and infrastructure predispose many poor to schistosomes and diseases and complications they bring. Poor sanitation practices lead to contamination of fresh water bodies. Adoption of proper sanitation practices such as effective disposal of human waste and treatment of water before using it reduces Schistosoma infection (Selendy, 2011).

Apart from that, exposure to Freshwater that is contaminated also increases the chances of the distribution of the parasite. Activities such as Swimming, bathing, and wading in contaminated water increase the chances of being infected with schistosomes. Other practices that expose people to fresh water include sand harvesting and fishing (Sukhdeo, Lewis, & Campbell, 2002). After the expulsion of the parasites’ eggs from the secondary host’s body, they hatch releasing miracidia, which is a free swimming, and which lives for only a few hours. They thus have to seek primary hosts, the fresh water snails, as fast as possible. The miracidia then attaches itself to the soft tissues of the snail and form a sporocyst near the penetration site. After the production of Daughtersporocysts, cercariae are produced. They are usually fork-tailed and attracted to skin secretions. The cercariae then penetrate the skin of the secondary hosts when they come in contact with the contaminated water (Dudek, 2006).Other stages of development continue inside the secondary host. When more people become exposed to water that is contaminated with Schistosoma cercariae, the distribution rate of Schistosoma increases. It, however, should be noted that the rate of distribution will also depend on the number of snails that are infected and the time of the year.

Agricultural practices especially those that involve irrigation also increases the distribution of schistosomes. As earlier mention, irrigation schemes offer a stable environment where the schistosomes can thrive. Irrigating crops using flood irrigation exposes the people who work in such fields to Schistosoma infection especially when they work barefoot. Increased agricultural practices in Egypt for instance and construction of dams for irrigation lead to increase rates of infection (Eline & Madsen, 2006).The canals have increasingly played host to a great number of Bulinus and Biomphalaria snails which are preferred by the schistosomes as primary hosts thus increasing their distribution (Eline & Madsen, 2006). Various factors in the irrigation schemes and dams created increases the chances of snails and Schistosoma occupying such areas; they include Shallow shores in the constructed dams with the fluctuation rates of the water levels being low. Fluctuating water levels results to variation in the population of snails. Secondly, presence of storage dams in the schemes also contributes to the existence of snails and Schistosoma parasites. Apart from that, absence of self-draining hydraulic structures makes the water flow at a slower speed thus leading to an increased population of the snails (Kumar, 1998). Inappropriate water management practices in the irrigation schemes might also create conditions that would favor the survival of snails the parasites’ eggs. The practices include leaving standing pools in the canals for a prolonged period. Lastly, agricultural runoff in the canals increases the population of snails in them (Davies, 2004). Changes in agricultural management and improving various agricultural practices that expose farmers and workers to direct contact with fresh water will help in the management of schistosomes in areas where irrigation is practiced.

Environmental factors that influence snail distribution also plays a huge part in influencing the distribution of the schistosomes. Since the snails form the primary hosts for the schistosomes. Environmental factors such as temperature and water PH influence the distribution of snails. Most snail species prefer warm water with moderate PH levels. An increase in the population of snails due to favorable environmental conditions increases the distribution of the Schistosoma parasites (Elder, Ribes, & Baker, 2004). Snail population and viability is influenced by various factors that include water levels, the current speed of the water body, water temperature and the Elevation of the area. High water levels lead to a decrease in the population of snails. Research has shown that flooding leads to a decrease in the population of snails while during the season of less flooding, the snail population increases and their life expectancy also doubles. The current speeds also influence the density of snail population. When the current speed is high, the snail population at such points is low but increases downstream where they are swept and where the current speed is low. Snails also tend to increase their reproductive activities in warm water and thus higher population densities can be observed I water bodies that have warm temperatures (Davies, 2004).It, however, should be noted that different Schistosoma species prefer different snail species.

Lastly, Demographic factors such as immigration patterns and high human population especially around fresh water bodies also influence the distribution of schistosomes. Areas where the population is high and where the people are poor are likely to have a higher rate of Schistosoma distribution. As a result of poor sanitation practices that usually comes about due to the increased number of people who put a lot of pressure on the existing sanitation infrastructure. Migration and immigration patterns also contribute to the distribution of schistosomes in that an infected person may transfer them to areas where they are not present (Selendy, 2011).

Diagnosis and Control Measures

Schistosoma Infection is usually diagnosed by the detecting the worm eggs in urine and faecal sample. The characteristics are sufficiently stated thus facilitating correct diagnosis of the specificSchistosoma.On some occasions; rectal biopsy microscopy has been used to diagnose infections of S. haematobium. Controlling the spread of schistosomiasis in the community is very difficult due to the fact that the parasites have become thoroughly integrated within the environment in which they inhabit. However, various practices can be employed to help manage their distribution. The measures and practices include integrated vector control measures, parasite control measures, personal protection, detecting of cases rapidly and fast referral and increasing awareness in the community (Davies, 2004).

