Relação parasito-hospedeiro entre pacu, Piaractus mesopotamicus e o ectoparasito Dolops carvalhoi (Lemos de Castro, 1949): respostas fisiológicas, morfofuncionais e do sistema imune não-adaptativo do hospedeiro frente à infestação experimental

Abstract

The freshwater fish louse Dolops carvalhoi is an ectoparasite of pacu, Piaractus mesopotamicus capable of causing substantial damage to its hosts. The aim of this study was to investigate some aspects of the host-parasite interaction between these two species. For this purpose the physiological and morphological responses and the participation of the non-adaptive immune system of pacu to the experimental infestation with D. carvalhoi were analyzed. Three different experimental protocols were designed in order to find out the effects of the parasites on their hosts, the host responses that may interfere with parasite establishment and the influence of chronic exposure of hosts with juvenile D. carvalhoi to a second acute stressor of confinement. The results showed that even low level of adult parasite infestation is capable to induce a stress response in the hosts. No changes were observed regarding the plasma osmoregulatory parameters, while an increased number of gill chloride cells and enhanced N+/K+-ATPase activity suggested that gill adaptive adjustments may be responsible for the maintenance of hydromineral homeostasis in P. mesopotamicus during the stressful situation imposed by parasites. The infestation also induced changes in the frequency of circulating blood defense cells and in the epidermis morphology of the hosts. These responses were considered to be adaptive and they points out to the activation and participation of the non-specific immune system during the time course of infestation. Hosts which have experienced a previous D. carvalhoi infestation displayed a lower infestation density and different patterns of physiological, morphological and immunological responses when they were submitted to a re-infestation. Macrophages were activated in both infested and previously infested fish. The increased density of mucous cells in the epidermis of previously infested fish suggests the participation of mucus in the defense mechanisms and possibly in the parasite detachment. Chronic infestation with juvenile parasites did not induce a stress response in the hosts. On the other hand, they were capable of modulating the stress response of fish to a second acute stressor. In conclusion, infestation with D. carvalhoi induced physiological and morphological responses in P. mesopotamicus, which were considered to be adaptive, and activated the non-adaptive immune mechanisms of the hosts in order to cope with the stress and damages imposed by parasites.