Fisheries play a crucial role in aquatic ecosystems, a resource with ecological and economic importance. Measuring parasite prevalence is necessary to assess these fish species\' health and well-being. Within the Sistan region, this study sought to estimate the prevalence and intensity of infection with plerocercoid Ligula intestinalis (L. intestinalis) in seven fish species and identify factors such as fish weight, seasonality, and fishing location that might influence infection rates. 2800 fish from seven species were obtained in the Sistan region throughout four seasons. Fish samples from three weight categories (<200gr, 200-400gr, 400gr<) were examined parasitologically. Ligula intestinalis was detected in 96/2800 (3.4%) of fishs. There was a variation in prevalence among fish species. Alburnus charusini (A. charusini), Hypophthalmichthys molitrix (H. molitrix), Hypophthalmichthys nobilis (H. nobilis), and Schizocypris altidorsalis (S. altidorsalis) showed no infection. Schizothorax zarudnyi (S. zarudnyi) showed a prevalence of 1.5% (6/400), Cyprinus carpio (C. carpio) 15% (60/400), and Ctenopharyngodon idella (C. idella) 7.5% (30/400). It was found that infection prevalence was significantly influenced by fish weight in C . carpio and C . idella. A higher prevalence was found in fish with lower weight. Infection rates significantly fluctuated with season across species, but fishing locations had no significant difference. The present study highlights the significant prevalence of L. intestinalis infections in fish populations within Iran\'s Sistan region, underscoring the need for ongoing monitoring and research to inform effective fishery management strategies. Future studies should focus on long-term monitoring efforts and consider the broader global context of fish parasitology to understand parasite-host interactions better and inform conservation measures worldwide.
UNASSIGNED:

Three subspecies of the ringed seal (Pusa hispida) are found in northeastern Europe: P. h. botnica in the Baltic Sea, P. h saimensis in Lake Saimaa in Finland, and P. h. ladogensis in Lake Ladoga in Russia. We investigated the poorly-known cestode helminth communities of these closely related but ecologically divergent subspecies using COI barcode data. Our results show that, while cestodes from the Baltic Sea represent Schistocephalus solidus, all worms from the two lakes are identified as Ligula intestinalis, a species that has previously not been reported from seals. The observed shift in cestode communities appears to be driven by differential availability of intermediate fish host species in marine vs. freshwater environments. Both observed cestode species normally infect fish-eating birds, so further work is required to elucidate the health and conservation implications of cestode infections in European ringed seals, whether L. intestinalis occurs also in marine ringed seals, and whether the species is able to reproduce in seal hosts. In addition, a deep barcode divergence found within S. solidus suggests the presence of cryptic diversity under this species name.

There have been several studies on heavy metals in Lake Naivasha. However, none of them has reported the levels of mercury (Hg), arsenic (As), and chromium (Cr). Moreover, there are no studies on the heavy metals\' concentrations in the straightfin barb (Enteromius paludinosus, Peters 1852), a fish species that hosts a parasite (Ligula intestinalis), the latter having been reported to have a high ability to absorb heavy metals from its host. This paper therefore addresses the accumulation of heavy metals, namely arsenic (As), chromium (Cr), lead (Pb), and mercury (Hg) in the tissues of straightfin barb, Enteromius paludinosus (Peters 1852) from the mouth of River Malewa in Lake Naivasha, Kenya. A total of 1307 fish were collected during the month of November 2017. Water samples, sediment samples, 25 fish muscle tissues, and its endoparasite, the cestode Ligula intestinalis, were isolated, and heavy metal concentrations were determined using the thermal-electron atomic absorption spectrophotometer at the Lake Nakuru Water Quality Testing Laboratory. The concentrations of heavy metals in the sediment were below the lowest effect level in sediment, threshold effect concentration in sediment, severe effect concentration in sediment, and the shale values of sedimentary rocks thus showing no sign of pollution. In the muscle tissues of the fish, As, Cr, Pb, and Hg showed high levels with mean concentrations of 5.0696, 22.0854, 45.2108, and 1.5458 mg/kg ww, respectively. Bioconcentration factors further supported the observation that trace element accumulation was higher in fish compared with sediment and water. The target hazard quotients of As, Cr, Pb, and Hg obtained for both the female and male were > 1 indicating a possible health risk associated with the consumption of E. paludinosus. The bioaccumulation factors (BAFs) for L. intestinalis were 2.4093, 2.1873, 5.8601, and 5.1395 for As, Cr, Pb, and Hg, respectively, indicating the potential of the cestode in the accumulation of heavy metals from the host; hence, it can be used as an accumulation bioindicator.

