JOURNAL OF INVERTEBRATE PATHOLOGY 43, 197-206 (1984) A Picornavirus-like Pathogen of Cotylogaster occidentalis (Trematoda: Aspidogastrea), an IntestinaC Parasite of Freshwater Mollusks HON S. IP AND SHERWINS. DESSER Department of Zoology, University of Toronto, Toronto, Ontario M5S IAI, Canada Received March 7, 1983; accepted May 31, 1983 Nonoccluded, icosahedral picomavirus-like (PVL) particles, 23 nm in diameter, forming para- crystalline arrays were seen in the cytoplasm of various cells in Cotylogaster occidentalis. Viral inclusions were visible in live specimens and in sections prepared for light and electron microscopy. All worms examined over a 2-year period were found to be infected. Infections were naturally acquired and susceptibility was not associated with any particular developmental stages. Devel- opment of viral inclusions involved an increase in the inclusion volume, progressive accumulation and condensation of materials into the interior of the inclusions, and formation of multilamellar membrane networks. Virus particles were observed in the stroma of the inclusions in association with multilamellar spherical ‘bodies. Mature PVL particles aggregated into polygonally shaped paracrystalline arrays. When such arrays occurred in the surface tegument, local disruption of the tegumentary membrane may liberate these particles into the environment. PVL particle production did not exhaust glycogen content of infected cells and did not appear to affect short-term survival of the parasite outside the molluscan host. KEY WORDS: picomavirus-like particles; Cotylogaster occidentalis; virus; trematode. INTRODUCTION Small RNA viruses have been reported from bacteria, plants, invertebrates, and vertebrates. Despite the wide range of po- tential hosts, they have not been reported from invertebrates other than arthropods (Brown and Hull, 1973). Particles or inclu- sion bodies of a suspected viral nature have been reported from the parasitic platyhel- minths. Mueller and Strano (1974a) found particles resembling C-type viruses in the excretory duct of Sparganum proliferum, an unusually proliferative larval cestode in man, and suggested that a viral infection may explain the “metatastic” growth of this parasite. The viral transformation hy- pothesis was abandoned, however, when Mueller and Strano (1974b) found similar particles in normal spargana of Spirometra mansonoides in various animal hosts. Daly et al. (1975) showed that normal spargana from man also contained C-type particles, C-Type particles were absent from various species in the related families Cyclophyl- lidea (Dougherty et al., 1975) and Caryo- phyllidea (Edwards and Mueller, 1978).The viral etiology of the C-type particles is questionable, however, as they did not take up nucleic acid precursors [3H]thymidine or [3H]uridine (Dougherty et al., 1975). Possible viral particles in intranuclear in- clusions, with center-to-center spacing of 200 A, have been reported by Beverly- Burton and Sweeny (1972) from various tis- sues of the trematodes Quinqueserialis quinqueserialis and Notocotylus urba- nensis. Tubular virus-like inclusions, 350 A in diameter and over 3.5 pm long, were present in the cecal epithelial cells of a di- genean, Paragonimus keflicotti (Byram et al., 1975).Dike (1971)found paracrystalline arrays of virus-like particles in the gastroin- testinal epithelium of a single specimen of the blood fluke, Schistosoma mansoni. In addition, virus-like particles have been re- ported in the nucleus and cytoplasm of gland cells of free-living (Reuter, 1975) and 197 0022-201 l/84 $1.50 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.