Page 117 - Medical Parasitology_ A Textbook ( PDFDrive )

110 10 Nematodes: Roundworms

Other Types of Nematodes Infecting Humans

Angiostrongylus cantonensis

Common name Rat lungworm

Distribution
Angiostrongylus cantonensis causes eosinophilic meningoencephalitis (cerebral
angiostrongyliasis) in humans. Cases have been reported in Taiwan, Thailand,
Indonesia, the Pacific islands, India, Egypt, Cuba and the USA

Habitat
In human, larvae cause infection in the brain. The adult worm is present in the
branches of pulmonary artery in rats.

Morphology
Adult worm is about 20 mm long. Eggs of Angiostrongylus resemble those of
hookworms.

Life Cycle
Rats are its natural host. Intermediate hosts are mollusks, slugs and snails. The infective
form is the third-stage larvae. Adults in the branches of pulmonary artery of rats produce
eggs which hatch in the lungs of rats and the larvae which migrate up the trachea are
swallowed and expelled in the faeces. The larvae infect mollusks, slugs and snails. Crabs
and freshwater prawns have been found to be carrier hosts. The larva undergoes 2 moults
in the intermediate host. In about 2 weeks, the infective third stage larvae develop, which
can survive in the body of the intermediate host for about a year. Rats become infected
when they eat the mollusks. In the rat, the larvae penetrate the gut wall to enter the venules
and are carried in circulation to the brain, where they develop into young adults in about
a month. These penetrate the cerebral venules and reach the pulmonary artery, where they
lodge, mature and start laying eggs. Human infection is acquired by eating infected mol-
lusks and other intermediate hosts containing the third stage larvae. Infection may also
occur through ingestion of raw vegetables contaminated by the mucus of infected mol-
lusks. The larvae penetrate the gut and are carried to the brain, but in human, they are
unable to develop further. They die and induce an inflammatory reaction in the brain and
meninges to produce meningoencephalitis. The incubation period is about 2–3 weeks.

Pathogenesis and Clinical Features
Angiostrongylus is the most common cause of eosinophilic meningitis in humans.
Patients present with intense headache, fever, neck stiffness, convulsions and pare-
ses. The worm may also cause eye invasion. In severe infection, it can cause damage
to the CNS.

Diagnosis
Peripheral eosinophilia, high cerebrospinal fluid (CSF) eosinophilia. Larvae and
immature worms may be seen in CSF.
History of exposure to snail hosts helps in diagnosis.