Scanning Electron Microscopy of an Opecoelid Cercaria and Its Encystment and Encapsulation in an Insect Host

Abstract

Cercariae, metacercariae, and metacercarial cysts of an opecoelid cercaria resembling the larva of Allopodocotyle lepomis were studied by scanning electron microscopy to determine changes which occur in surface structure during the transformation of cercaria to metacercaria and during early metacercarial development. In the cercaria, 2 types of ciliated sensory organs, papillae and pits, are situated in rows around the suckers and on the body surface. The oral and ventral suckers bear tubercles and are bordered by long, cytoplasmic extensions which constitute a hairlike fringe. Cercarial sensory organs, tubercles, and fringe disappear within 12 to 30 hr after penetration of mayfly naiads, but a second type of pitted sensory organ appears after 7 days. Microvilli appear on the surface of the metacercaria 2 to 4 hr after penetration but disappear 5 to 7 days later. A cyst wall of parasite origin is formed 12 to 24 hr after penetration and hemocytes attach to it to form the host capsule. The hemocytes are nearly spherical on attachment but later become flattened. The sucker tubercles and fringe probably function in attachment and penetration while the microvilli increase the adsorptive surface of growing metacercariae and may function in cyst wall formation. Study of parasite surfaces facilitates understanding of the host-parasite interface. Scanning electron microscopy has been used to study Schistosoma mansoni sporocysts (Hansen and Perez-Mendez, 1972), cercariae (Hockley, 1968; Robson and Erasmus, 1970; Short and Cartrett, 1973), and adults (Miller et al., 1972; Silk et al., 1970). It has also been employed in the study of rediae and cercariae of Parorchis acanthus by Rees (1971), cercariae of Zoogonoides viviparus by Koie (1971b), and sporocysts of Cercaria buccini by Koie (1971a). The present study deals with larval surface structure, tegumental changes during the transition from cercaria to metacercaria, and the topography of the newly encysted metacercaria and the host capsule. MATERIALS AND METHODS Opecoelid cercariae were obtained from naturally infected river snails, Nitocris dilatatus, collected Received for publication 19 September 1974. * This report is based on a portion of a dissertation submitted by S-j. L. to the Graduate School of West Virginia University in partial fulfillment of the requirements for the Ph.D. degree. The investigation was supported by NSF Research Grant GB-23536 and a grant from the West Virginia University Medical Corporation. t Present address: Department of Microbiology, University of Maryland, College Park, Maryland 20742. from Laurel Fork River, Randolph County, West Virginia. Snails were placed in finger bowls containing stream water, and infected snails were selected on the basis of cercarial shedding. The cercaria used in this study possesses 6 or 7 pairs of cephalic glands and a stylet 16 to 18 um long. It is morphologically identical to the cercaria of Allopodocotyle lepomis (Plagioporus 1.) as described by Dobrovolny (1939), except that he reported 5 pairs of cephalic glands, and it has been tentatively identified as that species. Metacercariae were obtained from mayfly naiads, Litobrancha recurvata, exposed to 150 to 300 cercariae at 22 to 24 C for 1 to 12 hr. Naiads held longer than 12 hr were maintained in water over a mud substrate at 15 C. Unencysted or encysted metacercariae were removed from the hemocoel 2 hr to 2 weeks after infection. The naiads were collected from a small stream in Preston County, West Virginia, which lacked operculate snails. Cercariae and metacercariae recovered from mayfly naiads 4, 12, 30, 60 hr and 7 and 14 days after exposure to infection were fixed in cold (4 C) or hot (60 to 70 C) 3% glutaraldehyde in 0.1 xi phosphate buffer (pH 7.3) for 15 min and transferred to cold, fresh fixative for 15 min. Worms were then washed for 30 min in 0.1 M phosphate buffer and dehydrated through an ethanol series and amyl acetate. Dehydrated worms were subjected to critical point drying (Anderson, 1951), coated with carbon and goldpalladium, and viewed with a Cambridge S4-10 scanning electron microscope. Measurements are in micrometers ( tm).