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This week we begin to explore a new phylum, the Platyhelminths, or flatworms. This phylum contains two major classes of parasitic worms, the cestodes (Cestoda) and the trematodes (Trematoda). These animals are physiologically much simpler than the nematodes, as they do not have a digestive tract or a circulatory system. In this ongoing series, we first cover the trematodes (blood flukes and liver flukes) and then we will cover the cestodes (tapeworms). Schistosomiasis, which we are going to cover this week, is responsible for a substantive burden of disease across large areas of Africa, Asia, and South America.
The Worm. Schistosomiasis, also known as bilharzia, is caused by trematodes of the genus Schistosoma, also commonly referred to as blood flukes. There are three main species that cause human infection, Schistosoma mansoni, S. haematobium, and S. japonicum. S mekongi and S. intercalatum also infect humans, but are of lesser importance. There are also several additional species that infect other animals, most often livestock. In this discussion we will focus on the three primary schistosome species that cause most human infections.
The life cycle of these three (as well as the other two species that infect humans) follow the same general developmental stages. Schistosoma ova are expelled in the definitive host feces for all three schistosome species, and in the urine for S. haematobium and S. japonicum. In fresh water, these eggs will hatch and release the miracidia, which are motile in water. Subsequently, these miracidia seek out the intermediate host, which is one of several species of freshwater snail depending on the specific environment, geographic location, and schistosome species. The miracidia penetrate the foot of the snail and develop into sporocysts, which then produce daughter sporocysts that make their way to the snail's hepatopancreas. In the hepatopancreas these second generation sporocysts develop into cercariae, which is the larval stage that is infective to humans. The cercariae have evolved several remarkable strategies to find and infect their definitive hosts. Anatomically, the cercariae are equipped with a long forked tail that provides for prodigious swimming capacity after they exit the snail. The cercariae migrate out of the snail according to a diurnal pattern that follows light cues. The cercariae swim upward, following wave vibrations, chemical trails, and obstructed light patterns in the water, all of which can indicate the presence of their human (or other mammal) host. Upon finding a host, the cercariae penetrate the skin, lose their tail, and transform into schistosomulae, which migrate through the skin seeking out capillary beds and venules. Once they gain the venous circulation, the schistosomulae are transported to the lungs, where they develop further, and then move on to the liver, where, in the sinusoids, males and females pair off. The helminths feed on red blood cells in the sinusoids. Among mating pairs the female is enclosed in the gynaecophoric channel of the male:
After pairing these male-female schistosome pairs remove from the liver to the mesenteric veins where they will mate for life. The schistosomes require approximately 2 months to become fully mature, which marks the point when females begin to release eggs. These eggs, when mature, are able to pass through the endothelium of the veins and the epithelium of the intestine (or bladder) where they pass out of the host in the feces (or urine). The graphic below by the Centers for Disease Control and Prevention (CDC) nicely depicts the general life cycle of schistosomes:
The Snail. Freshwater snails are the intermediate hosts of schistosomes. There are four main genera of these freshwater gastropods that are capable of serving as the intermediate hosts for schistosomes. Biomphalaria are the intermediate hosts for S. mansoni:
These snails are aquatic but air-breathing. They prefer slow moving or stagnant surface water systems and are distributed in tropical areas in South America and Africa. Bulinus snails are the intermediate hosts for S. haematobium:
These snails are aquatic, air-breathing, and also prefer stagnant or slow moving water systems. These snails are distributed throughout Africa and the Middle East. Oncomelania snails are the intermediate hosts for S. japonicum:
These snails are aquatic, breathe with gills and prefer fast moving, lotic water systems. They are distributed throughout east and southeast Asia.
