Natural Infection of Angiostrongylus Cantonensis in Malaysian Rodents and Intermediate Hosts, and Preliminary Observations on Acquired Resistance

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Abstract
Summary Angiostrongylus cantonensis was found in rats in the Kuala Lumpur area of Malaya. Infection rates are reported for field rats (Rattus argentiventer, R. jalorensis, and R. exulans); the common house-rat (R. r. diardi); the seaport rat (R. norvegicus); and to a lesser degree certain species of primary rainforest rats (R. muelleri, R. bowersi, and R. annandalei). The parasite was most common in field rats trapped in an oil-palm estate, but it also was commonly found in these hosts captured in scrub, high grass, ricefields, mangrove swamps, and in villages. Rarity of the worm in primary forest rats is a phenomenon attributed to ecological and distributional patterns of mollusk and rodent hosts and to a natural resistance in certain forest rats. Third-stage larvae of A. cantonensis were found in land slugs (Microparmarion malayanus, Girasia peguensis and Laevicaulis alte), in land snails (Macrochlamys resplendens and Achatina fulica), and in certain fresh-water snails (Pila scutata, Bellamya ingallsiana and Indoplanorbis exustus). No infection was found in 774 freshwater shrimp examined (Macrobrachium sp.), nor did feeding experiments succeed with these potential intermediate hosts. Experiments in white rats and a laboratory strain of Rattus r. jarak have shown that they develop a strong immunity after repeated infection with small numbers of worms. Naturally infected R. jalorensis also showed a strong resistance to reinfection. Attempts were unsuccessful to establish normal infections in the forest rats R. rajah and R. whiteheadi in the laboratory. Only a few stunted worms, restricted to the brain, were found after several exposures. These two species have not been found infected in nature. Natural immunity in white mice and acquired immunity in R. muelleri (a ground-dwelling forest rat) are also characterized by development of stunted abnormal worms in contrast to normal but fewer worms in immunized white rats and R. jalorensis.