Parasites in aquaculture: how to manage them sustainably?

Parasites are a major and recurrent problem in aquaculture, with heavy economic repercussions. Some studies (Shinn et al. 2015) estimate the annual loss for the industry around one billion dollars worldwide. Before gathering with torches and pitchforks to hunt down the beast, this brief article provides reflections about parasites in aquaculture and what should be done to set up efficient means of control in order to contain this costly issue.

A parasite is defined as an organism which lives on or inside a host to use its energetic resources at his own benefit during a part or the totality of his life cycle. This definition includes a lot of species. Some authors (Hechinger et al. 2015) estimate that parasitic organisms represent between one-third to half of the organisms on our planet. Other non-parasitic species are called “Free-living”. 

The ecological roles of parasites are still not fully understood. However they appear to have a regulatory role on the host population “in a density-dependent manner” (Sure B. et al., 2017) and to occupy a significant part of the food web, as they can represent in some estuary ecosystems a biomass equivalent to the biomass of predator (Kuris et al. 2008). If parasites are important actors of aquatic biocenosis, their presence inside the farms and management has become a major challenge for modern aquaculture. 

Aquaculture production worldwide often occurs in open or semi-open systems (cages, raceways, ponds etc.) where the level of containment and biosecurity applicable could never reach what can be done in modern land animal farming. In such conditions, the exchange of parasites between wild and farmed populations is inevitable over time and eradication appears to be a pipe dream. Especially when high-density and concentrated production units act like amplifiers, making the proliferation and associated economic loss hard to control. Instead, as stated by the proverb, before starting a battle it is better first to “know your enemy”: The knowledge of its life cycle allows pointing out the weakness on which it is effective to act for a long-term control of its expansion.

If direct treatments can be a key element of parasite management, their use must be done in a rational way because they induce a series of interlinked and lasting secondary effects either on the environment (pollution, resistance, etc.), on the host (residue) and on the consumer’s perception. Under such circumstances treatments must be limited and backed up with other synergistic actions (prophylaxis, disinfection etc.) to magnify its effect. To be efficient, just like the second part of the proverb, farmers need to “know themselves” or more especially their production tool to highlight their weak points.

Regarding the host, the origins of economic loss are diverse: Mortalities come first to the mind, they are often the results of secondary infections taking advantage of the damages caused by parasites. However, growth loss and reduction of feed efficiency also play an important contribution to the economic impact (Sommerville et al. 2009). If acting on the parasite population and the environment is essential, it is also important to work on the host's natural defense and its ability to overcome the stress caused by parasites. In a “parasite-centered” approach it is easy to forget the host. To make its contribution,Techna has developed a range of products called EpiShield, designed to support the fish and its natural defenses during the infection period. Once exposed, fish have the ability to generate an efficient line of defence against the parasite: The first infection being often the most dangerous. 

Aiming at the maintenance of the fish in its resilience zone, EpiShield products support the host in the triptyque “Host, pathogen and environment”, to potentialise the other actions carried out to keep the parasite population under control.  The dedicated solution of this range focuses on three actions: 

• Favour the integrity of the organ targeted by the parasite

• Participate to the reduction of secondary infection

• Support the wound healing process 

Sparicotyles (Sparicotyle chrysophrii) is a monogenean parasite infecting the gills of gilthead sea bream (Sparus aurata). Their presence on site is through the year, even though seasonality patterns exist, with a stronger prevalence during the cold season and higher susceptibility of young fish (Antonelli et al. 2010). To mitigate the impact of sparicotyles on the farm, several recommendations have been made (Fioravanti M. L., et al., 2020l), such as:

• Evaluation and follow-up of the infection through a scoring system. 
• Segregation and cleaning of the equipment, especially when shared between fish sizes. 
• Cleaning and changes of nets, to remove the entangled eggs.
• Put the new fish in remote cages, to reduce the cross contamination with the rest of the site. 
• Removal of dead fish

These recommendations help, first to understand the evolution of the parasite population, a prerequisite to proper management and second to break its cycle by removing entangled eggs which participate in recruitment on-site. 

In addition, Techna’s specific solution against sparicotyle, EpiShield - Gill, is developed to participate in gill's integrity and their natural defences. Gills are a central organ, as they are the source of oxygen for the organism: without oxygen, the metabolism cannot function. Using the benefits of plant extracts, such as garlic essential oil and specific micronutrients, EpiShield - Gill provides complementary effects to support the immune response and wound healing process. This solution can be either used in anticipation of the risk, for example on newly introduced fish in spring or to provide a protective effect or to support bigger animals during the winter season. 

Parasites are part of the aquatic ecosystem on which most of the aquaculture farms are located. Their eradication is not possible in open or semi-open systems, the solution lies in the comprehension of their life cycle and propagation environment to deploy a series of complementary actions to stem their proliferation. Techna’s range of products EpiShield is designed to participate in this sustainable management, by reinforcing the host in order to let him fully express his natural defenses.

For more information, please do not hesitate to ask our experts at Techna!

Bibliography:

• Antonelli, Laetitia & Quilichini, Yann & Marchand, Bernard., 2010. Sparicotyle chrysophrii (Van Beneden and Hesse 1863) (Monogenea: Polyopisthocotylea) parasite of cultured Gilthead sea bream Sparus aurata (Linnaeus 1758) 
• Fioravanti M.L., Mladineo I., Palenzuela O., Beraldo P., Massimo M., Gustinelli A., Sithà-Bobadilla A., 2020. Fish farmer’s guide to combating parasitic infections in European seabass and gilthead seabream aquaculture. 
• Hechinger, R.F., 2015. Parasites help find universal ecological rules.
• Kuris, A.M., Hechinger, R.F., Shaw, J.C., Whitney, K.L., Aguirre-Macedo, L., Boch, C.A., Dobson, A.P., Dunham, E.J., Fredensborg, B.L., Huspeni, T.C., Lorda, J., Mababa, L., Mancini, F.T., Mora, A.B., Pickering, M., Talhouk, N.L., Torchin, M.E., & Lafferty, K.D., 2008. Ecosystem energetic implications of parasite and free-living biomass in three estuaries. 
• Sommerville, C., 2009. Controlling parasitic diseases in aquaculture: new developments. In G. Burnell & G. Allan, eds. New technologies in aquaculture: improving production efficiency, quality and environmental management,
• Shinn, Andrew & Pratoomyot, Jarunan & Bron, James & Paladini, Giuseppe & Brooker, Esther & Brooker, Adam., 2015. Economic impacts of aquatic parasites on global finfish production.
• Sures B, Nachev M, Pahl M, Grabner D, Selbach C., 2017.  Parasites as drivers of key processes in aquatic ecosystems: Facts and future directions.