Schistosoma japonicum is a parasitic trematode (blood fluke) that causes a disease called Schistosomiasis in humans and other animals. S. japonicum is similar to Schistosoma mansoni but it infects both large and small bowel and produces a greater number of eggs than S. Mansoni; hence its disease, therefore, tends to be more severe, and rapidly progressive with liver involvement being more common and neurological involvement in about 5% of cases.
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Schistosoma japonicum is found only in Asia (Southeast Asia and the western Pacific, especially China, Indonesia, the Philippines, but no longer in Japan) and is the only one of all the Schistosoma species for which domestic animals such as pigs and water buffalo, act as important reservoirs.
Morphology and Characteristics
- Schistosoma japonicum male adult worm is shorter compared to the female. The adult male worm measures about 720 mm in length while the female measures about 1026 mm in length
- Adult schistosoma japonicum worms reside in the Mesenteric veins that drain the intestinal tract
- Schistosoma japonicum also causes intestinal schistosomiasis as Schistosoma mansoni
- The female encloses in a tegumental fold called the ventral groove (or canalis gynaecophorus) of the relatively thick male.
Schistosoma japonicum Egg
The Schistosoma japonicum egg is elliptical in shape and measures with a tiny lateral spine that may be lacking sometimes. The egg measures about 70100 micrometers by 5065 micrometers.
The mode of transmission of Schistosoma japonicum infection occurs when you come in contact with water that contains the free-swimming larvae called cercariae (the infective stage) which then penetrate the skin to cause infection when they shed their tails and are then taken via the blood to other parts of the body. Though animals can contribute as hosts in the Schistosoma life cycle, humans are the most important reservoirs but for Schistosoma japonicum, animals contribute significantly to the dissemination of its eggs.
The symptoms and signs of Schistosoma japonicum infection occur as a result of the passage of the eggs through the intestinal mucosa, which damages the walls of both the small and large intestines (superior and inferior mesenteric venules) because of the immune response to the antigenic substances released by the eggs.
In some rare cases, schistosoma japonicum eggs can be carried to other body parts and cause abnormalities relating to the affected organs this is known as Metastatic schistosomiasis. When the eggs are in the central nervous system, the CNS (brain, spinal cord), they cause Cerebral Schistosomiasis which compresses the spinal cord and causes acute paraplegia. Hepatosplenic Schistosomiasis leads to fibrosis of the liver and spleen. This is caused by the deposition of eggs of schistosoma japonicum around the portal vein branches in the liver (a condition known as pipestem fibrosis discovered by Symmers) and results in circulatory anomalies, ascites, portal hypertension, hemorrhages of the gastrointestinal tract, and splenomegaly.
Stages of Schistosoma japonicum infection
- Stage 1 (stage of Invasion): This stage is common to all schistosoma species and shows the first clinical signs and symptoms of acute infection; usually the symptoms appear within 24 to 48 hours after exposure. This stage is characterized by itching at the sites of entry of the cercariae, the itch is commonly known as Swimmers itch.
- Stage 2 ( Toxemia stage/ Katayama syndrome) this stages present with an early allergic manifestation resulting from egg deposition. The symptoms and signs of this stage forms a syndrome known as Katayama Syndrome or Katayama Fever and they include Fever, headache, chills, muscle pains, Malaise, diarrhea and vomiting. There may be generalized lymphadenopathy, hepatosplenomegaly, urticaria and leukocytosis with marked eosinophilia.
- Stages 3 and 4 (Chronic stage) this occurs about 3 months to several years after infection and is due to deposition of eggs in the tissues. There may be lethargy, recurrent bloody diarrhea and in some cases, intestinal polyps (bilharziomas) and progressive fibrosis of the intestinal wall leading to formation of strictures; intestinal obstruction is very rare. Other manifestations may include granulomatous hepatitis followed by progressive peri-portal fibrosis (also called pipe stem fibrosis) resulting in portal hypertension with associated splenomegaly, ascites and esophageal varices that occasionally may bleed.
Symptoms of Schistosoma japonicum infection
- Muscle aches
- Abdominal pain
- Nausea and vomiting
- Gastrointestinal hemorrhage
- Massive splenomegaly
- Profuse diarrhea
- There may be generalized lymphadenopathy
Schistosoma japonicum Life cycle
- The Life cycle of Schistosoma japonicum starts when the eggs are released by the adult worms in stool. The ova are used for diagnosis by identifying the rudimentary lateral spine
- The released Schistosoma japonicum eggs then hatch into immature ciliated larvae that are called Miracidia that penetrate only Onchomelania genus of freshwater snails in order to carry out asexual reproduction and develop into Sporocysts.
- Sporocysts then develop into mature larvae called Cercariae which are the infective stages of Schistosoma japonicum
- Cercariae now penetrate the human skin to cause Schistosomiasis. When in the body tissue, cercariae lose their tails to be called Schistosomutae. These Schistosomutae enter the superior mesenteric artery pass into the portal circulation and reach the liver and mature into adults; the adults migrate against the portal flow to reside in the mesenteric venules.
- Adult worms then lay eggs daily that are then released in stool and the life cycle continues
The life cycle of Schistosoma species takes about 12 to 14 weeks to complete. The intermediate host of Schistosoma japonicum is a freshwater snail of the genus Onchomelania they are amphibious snails that are also found on moist ground and plants such as in rice paddies; the infective stage is the larval stage that is called Cercariae.
Diagnosis of Schistosoma japonicum infection is made by identification of the elliptical egg with a small lateral spine which may be absent. Therefore, the Schistosoma japonicum egg is the diagnostic stage.
The drug of choice for the treatment of Schistosoma japonicum infection is Praziquantel using a dosage of 40mg/Kg of a single dose.
- Regular drug therapy of affected group of population
- Elimination of snails populations in freshwater bodies
- Provision of safe and adequate water supply
- Practice good environmental hygiene such as avoidance of improper disposal of human feces
- Avoid coming in contact with natural or artificial bodies of water (freshwater) either through fishing or swimming
- Liver cirrhosis
- Portal hypertension
- Ascites (Abdominal enlargement due to accumulation of fluid in the peritoneal space)
- In rare cases, there may be intestinal Stricture