Schistosoma: Blood Flukes

Schistosoma spp. (Blood Flukes)

• Schistosomes are dioecious trematodes that cause Schistosomiasis (bilharziasis).
• Schistosomiasis is a water-borne disease and a public health concern affecting millions in Africa, Asia, and Latin America.
• Over 100 million people are infected with S. haematobium, S. mansoni, and S. japonicum.
• S. mekongi and S. intercalatum are two other Schistosoma species that parasitize humans.
• The male worm is broader than the female, with lateral borders rolled ventrally into a gynecophoric canal to hold the female.
• Schistosomes were formerly called Bilharzia, named after Theodor Bilharz, who first observed the worm in the mesenteric veins of an Egyptian in Cairo in 1851.
• Schistosomes are unisexual, lack a muscular pharynx.
• Intestinal caeca reunite after bifurcation to form a single canal.
• Non-operculated eggs are produced, and there is no redia stage in larval development.
• Cercariae have forked tails and infect by penetrating the unbroken skin of definitive hosts.

Schistosoma Haematobium

• Adult worms live in the vesicle and pelvic plexuses of veins.
• The eggs are ovoid, about 150 µm by 50 µm, nonoperculated, with a brownish yellow.
• The male is 10-15 mm long and 1 mm thick, covered by a finely tuberculated cuticle.
• It has 2 muscular suckers (oral sucker is small, ventral sucker is large and prominent).
• The ventral sucker extends into the gynecophoric canal, holding the female worm..
• The adult female is long and slender, 20 mm by 0.25 mm, with cuticular tubercles confined to the ends.
• The gravid worm contains 20–30 eggs in its uterus, passing up to 300 eggs daily.
• The transparent eggs have a terminal spine at one pole.
• The life cycle involves 2 hosts: humans (definitive) and fresh water snails (intermediate).
• The infective form is the Cercaria larva.
• The eggs passed in urine are embryonated.
• They hatch in water under suitable conditions, releasing free-living ciliated miracidia.
• Miracidia penetrate the tissues of suitable intermediate host snails.
• The intermediate hosts include snails of the Bulinus species in Africa; in India.
• Inside the snail, miracidia lose their cilia.
• It takes around 4-8 weeks to successively pass through the stages of the first and second generation sporocysts.
• Asexual reproduction produces a large number of cercariae within the second generation sporocyst.
• The cercaria has an elongated ovoid body and forked tail (furcocercous cercaria).
• Cercariae escape from the snail and swim in water for 1-3 days.
• They penetrate unbroken skin, facilitated by lytic substances from penetration glands.
• On entering the skin, cercariae shed their tails, becoming schistosomulae.
• Schistosomulae enter the peripheral venules then migrates to the right side of the heart, pulmonary circulation, left side of the heart, and then the liver.
• In the intrahepatic portal veins, schistosomulae grow and become sexually differentiated adolescents about 20 days after penetration.
• They start migrating against the blood stream into the inferior mesenteric veins, reaching the vesicle and pelvic venous plexuses to mature, mate, and lay eggs.
• Eggs start appearing in urine typically 10-12 weeks after cercarial penetration.
• Clinical illness depends on the stage of infection: skin penetration and incubation, egg deposition and extrusion, tissue proliferation and repair.
• Clinical features during incubation : local cercarial dermatitis or general anaphylactic/toxic symptoms.
• Cercarial dermatitis: transient itching and lesions at cercariae entry site (swimmer's itch).
• It is more frequent in visitors to endemic areas than locals.
• Anaphylactic or toxic symptoms include fever, headache, malaise, and urticaria.
• Leucocytosis, eosinophilia, enlarged tender liver, and a palpable spleen accompany it.
• Diagnosed via, urine microscopy, detection of antigens (CAA & CCA) via ELISA, complement fixation tests and antibody tests.
• Fairley's Test detects schistosomiasis.
• Imaging can be used to assess bladder and ureteral calcification, or via the USG, IVP and cystoscopy.

Laboratory Diagnosis

• Urine microscopy detects eggs with characteristic terminal spines in centrifuged urine deposits.
• Histopathology diagnoses via eggs in bladder mucosal biopsy.
• Serology tests determine presence of antigen.
• Another diagnostic method is the detection of specific schistosome antigens in serum or urine.
• Serological tests are described to detect specific antibody, but is not useful as is cannot differentiate between present or past infection.
• X-rays of the abdomen reveal bladder and ureteral calcification.
• Ultrasonography (USG) is helpful for diagnosing S. haematobium infection.

Treatment

• Praziquantel (40mg/kg for 1 day) is the drug of choice.
• Metriphonate (7.5 mg/kg weekly for 3 weeks) is an alternative for schistosomiasis due to S. haematobium.

Schistosoma Mansoni

• Widely distributed in Africa, South America, and the Caribbean islands.
• Adult worms live in the inferior mesenteric vein.
• Morphology and life cycle resembles S. haematobium.
• The adult worms are smaller, with integuments studded with coarse tubercles.
• The gravid female's uterus contains very few eggs (usually 1-3).
• The prepatent period is 4-5 weeks.
• Its egg has a characteristic lateral spine near the rounded posterior end, with a non operculated and yellowish brown shell.
• The intermediate host is Biomphalaria.
• In humans the schistosomulae mature in the liver, then blood stream into the venules of the inferior mesenteric.
• Eggs penetrate the gut wall and are shed in feces.
• Following skin penetration by cercariae, a pruritic rash called cercarial dermatitis or swimmer's itch occurs.
• Cercarial dermatitis may develop locally.
• Acute schistosomiasis or Katayama fever occurs during maturation and at the beginning of oviposition: which can occur 4–8 weeks after skin invasion.
• Intestinal symptomatology occurs, because eggs are deposited in the samall intestine and is known as intestinal bilharziasis or schistosomal dysentery.
• Patients develop colicky abdominal pain and bloody diarrhea.
• The eggs deposited in the gut wall cause inflammatory reactions, micro-abscesses, granulomas, hyperplasia and eventual fibrosis.

Diagnosis

• Stool microscopy can be used to detect eggs
• Can use Concentration methods when infection is light.
• Biopsy of rectal mucosa helps to reveal eggs under the microscope.
• Serological diagnoses are carried out by detecting schistomal antigen.
• Blood examinations and USGs are performed.

Schistosoma japonicum

• Common name: Oriental blood fluke.
• Distribution: Far East (Japan, China, Taiwan, Philippines).
• It lives in the venules of the superior mesenteric vein draining the ileocecal region.
• In the gravid female, the uterus can contain up to 100 eggs at one time where as up to 3,500 eggs may be passed daily by a single worm.
• Its prepatent period is 4 – 5 weeks.
• The eggs are more spherical than those of S. haematobium and S. mansoni and have no spine, but shows, instead, a small knob.
• Man is the definitive host
• Snails in the genus Oncomelania are the intermediate host
• Life cycle is similar to S. haematobium life cycle.
• The infective form to humans is via fork tailed cercaria from the snails.
• Eggs deposited in the superior mesenteric venules penetrate the gut wall and are passed in feces.

Pathogenicity

• Disease caused is known as shistosomiasis japonicum aka Oriental schistosomiasis otherwise known as Katayama disease.
• acute illness symptoms are fever, abdominal pain, diarrhoea and allergic manifestations, known as Katayama fever
• Eggs are seen in feces.