ANGIOSTRONGYLIASIS

Angiostrongyliasis is an infection caused by a few species of nematodes within the genus Angiostrongylus. While A. cantonensis was first described in rats from China in 1935, it is now known that the rat lungworm exists in rodents and molluskan intermediate hosts throughout scattered tropical areas of Southeast Asia, Australia and the Pacific islands, the Caribbean, and Africa. Human infections have been reported primarily from Taiwan, China, Thailand and Southeast Asia, Japan, Oceania, Tahiti, New Caledonia, West Africa, and Cuba. The rate of infection in rats in some regions may be as high as 85%. The first human infection was reported in 1944, in a Taiwanese boy who had meningitis and parasites in his cerebrospinal fluid. Eosinophilic meningitis and, less often, eye involvement are hallmarks of this disease.

Angiostrongylus costaricensis (also called Parastrongylus costaricensis) is a parasite of mesenteric arteries of rodents, especially rats, and is an occasional abdominal parasite in humans, primarily affecting the cecum and ileum and causing pain and radiographic findings that may sometimes simulate acute appendicitis or Crohn's disease. It has been reported from Costa Rica and other countries of Central America and rarely from scattered areas of South America and the Caribbean.

Angiostrongyliasis cantonensis

Angiostrongyliasis caused by A. cantonensis is primarily a disease of rodents, where adult worms live in their pulmonary arteries. Eggs deposited by female lung worms lodge in the smaller arterial vessels. First-stage larvae hatch and break out into the alveoli and then migrate to the trachea and pharynx. They are then swallowed and are passed in the host feces. Further development occurs when larvae are ingested by suitable immediate hosts, including slugs and both terrestrial and aquatic snails. Other invertebrates serve as intermediate hosts and further distribute the larval stage of the parasite. Within mollusks, the larvae develop and molt twice to become infective third-stage larvae. The life cycle is complete when the rodent or other definitive host eats infected mollusks, and third-stage larvae are released during transit through the rodent intestine. These larvae migrate to the brain and meninges, molting twice to become adults. This takes approximately one month, at which time young adults migrate to the pulmonary arteries, mate, and begin ovipostion.

Humans are accidental definitive hosts in whom the life cycle of the parasite is similar to that in rodents, except that larvae in humans do not mature to adults. Ingestion of raw or undercooked mollusks containing infective third stage larvae results in infection. The incubation period is approximately three weeks, but may be longer. As larvae penetrate into the central nervous system, they cause damage mechanically and also through inflammation of the brain parenchyma and meninges. Spinal cord involvement is uncommon. Wandering larvae and young adults are approximately 2 mm in length and 1 to 5 µm in diameter.

The commonest clinical presentation is headache, which may be intermittent initially, but soon becomes more persistent and severe with nausea and vomiting. Seizures and change in 1evel of consciousness are uncommon except in children or in severe infections. Fever, when present, is low grade and of short duration. Neck stiffness is present on examination in some patients and there may be cranial nerve involvement or paresthesias of the trunk and extremities. Ocular disease is uncommon, but occurs when larvae penetrate the globe and reside in the anterior chamber or vitreous. Pain, blepharospasm, iridocyclitis, optic neuritis, and retinal detachment have all been reported. Most patients recover completely, with the meningitis resolving first.

Autopsy studies in fatal cases show the meninges to be thickened by an inflammatory exudate. The underlying brain and spinal cord show focal petechial hemorrhages, with sections of the brain cortex showing necrotic tracks with surrounding inflammatory cells. Charcot-Leyden crystals in large numbers in the meninges are characteristic of the eosinophil-mediated killing of larval parasites.

Laboratory findings in patients show abnormal cerebrospinal fluid (CSF) which is turbid, having an increased white blood cell count and predominance of eosinophils. Developing larvae are sometimes isolated in the CSF. A mild peripheral blood eosinophilia is also seen. An ELISA serologic test is available for confirmation.

Only a few reports are available on imaging in patients with eosinophilic meningitis caused by A. cantonensis. The findings on CT and MRI are most often nonspecific. A small space-occupying lesion may be seen that can be ring-enhancing and show variable edema, or there may be a more diffuse abnormality with linear hyperintense foci on T2- or gadolinium-enhanced Tl-weighted images of the cerebral and cerebellar cortex . This reflects the variable pathology of CNS involvement. Chest radiographs in children during acute disease show patchy opacification in a segmental lower lobe distribution, unlike chronic eosinophilic pneumonia which often has an upper lobe predominance. An excellent review of this infectious disease is provided by Jindrak (1995).

Angiostrongyliasis (Parastronglylosis) costaricensis

The abdominal parasite Angiostrongylus (or Parastrongylus) costaricensis, was first described in 1971, although the disease had been known previously in Costa Rican children and has since been reported elsewhere in Central and South America, with rare single cases reported from the Democratic Republic of the Congo and the United States. Humans acquire this infection in the same manner as A. cantonensis but the third-stage larvae migrate to the mesenteric arteries instead of the CNS. The incubation period estimated by experimental and clinical data is approximately one to two months. This follows ingestion of infected mollusks directly or of their secretions containing infective larvae on vegetable foods. Female worms deposit eggs that wedge in capillaries in the bowel wall. These eggs hatch, allowing first-stage larvae to burrow through the gut wall and, in rodents, into the lumen. The larvae are then passed in the host feces to continue the life cycle in the intermediate host. As with A.cantonensis, humans are accidental definitive hosts, but unlike that lung worm, A. costaricensis does mature to the adult form in the human host. The ovipositing female is thus responsible for much of the pathology and clinical findings in this abdominal disease, since in humans it is the egg burden in the bowel wall and the associated inflammation that cause pathology, with few eggs hatching. The cecum is the most common site of involvement and both eggs and migrating larvae cause intense inflammatory reaction within the bowel wall.

Abdominal angiostrongyliasis (or parastrongylosis) occurs more often in the pediatric population, with right lower quadrant pain being the most common complaint, mimicing acute appendicitis or Crohn's disease. A tender mass may be palpated in the right iliac fossa, with symptoms including anorexia, vomiting, and fever. A moderate eosinophilia may be present and some patients may present with recurrent gastrointestinal bleeding. Intestinal perforation can occur due to transmural necrosis, the result of ischemia and small vessel thrombosis due to an eosinophilic obliterative arteritis.