BACKGROUND: The position of Digramma interrupta remains disputable as it was raised by Cholodkovsky from Ligula alternans. This study aimed to survey the evolutionary relationships and the taxonomic position of D. interrupta and L. intestinalis. It also intended to support or reject the validity of D. interrupt as an independent genus and its correlation with L. intestinalis on the basis of their morphological characteristics and a study on molecular data.
METHODS: Overall, 1301 fish varieties, including 883 Alburnoides bipunctatus and 418 Abramis brama, were collected from north and north-western parts of Iran. A. bipunctatus samples were obtained from fresh water sources of the Maragheh dam (northwest) and the Ramesar Lake (north). Moreover, samples of A. brama were captured from the Aras Dam (northwest) and the Bandar-e-Anzali lagoon (north). PCR was used to generate a fragment spanning two independent ITS-inclusive parts: ITS1-5.8S and ITS2 with two pairs of primers.
RESULTS: Nucleotide variation between L. intestinalis and D. interrupta samples amounts to about 3% to 7%. Between samples of L. intestinalis and GenBank data, and also between D. interrupta specimens and GenBank data, the diversity was seen for about 1% to 3%. Moreover, about 1% to 4% nucleotide variation was seen only in L. intestinalis samples caught from the same host, which could be supplementary to the presence of a species and/or strains in this genus.
CONCLUSIONS: Maybe D. interrupta was just a rare diplogonadic form of the Ligula species, not a different genus and not synonymous with the Ligula genus, but only another species of the Ligula genus.

Ligula intestinalis is a tapeworm using copepods and cyprinid fish as intermediate hosts and fish-eating birds as final hosts. Since some parasites can increase their own fitness by manipulating the behavior of the intermediate host, we explored if this parasite affected predator avoidance, swimming activity and depth preference of the fish intermediate host, Engraulicypris sardella. We found that when L. intestinalis had reached a developmental stage that is able to establish in the bird host, it had a significant impact on E. sardella behavior, while the tapeworm that was not fully developed had little effect and fish hosts showed a behavior more similar to uninfected fish. These results are discussed with respect to two different processes: the manipulation hypothesis and the energy drain hypothesis.

Freshwater ecosystems are often impacted by biological invasions, including the introduction of exotic parasites capable of infecting native species. Here, we report the occurrence of the introduced tapeworm Ligula sp. from common bully, Gobiomorphus cotidianus, and quinnat salmon, Oncorhynchus tshawytscha, in Lake Hawea, South Island, New Zealand. This parasite has a complex life cycle, reaching its adult stage in fish-eating birds. Worms recovered from the body cavity of fish hosts reached huge sizes (60-300 mm long); however, their low prevalence in fish populations suggests that infections are rare or localised. Molecular analysis (internal transcribed spacer (ITS)1 and ITS2 sequences) confirms that these specimens belong to the genus Ligula and suggests tentative routes of invasion into New Zealand. Monitoring the spread of this parasite is important, as it can impact fish populations and also, when infection levels are high, those of piscivorous birds.

We describe here the mass death of predatory carp, Chanodichthys erythropterus, in Korea induced by plerocercoid larvae of Ligula intestinalis as a result of host manipulation. The carcasses of fish with ligulid larvae were first found in the river-edge areas of Chilgok-bo in Nakdong-gang (River), Korea at early February 2016. This ecological phenomena also occurred in the adjacent areas of 3 dams of Nakdong-gang, i.e., Gangjeong-bo, Dalseong-bo, and Hapcheon-Changnyeong-bo. Total 1,173 fish carcasses were collected from the 4 regions. To examine the cause of death, we captured 10 wondering carp in the river-edge areas of Hapcheon-Changnyeong-bo with a landing net. They were 24.0-28.5 cm in length and 147-257 g in weight, and had 2-11 plerocercoid larvae in the abdominal cavity. Their digestive organs were slender and empty, and reproductive organs were not observed at all. The plerocercoid larvae occupied almost all spaces of the abdominal cavity under the air bladders. The proportion of larvae per fish was 14.6-32.1% of body weight. The larvae were ivory-white, 21.5-63.0 cm long, and 6.0-13.8 g in weight. We suggest that the preference for the river-edge in infected fish during winter is a modified behavioral response by host manipulation of the tapeworm larvae. The life cycle of this tapeworm seems to be successfully continued as the infected fish can be easily eaten by avian definitive hosts.

BACKGROUND: The tapeworm Ligula intestinalis (Diphyllobothriidea) is one of the most fascinating cestode parasites because it may cause parasitic castration of its second intermediate host, teleost freshwater fishes, due to inhibition of production of fish gonadotropic hormones. Large-sized (length up to 1 m) larvae called plerocercoids develop several months in the body cavity of freshwater fish and affect host behavior to facilitate transmission to the final host, a fish-eating bird. Vitellogenesis, i.e. formation of vitellocytes, is a key process in formation and nutrition of female gametes, oocytes in many flatworms, mainly parasitic Neodermata. The present study provides the first ultrastructural evidence in flatworms (Platyhelminthes) of the process that is interpreted as cytoplasmic-like cell death, i.e. a special case of programmed cell death (paraptosis) in vitellocytes of L. intestinalis.
RESULTS: As molecular markers for paraptosis are not yet available, its identification was based on morphological criteria. Electron microscopy analyses revealed evident structural changes in vitellocytes associated with progressive cytoplasmatic vacuolation, swelling of the granular endoplasmic reticulum and mitochondria. In addition, the present study has shown that vitellocytes of L. intestinalis share numerous features in common with the members of other earliest evolved eucestodes.
CONCLUSIONS: The present study indicates that paraptotic-like cell death may occur in parasitic flatworms (Neodermata). The presence of GER-bodies in mature vitellocytes indicates close relationship between the Diphyllobothriidea, Caryophyllidea and Spathebothriidea, which are considered as the earliest evolved groups of the Eucestoda. Beyond the general similarities, however, a number of differences exist between the morphology, chemical composition and amount of these inclusions which could be due to the variations in their embryonic development, life cycle strategies and definitive host groups.