Schistosomiasis
The Parasite
Parasitic worm – Platyhelminthes
Affects ¼ of the world’s population
Þ 20 million ppl
greatest infection group Þ 9 to 15 yo
Hallmark of disease is urinating blood
Live over 45 yrs in human host
Species: S. mansoni, S. japonicum, S. haematobium; minor: S. intercalatum, S. mekoni
Morphological features: adult worms 1-2cm long, separate sexes (male is to hot dog bun as female is to hot dog)
geographic distribution:
- S. mansoni: Puerto Rico, Haiti, Dominican Republic, S. America (Brazil), Africa (Nile delta) and Middle East
- S. haematobium: Africa (Arabian peninsula) and Middle East
- S. japonicum: China, Philippines, Japan, Indonesia, etc (Far East)
Life Cycle: cercaria in water infect human (definitive host) Þ schistosomula Þ adults Þ produce eggs (sexual reproduction) which are excreted into water Þ hatch in decreased osmolality Þ miracidium Þ infect snail (intermediate host) Þ asexual reproduction Þ sporocysts Þ cercaria (float near top of water)
Invasion
Infective stage: cercaria
- free living actively motile, fresh water, ambient temp, aerobic, limited by trilaminate membrane, two suckers
- enzyme containing penetration glands
Þ looks for lipids in human skin
resemble a two-tailed sperm
once it invades, it loses its tail Þ schistosomula
First parasitic stage: schistosomula
parasitic in salty environment of the host (dies in fresh water), stationary in subcutaneous tissues for 2-3 days, tolerates temp change to 37 C, uses stored glycogen, limited by a heptalaminate memb (necessary for life in the bloodstream)
uses Kreb’s cycle: takes up glucose from the blood (unlimited supply) therefore wastes a lot of energy
adjusts to increased osmolality
migrates to lungs Þ portal veins of liver Þ becomes adult (mansoni and japonicum); haematobium goes to bladder
adults mate (hot dog in bun arrangement) Þ go downstream (mesentaries) Þ excrete eggs 6-8 wks later Þ resides in veins
Requirements for transformation:
- temp change
- salt change
- loss of tail
- migration of membrane vesicles from subtegumental cells to nearest surface
Pathogenesis
Acute schistosomiasis
cercarial dermatitis (produces rash): immediate and delayed hypersensitivity
Katayama fever: immune complex disease
Intermediate schistosomiasis:
intestinal inflammation and encephalitis
Chronic schistosomiasis
basic pathological lesion: egg granuloma
mechanism: delayed hypersensitivity
regulation: multiple mechanisms
pathophysiological consequences: inflammation and fibrosis in liver, inflammation and bleeding in urinary bladder
mansoni:
300 eggs/day
look for eggs in stool (Kato method)
symptoms: crampy abdominal pain, bloody stools, hemoptysis
signs: hepatosplenomegaly, esophogeal varices, colonic polyposis (in Egypt)
rare: core pulmonale, transverse myelitis
japonicum:
3000 eggs/day Þ also attacks brain
look for eggs in stool (Kato method)
haematobium:
migrate to vesicular veins Þ bladder Þ lay eggs Þ bleeds Þ urinate blood
diagnose with dipstick in urine
swimmer’s itch: cercariae invade Þ don’t adapt to human environment Þ die Þ skin becomes itchy
patho of liver disease:
egg deposition in liver
- worms aren’t a problem, eggs are
Þ immune response Þ mericidium Þ migrate to GI tract
or eggs lodge in the liver Þ form granulomas Þ portal vein backs up Þ splenic vein backs up Þ blood diverted to esophogeal varices so become more pronounced
delayed hypersensitivity reaction
fibrosis Þ hepatosplenomegaly
hemodynamic changes: presinusoidal block, portal hypertension (bugs migrate to lungs Þ transverse myelitis), arterialization
chronic episodes of bleeding form punch-thin fibrosis of the liver
host genetics (mansoni)
- haploid genome size 2.7 x108 bp
- 7 pairs of autosomal chrom; 1 pair of sex chrom
- male = ZZ; female = ZW
Management
Kill off the snails (hermaphroditic) Þ necrosamide
Construction of modern sanitation and water purification facilities would put an end to this endemic parasite
Chemotherapy: praziquantal is drug of choice for all 3 species (single dose eradicates 90%); oxamniquine is sometimes used for mansoni