Plerocercoid

Abstract: A technique has been elaborated that enabled the plerocercoid larvae of Schistocephalus solidus to be removed from the body cavity of Gasterosteus aculeatus without bacterial contamination. Larvae were cultured in plugged test-tubes under completely aseptic conditions in a variety of balanced salines, glucose salines and nutrient peptone broth. The most successful results were obtained with peptone broth at room temperatures (16-19° C) in which plerocercoids remained active and showed normal behaviour for periods up to 300 days. In ¾ strength Locke's solution, which was found by experiment to be approximately isotonic with Schistocephalus (δ = -0.44 ± 0.02° C), the mean period of normal behaviour was 114 days. In the remaining saline and saline-glucose media, the mean viability and period of normal behaviour was considerably less. In the plerocercoid, histological examination revealed that the genitalia are in an immature condition. During cultivation at room temperatures, the genitalia remained in this undifferentiated condition and showed no signs of undergoing spermatogenesis, oogenesis or vitellogenesis. Plerocercoids were induced to develop into sexually mature adults by raising the temperature of cultivation in peptone broth to 40° C. (i.e. the body temperature of the final host in the natural life cycle). Oviposition took place after 48-60 hr. at this temperature, and histological examination revealed that spermatogenesis, oogenesis, vitellogenesis and shell formation had taken place in a normal manner. The viability of artificially matured Schistocephalus was 4-6 days in vitro --a period equivalent to the viability of the adult in vivo . The eversion of the cirris was observed in each proglottid after 40 hr. cultivation at 40° C. During the sexual process the cirris everted and invaginated at the rate of about once per second. Cross-fertilization between segments of the same worm or with segments of another worm was not observed. Except for one specimen in ¾ strength Locke's solution which underwent spermatogenesis and partial vitellogenesis, larvae cultured in salines or glucose salines at 40° C. died within 1-3 days without further development. Attempts to hatch out the eggs produced by the cultivation of larvae in peptone broth at 40° C. proved unsuccessful. Histological examination revealed that spermatozoa had not been taken into the vagina. It was concluded that the eggs were not fertilized owing to the failure of normal copulation to take place.

Abstract: Summary 1A study of host-parasite relationships has been made on the cestode Schistocephalus solidus Muller by examining conditions relating to the infection and growth of the plerocercoid in the stickleback, and- its transfer to other hosts for maturation; the infection of Cyclops by the coracidimn with its subsequent change to a procercoid; and the passage of the procercoid into the body cavity of the fish to become a plerocercoid. It has been possible, by establishing suitable techniques, to carry out the whole life cycle in the laboratory and so facilitate these studies. 2Eggs were obtained from plerocercoids and observations made on their development. The time of hatching of the coracidium varies considerably, both with temperature and with intrinsic factors. The average time is about three weeks but hatching may occur up to six months after the eggs have been shed. 3From such eggs, free coracidia and, subsequently, procercoids in Cyclops and nauplii, were secured and studied. The growth rate of procercoids was determined together with the effect of infection on the growth of nauplii. 4Infections in fish were secured from infected Cyclops but not in sufficient numbers to justify more than tentative conclusions about the growth rate of plerocercoids. The time required for penetration of the fish gut wall by the procercoid can be as short as two hours. 5Changes in morphology of the worm in its transformation from a coracidium to a plerocercoid were surveyed and it was shown how the formation of the musculature precedes the initiation of strobilation which occurs rapidly and completely, followed immediately by the formation of the genital rudiments. 6The excretory system of the plerocercoid could be displayed effectively by a method of embedding in transparent plastic. 7Infected fish from one source were collected throughout the year and the weights of the fish and of the worms within them were ascertained. It was found that in the autumn there was an increase in the number of large fish in the marginal zone–possibly related to an increase of plankton–but during the months from winter to summer the mean size of fish was less and remained relatively stable in this zone. The mean weights of plerocercoids however, increased during this period as did the temperature of the water. Some indications of the growth rate of the plerocercoid were observed from the collected data. 8An examination of the stomach contents of the fish showed a wide variety of diet with little or no change throughout the year and also indicated that Cyclops form an almost insignificant feature in this diet.