The Disease. The clinical spectrum of schistosomiasis is extraordinarily complex and encompasses both acute and chronic disease, which differ by schistosome species. Most low volume acute infections are asymptomatic, and will not present with either acute or chronic disease. However, high volume acute infections, especially in naive hosts, will frequently manifest acute clinical disease. A syndrome consisting of some combination of cough, fever, malaise, hyper-eosinophilia, lymphadenopathy, and hepatosplenomegaly appear anywhere between 2 to 12 weeks following exposure to infectious cercariae.This syndrome is known as Katayama fever (after a valley in Japan, from which early descriptions of the disease were reported). This acute syndrome likely results from the migration of the schistosomulae through multiple organ tissues (see life cycle above).
Chronic infection and subsequent disease is responsible for the great global burden attributable to schistosomiasis. Chronic disease is primarily due to years of the deposition of eggs by reproductively active females in the tissues of the host. As females release their eggs, only approximately 50% make their way into the lumen of the intestine (in S. mansoni and S. japonicum infections) or urinary tract (in S. haematobium) and exit the host. The remaining eggs stay embedded in host tissues. Chronic symptoms are associated with both the expunged and embedded eggs and manifest according to Schistosoma species.
Schistosoma mansoni and Schistosoma japonicum:
About half of these will make their way into the lumen of the intestine and are expunged from the host. The breach of intestinal epithelium by the the eggs results in bleeding into the intestinal space leading to blood in the stool and diarrhea, which are common symptoms in chronic infections. In high volume chronic infections, intestinal blood loss can be substantive and lead to anemia. The consequences of "hidden" schistosomiasis in young children with heavy infections can be particularly severe with impaired physical and cognitive development.
The approximately 50% of schistosome eggs that are not expunged stay embedded in the intestinal epithelium or liver parenchyma. These embedded eggs stimulate potent inflammatory responses in the host and lead to granulomas and fibrotic lesions.
_histopathology.JPG)
Portal fibrosis leads to hepatomegaly and splenomegaly. Subsequent obstruction of the portal circulation can lead to both portal hypertension and esophageal varices, which in turn can manifest in the transport of schistosome eggs to the lungs and hematemesis, respectively. The re-deposition of eggs in the lungs can also stimulate inflammation locally, resulting in pulmonary granulomas and pulmonary heart disease.
Chronic high volume S. japonicum infections seem to present greater risk for liver cirrhosis and hepatic cancer, as well as colon cancer, because this species produces up to 10 times more eggs than S. mansoni, which subsequently lead to a far greater number of granulomas.
Schistosoma haematobium:
The paired adults of this species are located in the venous plexus of the bladder. As with the other Schistosoma species, only about 50% of the eggs are successfully expunged from the host. These eggs are released into the vasculature, where they penetrate the endothelium of the blood vessel and then the wall of the bladder. In these infections, the breach of the bladder wall by the emerging eggs leads to hematuria, polyuria and dysuria.
Again, approximately 50% of the schistosome eggs are not expunged and remain embedded in the wall of the bladder.
As with the other Schistosoma species, these embedded eggs are potent stimulators of inflammation, which leads to fibrotic lesions in the bladder and calcification:
In high volume chronic infections, urinary fibrosis can obstruct the urinary tract leading to urinary stasis and secondary bacterial infections.
Chronic high volume S. haematobium infections are also associated with a relatively high risk of squamous cell carcinoma of the bladder.
The Epidemiology and the Landscape. The are currently over 200 million people throughout the world infected with at least one species of Schistosoma helminth. There are 76 endemic countries, but the greatest burden (≥ 1 million infections) occurs in 30 African countries and Brazil. Approximately 20 to 50 million people experience substantive disability due to their infections. The map below, published in Acta Tropica (Volume 120, Supplement 1, September 2011, Pages S121–S137), depicts this global distribution including those areas where schistosomiasis has been eliminated:
The distribution of schistosomiasis is primarily a function of the distribution of the intermediate host snails. The maps below, which were produced by the Schistosomiasis Research Group at Cambridge University, depict the geographic distribution of species-specific schistosomiasis:
Notice how the geography of schistosomiasis caused by S. mansoni conforms to the distribution of Biomphalaria snails described above.
