Parastrongy

• Parastrongylus cantonensis

  Rat Lung Worm

  
• Parastrongylus cantonensis

  • First identified by Chen in Canton, China in 1935 from domestic rats
  • Definitive Host: Rats
  • Intermediate Host: Snails or Slugs
  • Paratenic Host: Crabs, freshwater shrimps
  • Accidental Host: Humans
  • Has lung migration

  
• First human infection documented by Nomura and Lin in Taiwan in 1945

  
• Documented in at least 30 countries
• Worldwide distribution
• Adult Male

  • 16-22mm by 0.25-0.35mm
  • Posterior end: well-developed caudal bursa: kidney-shaped and single lobed
• Adult Female

  • 19-33mm by 0.28-0.50mm
  • Uterine tubules wound spirally around the intestine observed through the transparent cuticle: Barber's pole pattern
  • Blunt posterior end
  • Lay 15,000 eggs per day
• Ova

  • 46-48um by 68-74um
  • Elongated ovoidal eggs with delicate hyaline shells
  • Unembryonated when oviposited
  • Looks like the ova of a hookworm
  • Unembryonated egg in the environment
• Rhabditiform Larva

  • Found in lungs of rodents
  • Distinct small knob near the tip of the tail
• Filariform Larva

  • Two well-developed chinitous rods below the buccal cavity
  • Rods have expanded knob-like tips
• Life Cycle


  1. Rat ingest L3 from environment (rare for humans to acquire in the environment) and the L3 will infect the rats in the pulmonary arteries of the rat and it will mature to male and female Parastrongylus cantonensis

  2. It will then go to the intestine and lay eggs. The eggs will be passed out in the fecal material of the rats

  3. The eggs will undergo embryonation and eaten by snail or slug

  4. It will molt inside the snail/slug and eventually to L1

  5. In cases of humans, they can harbor this parasite through eating snails (infective stage: L3), or eating crabs shrimp with encysted larvae

  6. The larva will be ingested by humans and reach small intestine, but will not develop to adult male or female. It will only go to the circulation and will go to different parts of the body, eyes, lungs and other visceral parts of the body

  7. When humans eat the definitive host, nothing will happen because it is already an adult

  
• Humans


  • Ingestion of snails, slugs
  • Ingestion of paratenic hosts: crabs, freshwater shrimps
• Eosinophilic meningoencephalitis


  • BRAIN: primary site of infection
  • Eosinophils on CSF
  • Acute, severe, intermittent occipital or bitemporal headache (increase of intracranial pressure)
  • Stiffness of neck, paresthesia, vomiting, fever, nausea, blurred vision, diplopia, myalgia, fatigue
  • Confusion, incoherence, disorientation, memory lapses, coma
  • Intraocular hemorrhage, blindness
• Eosinophilic Enteritis


  • eosinophilic inflammation of the mesenteric arterioles of the ileocecal region mimicking appendicitis
  • Can cause abdominal pain
• Inflammatory Reaction and Local tissue necrosis

  • due to dead worms
  • Local tissue necrosis causes the toxin release when the worms die

  
• Relatively difficult to diagnose since the primary site of infection is the brain
• Usually the manifestations are neurologic
• Possible outbreak when humans eat snails/slugs or undercook crabs and fresh water shrimp

  
• Presumptive diagnosis

  • Travel and exposure history
  • Clinical symptoms
  • Medical history
  • Laboratory findings
  • Brain imaging
  • Serological tests
• Blood Tests

  • Eosinophilia, elevated eosinophils in CSF similar with other CNS parasitism
• CT Scan

  • Cerebral edema, hydrocephalus, meningeal lesions
• No specific tests in terms of ancillary test in this parasitic infection as well as the manifestations because it has the same manifestation as other causative agents – that it is why it is difficult to diagnose

  
• Treatment

  • No antihelminthic treatment
  • Mebendazole and albendazole may be effective
  • Surgical removal of the worm (eyes)
  • Prednisone in severe cases of cranial nerve involvement
  • Analgesics and lumbar puncture for headache
  • Resolve spontaneously: adult worm cannot survive for a long time inside human body

  
• Prevention and Control

  • Awareness and education on proper eating habits and food preparation: discourage eating raw or poorly cooked mollusks or unwashed vegetables
  • Hand washing after gardening
  • Use of molluscicides: Metaldehyde or iron phosphate food bait pellets – kills snails and slugs
  • Copper barriers against snails and slugs