Abstract: Infection of British freshwater fishes with the plerocercoid larva of Ligula intestinalis is most widespread and common in the roach. It is less common in gudgeon, rudd, bream, and minnow, and rare in dace. Roach fry may become infected within the first 3 months of life. Infections in roach exceeding 13 cm in length are infrequent. Data concerning the size distribution of plerocercoids in fish of different size are given. In infected roach, the liver weight, packed cell volume of erythrocytes and hemoglobin are reduced. Gonadal retardation is a universal consequence of infection. It is severe in roach, rudd, bream, dace, and minnow but is less marked in gudgeon. Plerocercoid larvae of pseudophyllidean cestodes of the family Ligulidae have been recorded from the body cavity of numerous species of freshwater fishes in many parts of the Northern Hemisphere. Early accounts concerning the genus Ligula were reviewed by Cooper (1918). He recognized only the single species L. intestinalis (L.) and listed 63 species of host fish from many different families. More recently, Russian workers have suggested that at least two genera and five species, including L. intestinalis, are involved and that the plerocercoid larva of each species is restricted to a relatively narrow range of host species (Bykhovskaya-Pavlovskaya et al. 1962). In Britain the plerocercoids of Ligula intestinalis (L.) have been previously recorded from the roach (Rutilus rttilus) (Baylis 1928, 1939; Smyth, 1947; Kerr, 1948), gudgeon (Gobio gobio) (Baylis, 1939), brown trout (Salmo trutta) (Baylis, 1939) and minnow (Phoxinus phoxinu s) (Archer, pers. comm.). The only other member of the family Ligulidae which has been found in British fish is Schistocephalus solidus (Muller), the plerocercoid larva of which is restricted to the stickleback (Gasterosteus aculeatus). Despite numerous records of plerocercoids of Ligula intestinalis parasitizing fish, there have been relatively few accounts of the incidence and intensity of the infections encountered. Kosheva (1956) and Zitnian (1964) have provided data concerning infections in bream (Abramis brama) and roach respecReceived for publication 20 November 1967. * Present address: Department of Biology, Rice University, Houston, Texas. tively in Eastern Europe; Pitt and Grundmann (1957) described infections of the yellow perch (Perca flavescens) in the USA, and Van Cleave and Mueller (1934) discussed its incidence in the fishes of Oneida Lake, New York. The presence of Ligula intestinalis pleroc rcoids has been shown by many workers to be associated with certain pathological effects in the fish host (see Dogiel et al., 1961). Most of these reports originate in the USSR where ligulosis presents a serious problem in p sciculture. Blood changes associated with L. intestinalis infection, in particular an increase in the monocyte and polymorphonuclear leukocyte counts, have been described in the gudgeon by Sadkovskaya (1953) and in the crucian carp (Carassius carassius) by Shpolyanskaya (1953). The latter author also noted a fall in blood hemoglobin; a similar decrease was observed by Kosheva (1956) in bream infected with Ligula intestinalis and with the related species Digramma interrupta. Kosareva (1961) found that in roach, bream, and white bream (Blicca bjoernka) infected with L. intestinalis and D. interrupta the liver glycogen levels tended to be lower than in noninfected fish. Kosheva (1956) stated that infections with either species of tapeworm tended to depress the fat content of bream, and Pitt and Grundmann (1957) found that the rate of growth of yellow perch infected with L. intestinalis was lower than that for noninfected individuals. A commonly noted feature of infection with L. intestinalis plerocercoids is the inhibition of host gonad development. The most specific references to this condition are those of Kerr (1948), Kirschenblat (1951), and

Abstract: To determine whether suppression of GH and/or PRL in the presence of normal growth rates would affect the timing of puberty onset, weanling rats were infected with plerocercoid larvae of the tapeworm Spirometra mansonoides. Plerocercoid growth factor, which is released by S. mansonoides, stimulates growth while depressing endogenous GH. Vaginal opening and ovulation were significantly delayed in infected animals compared to those in untreated or vehicle-injected controls (P < 0.01). Although growth rates were comparable in all three groups, plerocercoid-treated (10 plerocercoids/rat) animals showed significantly depressed circulating PRL levels at 26 days of age but not at 32 days of age, and depressed GH levels at both ages. Vaginal opening and, to a lesser extent, ovulation could be normalized in the plerocercoid-treated animals by the administration of PRL or GH or 24–26 days of age. The effects of plerocercoids and hormonal replacement were not dependent upon the adrenals. Despite normal body weight g...