Bilharziasis: Causes and Pathogenesis

Bilharziasis

• Chronic specific granulomatous infection caused by Schistosoma species
• Endemic in Egypt

Life Cycle and Pathogenesis

• Acute schistosomiasis:
  • Severe febrile illness, peaks after 2 months
  • Dominated by TH1 cells, producing IFN-γ, stimulating macrophages to secrete cytokines (TNF, IL-1, and IL-6) → fever
• Chronic schistosomiasis:
  • Associated with dominant TH2 response, although TH1 cells persist
  • Parasite egg proteins stimulate mast cells to produce IL-4 → TH2 differentiation
  • Both TH1 and TH2 cells contribute to granuloma formation around eggs

Bilharzial Lesions

• Produced by cercaria:
  • Acute allergic dermatitis, maculopapular skin rash
  • Microscopic picture: dilated capillaries, neutrophils, eosinophils, and macrophages
• Produced by adult worms:
  • Live in veins around bladder (S. hematobium) or colon (S. mansoni)
  • Dead worms → severe inflammation, necrosis, and thrombophlebitis
  • Living worms → brown bilharzial pigment engulfed by phagocytic cells of liver, spleen, and other tissues
• Produced by ova:
  • Recurrent bleeding (due to injury by ova spines) leads to anemia
  • Ova may be trapped in bladder, intestine, or carried to lungs, liver, or other sites
  • Trapped ova produce "egg antigens" leading to T-lymphocyte sensitization, lymphokine release, and inflammatory response → granuloma formation

Granuloma Development

• Three successive phases:
  • Cellular granuloma: ova surrounded by macrophages, eosinophils, neutrophils, lymphocytes, and giant cells
  • Fibro-cellular granuloma: cellular granuloma surrounded by fibroblasts and capillaries
  • Fibrous (healed) granuloma: smaller, dense collagen, with inflammatory cells disappearing

Liver Bilharziasis

• Common in patients with intestinal bilharziasis
• Portal tracts show major changes:
  • Ova carried as emboli through portal vein and trapped in small and large portal tracts
  • Granulomas → fibrosis
• Dead worms → trapped inside venules of coarse tracts, leading to thrombophlebitis
• Angiomatoids: dilated capillaries in fibrotic portal tracts, representing dilated collateral channels between hepatic artery and portal veins
• Hepatic lobules show minimal insignificant lesions, with preserved architecture

Types of Bilharzial Periportal Fibrosis

• Fine bilharzial periportal fibrosis:
  • Bilharzial affection predominantly involves fine tracts
  • Lesions produced by ova
  • Gross picture: normal-sized liver, firm, finely granular surface, and dark brown cut surface
  • Microscopic picture: ova, granulomatous inflammation, and fibrosis in portal tracts
• Coarse bilharzial periportal fibrosis:
  • Bilharzial affection predominantly involves coarse tracts
  • Lesions produced by ova and/or dead worms
  • Gross picture: reduced liver size, firm, irregular surface, and dark brown cut surface
  • Microscopic picture: ova, granulomatous inflammation, and fibrosis in portal tracts
• Mixed fine and coarse bilharzial periportal fibrosis:
  • The usual pattern

Effects and Complications

• Portal hypertension:
  • Caused by portal fibrosis and angiomatoids
  • Effects: splenomegaly, ascites, and opening of porto-systemic venous collaterals
• Portal vein thrombosis: may occur due to vascular stasis
• Mild disturbances of liver functions: lowering of plasma proteins
• Ascites: due to portal hypertension and lowering of plasma proteins
• Ammonia encephalopathy:
  • Normally, ammonia is converted to urea in the liver
  • Hepatic portal fibrosis → ammonia escape through open porto-systemic collaterals → reach systemic circulation → brain → encephalopathy → coma
• Causes of death in hepatic bilharziasis:
  • Bleeding (hematemesis)
  • Superimposed chronic viral hepatitis (relatively common) leading to cirrhosis and liver failure