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Helminthic Zoonoses: Echinococcosis & Trichinellosis

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ECHINOCOCCOSIS (Hydatid Disease)

Etiology

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Epidemiology

• Global distribution: highest prevalence in Eurasia, Africa, Australia, and South America, particularly in rural sheep-raising regions
• Community-based surveys show infection rates up to 6.6% in endemic areas
• In the US, ~200 cases/year, mostly imported; indigenous cases among Basque sheep farmers in western states and Native Americans in the southwest
• WHO estimates >1 million individuals infected worldwide; classified among 20 neglected tropical diseases (NTDs)

• Endemic in arctic and alpine zones of the Northern Hemisphere
• Highly endemic in western China, Tibet, central Asia, and alpine Europe
• Emerging in western Canada

• Definitive hosts: Dogs, wolves, foxes (harbor adult tapeworms in small bowel)
• Intermediate hosts: Sheep, cattle, camels, moose, caribou, rodents (harbor cysts in viscera)
• Human infection: Accidental — by ingesting Echinococcus eggs from dog/fox feces via contaminated hands, food, or soil
• After ingestion, eggs hatch → oncospheres penetrate intestinal wall → travel via portal blood → lodge in liver (first filter), then lungs, brain, bone, heart, kidney
• In the liver/lungs, larvae form cysts; in canines that eat infected viscera, scolices mature into adult tapeworms (3–5 mm, only 3 proglottids in E. granulosus)

• Pastoral: Most common; dogs fed raw sheep/cattle viscera; shepherds infected by handling dogs (eggs on fur → hands → ingestion)
• Sylvatic: Alaska and western Canada; wolves + moose/caribou

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Pathogenesis

• Cyst has three layers:
  1. Pericyst (adventitia): Host-derived fibrous outer layer
  2. Laminated layer: Acellular, outermost parasite layer
  3. Germinal (inner) layer: Produces brood capsules and protoscolices
• Cyst fills with clear fluid, brood capsules, protoscolices ("hydatid sand") and may develop daughter cysts
• Cyst grows slowly: ~1 cm/5–6 months; can exceed 10 cm over years
• Damage is primarily mechanical from cyst enlargement; rupture causes hypersensitivity reactions (urticaria, anaphylaxis) and dissemination

• Grows as a budding, infiltrative tumor-like mass rather than a contained cyst — hence the name "alveolar"
• Destroys liver tissue progressively; may disseminate hematogenously to lung and brain
• Fatal if untreated

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Signs and Symptoms

• Liver (70% of cases): Upper abdominal pain/discomfort, hepatomegaly, palpable mass, jaundice, biliary colic, portal hypertension, ascites, inferior vena cava compression
• Lung (2nd most common): Cough (salty taste, vomiting of hydatid membranes if rupture into bronchi), chest pain, hemoptysis, pleural effusion, pneumothorax, eosinophilic pneumonitis
• Brain: Seizures, paralysis
• Spine: Neurologic symptoms
• Bone: Pathologic fractures
• Heart: Cardiac mass, pericardial effusion, arrhythmias, ventricular rupture
• Rupture: Fever, pruritus, urticaria, anaphylactic shock, dissemination
• Biliary involvement: Mimics cholecystitis; obstructive jaundice

• Epigastric pain, hepatomegaly, obstructive jaundice
• Resembles liver cancer — progressive, tumor-like mass expanding over decades
• May metastasize to lung and brain
• Weight loss, malaise, eventual liver failure

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Diagnosis

• Ultrasound — procedure of choice; WHO classification defines 6 cyst stages in 3 groups:
  • Active (viable): CE1 (unilocular), CE2 (multivesicular with daughter cysts)
  • Transitional: CE3a ("water lily sign" — floating membranes), CE3b (solid with daughter cysts)
  • Inactive (nonviable): CE4, CE5 (solidification + calcification)
• CT — detects smaller cysts outside liver, precise localization; >50% of hepatic cysts show calcific rim on X-ray
• MRI — best for postsurgical residual lesions, cardiac, and extrahepatic sites; T2-weighted series preferred when ultrasound not feasible

• Indirect hemagglutination, latex agglutination, EIA (screen) → immunoblot (confirm)
• Sensitivity: ~90% for hepatic, ~60% for pulmonary cysts; lower for brain/splenic cysts
• Serology typically negative in inactive cysts (CE4/CE5); titers rise after treatment (cyst disruption releases antigen)
• Cross-reactivity with T. solium (cysticercosis) — immunoblot needed for confirmation

