Angiostrongyliasis is an infection by a nematode from the Angiostrongylus genus of kidney and alimentary tract roundworms. Angiostrongyliasis is infection with Angiostrongylus cantonensis (cerebral) or Angiostrongylus costaricensis (abdominal). Both are parasites of rats that can infect humans.
A. cantonensis and its vectors are endemic to Southeast Asia and the Pacific Basin. The infection is becoming increasingly important as globalization allows it to spread to more and more locations, and as more travelers encounter the parasites. The parasites probably travel effectively through rats traveling as stowaways on ships, and through the introduction of snail vectors outside endemic areas.
Although mostly found in Asia and the Pacific where asymptomatic infection can be as high as 88%, human cases have been reported in the Caribbean, where as much as 25% of the population may be infected. In the US, cases have been reported Hawaii, which is in the endemic area. The infection is now endemic in wildlife and a few human cases have also been reported in areas where the parasite was not originally endemic, such as New Orleans and Egypt.
A. cantonensis has many vectors, with the most common being several species of snails, including the giant African land snail (Achatina fulica) in the Pacific islands and snails of the genus Pila in Thailand and Malaysia. The golden apple snail, A. canaliculatus, is the most important vector in areas of China. Freshwater prawns or crabs can also act as vectors.
Humans are infected by the 3rd stage larvae, either by eating undercooked intermediate hosts or by consuming vegetables. The incubation period in humans is usually from 1 week to 1 month after infection.
The larvae migrate to the gray matter of the brain, where they molt before ultimately lodging in a pulmonary artery. After maturation, the worms lay eggs in the lung that then hatch. Larvae are coughed up, swallowed, and passed in the feces. The life cycle is completed when larvae infect snails, which in turn are eaten by humans.
Infection first presents with severe abdominal pain, nausea, vomiting, and weakness, which gradually lessens and progresses to fever, and then to CNS symptoms and severe headache and stiffness of the neck.
The CNS symptoms begin with mild cognitive impairment and slowed reactions, and in very severe cases can progress to unconsciousness. Patients may present with neuropathic pain early in the infection.
Occasionally patients present with cranial nerve palsies, usually in nerves 7 and 8, and rarely larvae will enter ocular structures.
The diagnosis of Angiostrongyliasis is based on the demonstration of A. cantonensis in cerebrospinal fluid, but is complicated due to the difficulty of presenting the angiostrongylus larvae themselves. Diagnosis will therefore usually be made based on the presence of eosinophilic meningitis and history of exposure to snail hosts. Eosinophilic meningitis is generally characterized as a meningitis with >10 eosinophils/uL in the CSF or at least 10% eosinophils in the total CSF leukocyte count. Occasionally worms found in the cerebrospinal fluid or surgically removed from the eye can be identified in order to diagnose Angiostrongyliasis.
Lumbar puncture should always be done is cases of suspected meningitis. In cases of eosinophilic meningitis it will rarely produce worms even when they are present in the CSF, because they tend to cling to the end of nerves. Larvae are present in the CSF in only about 2-10% of cases. However, as a case of eosinophilic meningitis progresses, intracranial pressure and eosinophil counts should rise. Increased levels of eosinophils in the CSF is a trademark of the eosinophilic meningitis.
Brain lesions, with invasion of both gray and white matter, can be seen on a CT scan or MRI. However MRI findings tend to be inconclusive, and usually include nonspecific lesions and ventricular enlargement. Sometimes a hemorrhage, probably produced by migrating worms, is present and of diagnostic value.
In patients with elevated eosinophils, serology can be used to confirm a diagnosis of Angiostrongylias rather than infection with another parasite. There are a number of immunoassays that can aid in diagnosis, but serologic testing is available in few labs in the endemic area, and is frequently non-specific. Some cross reactivity has been reported between A. cantonensis and trichinosis, making diagnosis less specific.
The most definitive diagnosis always arises from the identification of larvae found in the CSF or eye, however due to this rarity a clinical diagnosis based on the above tests is most likely.
Treatment of angiostrongyliasis is not well defined, but most strategies include a combination of antiparasitics to kill the worms, steroids to limit inflammation as the worms die, and pain medication to manage the symptoms of meningitis.
Anti-helminthics are often used to kill off the worms, however in some cases this may cause patients to worsen due to toxins released by the dying worms. Albendazole, ivermectin, mebendazole, and pyrantel are all commonly used, though albendazole is usually the drug of choice. Studies have shown that anti-helminthic drugs may shorten the course of the disease and relieve symptoms. Therefore anti-helminthics are generally recommended, but should be administered gradually so as to limit the inflammatory reaction.
Anti-helminthics should generally be paired with corticosteroids in severe infections to limit the inflammatory reaction to the dying parasites. Studies suggest that a two week regimen of a combination of mebendazole and prednisolone significantly shortened the course of the disease and length of associated headaches without observed harmful side-effects. Other studies suggest that albendazole may be more favorable, because it may be less like to incite an inflammatory reaction. The Chinese herbal medicine long-dan-xie-gan-tan (LDGXT) has also been shown to have a similar anti-inflammatory effect, and in mild cases may be used alone to relieve symptoms while infection resolves itself.
Symptomatic treatment is indicated for symptoms such as nausea, vomiting, headache, and in some cases, chronic pain due to nerve damage or muscle atrophy.
Large nodules composed of scar tissue, eggs, and larvae cause lower right quadrant pain. Surgical excision of the nodules, together with the appendix, relieves these symptoms.
In cases of eye infection, the worms can sometimes be removed surgically.
Transmission of the parasite is usually from eating raw or undercooked snails. Infection is also frequent from ingestion of contaminated water or unwashed salad that may contain small snail and slugs, or have been contaminated by them. Infective larvae emerge from the snails or slugs on the slime trail; humans become infected by eating material contaminated with the slime trail. Therefore it is very important to avoid raw snails, wash and cook vegetables thoroughly, and avoid open water sources that may be contaminated.
To avoid infection when in endemic areas, travelers should:
At the time of writing (2009) there are no vaccines in development for angiostrongyliasis.
Frequently the infection will resolve without treatment or serious consequences, but in cases with a heavy load of parasites the infection can be so severe that even with treatment there will be death or permanent damage to the CNS resulting in a variety of negative outcomes depending on the location of the infection, as well as chronic pain.
In humans, Angiostrongylus is the most common cause of eosinophilic meningitis: Migrating infective larvae or young adult worms die in and around blood vessels and arteries in the brain and provoke an eosinophilic meningoencephalitis, with a low mortality (less than 1%).