Nematode Zoonoses - ANGIOSTRONGYLIASIS

SYNONYMS:

Angiostrongylosis, eosinophilic meningitis or meningoencephalitis (A. cantonensis), abdominal angiostrongylosis (A. costaricensis).

ETIOLOGY:

Two metastrongylids, Angiostrongylus (Morerastrongylus) costaricensis and A. cantonensis, are the etiologic agents. The first species is responsible for abdominal angiostrongyliasis, and the second one for eosinophilic meningitis or meningoencephalitis.  The definitive hosts of both species are rodents; man is an accidental host. Both species require mollusks as intermediate hosts for the completion of their life cycle. The main definitive host of A. costaricensis is the cotton rat, (Sigmondon hispidus), in which the adult nematode lodges in the mesenteric arteries and their branches on the intestinal wall. The first-stage larva emerges from eggs laid in the arteries, penetrates the intestinal wall, and is then carried with the fecal matter to the exterior. In order to continue their development, the first-stage larvae have to be ingested by a slug,(Vaginulus ameghini), in which they change successively into second- and third-stage larvae. When the infective third-stage larva is ingested by a rodent, it seeks the ileocecal region, where it penetrates the intestinal wall and locates in the lymphatic vessels (both inside and outside the abdominal lymph nodes). In this location the larvae undergo two molts before migrating to their final habitat, the mesenteric arteries of the cecal region. Oviposition begins after about 18 days. and the first-stage larvae appear in the feces 24 days after infection (prepatent period). In man, an accidental host. the parasite can reach sexual maturity and produce eggs, but the eggs usually degenerate, causing a granulomatous tissue reaction. The development cycle of A. cantonensis is similar to that of A. costaricensis. The intermediate hosts are various species of land snails, slugs, and freshwater snails. The definitive hosts can become infected by ingesting infected snails, or plants and water contaminated by them with the third larvae. In addition, infection can occur as a result of consuming transfer hosts (paratenic hosts), such as crustaceans, fish, amphibians, and reptiles, which in turn have eaten infected mollusks (primary intermediate hosts). The definitive hosts of A. cantonensis are primarily various species of the genus Rattus. When they enter a rat's body, the third-stage larvae (which developed in a mollusk) penetrate the intestine and are carried by the circulatory system to the brain, where they undergo two more molts and become young adult parasites. From the cerebral parenchyma they migrate to the surface of the brain. They remain for a time in the subarachnoid space and later migrate to the pulmonary arteries, where they reach sexual maturity and begin oviposition. The eggs hatch in the pulmonary arterioles, releasing the first larva, which migrates up the trachea, is swallowed, and is eliminated with the feces. Mollusks are infected by ingesting fecal matter of infected rodents. In man, who is an accidental host, the larvae and young adults of A. cantonensis generally die in the brain, meninges, or medulla oblongata. The nematode can occasionally be found in the lungs.

GEOGRAPHIC DISTRIBUTION AND OCCURRENCE:

Abdominal angiostrongyliasis, caused by A. costaricensis, is a parasitosis described a few years ago in Costa Rica; it is one of the most recently recognized zoonoses. Human disease has also been confirmed in Honduras, El Salvador, and Brazil. Suspected clinical cases have occurred in Nicaragua and Venezuela. In Panama, the adult parasite was found in five species of rodents belonging to three different families. In the past few years, the parasite has been found in several specimens of Sigmodon hispidus (hispid cotton rat) in Texas, USA. Ozyomys caliginosus (rice rat) in Colombia; and slugs in Guayaquil, Ecuador. The parasitosis is probably much more widespread than is currently recognized. A. costaricensis has not been recorded outside the Americas. Human cases of parasitism by A. cantonensis have occurred in Thailand, Vietnam, Kampuchea, the Philippines, Indonesia, Taiwan, Japan, Australia, and several Pacific islands. The parasite is much more widely distributed, and its existence in rats has been confirmed in southern China, India, Malaysia, Sri Lanka, Madagascar, Mauritius, and Egypt. Until recently, the geographic distribution of A. cantonensis was thought to be limited to Asia, Australia, the Pacific islands, and Africa. However, in recent years its presence has been confirmed in Cuba, where infected rats (Rattus norvegicus) and mollusks have been found; likewise, five human cases of meningoencephalitis have been attributed to A. cantonensis in that country. It is believed that the parasite was introduced to the island some years ago by rats from a ship from Asia. In a study carried out on rat species (R. norvegicus, R. rattus, and R. exulans) on the Hawaiian and Society Islands, the parasite was found in more than 40% of the specimens captured. In Egypt, 32.7% of 55 specimens of R. norvegicus harbored the parasite. In the province of Havana, Cuba, 12 out of 30 captured R. norvegicus were infected. In view of the worldwide distribution of R. norvegicus and R. rattus, these rodents were examined for the parasite in Puerto Rico, London, and New Orleans, but the results were negative. Eosinophilic meningitis associated with infection by A. cantonensis has been recorded in several hundred patients in endemic areas.

