The intensification of aquaculture in the Mediterranean is accompanied by an increase in parasites affecting fish such as gilt-head bream and sea bass.
Sparicotyle (synonymous with Microcotyle) chrysophrii has become the main pathogenic ectoparasite in gilt-head bream (Sparus aurata) aquaculture. It has been reported that this parasite has great versatility to spread to other species.
Although the incidence of S. chrysophrii is well monitored in marine aquaculture systems throughout the Mediterranean, there is a lack of data availability on its epidemiology.
Experts expect the proliferation of this parasite to increase under global warming conditions, as well as the lack of data on the implementation of integrated pest management strategies at the national and Mediterranean levels.
In this regard, researchers from the Biology Centre Czech Academy of Sciences, University of Udine, Institute of Marine Biology, Biotechnology & Aquaculture, and Universitat Autònoma de Barcelona published a scientific review compiling findings from reports, critically analyzing existing knowledge, and proposing research topics on the ectoparasite Sparicotyle chrysophrii affecting gilt-head bream (Sparus aurata).
The study describes the morphology and physiology, life cycle, epidemiology, environmental factors, host range, fish-to-fish transfer in aquaculture farms, transfer to wild fish, infection models, host response, and disease management.
Below is a summary of the scientific article, which you can download at the end of this post.
According to the study, the geographical range of the parasites, affecting wild or cultured fish, is limited to the Mediterranean basin and the Red Sea, except for the Canary Islands, the archipelago off the northwest coast of Africa in the Atlantic Ocean.
Environmental factors such as seawater temperature, pH, photoperiod, and salinity are crucial parameters for the spread of monogeneans in general, particularly during their larval stages.
The study authors cite that sea temperatures between 14 and 30°C are closely associated with metabolic rate and, consequently, the development of parasites and host immunity. This indicates that temperature is the most significant variable.
“Larvae exhibit a considerably wide optimal temperature range (measured as hatching success exceeding 88%); from 14°C to 22°C, which mainly corresponds to spring and autumn in the Mediterranean, which could partly explain their widespread presence in the Mediterranean. Temperatures of 22°C and 26°C induce a faster appearance of early eye-spot oncomiracidia (96 h), and also shorten the incubation and hatching period,” they emphasized.
According to the study, the host range for Sparicotyle chrysophrii is limited to fish species from the Sparidae family. The typical host is gilt-head bream, but the monogenean has also been isolated from sharp-snout seabream (Diplodus puntazzo) and to a lesser extent from the bogue (Boops boops).
Additionally, a recent study from the ParaFishControl project of H2020 demonstrated the ability of the monogenean to infect another sparid, the blotched picarel (Spicara maena), in the area of intensive aquaculture activities in the Zadar archipelago, Adriatic Sea.
Fish-to-Fish Transfer in Aquaculture Farms
“Biological characteristics of S. chrysophrii, such as hermaphroditism allowing all adult parasites to produce fertilized eggs; high fecundity of adults; and short generation time resulting in multiple fish infections in the same cage or neighboring cages during a single production cycle (around 8 to 10 months in the cage), enable a high incidence of sparicotylosis,” the researchers highlight.
Furthermore, they describe that the infectivity rate of parasites in aquaculture farms is reinforced by the limited dispersion of eggs within the cages due to biofouling on the net and the high fish density in the same water volume.
According to the researchers, “The resulting levels of infection and reinfection in the farms have contributed to the establishment of sparicotylosis as an endemic disease in many gilt-head bream-producing farms and in an increasing number of geographic areas“.
Transfer of Cultured Fish to Wild Fish
Previous studies have reported the transfer of S. chrysophrii between wild sparids and farmed gilthead bream, highlighting it as the most important risk factor for the spread of the monogenean in aquaculture.
“Potentially, an additional pathogen load is transmitted by escaped farmed gilthead bream, which, once accidentally released from sea cages due to net damage (mechanical tearing, extreme weather conditions, gilthead bream grazing on biofouling), gather at their original farming site or join neighboring farming sites,” they describe.
According to the researchers, infection models depend on the introduction and cohabitation of naturally infected gilthead bream (donor fish) with uninfected hosts (receptive fish) for 10 weeks, with special attention given to controlling the infection level in the uninfected fish.
“Alternatively, infection can be achieved through direct transmission of eggs by utilizing their twin filaments and positive buoyancy that aids in their attachment to various substrates. To achieve this, eggs are extracted from tanks with donor fish using a 250 μm mesh held by a floating PVC ring for 2-3 days in the upper water column,” they reported.
Pathology of Sparicotyle chrysophrii
According to the researchers’ report, the pathogenesis begins at the attachment site when parasite groups pinch and squeeze the gill filaments, and haptoral movements cause damage to the gill epithelium.
Bacteria introduced into the wound and damaged tissues cause an inflammatory reaction, manifested by epithelial hyperplasia, capillary congestion, inflammatory infiltrate, development of lamellar adhesions, and progressive reduction of interlamellar spaces.
“Therefore, the main clinical signs are induced by the parasite’s attachment clamps on the gill epithelium and subsequent blood feeding, exacerbated by the colonization of secondary bacterial infections, particularly T. maritimum,” they emphasized.
The control of monogenean presence and proliferation within an aquaculture farm is carried out through an integrated control system that encompasses different strategies:
- Surveillance of parasite presence using diagnostic techniques,
- Periodic cleaning of relevant substrates: nets or ropes,
- Cleaning, disinfection, and equipment change for work with fry and larger fish,
- Biomass control in the cages, and
- Prompt removal of daily mortalities.
Treatments against sparicotylosis can be performed in cages, tanks, and ponds, mainly as baths and occasionally as oral treatments, combined with other preventive strategies to enhance their efficacy.
Formalin continues to be the first choice for most fish farmers in the Mediterranean, although its prohibition in Italy is likely to be replicated in other European Union countries.
Other synthetic and natural commercial compounds, such as distilled water, limoseptic®, hydrogen peroxide, chlorine, and praziquantel (PZQ), were tested against sparicotylosis as bath treatments, resulting in varying efficiency.
Although fish farmers monitor the incidence of S. chrysophrii with the first seasonal increase in seawater temperature, data is unavailable, and published scientific studies are outdated.
On the other hand, scientists highlight that the implementation of genomic selection in selective breeding programs can be an efficient methodology for genetic improvement of resistance to S. chrysophrii.
Reference (open access)
Mladineo, I, Volpatti, D, Beraldo, P, Rigos, G, Katharios, P, Padros, F. Monogenean Sparicotyle chrysophrii: The major pathogen of the Mediterranean gilthead seabream aquaculture. Rev Aquac. 2023; 1- 22. doi:10.1111/raq.12839