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The tapeworm Ligula intestinalis occurs in the body cavity of its cyprinid second intermediate host, in this study the roach Rutilus rutilus, and inhibits host gonadal development. The mechanism by which infected fish are prevented from reproducing is unknown. Comparison of parameters, such as body length and weight, and condition factor and age, between infected and uninfected individuals, indicated only minor effects of parasitism on growth and condition. In contrast, seasonal gonadal development, as observed in uninfected fish, did not occur in infected fish, and gonads remained small and blocked at the primary oocyte stage in female roach. As immature ovaries and testes are still present, the parasite is presumed to act upon the brain -pituitary -gonadal axis of the fish to inhibit further development of reproductive organs. We investigated the Ligula/fish interaction at the level of the pituitary gland by determination of gonadotrophin (LH) content using a heterologous RIA for carp (Cyprinus carpio) LHb subunit. The results indicated that the pituitary glands of infected roach contained approximately 50% less LH than non-infected fish. After the cloning and sequencing of roach LHb subunit, we measured roach LHb mRNA levels by real-time RT-PCR. A corresponding 50% reduction in LHb mRNA pituitary levels was determined. These results reflect a significant and measurable effect of parasitism on the pituitary gland, and lend support to the hypothesis that excretory/secretory products released from the parasite interact with the brain -pituitary -gonadal axis of the fish host and thus inhibit gonadal development. Reproduction (2005) 130 939-945 IntroductionPrevious studies have revealed that several parasitic infections can affect host reproduction. For example, Joose & van Elk (1986) noted that Trichobilharzia ocellata induces gigantism and the cessation of egg production in its molluscan host, a snail Lymnea stagnalis, and Crews & Yoshino (1989) observed that Schistosoma mansoni similarly suppresses reproduction and gonadal growth in another snail species, Biomphalaria glabrata. Furthermore, in vertebrate hosts, Taenia taeniaeformis appears to directly affect the testis in the rat (Lin et al. 1990) and Taenia crassiceps induces feminisation in infected mice (Larralde et al. 1995).Ligula intestinalis, which is found in the body cavity of certain cyprinid fish, inhibits reproduction in both male and female fish. The gonads, however, are present but remain in an immature state, irrespective of fish age or season. Although this phenomenon has been reported several times (e.g. Arme & Owen 1968, Mahon 1976, Sweeting 1977, Bean & Winfield 1989, the mechanism of the action of this parasite remains unknown. Previous studies have indicated effects of infection at the pituitary gland level. Kerr (1948) and Arme (1968) noted that in ligulosed roach, Rutilus rutilus, the putative gonadotrophs are much reduced in number, compared with non-ligulosed individuals, are only lightly granulated, and have an irregular nucle...

Abstract. The pathological effects of Bothriocephalus acheilognathi on the gut of experimentally infected carp, Cyprinus carpio L., have been studied. Light and electron microscopical observations revealed that damage to the host gut was not restricted to the area engulfed by the bothria, and loss of gut microvilli and the separation and shedding of enterocytes occurred against the site of attachment. Lymphocytes, macrophages and eosinophils accumulated in the wall of the infected gut and migrated into the lumen where they adhered to the parasite tegument. The role of parasite‐derived and host‐produced factors in leucocyte migration and attachment to the worm are discussed.

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