Tetrahymenid ciliates as parasites of the gray garden slug

Abstract

The role of two species of holotrichous ciliates, Tetrahymena limacis and T. rostrata, as parasites of the gray garden slug, Deroceras reticulatum, was examined. Infectivity tests with each ciliate species demonstrated that T. rostrata was pathogenic for D. reticulatum, producing subacute, lethal infections. Infection with T. limacis was not fatal. The only noticeable effect was a significant difference in the size and weight of the newly hatched slugs that had been exposed to a heavy concentration of ciliates. Histological examination of naturally and experimentally infected slugs revealed that T. limacis was confined to the lumen of the alimentary tract, primarily to the lumina of the liver lobules. No distinct cytopathologic effects were observed. In contrast, infection with T. rostrata was marked by extensive damage and inflammation, particularly in the kidney and other pulmonary organs and tissues. Ciliates were shown to enter a dorsal integumental pouch posterior to the mantle of the slug and penetrate to the loose, connective tissue of the body wall. Upon migration to the kidney via the venous system, the ciliates rapidly multiplied and eventually invaded most of the other slug tissues and organs. Through parasitization of the albumen gland, T. rostrata was incorporated into eggs during their formation in the genital tract of D. reticulatum. A study of ciliare-infected eggs revealed that T. rostrata was transmitted trans-ovum. Embryos became infected when ciliates were apparently ingested during the aspiration of albumen. While some of the infected embryos freed themselves of ciliates after hatching, the others succumbed within the eggs or died soon after hatching. Six new host records were discovered for T. limacis, and three were discovered for T. rostrata in the course of observations of slug parasitization in the field. Mixed infections were also found, and the free-living phase of each ciliate species was recovered from soil samples. Ancillary observations of Colpoda steinii confirmed the general opinion that this ciliate is a harmless edaphic species, which occasionally occurs in the intestinal tracts of various snails and slugs.