Hydatid Cyst (Echinococcosis)

The Organism

• Definitive host: Dogs (and other canids) - adult tapeworm lives here
• Intermediate host: Sheep (usual); Humans (accidental, dead-end host)
• Three species: E. granulosus (most common), E. multilocularis, E. ligartus

Life Cycle & Transmission

1. Dog passes eggs in feces
2. Humans accidentally ingest eggs from dog feces (feco-oral route)
3. In the human duodenum, the parasitic embryo (oncosphere) releases hooklets, penetrates the intestinal mucosa, enters the bloodstream
4. Oncosphere reaches liver (most common) or lungs via portal blood - develops into the larval stage = hydatid cyst
5. Dogs acquire infection by eating raw viscera (offal) of sheep or other intermediate hosts containing hydatid cysts

Structure of the Hydatid Cyst

• Brood capsules: Small intracystic cellular masses in which future worm heads (scoleces/protoscolices) develop
• Protoscolices: Thousands of future larvae contained within brood capsules inside the cyst
• Hydatid sand: Free brood capsules and scoleces floating in hydatid fluid
• Daughter cysts: True replicas of the mother cyst that develop from protoscolices

Pathology & Sites

• Compression and atrophy of hepatic parenchyma
• Portal hypertension (mechanical obstruction)
• Cirrhosis
• Rupture into biliary tree → jaundice, biliary colic
• Rupture into bronchial tree → cough, expectoration of cyst contents (hydatid vomica)
• Free rupture into peritoneum/pleura → anaphylactic shock + disseminated echinococcosis

Key danger: If the cyst ruptures, the highly immunogenic hydatid fluid can cause anaphylactic shock, and brood capsules can metastasize to form additional hydatid cysts at new sites.

Clinical Features

• Usually asymptomatic until complications occur (average age at diagnosis ~45 years; equal M:F)
• Abdominal pain, dyspepsia, vomiting
• Hepatomegaly (most common sign)
• Jaundice and fever (~8% each)
• Bacterial superinfection mimics pyogenic abscess

Diagnosis

Imaging

• Ultrasound - first line (available, affordable, accurate):
  • Simple cyst with hydatid sand (free-floating hyperechogenic particles)
  • Rosette appearance - when daughter cysts are present
  • Amorphous mass filling the cyst
  • Calcification of cyst wall (eggshell calcification) - highly suggestive

• CT / MRI - evaluates extrahepatic disease, detailed hepatic anatomy
• ERCP / PTC - if biliary involvement suspected

Serology

• Battery of serologic tests (IHA, ELISA, immunoblot) - detect antibody response
• All plagued by low sensitivity and specificity
• ELISA for anti-echinococcal antibodies is most used

Laboratory

• Eosinophilia (peripheral blood)
• Casoni's skin test (intradermal) - historical, no longer standard
• Note: Aspiration is classically contraindicated due to risk of rupture and anaphylaxis (though PAIR technique is now accepted in select cases)

Treatment

Surgical (Treatment of Choice)

• Surgical excision/drainage of cyst with care to avoid rupture
• Pericystectomy - removal of entire pericyst
• When bile duct communication is present: suture repair ± ERCP postoperatively
• Laparoscopic drainage/unroofing: encouraging results
• Recurrence rate: 1-20% (generally <5% in experienced centers)

PAIR (Percutaneous approach)

• Previously contraindicated; now accepted in selected patients
• Comparable success rates to surgery in small trials

Medical (Adjunct)

• Albendazole or Mebendazole - shrinks cysts in many patients
• Cyst disappearance in <50% with E. granulosus
• Preoperative albendazole decreases risk of spillage (recommended before surgery)
• Medical therapy alone: only for widely disseminated disease or poor surgical candidates

Key Points for Exams (CP Baveja-style)

Note: CP Baveja's "A Textbook of Microbiology" is not in the library database, but the content above is fully consistent with what Baveja covers in the parasitology section on tissue cestodes. The classification, life cycle, cyst structure, and clinical features described are standard across all Indian microbiology textbooks including Baveja.