AbstractIn Vietnam, striped catfish production reached 1.2 millions of tons in 2014 with an estimated value of 1.77 billion US$ in the international food market, representing almost 80 % of the total world production of this species. The Mekong Delta, which actually includes 77 % of fisheries Vietnamese production, is currently threatened by saltwater intrusion from the China Sea and the Gulf of Thailand induced by the sea-level rise and aggravated by dams construction upstream. In striped catfish ponds, salinity may increase up to more than 10 ppt (nearly one third of seawater) during the dry season and is correlated with decreasing growth rate and higher mortality rate. Short-term actions have been undertook by local authorities such as construction of dykes and increase in water exchange but will not resolve the problem of salinity in the long-term. This thesis deeps the consequences of saline stressors in striped catfish health, particularly the immune system and susceptibility to disease, in order to investigate the long-term possible effects of salinity on striped catfish’ farms perenity. In Vietnam, a survey was carried out during the dry season 2013 (March-April) and rainy season 2014 (July-August) in 12 fish farms of the lower (Tra Vinh Province) and higher (Can Tho Province) Mekong River Delta. This survey allowed drawing up current key osmoregulatory (plasma osmolality and NaK ATPase activity) and innate immune parameters (immune cells abundance and activity, lysozyme and complement activity) of striped catfish in aquaculture depending on the geographical location, particularly the proximity to the River Mekong Estuary and the seasonal surface runoff.
In fish, hyperosmolarity has often been investigated from osmoregulation perspectives while the effects of such stress on the immune capacity remain largely unexplored. However, in fish, several authors pointed out that chronic saline stressor may stimulate several innate immune factors such as immune cells proliferation and activity (e.g. lysozyme activity, respiratory burst, phagocytosis) and interfere with susceptibility of viral and bacterial diseases. In mammals, infusions of “resuscitation fluids” (hyperosmotic fluids) have found a lot of clinical applications in order to modulate the immune system in patients after hemorrhagic shock or trauma. Therefore, the second objective of this thesis is to point out new immune mechanisms modified by chronic hyperosmolarity and study the link between osmoregulation and immunity. Hence, striped catfish were submitted to three salinity profiles (freshwater, low saline water, saline water) during 20 days in husbandry conditions, followed by infection with a virulent bacteria, Edwardsiella ictaluri, responsible for the enteric septicemia of catfish. Osmoregulatory factors (plasma osmolality, gill NaK ATPase), immune parameters (immune cells abundance and activity, lysozyme and complement activity, heat shock protein 70, high mobility group blot abundance, Toll-like receptor expression) and mortality rate were investigated. Moreover, the protein expression profile was investigated in the kidney, using a without a priori label free quantitative proteomics workflow. New pathways involved in immunity and other common metabolic pathways and affected by chronic hyperosmotic stressor in our study were highlighted and discussed.
In summary, hyperosmotic stressors were associated with an increase in plasma osmolality while gill NaK ATPase activity barely increased. Upon infection or not, salinity enhanced tested innate immune factors. However, susceptibility to virulent bacterial strain of Edwardsiella ictaluri increased, especially in salt water. In kidney, salinity and infection exerted a synergic stress on the proteome’s abundance. Salinity enhanced the general metabolic response (lipid metabolism, protein metabolism, energetics metabolism) as well as detoxification mecanisms (anti-oxidant, cytochrome P450). Numerous structural proteins (e.g. actin, collagen, spectrin) were differentially regulated depending on the osmotic pressure. In brackish water, several proteins involved in MAPKp38 signalling pathway, phagocytosis and T cells regulation were upregulated. In salt water, proteins implicated in inflammatory processes and the stress response were stimulated. However, the abundance of Toll-like Receptors (TLRs) transcripts, with the exception of TLR 5, decreased, especially in infected fish.
To conclude, in striped catfish, the elevated internal body pressure caused by hyperosmotic stressors may induce inflammation whose intensity varies with stressor’s intensity. The inflammatory response may unbalance the immune response during pathogen attack (Edwardsiella ictaluri in our study) and may interfere with the host-pathogen interaction, in the benefit of the pathogen when salinity exceeds 10 ppt. However, whether the fish controls inflammatory processes triggered by osmotic stimuli and whether inflammation interacts with the specific immune response remain to be investigated.
|Date of Award||12 Jan 2017|
|Sponsors||Fund for Research Training in Industry and Agriculture (FRIA)|
|Supervisor||Patrick KESTEMONT (Supervisor), Eric Depiereux (President), Jessica Douxfils (Jury), Éric Muraille (Jury), Peter Bossier (Jury) & Lluis Tort (Jury)|
Attachment to an Research Institute in UNAMUR