Notice, here, how the geography of schistosomiasis caused by S. haematobium conforms to the distribution of Bulinus snails described previously.
Finally, notice how the geography of schistosomiasis caused by S. japonicum conforms to the distribution of Oncomelania snails described previously.
The landscape epidemiology of schistosomiasis is a confluence of the schistosome relationship with its preferred intermediate host, local snail habitat and water environment, and, finally, the human activity (occupational, domestic, and/or recreational) that creates contact points with this habitat. So, as with so many human infections, the disease ecology is really defined by the intersection of the physical landscape and the human social landscape.
This week we begin to explore a new phylum, the Platyhelminths, or flatworms. This phylum contains two major classes of parasitic worms, the cestodes (Cestoda) and the trematodes (Trematoda). These animals are physiologically much simpler than the nematodes, as they do not have a digestive tract or a circulatory system. In this ongoing series, we first cover the trematodes (blood flukes and liver flukes) and then we will cover the cestodes (tapeworms). Schistosomiasis, which we are going to cover this week, is responsible for a substantive burden of disease across large areas of Africa, Asia, and South America.
The Worm. Schistosomiasis, also known as bilharzia, is caused by trematodes of the genus Schistosoma, also commonly referred to as blood flukes. There are three main species that cause human infection, Schistosoma mansoni, S. haematobium, and S. japonicum. S mekongi and S. intercalatum also infect humans, but are of lesser importance. There are also several additional species that infect other animals, most often livestock. In this discussion we will focus on the three primary schistosome species that cause most human infections.
S. mansoni male-female pair
The life cycle of these three (as well as the other two species that infect humans) follow the same general developmental stages. Schistosoma ova are expelled in the definitive host feces for all three schistosome species, and in the urine for S. haematobium and S. japonicum. In fresh water, these eggs will hatch and release the miracidia, which are motile in water. Subsequently, these miracidia seek out the intermediate host, which is one of several species of freshwater snail depending on the specific environment, geographic location, and schistosome species. The miracidia penetrate the foot of the snail and develop into sporocysts, which then produce daughter sporocysts that make their way to the snail's hepatopancreas. In the hepatopancreas these second generation sporocysts develop into cercariae, which is the larval stage that is infective to humans. The cercariae have evolved several remarkable strategies to find and infect their definitive hosts. Anatomically, the cercariae are equipped with a long forked tail that provides for prodigious swimming capacity after they exit the snail. The cercariae migrate out of the snail according to a diurnal pattern that follows light cues. The cercariae swim upward, following wave vibrations, chemical trails, and obstructed light patterns in the water, all of which can indicate the presence of their human (or other mammal) host. Upon finding a host, the cercariae penetrate the skin, lose their tail, and transform into schistosomulae, which migrate through the skin seeking out capillary beds and venules. Once they gain the venous circulation, the schistosomulae are transported to the lungs, where they develop further, and then move on to the liver, where, in the sinusoids, males and females pair off. The helminths feed on red blood cells in the sinusoids. Among mating pairs the female is enclosed in the gynaecophoric channel of the male:
After pairing these male-female schistosome pairs remove from the liver to the mesenteric veins where they will mate for life. The schistosomes require approximately 2 months to become fully mature, which marks the point when females begin to release eggs. These eggs, when mature, are able to pass through the endothelium of the veins and the epithelium of the intestine (or bladder) where they pass out of the host in the feces (or urine). The graphic below by the Centers for Disease Control and Prevention (CDC) nicely depicts the general life cycle of schistosomes:
Biomphalaria snail
Bulinus snail
These snails are aquatic, air-breathing, and also prefer stagnant or slow moving water systems. These snails are distributed throughout Africa and the Middle East. Oncomelania snails are the intermediate hosts for S. japonicum:
Oncomelania snail
These snails are aquatic, breathe with gills and prefer fast moving, lotic water systems. They are distributed throughout east and southeast Asia.
