The Occurrence of Angiostrongylus cantonensis in Frogs of New Caledonia with Observations on Paratenic Hosts of Metastrongyles

Abstract

Twenty-three of 43 frogs (Hyla aurea) from market gardens in Noumea, New Caledonia were found to harbor infective-stage larvae of Angiostrongylus cantonenisis in their tissues. The larvae were encapsulated in the wall of the alimentary tract, the mesentery, liver, and somatic musculature; they were infective for rats. Sea snakes (Laticauda colubrina) were experimentally infected with third-stage A. cantonensis larvae and in these hosts the larvae survived and remained infective for rats for at least 1 month. The use of paratenic hosts by A. cantonensis and other metastrongyles is discussed. Very little is known as to the extent and significance of the role played by paratenic hosts in the transmission of metastrongylid infections in nature. The Hobmaiers (1935) were the first to note the use of such hosts among the metastrongyles when they demonstrated that amphibians, reptiles, birds, and mammals could serve as experimental hosts for larvae of the cat lungworm, Aelurostrongylus abstrusus. Subsequently, it has been shown that other metastrongyle larvae will enter the tissues of various vertebrates that possibly may serve as paratenic hosts in nature (Gerichter, 1949; Seneviratna, 1959; Anderson, 1962). The discovery that Angiostrongylus cantonensis, a parasite of rats, may infect man and cause eosinophilic meningitis has served to focus attention upon the biology of this nematode in particular, and upon metastrongyles in general. Thus far, infectivestage A. cantonensis larvae have been reported in nature from such paratenic hosts as planarians (Alicata, 1962), shrimp (Alicata and Brown, 1962; Rosen et al., 1967) and land crabs (Alicata, 1964b). In addition to these natural infections there have been successful experimental infections of shrimp (Wallace and Rosen, 1966), fish (Wallace and Rosen, 1967) and pigs and calves (Alicata, 1964a). Observations reported in this paper further extend the paratenic host range for A. cantonensis to amphibians and reptiles. Received for publication 16 February 1968. * Present address: Division of Infectious and Tropical Diseases, School of Public Health, UCLA, Los Angeles, California 90024. RESULTS 1. Natural infection in frogs In the course of surveying market gardens in Noumea, New Caledonia for mollusks and other invertebrates harboring nematode larvae frogs infected with A. cantonensis were found on several occasions. Hyla aurea (Less.) was the frog collected. This species was originally introduced from Australia and is the only known species of frog to occur in New Caledonia. It is relatively small, 5 to 7 cm long, and may weigh from 5 to 27 g. Examination of the stomach contents of these amphibians revealed that while insects probably comprise the bulk of their diet the remnants of mollusk shells as well as the bodies of smaller frogs were not uncommon findings. From mid-1966 to April 1967 a total of 43 frogs (29 females, 14 males) were examined for infection with A. cantonensis. In each instance, except as subsequently noted, the entire frog was examined for the presence of larvae by digestion in 0.5% pepsin in 0.7% HC1 and by pressing the tissues between thick glass slides. The frog was killed, skinned and the mesentery was examined for the presence of encapsulated larvae. Selected organs and tissues were then removed and divided into four groups for digestion: (1) alimentary tract and mesentery, (2) liver, (3) rear legs, and (4) rest of the carcass. Following digestion for 2 to 3 hr the digests were allowed to sediment in a modified Baermann funnel for an additional 2 to 3 hr prior to examination for larvae. Tissues that were examined for larvae