• Protoscolices (100–150 μm, with hooklets) and free hooklets ("hydatid sand") in aspirated/excised cyst material
• Histopathology: Laminated membrane + brood capsules = CE; infiltrative back-to-back brood capsules without thick laminated wall = AE

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Differential Diagnosis

• Liver: Hepatocellular carcinoma, liver abscess (pyogenic or amoebic), nonparasitic simple cyst, polycystic liver disease, cholangiocarcinoma
• Lung: Primary lung tumor, lung abscess, tuberculoma, aspergilloma
• Brain: Primary or metastatic brain tumor, neurocysticercosis, brain abscess
• Bone: Primary bone tumor, metastatic disease
• AE specifically: Hepatocellular carcinoma (very closely mimics on imaging)

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Treatment

• Inactive CE4 and CE5 cysts without complications can be monitored without intervention

• Drug of choice; oral 10–15 mg/kg/day continuously (without interruption) for ≥30 days
• Effective for small/medium CE1 and CE3a cysts; limited efficacy on cysts >10 cm or CE2/CE3b
• AE: Albendazole continued indefinitely (suppresses growth but rarely curative)
• After apparently curative AE surgery: albendazole for ≥2 years
• Adverse effects: hepatotoxicity, leukopenia, thrombocytopenia, alopecia; avoid in pregnancy

• For uncomplicated CE1 and CE3a cysts
• Scolicidal agent: hypertonic saline (preferred); anaphylaxis risk 0.03%, reversible allergic reactions 1.7%
• Must be combined with albendazole (≥30 days after puncture)
• CE2 and CE3b tend to relapse after PAIR

• Indications: complicated cysts (rupture, biliary fistula, compression of vital structures, superinfection, hemorrhage), CE2/CE3b, cysts at high risk of rupture, AE
• Perioperative albendazole (10–15 mg/kg/day): start 1 week before, continue 4 weeks after surgery
• Some add praziquantel (50 mg/kg/day) perioperatively
• AE may require liver transplantation if resection is incomplete
• Open, conservative, or laparoscopic techniques

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Prevention

• Deworm dogs regularly and dispose of infected carcasses/offal by burning or burial
• Prohibit feeding of raw viscera to dogs
• Handwashing after contact with dogs (especially in endemic areas)
• Control dog populations near sheep flocks
• Education of shepherds and hunters in endemic areas
• Vaccination of intermediate hosts (sheep) with EG95 vaccine has shown promise in some programs

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TRICHINELLOSIS (Trichinosis)

Etiology

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Epidemiology

• T. spiralis is enzootic worldwide in omnivorous/carnivorous animals (bears, boars, rats, pigs)
• US trend: Declined from 400–500 cases/year in the 1940s to ~10–20 cases/year currently, due to education, pig-raising regulations, and pork freezing/processing requirements
• Global burden: A review of 1986–2009 outbreaks documented 65,818 cases from 41 countries, with 42 deaths; 86% from Europe (nearly 50% from Romania pre-2000)
• Transmission routes:
  • Domestic cycle: Undercooked pork (most common worldwide) — pigs infected by eating raw meat scraps/rats
  • Sylvatic cycle: Wild game — bear, wild boar, walrus, horse (now the majority of US cases)
  • Dogs (Asia/Africa), horses (Italy/France)
  • Herbivores (cattle, horses) not natural hosts but implicated when contaminated with pork
  • Curing and smoking do not reliably kill larvae
• T. nativa (Arctic): present in walrus and bear meat; resistant to freezing

• Humans ingest encysted larvae in undercooked meat
• Gastric acids/proteases release larvae → invade small bowel mucosa → mature to adults in 2 days
• Adults embed in columnar epithelium; females (3 mm) produce newborn larvae within 5 days of mating; adults expelled by immune response within 3–5 additional weeks
• Newborn larvae (with a swordlike stylet) penetrate lamina propria → lymphatics/blood → migrate throughout body
• Larvae survive only in striated skeletal and cardiac muscle cells → transform muscle cells into "nurse cells" (unique to Trichinella) where they encyst and can survive for decades
• Calcification occurs eventually
• Humans are dead-end hosts — larvae cannot re-develop into adults in the same host

