Schistosomiasis

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schistosoma is distinct from other trematoda

  • trematode中schistosomaは他のtrematodeと異なる特徴を持つ
schistosoma other trematode
transmitted to human
by percutaneous invasion of cercariae
transmitted to human
by oral ingestion of metacercariae
male and female separated 雌雄別体 hermaphrodite雌雄同体
does not require 2nd intermediate host
(no metacercariae stage)
requires 2nd intermediate host

distribution

Schistosomiasis distribution adapted from Ferarri, Lancet Neurol 2011.jpg

lifecycle

Intermediate host
Category snails
Latin Oncomelania nosophora 宮入貝 in Japan
Inward infection
  1. eggs hatch in freshwater to miracidia
  2. miracidia invade snail tissue
Organ snail tissue
Reproduction asexual
Course
  1. miracidia grow into sporocysts in snail tissue
  2. sporocysts release cercariae in snail tissue
Detection
Outward transmission cercariae swim into freshwater in reaction to light (sunlight)
Definitive host
Category human being
Latin
Inward infection
  1. cercariae penetrate skin of human who put body into contaminated freshwater
  2. cercariae lose their tail during skin penetration and become schistosomulae
  3. schistosomulae enter into circulation and migrate to specific organ to grow up to adults
Organ
  • venous plexus of urinary bladder in case of S. haematobium
  • portal vein through liver in case of other spp.
Reproduction sexual (different from other trematodes)
Course
  1. adults differentiate to male and female
  2. both mate in portal vein and live veins of the organ for decades
  3. mated female produce egg inside veins
    • eggs obstruct veins and induce inflammatory reaction to cause disease
Detection
  • microscopic detection of eggs in urine (S. haematobium) or feces (other spp.)
    • Kato-Katz technique for feces
  • serological detection of protein of eggshell
    • circumoval precipitin test (COPT)
Outward transmission
  1. eggs are released into freshwater from urine or feces
  2. eggs hatch in freshwater and invade snail tissue

Schistosoma japonicum

  • Katsurada Fujiro found 26 May 1904 from a cat in Yamanashi(水腫腸満)
  • Fujinami Kan found 30 May 1904 from a murder victim in Katayama, Hiroshima(片山病)
  • found egg in 1904 in Saga(奇病)


pathogenesis

  • adult female paired by male migrate to messenteric vein from portal vein and lay eggs into messenteric venule, which are secreted from messenteric venule to intestinal lumen
  • eggs also reach urinary bladder or genital tract and generate inflammation, followed by necrosis of tissue and perforation to intestine, resulted in extracorporeal distribution of egg via stool
  • eggs are also distributed in liver and cause inflammation resulting in cirrhosis
  • inflammation is exclusively due to egg
  • adult pairs do not cause disease per se

co-infection

  • HIV
  • Typhoid
    • Salmonella typhi has crossover antigenicity with Schistosoma and makes immune-tolerance against Schistosoma infection
    • Also Salmonella hide between male and female schistosomes in the gynecophoric canal
  • HepB/C

treatment

  • praziquantel was discovered only in mid-1970s in Germany
  • praziquantel only kills adult schistosoma

control

  • In Japanese history before introduction of praziquantel, one of ways to control was eliminate intermediate hosts, snails - Oncomelania nosophora 宮入貝
  • After introduction of praziquantel
  1. Malacological monitoring of snails in endemic area
  2. Improve water irrigation in order to avoid colonization of snails
  3. Effective screening of people in endemic area to detect actively infected patients
  4. Periodical mass drug administration to people in endemic area
  • Praziquantel 30mg/kg twice for MDA
  • Praziquantel 60mg/kg in two divided dose for detected patients