Histopathological effects of the acanthocephalan Leptorhynchoides thecatus in the ceca of the green sunfish, Lepomis cyanellus.

Abstract

Green sunfish (Lepomis cyanellus) possess an alimentary canal that has seven pyloric ceca, some or all of which may be parasitized by the acanthocephalan Leptorhynchoides thecatus. The microscopic anatomy of ceca with worms and the microscopic anatomy of ceca without worms in parasitized fish were compared to each other and to that of ceca from unparasitized fish. Parasites occluded ceca and caused significant distention (two-tailed Student's t-test, P < 0.05). Frequently, the proboscis was positioned in the lamina propria, but sometimes attachment was in the muscularis mucosa. Occasionally, the cecal wall was perforated. Epithelium was destroyed at the site of attachment and cell debris occurred in the lumen. The abundance of goblet cells in the mucosa of infected ceca was significantly greater than in ceca without worms whether in parasitized or unparasitized fish. Circular and longitudinal muscle layers in the muscularis mucosa were significantly thicker in parasitized than in unparasitized ceca. Additionally, muscle layers in unparasitized ceca of infected fish were significantly thicker than those in uninfected fish, revealing an effect of parasitism more general than necrosis at the site of attachment. Numerous papers have reported pathological effects of acanthocephalans on their vertebrate definitive hosts. Most of these papers described granulomas, necrosis, inflammation, and occasional perforation of the gut wall. Quantitative data that compared infected with uninfected individuals are scanty. Bullock (1967) found no difference in the number of mucous cells, granular cells, or rodlet cells in uninfected mosquito fish, Gambusia affinis (Baird & Girard, 1853), and those infected with Octospiniferoides chandleri Bullock, 1957. Although necrosis occurred at the attachment site of the parasites, the mucosal epithelium was normal immediately outside of the wounds (Bullock, 1967). Changes in addition to those at the attachment site were described in rats infected with Moniliformis moniliformis (Bremser, 1819). The diameter of the intestine and the thickness of its muscle layers were greater throughout in infected than in uninfected rats (Singhvi & Crompton, 1982). Structural damage at the point of attachment by Leptorhynchoides thecatus (Linton, 1891) was described in largemouth bass, Micropterus salmoides (Lacepede, 1802), by Venard & Warfel (1953) and in smallmouth bass, Micropterus dolomieui Lacepede, 1802, by Esch & Huffines (1973), but effects of infection 1 This research was conducted while I. de Buron was a recipient of a Lavoisier stipend from the French Ministere de La Recherche et de l'Industrie. Dr. Vincent A. Connors offered helpful comments and assistance. 2 Present address: Division of Biological Sciences, University of Montana, Missoula, Montana 59812, U.S.A. TRANS. AM. MICROSC. SOC., 113(2): 161-168. 1994. ? Copyright, 1994, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.51 on Sat, 18 Jun 2016 05:44:46 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. were not quantified. Furthermore, effects of parasitism by L. thecatus, other than necrosis at attachment sites, have not been reported. Following initial establishment in the anterior part of the intestine and pyloric ceca of green sunfish (Lepomis cyanellus Rafinesque, 1819), L. thecatus localizes in the pyloric ceca (Uznanski & Nickol, 1982). The number of lymphocytes, the number of goblet cells, the diameter of the cecum, and the thickness of layers in the muscularis mucosa were recorded for ceca containing worms and compared with values for the same characteristics in ceca without worms in infected fish and in uninfected fish. MATERIALS AND METHODS Green sunfish were seined from an oxbow lake in the Elkhorn river drainage of Holt County, Nebraska. Fish were killed immediately by severing the spinal column. Digestive tracts were removed and injected with Bouin's solution in alcohol. After 24 h of fixation, ceca from four fish infected with Leptorhynchoides thecatus and four uninfected fish were prepared for serial sectioning. All fish were 9.6 (+0.5) cm standard length. Serial transverse sections (7 Aum) were stained with Masson's trichrome solution, modified by substituting toluidine blue for fast green. Because histological features do not vary along a single cecum or among ceca of uninfected fish (Williams & Nickol, 1989), parasitized and unparasitized ceca were selected randomly for examination. For each cecum examined, the diameter, thickness of longitudinal (outer) and circular (inner) muscle layers of the muscularis mucosa, and the numbers of lymphocytes and goblet cells were determined. Blood cells were identified according to the descriptions of Jakowska (1956), Bullock (1963), Harder (1975), Ellis (1977), and BastideGuillaume (1986). In parasitized ceca, measurements and counts were made at the edge of the trunk of the acanthocephalan and at the point of attachment of the proboscis. The thickness of muscle layers was measured with an ocular micrometer at four points in each transverse section observed. The diameter of the cecum was taken at the greatest diameter of the section being examined. Counts of lymphocytes and goblet cells were made along a 100-p,m length of the supranuclear portion of the mucosal epithelium in folds not damaged by the parasite. Although this investigation emphasized study of ceca, qualitative observations were made along the intestine of one infected fish. Comparisons between means were made by two-tailed Student's t-tests and significance was accepted at P < 0.05.