Evaluation of bromocriptine and plumbagin against the monogenean Rhabdosynochus viridisi: Computational drug repositioning and in vitro approaches

Monogeneans are parasitic platyhelminths that can harm the health of farmed fish. Few treatments are available against monogeneans, and the incentive to develop new antiparasitic agents is similar or even lower than the incentive for neglected parasitic diseases in humans. Considering that searching for and developing new antimonogenean compounds may require enormous investments of time, money, and animal sacrifice, the use of a computer-guided drug repositioning approach is a reasonable alternative. Under this context, this study aimed to evaluate the effectiveness of plumbagin and bromocriptine against adults and eggs of the monogenean Rhabdosynochus viridisi (Diplectanidae). Plumbagin is a phytochemical compound that has recently emerged as a potent antimonogenean; however, further investigation is required to determine its effects on different monogenean species. Bromocriptine was selected through a computational approach that included molecular docking analyses of 77 receptors of monogeneans (putative drug targets) and 77 ligands (putative inhibitors). In vitro experiments showed that bromocriptine does not exhibit mortality at concentrations of 0.1, 1, and 10 mg/L whereas plumbagin at 2 and 10 mg/L caused 100% monogenean mortality after 3 h and 30 min, respectively. The most effective concentration of plumbagin (10 mg/L) did not completely inhibit egg hatching. These findings underscore plumbagin as a highly effective agent against adult monogeneans and highlight the need for research to evaluate its effect(s) on fish. Although computational drug repositioning is useful for selecting candidates for experimental testing, it does not guarantee success due to the complexity of biological interactions, as observed here with bromocriptine. Therefore, it is crucial to examine the various compounds proposed by this method.

The Efficacy of Geraniol and ß-Citronellol against Freshwater and Marine Monogeneans

Monogeneans are parasitic flatworms that may be a threat for finfish aquaculture. In this study, the anthelmintic activity of two terpenes, geraniol and β-citronellol, was tested in vitro against ancyrocephalin and diplectanid monogeneans. Experiments were performed in both water and a culture medium. We observed that monogeneans in culture medium may be more tolerant to treatments compared with bioassays performed only in water. Concentrations of 300 mg/L of both compounds were required to kill 100% of monogeneans at 1 h postexposure. The toxicity of β-citronellol to fish was not evaluated. However, geraniol at 300 mg/L and 150 mg/L killed juvenile Nile Tilapia Oreochromis niloticus and White Snook Centropomus viridis, respectively, after a few minutes. Therefore, the present work suggests that other alternatives should be studied for use against monogeneans in aquaculture.

Susceptibility of Pacific white snook Centropomus viridis to Vibrio species

To examine the pathogenicity of Vibrio strains, several doses of Vibrio harveyi (CAIM 1622 and CAIM 1508), Vibrio ponticus (CAIM 1751) and Vibrio anguillarum (CAIM 8) were used to challenge Pacific white snook Centropomus viridis Lockington, 1877 juveniles, and survival, gross signs and histological lesions were observed. Susceptibility of pathogenic vibrios CAIM 1508 and CAIM 1751 to antibiotics used in aquaculture was also evaluated. The growth ability of the tested strains was not related to their pathogenicity. One of the V. harveyi strains (CAIM 1508) was the most virulent, causing per-acute septicaemia in C. viridis even at a low dose (1.4 × 104 CFU g-1). Although the V. ponticus strain (CAIM 1751) was less virulent, this is the first report of it as a pathogen of white snook. Fish challenged with V. ponticus displayed external, generalized haemorrhaging. Necrosis of the digestive tract and intravascular haemosiderosis were the most remarkable histological lesions in fish challenged with both strains. Multifocal necrosis of the internal organs and bacterial masses was also observed. The lowest minimum inhibitory concentration of the pathogenic strains (CAIM 1508 and CAIM 1751) was calculated for enrofloxacin (20 and 10 µg ml-1, respectively), and both bacteria were resistant to amoxicillin, ampicillin and trimethoprim-sulfamethoxazole.

Embryonic development of the bullseye puffer Sphoeroides annulatus (Tetraodontidae): A morphofunctional approach to ontogenetic steps

The embryonic development of the bullseye puffer, Sphoeroides annulatus, was characterized on the basis of the theory of saltatory ontogeny. This theory predicts a correlative relationship between the ontogeny-type in an altricial-precocial spectrum and the habitat that a species occupies within an unstable-stable environmental spectrum. Because S. annulatus inhabits a variety of unstable environments along a wide latitudinal range, the hypothesis that this species presents one of the most altricial embryonic developments among tetraodontids was tested. Based on major developmental events that marked the ontogenetic thresholds nine embryonic steps were identified. Developmental features such as small adhesives eggs, lack of vitelline circulation, small free embryos swimming up at hatching guided by positive phototaxis, and small first-feeding larvae actively swam in the water column, suggest that S. annulatus belongs to the reproductive guild of the nonguarders-lithopelagophils. Moreover, a comparative analysis of the developmental sequences, egg size, and first-feeding larvae size between tetraodontids confirms the hypothesis of this study and supports the evolutionary principle of the altricial-precocial spectrum postulated in the theory of saltatory ontogeny.

Immunostimulation and trained immunity in marine fish larvae

Little is known about the strategies for immunization and immunostimulation in marine fish larvae; however, both strategies have the potential to improve survival during the first days of larval culture. The biological variety of marine fish larvae complicates the standardization of the application of these strategies, although the mechanisms of early development are similar in marine species of commercial importance and those with aquaculture potential. Immunostimulation experiments performed with fish larvae provide evidence that helps to further understand the mechanisms of trained immunity, which can be used to understand responses to immunostimulation treatments. Therefore, during the stages of marine fish larviculture, strategic planning is required for the standardization of immunostimulation methods. A standardized method could improve our understanding of the effects of immunostimulating agents on the maturation of immune systems in marine fish larvae; this information would enhance the ability to achieve early training of innate immunity and determine its potential to improve the survival of cultured larvae.