Primary human species are;
- Schistosoma mansoni
- Schistosoma haematobium
- Schistosoma japonicum
- (S.mekongi & S.intercalatum encountered less frequently)
Schistosomes are often referred to as 'blood' trematodes as they differ from other trematodes because they infect humans by penetrating intact skin to gain entry to the circulatory system rather than infection through ingestion. In other words, Schistosomes infect humans through direct larval (cercariae) penetration rather than the ingestion of metacercariae.
Schistosomes are also unique among the flukes in that there are both a male and female organisms.
Schistosoma eggs also lack an operculum which characterizes other fluke eggs.
Their life cycle is as follows;
- Eggs in feces or urine are passed into water
- Larvae are liberated and penetrate the intermediate host snail where they further develop.
- Cercariae emerge from the snail while in the water
- Cercariae penetrate the skin of humans in contact with the water
- Larval migration begins through the circulatory system where they may enter alveoli to produce hemoptysis. Organisms mature in the liver before entering specific veins specific to the infecting species. (S.haematobium in veins of the bladder, S.japonicum in veins of small intestine & S.mansoni in veins of the large intestine)
- Eggs are passed to continue the cycle.
Symptoms include cercarial dermatitis, acute schistosomiasis (Katayama fever) and related tissue egg deposition. Acute schistosomiasis begins when the adult female begins laying eggs.
In the circualtory system it is believed the organisms either becomes covered with host soluble blood group antigens, lipoproteins, or develops antigens similar to the host's so that it excapes the host's immune response. For this reason, adult worms in the veins evoke little immune response.
Symptoms may vary in intensity but can include malaise, fever, abdominal tenderness or hepatic pain.
Infection with S.mansoni or S. japonicum may cause diarrhea. S. haematobium causes hematuria.
Schistosomiasis should be considered with any patient from endemic areas who has had exposure to untreated water and presents with symptoms previously mentioned. Diagnosis is confirmed with the identification of Schistosome eggs recovered the patient.
All eggs are embryonated when passed
All eggs are easily differentiated by their appearance
- S.mansoni eggs are large (110-170 µm), oval and have a lateral, 'rose thorn' spine.
- S.haematobium eggs are large, oval (110-170 µm) and have a terminal (end) spine.
- S.japonicum eggs are smaller (55-90 µm), round and have a 'crooked finger' spine.
S.mansoni & S.japonicum from fecal specimens although on occasion both may be recovered from urine as well.
Praziquantel is the drug of choice in treating schistosomiasis. O & P examinations should be conducted periodically for up to a year post treatement to ensure erradication.
(note lateral 'rose-thorn' spine on egg)
Update; I recently took some photos of S.haematobium from a preparation obtained from our Pathology department. Unfortunately I don't have much information on this patient's history nor the stain used. However, it does make for a pretty photo!
(Note terminal spine)
(Click on photo to enlarge for better viewing)
Schistosoma haematobium (X400)
(Terminal spine at bottom barely visible)
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