Metacercariae

BackgroundAdult Clonorchis sinensis live in the bile duct and cause clonorchiasis. It is known that the C. sinensis metacercariae excyst in the duodenum and migrate up to the bile duct through the common bile duct. However, no direct evidence is available on the in vivo migration of newly excysted C. sinensis juveniles (CsNEJs). Advanced imaging technologies now allow the in vivo migration and localization to be visualized. In the present study, we sought to determine how sensitively CsNEJs respond to bile and how fast they migrate to the intrahepatic bile duct using PET-CT.Methodology/principal findingsCsNEJs were radiolabeled with (18)F-fluorodeoxyglucose ((18)F-FDG). Rabbits with a gallbladder contraction response to cholecystokinin-8 (CCK-8) injection were pre-screened using cholescintigraphy. In these rabbits, gallbladders contracted by 50% in volume at an average of 11.5 min post-injection. The four rabbits examined were kept anesthetized and a catheter inserted into the mid duodenum. Gallbladder contraction was stimulated by injecting CCK-8 (20 ng/kg every minute) over the experiment. Anatomical images were acquired by CT initially and dynamic PET was then carried out for 90 min with a 3-min acquisition per frame. Twelve minutes after CCK-8 injection, about 3,000 (18)F-FDG-labeled CsNEJs were inoculated into the mid duodenum through the catheter. Photon signals were detected in the liver 7-9 min after CsNEJs inoculation, and these then increased in the whole liver with stronger intensity in the central area, presenting that the CsNEJs were arriving at the intrahepatic bile ducts.ConclusionIn the duodenum, CsNEJs immediately sense bile and migrate quickly with bile-chemotaxis to reach the intrahepatic bile ducts by way of the ampulla of Vater.

Human clonorchiasis has been increasingly prevalent in recent years and results in a threat to the public health in epidemic regions, motivating current strategies of vaccines to combat Clonorchis sinensis (C. sinensis). In this study, we identified C. sinensis paramyosin (CsPmy) from the cyst wall proteins of metacercariae by proteomic approaches and characterized the expressed recombinant pET-26b-CsPmy protein (101 kDa). Bioinformatics analysis indicated that full-length sequences of paramyosin are conserved in helminthes and numerous B-cell/T-cell epitopes were predicted in amino acid sequence of CsPmy. Western blot analysis showed that CsPmy was expressed at four life stages of C. sinensis, both cyst wall proteins and soluble tegumental components could be probed by anti-CsPmy serum. Moreover, immunolocalization results revealed that CsPmy was specifically localized at cyst wall and excretory bladder of metacercaria, as well as the tegument, oral sucker and vitellarium of adult worm. Both immunoblot and immunolocalization results demonstrated that CsPmy was highly expressed at the stage of adult worm, metacercariae and cercaria, which could be supported by real-time PCR analysis. Both recombinant protein and nucleic acid of CsPmy showed strong immunogenicity in rats and induced combined Th1/Th2 immune responses, which were reflected by continuous high level of antibody titers and increased level of IgG1/IgG2a subtypes in serum. In vaccine trials, comparing with control groups, both CsPmy protein and DNA vaccine exhibited protective effect with significant worm reduction rate of 54.3% (p<0.05) and 36.1% (p<0.05), respectively. In consistence with immune responses in sera, elevated level of cytokines IFN-γ and IL-4 in splenocytes suggested that CsPmy could induce combined cellular immunity and humoral immunity in host. Taken together, CsPmy could be a promising vaccine candidate in the prevention of C. sinensis regarding its high immunogenicity and surface localization.

Some parasites are able to manipulate the behaviour of their hosts to their own advantage. One of the most well-established textbook examples of host manipulation is that of the trematode Dicrocoelium dendriticum on ants, its second intermediate host. Infected ants harbour encysted metacercariae in the gaster and a non-encysted metacercaria in the suboesophageal ganglion (SOG); however, the mechanisms that D. dendriticum uses to manipulate the ant behaviour remain unknown, partly because of a lack of a proper and direct visualisation of the physical interface between the parasite and the ant brain tissue. Here we provide new insights into the potential mechanisms that this iconic manipulator uses to alter its host's behaviour by characterising the interface between D. dendriticum and the ant tissues with the use of non-invasive micro-CT scanning. For the first time, we show that there is a physical contact between the parasite and the ant brain tissue at the anteriormost part of the SOG, including in a case of multiple brain infection where only the parasite lodged in the most anterior part of the SOG was in contact with the ant brain tissue. We demonstrate the potential of micro-CT to further understand other parasite/host systems in parasitological research.

Prohemistomum vivax (Cyathocotylidae) is a significant parasitic threat in Egyptian African catfish (Clarias gariepinus) aquaculture, causing tissue damage, high prevalence, and economic losses. This study evaluated the dose-dependent efficacy of praziquantel against naturally occurring P. vivax encysted metacercariae (EMC) under controlled laboratory conditions and explored its potential mode of action via molecular docking. A total of 105 naturally infected catfish (150-250 g) were randomly assigned to seven groups. Each treatment consisted of three independent replicate tanks (experimental units), with five fish per tank (n = 15 fish per treatment). Treatments included a control group (0 mg/L) and six praziquantel groups administered as single doses (0.5, 2, or 3 mg/L for 24 h) or double doses repeated after 7 days. Parasitological, histopathological, and pro-inflammatory gene expression analyses (TNF-α, IL-1β) were performed. The highest efficacy was achieved with double-dose 3 mg/L, resulting in 94.2 ± 2.1% parasite reduction (p < 0.001), with clear dose-dependent trends (single-dose: 28.8-67.9%; double-dose: 43.7-94.2%). Histopathological analysis showed reduced cyst burden and tissue lesions, while inflammatory markers were significantly downregulated in treated groups. To investigate praziquantel's mechanism of action, in silico docking targeted P. vivax cytochrome c oxidase subunit I (COI), a key mitochondrial enzyme. Praziquantel exhibited strong binding affinity (-6.1 kcal/mol; RMSD = 0.0 Å), forming stable hydrophobic interactions with conserved residues (Val55A, Ala58A, Leu88A, Phe91A, Trp142A). The localization of Trp142A within the active site suggests potential enzyme inhibition, supporting a mitochondrial mechanism of action. These findings confirm praziquantel's effectiveness against P. vivax EMC and identify COI as a potential molecular target. Given its critical role in human medicine, further research into resistance risks, environmental safety, and regulatory frameworks in aquaculture is recommended.