THE DISEASE IN MAN:

The clinical manifestations of abdominal angiostrongyliasis caused by A. cantonensis are moderate but prolonged fever, abdominal pain on the right side. and, frequently, anorexia, diarrhea, and vomiting. Leukocytosis is characteristic (20,000 to 50,000 per mm3), with marked eosinophilia (11 to 82%). Palpation sometimes reveals tumoral masses or abscesses. Rectal examination is painful, and a tumor can occasionally be palpated. Lesions are located primarily in the ileocecal region, the ascending colon. appendix, and regional lymph nodes, but they are also found in the small intestine. Granulomatous inflammation of the intestinal wall can cause partial or complete obstruction. Appendicitis was the preoperative

DIAGNOSIS:

In 34 cases, all but two of the children survived and recovered. The highest prevalence (53%) was found in children 6 to 13 years old, and twice as many boys as girls were affected. Ectopic localizations may occur; when the liver was affected in some Costa Rican patients, the syndrome resembled visceral larva migrans. Serologic studies carried out in Australia, in human populations living in localities where the infection occurs in rats and those living in other places where it does not, indicate that many human infections are asymptomatic.

THE DISEASE IN ANIMALS:

In rodents, A. costaricensis produces lesions that are located primarily in the cecum, as well as focal or diffuse edema of the subserosa, a reduction in mesenteric fat, and swelling of the regional lymph nodes. In highly parasitized animals, eggs and larvae may be found in various viscera of the body. No significant difference in weight between parasitized and nonparasitized animals has been confirmed. Rats infected by A. cantonensis may show consolidation and fibrosis in the lungs. However, the physical appearance of the animals does not reflect the degree of pathologic changes. For both parasites, the prevalence of the infection is greater in adult than in young rodents.

SOURCE OF INFECTION AND MODE OF TRANSMISSION:

Several species of rodents are known to serve as definitive hosts of A. costaricensis: Sigmodon hispidus, Rattus rattus, Zygodontomys microtinus, Liomys adspersus, 0ryzomys fulvescens, and 0. caliginosus; also, natural infection has been found in a coati (Nasua narica) and marmosets (Saguinus mystax). In a study carried out in Panama, the highest prevalence of the infection was found in the cotton rat, S. hispidus, which was also the most abundant rodent in the six localities studied. The cotton rat inhabits areas close to dwellings in both tropical and temperate America. It is omnivorous, feeding on both plants and small vertebrate and invertebrate animals, including slugs (V. ameghini). All these facts indicate that the cotton rat is a prime reservoir and that it plays an important role in the epidemiology of the parasitosis. Rodents are infected by ingesting infected mollusks. Another probable source of infection is plants contaminated with mollusk secretions ("slime") containing third-stage infective larvae of the parasite. The manner in which man contracts the infection is not well known. Infection probably occurs by ingestion of poorly washed vegetables containing small slugs or their secretions. It is believed that children can become infected while playing in areas where slugs are abundant by transferring snail secretions found on vegetation to their mouths. An increase in cases in children occurs in Costa Rica during the rainy season, when slugs are most plentiful. Humidity is an important factor in the survival of both the first- and third-stage larvae in the environment, since they are susceptible to desiccation. The parasite species in the Far East (A. cantonensis) has been found in at least ten different species of the genus Rattus and in Bandicota indica and Melomys littoralis. These rodents, natural definitive hosts, are infected by consuming mollusks or paratenic hosts that harbor third-stage larvae. The infection rate of the mollusks is usually high; both the prevalence and the number of larvae an individual mollusk can harbor vary according to the species. Man, who is an accidental host, is infected by consuming raw mollusks and also paratenic hosts such as crustaceans or fish. The ecology of angiostrongyliasis is closely related to the plant community, since it ultimately supports the appropriate mollusks and rodents. The frequency of the human parasitosis depends on the abundance of these hosts and the degree to which they are infected, and, also, in the case of A. cantonensis, on eating habits (consumption of raw mollusks, crustaceans, and fish).

DIAGNOSIS:

The spinal fluid characteristically shows elevated protein and an eosinophilic pleocytosis. Occasionally, the parasite can be recovered from spinal fluid. Peripheral eosinophilia with a low-grade leukocytosis is common. A serologic test is available from the Centers for Disease Control and Prevention; its sensitivity and specificity are not established. CT and MRI may show a central nervous system lesion.

TREATMENT:

No specific treatment is available; however, levamisole, albendazole, thiabendazole (25 mg/kg three times daily for 3 days), or ivermectin can be tried. Symptomatic treatment with analgesics or corticosteroids may be necessary. The illness usually persists for weeks to months, the parasite dies, and the patient then recovers spontaneously, usually without sequelae. However, fatalities have been recorded.

CONTROL:

At least theoretically, angiostrongyliasis could be controlled by reducing rodent and mollusk populations. Preventive measures at the individual level consist of washing vegetables thoroughly, washing hands after garden or field work, not eating raw or undercooked mollusks and crustaceans, and not drinking water that may be unhygienic.