The Disease. The clinical spectrum of schistosomiasis is extraordinarily complex and encompasses both acute and chronic disease, which differ by schistosome species. Most low volume acute infections are asymptomatic, and will not present with either acute or chronic disease. However, high volume acute infections, especially in naive hosts, will frequently manifest acute clinical disease. A syndrome consisting of some combination of cough, fever, malaise, hyper-eosinophilia, lymphadenopathy, and hepatosplenomegaly appear anywhere between 2 to 12 weeks following exposure to infectious cercariae.This syndrome is known as Katayama fever (after a valley in Japan, from which early descriptions of the disease were reported). This acute syndrome likely results from the migration of the schistosomulae through multiple organ tissues (see life cycle above).
Chronic infection and subsequent disease is responsible for the great global burden attributable to schistosomiasis. Chronic disease is primarily due to years of the deposition of eggs by reproductively active females in the tissues of the host. As females release their eggs, only approximately 50% make their way into the lumen of the intestine (in S. mansoni and S. japonicum infections) or urinary tract (in S. haematobium) and exit the host. The remaining eggs stay embedded in host tissues. Chronic symptoms are associated with both the expunged and embedded eggs and manifest according to Schistosoma species.
Schistosoma mansoni and Schistosoma japonicum:
The paired adults of these species are located in the mesenteric veins, where females release their eggs into the lumen.
About half of these will make their way into the lumen of the intestine and are expunged from the host. The breach of intestinal epithelium by the the eggs results in bleeding into the intestinal space leading to blood in the stool and diarrhea, which are common symptoms in chronic infections. In high volume chronic infections, intestinal blood loss can be substantive and lead to anemia. The consequences of "hidden" schistosomiasis in young children with heavy infections can be particularly severe with impaired physical and cognitive development.
The approximately 50% of schistosome eggs that are not expunged stay embedded in the intestinal epithelium or liver parenchyma. These embedded eggs stimulate potent inflammatory responses in the host and lead to granulomas and fibrotic lesions.
_histopathology.JPG)
Portal fibrosis leads to hepatomegaly and splenomegaly. Subsequent obstruction of the portal circulation can lead to both portal hypertension and esophageal varices, which in turn can manifest in the transport of schistosome eggs to the lungs and hematemesis, respectively. The re-deposition of eggs in the lungs can also stimulate inflammation locally, resulting in pulmonary granulomas and pulmonary heart disease.
Chronic high volume S. japonicum infections seem to present greater risk for liver cirrhosis and hepatic cancer, as well as colon cancer, because this species produces up to 10 times more eggs than S. mansoni, which subsequently lead to a far greater number of granulomas.
Schistosoma haematobium:
The paired adults of this species are located in the venous plexus of the bladder. As with the other Schistosoma species, only about 50% of the eggs are successfully expunged from the host. These eggs are released into the vasculature, where they penetrate the endothelium of the blood vessel and then the wall of the bladder. In these infections, the breach of the bladder wall by the emerging eggs leads to hematuria, polyuria and dysuria.
Again, approximately 50% of the schistosome eggs are not expunged and remain embedded in the wall of the bladder.
As with the other Schistosoma species, these embedded eggs are potent stimulators of inflammation, which leads to fibrotic lesions in the bladder and calcification:
In high volume chronic infections, urinary fibrosis can obstruct the urinary tract leading to urinary stasis and secondary bacterial infections.
Chronic high volume S. haematobium infections are also associated with a relatively high risk of squamous cell carcinoma of the bladder.
