Angiostrongyliasis caused by infection with the tissue nematode Angiostrongylus cantonensis (A. cantonensis) and is also know as ‘angiostrongyliasis of the nervous system’ and neuroangiostrongyliasis.’ Angiostrongyliasis is often associated with human eosinophilic meningitis and may alternatively be attributed to the tissue nematodes Angiostrongylus mackerrasse (A. mackerrasse), Angiostrongylus costaricensis (A. costaricensis), or Angiostrongylus malayiensis (A. malayiensis) in Australia, Costa Rica, and Malaysia in place of A. cantonensis, respectively.
History of Discovery
Angiostrongyliasis cantonensis was discovered in rats in Canton, China by Chen in 1944, and human infection with the lungworm was recognized by Nomura and Lin in Taiwan in 1945 when A. cantonensis was recovered from the cerebral spinal fluid of a fifteen-year-old boy with acute meningitis.
Known animal reservoirs of the agents causing Angiostrongyliasis include members of the rodent and marsupial animal groups. The most common reservoirs within these groups are rats and bandicoots, large, rat-like, insectivorous marsupials found in Australia and its neighboring islands. The vectors responsible for the transfer of Angiostrongylus agents from non-human host to non-human host are terrestrial and mollusks and freshwater crustaceans that may become infected by ingestion of infected mollusks. One of the most common vectors, often ingested raw in endemic regions, is the Giant African Land Snail (pictured above from http://ww.maine.gov/agriculture/pi/currentissues/GALS.htm).
The incubation period for A. cantonensis refers to the period of time between the ingestion of infected food and the onset of neurological symptoms. This incubation period varies widely from person to person and has been observed as lasting from three and thirty-six days. The vast majority, eighty-eight percent of cases were observed to have incubation periods between six and twenty-five days, with an average incubation period of 16 days.
A. cantonensis Morphology
Adult Angiostrongylus nematodes are slender worms that can grow to be up to 25mm long. First stage larvae are, on average, 0.27mm long and 0.014mm wide, while third stage larvae have mean dimensions of 0.557mm long and 0.025mm wide.
Adult A. cantonensis worms are found in the tissue of the pulmonary artery and right heart chamber of their definitive hosts, the rodents or marsupials. There, female worms lay eggs that grow into first stage larvae and eventually hatch in the lung vessels. These larvae proliferate in the vessels and migrate from the respiratory system to the pharynx, and from there pass into the gastrointestinal tract and exit the body in feces. These infected feces may then be ingested by an intermediate host, a mollusk. Larvae from the feces molt twice within the mollusk to develop into second and third stage larvae.
Definitive hosts first become infected with A. cantonensis after ingesting mollusks harboring third stage larvae. Once inside the rodent host, the third stage larvae migrate from the gastrointestinal system, into the circulatory system, and finally to the brain. In the brain the larvae molt two more times becoming fourth stage larvae and fifth stage larvae, otherwise known as the young adult worm. The young adult worms mature in the brain for approximately ten days causing symptoms associated with the infection and eosinophilic meningitis before migrating to the pulmonary arteries to achieve full maturity and restart the cycle.
Transmission to Humans
Humans are accidental or incidental hosts of A. cantonensis and may become infected after ingesting infected mollusks or crustaceans, the paratenic hosts, that are raw or improperly cooked. Humans may also be infected by ingesting fresh vegetables contaminated with the worm by carnivorous planarians that have fed on infected snails or slugs. Alternatively, vegetables may also be infected with the liquids secreted by the mollusks that are not removed by proper washing before human ingestion.
Angiostrongyliasis is often benign and runs a self-limiting course, but is characterized by abrupt onset of symptoms associated with eosinophilic meningitis, such as stiff neck, headache, and sensorial changes. Other common symptoms include nausea and vomiting, and fever throughout the infection, and abdominal pain, malaise, and constipation early on in the infection. Occasionally, a young adult worm may migrate from the brain into the eye, causing pain, and abnormalities associated with vision.
The two factors that contribute most greatly to the diagnosis of angiostrongyliasis is a history of ingestion of known hosts in endemic regions and evidence of antibodies and antigens in cerebral spinal fluid or eyes. There are four main diagnostic tests that can be used in order to detect these things.
• enzyme-linked immunoabsorbent assay (ELISA): detects serum antibodies against the antigen prepared from the parasite. Serum is considered to be positive if it has more than twice the normal antibody levels. False negatives are not uncommon.
• spinal tap: patient assumes sitting position for a minimum of 30 minutes prior to puncture with a 19 or 21 gauge needle while in a flat position. Initial fluid is observed for presence of parasites.
• others: often produce inconsistent and/or unreliable results
- indirect immunofluorescent antibody (IFA):
- indirect hemagglutination (IHA):
- counterimmunoelectrophoresis (CIE):
Management and Therapy
Despite the fact that there is no specific therapy for the Angiostrongyliasis because of its short course, there are a number of treatments for symptoms.
• analgesics and corticosteroids: alleviate radicular symptoms and headache
• spinal tap: reduces intracranial pressure and associated headache
• general and neurologic care: for patients suffering from complications such as infections
• thiabendazole: anthelminthic drug determined to have insignificant effect on clinical course of angiostrongyliasis
• mebendazole: 100mg, twice daily for five days for anthelminthic treatment
Angiostrongyliasis is widely distributed in regions where the rodents and marsupials known to carry the infection are known to exist. The highest numbers of human cases are found in Taiwan, Thailand, and in the Pacific Islands. The lowest numbers of human infections are found Vietnam, Malaysia, Indonesia, Japan, and Cuba, with virtually no cases reported in North America.
The demographics of patients most frequently affected vary with country. In Tahiti, adults are affected more frequently than children, and the sexes have equal rates of infection, while in Thailand, males are nearly three times as likely to become infected as females and the majority of cases occur in individuals who are between the ages of twenty and thirty-nine. Alternatively, in Taiwan, the vast majority of cases, eighty percent, are children under the age of twelve who play with or eat raw Giant Africa Land Snails during the months of high rainfall when they are most abundant.
Angiostrongylus cantonensis infections have been most frequently in the tropics and subtropics. The infection has been reported in rats without evidence of human infection in New Orleans and Egypt, and with human infection in countries as diverse as American Samoa, China, Hawaii, Indonesia, the Ivory Coast, Japan, Malaysia, South Vietnam, Tahiti, Taiwan, Thailand, Vanuatu, and Vietnam with unconfirmed cases in Australia, Cuba, Hong Kong, Mauritius, New Caledonia, Papa New Guinea, the Philippines, Puerto Rico, and Réunion, and other Pacific islands.
Since Angiostrongyliasis can easily be prevented by thorough cooking and washing of foods, public health prevention and education measures include education on proper food preparation techniques, and control of infection mollusks. Other prevention strategies may include reduction of rat populations.
Useful Web Links
Hung, Tsu-Pei and Chen, Eng-Rin. “Angiostrongyliasis (Angiostrongylus cantonensis)” Handbook of Clinical Neurology: Microbial Disease 52 (1988): 545 – 62.
Markell, E Edward K., John, David T., and Krotoski, Wojciech A. Markell and Voge’s Medical Parasitology, Eighth Edition. Philadelphia: W.B. Saunders Company, 1999.