IVC measures aimed at bringing down the population of vectors in areas where the parasites are predominant. The measures include killing of snails using chemicals, destroying places where the snails live by practicing better environment management such as better farming and irrigation practices (Davies, 2004).Parasite control measures, on the other hand, aimed at cutting down the parasites’ population by taking measures such as water treatment with chemicals such as iodine or chlorine before using it in order to kill immature schistosomes and their eggs. People who work in contaminated water can practice Personal protection. This would protect them from being exposed to the parasites. Such group of people includes fishermen, farmers and dam constructors. They should put on rubber boots in order to prevent the cercariae from penetrating their skin (Eline & Madsen, 2006).

Quick detection of Schistosoma infection cases and referral is also an effective way of controlling their spread. Cases should be reported to the nearest health facilities so as an effective treatment can be administered. Praziquantel is the main drug that is used in treating schistosomiasis. It is usually administered orally, and one is never expected to self-prescribe the drug. It must be administered at the health facility after the right diagnosis has been made. Lastly, the community should be enlightened on the causes of schistosomiasis. Apart from that, they should also be educated on the different modes through which the schistosomes are transmitted and the importance of maintaining good sanitary practices (Labspace, 2013).

Summary and Conclusion

The three main Schistosoma parasites that affect humans include Schistosomamansoni, Schistosomahaematobium, and Schistosomajaponicum. These Schistosoma species form an important part of animal and human parasites across South America, Asia and Africa. They are mostly found in rural areas where the main economic activities include agriculture and inland fishing. Their distribution is mainly dependent on the distribution of snails which are their intermediate hosts (Bogitsh, Carter, & Oeltmann, 2012).

Different Schistosoma species have their eggs ranging from being round to oval in shape. . S. mansoni usually produces oval eggs that come with a sharp lateral spine while S. japonicum has round eggs that come with a rudimentary lateral spine. S. haematobium also produces oval eggs, but they have a sharp terminal spine (Gunn and Pitt, 2012).

It is also important to note that more than 200 million people all over the world are estimated to be infected with schistosomes. Additionally, Infections have been observed and recorded throughout the human history with the first case of infection was recorded in ancient Egyptian papyri which dated between 2000BC and 1000 BC. The parasites also affected most of Napoleon’s men in the 18^th century while they were in North Africa (Price, 1980).

Schistosomamansoni causes two main types of Schistosomiasis namely Urinary Schistosomiasis and Intestinal Schistosomiasis while Schistosomajaponicum causes major tissue damages due to a large number of eggs it produces. It also requires snail and vertebrate hosts in order to survive. Schistosomahaematobium, on the other hand, is the only one that affects that affects the human urinary system since the Calcified eggs in the bladder wall increase the likelihood of blockage of the vessels. It also causes blood in urine and some cases in the stool (Gunn & Pitt, 2012).

Human practices play a huge role in the distribution of the Schistosoma parasites. Practices such as poor sanitation practices, agricultural practices involving irrigation, fishing and bathing in contaminated water increase the chances of Schistosoma parasites penetrating the body and being distributed to other areas. Various factors in the irrigation schemes and dams created increases the chances of snails and Schistosoma occupying such areas; they include Shallow shores in the constructed dams with the fluctuation rates of the water levels being low. Fluctuating water levels results to variation in the population of snails (Eline & Madsen, 2006).

Secondly, presence of storage dams in the schemes also contributes to the existence of snails and Schistosoma parasites. Apart from that, absence of self-draining hydraulic structures makes the water flow at a slower speed thus leading to an increased population of the snails. Inappropriate water management practices in the irrigation schemes might also create conditions that would favor the survival of snails the parasites’ eggs. The practices include leaving standing pools in the canals for a prolonged period. Lastly, agricultural runoff in the canals increases the population of snails in them (Davies, 2004).

Other human factors that influence the distribution of the parasite are the pollution levels and the poverty levels. The parasites tend to be widely distributed in regions where population is high, and most of the people are poor. This can be brought about as a result of poor sanitation practices that are associated with an increased number of people who are economically disadvantaged and who put a lot of pressure on the existing sanitation infrastructure. Migration and immigration patterns also contribute to the distribution of schistosomes since an infected person may transfer them to areas where they are not present (Anthamatten & Hazen, 2012).

Various control measures can, however, be used to curb the spread of Schistosomes and prevent their spreading. This includes use integrated vector control measures, parasite control measures, personal protection, detecting of cases rapidly and fast referral and increasing awareness in the community (Davies, 2004).

In conclusion, human beings play a big part in the distribution of schistosomes and a positive change in the sanitary practices, irrigation practices and proper control of the vectors that host the parasites may lead to complete eradication of the parasites.