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Pathogenesis

• Severity correlates with worm burden: <10 larvae/g muscle → asymptomatic; >50 larvae/g → potentially life-threatening
• The number of encysted larvae ingested is the key determinant of disease severity
• Phase 1 (Intestinal): Larval release + adult worm invasion of intestinal mucosa → local inflammation → diarrhea, abdominal pain
• Phase 2 (Migration/Systemic): Newborn larvae circulate and invade striated muscle → marked local and systemic hypersensitivity: fever, eosinophilia, periorbital edema
• Phase 3 (Muscle encystment): Nurse cell formation → myositis, myalgias, weakness
• Cardiac involvement: larvae invade myocardium → myocarditis, arrhythmias, heart failure
• CNS: encephalitis (rare)
• Acquired immunity expels adult intestinal worms but has little effect on muscle-dwelling larvae

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Signs and Symptoms

• Diarrhea, abdominal pain, nausea, vomiting, constipation
• Self-limited; resolves within ~10 days

• Fever
• Periorbital and facial edema (classic sign)
• Eosinophilia (>90% of symptomatic patients; can exceed 50% at 2–4 weeks)
• Subconjunctival, retinal, and subungual ("splinter") hemorrhages
• Maculopapular rash
• Headache, cough, dyspnea, dysphagia
• Myocarditis (tachyarrhythmias, heart failure) — major cause of death
• Encephalitis, pneumonitis (uncommon but potentially fatal)

• Myalgias, muscle tenderness, weakness — inflammatory myositis
• Most commonly involved muscles: extraocular muscles, biceps, jaw, neck, lower back, diaphragm
• Elevated CPK and LDH
• Symptoms peak ~3 weeks post-infection; gradual recovery over months
• T. pseudospiralis infection (non-encapsulating): prolonged polymyositis-like illness

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Diagnosis

• Eosinophilia: >90% of symptomatic patients; peaks >50% at 2–4 weeks
• Elevated muscle enzymes: CPK, LDH (most symptomatic patients)
• Serology (anti-Trichinella antibodies): EIA (most common), latex agglutination, indirect immunofluorescence
  • IgG antibodies most sensitive
  • Usually detectable 2–5 weeks post-infection (may be negative in first 2 weeks)
  • Repeat testing may be needed
  • Cross-reactivity with other nematodes; immunoblot for confirmation
• Muscle biopsy (definitive): ≥1 g of deltoid or gastrocnemius (near tendon insertions); coiled larvae seen in squash preparations (fresh tissue compressed between glass slides); pepsin digestion improves visualization; larvae may be missed on routine histology sections alone
• PCR: Detects Trichinella DNA; allows species identification from biopsy

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Differential Diagnosis

• Periorbital edema: Angioedema, allergic reaction, nephrotic syndrome, orbital cellulitis
• Myositis/myalgias + eosinophilia: Polymyositis, dermatomyositis, eosinophilic myositis, fasciitis
• Eosinophilia + fever: Other helminthic infections (toxocariasis, fascioliasis, strongyloidiasis), drug hypersensitivity, hypereosinophilic syndrome
• Myocarditis: Viral myocarditis, other causes
• CNS involvement: Viral encephalitis, other parasitic CNS infections (neurocysticercosis)
• Intestinal phase: Acute gastroenteritis (bacterial, viral), food poisoning

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Treatment

• Albendazole/mebendazole are active against intestinal adult worms and prevent further larval release; efficacy against fully encysted muscle larvae is limited
• Giving benzimidazoles during systemic phase may worsen symptoms (dying larvae → exacerbated inflammatory response); combine with corticosteroids in severe cases
• Corticosteroids alone used for severe myositis and myocarditis

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Prevention

• Thorough cooking of pork and wild game until meat is no longer pink (internal temperature ≥63°C/145°F)
• Freezing at −15°C for 3 weeks or −20°C for at least 3 days kills T. spiralis in pork
  • Exception: T. nativa (Arctic species in walrus/bear) is resistant to freezing — cooking is the only reliable method
• Curing, salting, smoking, and drying are NOT reliable
• Regulations: Ban on feeding unprocessed meat scraps/garbage to pigs (enforced in US, Canada, EU) has dramatically reduced domestic pork transmission
• Meat inspection programs (including the ECDC-endorsed approach)
• Consumer education regarding wild game preparation

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Summary Comparison Table

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