The Epidemiology and the Landscape. The are currently over 200 million people throughout the world infected with at least one species of Schistosoma helminth. There are 76 endemic countries, but the greatest burden (≥ 1 million infections) occurs in 30 African countries and Brazil. Approximately 20 to 50 million people experience substantive disability due to their infections. The map below, published in Acta Tropica (Volume 120, Supplement 1, September 2011, Pages S121–S137), depicts this global distribution including those areas where schistosomiasis has been eliminated:
The distribution of disability-adjusted life years associated with schistosomiasis reflects the distribution of endemicity:
Age-standardised disability-adjusted life year (DALY) rates from Schistosomiasis by country (per 100,000 inhabitants).
no data
less than 50
50-75
75-100
100-150
150-200
200-250
250-300
300-350
350-400
400-450
450-500
more than 500
The distribution of schistosomiasis is primarily a function of the distribution of the intermediate host snails. The maps below, which were produced by the Schistosomiasis Research Group at Cambridge University, depict the geographic distribution of species-specific schistosomiasis:
Global distribution of S. mansoni infections
Notice how the geography of schistosomiasis caused by S. mansoni conforms to the distribution of Biomphalaria snails described above.
Global distribution of S. haematobium infections
Notice, here, how the geography of schistosomiasis caused by S. haematobium conforms to the distribution of Bulinus snails described previously.
Global distribution of S. japonicum infections
Finally, notice how the geography of schistosomiasis caused by S. japonicum conforms to the distribution of Oncomelania snails described previously.
The landscape epidemiology of schistosomiasis is a confluence of the schistosome relationship with its preferred intermediate host, local snail habitat and water environment, and, finally, the human activity (occupational, domestic, and/or recreational) that creates contact points with this habitat. So, as with so many human infections, the disease ecology is really defined by the intersection of the physical landscape and the human social landscape.
Control and Prevention.
Snail Control
Early (and some ongoing) attempts at schistosomiasis elimination focused on gastropod control in various surface water sources. Typically, these control campaigns have involved the chemical treatment of freshwater sources to eliminate the local snail population, thus blocking transmission of the schistosomes at the intermediate host. However, two important problems make gastropod control unrealistic in many settings.
First, the chemical treatment of water sources can have much broader ecologic impact than what is intended by the public health initiative. Introducing toxic agents into surface water may kill the snails, but it can also harm other organisms in the aquatic ecosystem and result in unanticipated and detrimental ecologic effects. Some investigations have explored the possibility of introducing biologic mechanisms of control to supplant chemical treatment with more "natural" mechanisms. However, these approaches can also be dangerous if the natural biologic mechanisms involve the introduction of non-native invasive species into the aquatic environment. Nevertheless, the use of some plants in specific aquatic environments has been quite successful. For example, planting Sarcoca dodecandra, i.e. the gopo berry shrub, in water sources kills any snails present.
Second, whether chemical or natural, snails in most areas prove robust to elimination. Both their high reproductive capacity and diffuse distribution within the aquatic system usually allow some members of the population to survive, which can then re-populate the local environment fairly quickly.
Deworming
The World Health Assembly has adopted deworming treatment with praziquantel as the primary approach to schistosomiasis control. According to the goals of the World Health Assembly and the Schistosomiasis Control Initiative, between 75% and 100% of school-age children in endemic areas should be treated with praziquantel. In highly endemic areas, large-scale, population-based, systematic therapeutic and prophylactic treatment of the population has proven far more effective than selective treatment of diagnosis-confirmed individuals.
De-worming campaigns do offer some hope for area-specific elimination, since there is a safe, effective, easily administered, and fairly cheap anti-helminthic drug available (i.e. praziquantel). However, as one might expect, there are obstacles to overcome in de-worming. First, praziquantel is not free and, while the price dropped by over 90% in the 1990s, without adequate funding poor communities will not be able to prioritize the cost, especially since many infections are generally asymptomatic. Second, effective ways to deliver the de-worming medications to communities need to be implemented, which can be logistically challenging particularly in remote communities or during times of the year when travel may be restricted (i.e. during the rainy season). Third, the extensive use, or misuse, of these drugs will likely lead to antihelminthic-resistance in the schistosomes, thus making the drugs ineffective. Nevertheless, if adequate resources can be put behind de-worming campaigns, and if delivery systems can be adapted to actively engage community members in the delivery and monitoring of these de-worming medications to simultaneously circumvent logistical obstacles and reduce the development of resistance, then substantial reductions in schistosomiasis may still be possible.