Immune responses of resistant and sensitive common carp families following experimental challenge with Aeromonas hydrophila

Abstract

Parameters of non-specific immune response and level of specific and natural antibodies were investigated after an experimental challenge with Aeromonas hydrophila in genetically different common carp (Cyprinus carpio) families. Ten resistant and ten sensitive families were used for the experiment, which had been selected out of 96 families, based on the results of a preliminary challenge test. Blood samples were collected 12 h, one week and 21 days following the challenge. Phagocytic and respiratory burst activities of phagocytic cells, lysozyme activity of the blood plasma were determined. Level of specific antibodies against A. hydrophila and level of natural antibodies were measured in the samples taken on the 28th day. Non-infected fish from resistant and sensitive families were used as controls. Significant differences of phagocytic and lysozyme activities were measured between the challenged resistant and sensitive families. The level of specific antibodies between the same families was also found to be significantly different. There were no significant differences of the studied parameters between the control groups. Based on our results, phagocytic activity of leukocytes, plasma lysozyme activity and specific antibody titre were found to be higher in the resistant families than in the sensitive ones following infection with A. hydrophila.

Introduction

Motile Aeromonad Septicaemia (MAS) caused by the bacterium Aeromonas hydrophila is a worldwide problem for fish farmers [1]. This bacterium is a part of the normal intestinal bacterial flora of the fish and is able to cause disease by the effect of stress [2]. Septicaemia can spread among cultured fish very quickly resulting in high mortality rates [3]. A. hydrophila is extremely heterogeneous and there is currently no commercially available vaccine that is effective against all A. hydrophila strains [4]. Antibiotics and chemotherapeutic agents are often used to prevent or treat fish diseases of bacterial origin, but they can accumulate in the fish meat and in the aquatic environment, and there is an increasing risk of developing antibiotic resistant strains of bacteria.

Development of fish strains that are resistant to the disease is an alternative way of prevention, by breeding strains selectively for increased disease resistance. There are promising results of increasing disease resistance by selective breeding of some economically important fish species, such as Atlantic salmon (Salmo salar), rainbow trout (Onchorhyncus mykiss), Atlantic cod (Gadus morrhua), rohu carp (Labeo rohita) or common carp (Cyprinus carpio) [5], [6], [7], [8], [9], [10], [11]. However, knowledge about the immunological traits that can correlate with disease resistance and can be used as potential indirect markers for selection is limited. In previous studies, investigations on potential correlations between survival or resistance to various pathogens and innate (non-specific) immune response, e.g. phagocytic and respiratory burst activity of phagocytic cells [12], [13], [14], plasma lysozyme activity [14], [15], [16], [17], [18], serum haemolysin titre [17], [18], level of natural antibodies [19], [20], spontaneous haemolytic activity of serum [21] and proliferation of leukocytes in lymphoid tissues [22] were done. There are contradictions among results of these studies, as these traits showed a positive or negative correlation with disease resistance or no correlation. It was found that fish with various genetic backgrounds also showed various specific immune responses (level of specific antibodies) however, it did not always correlate with resistance against specific pathogens [16], [23], [24], [25].

The aim of our study was to investigate the non-specific and specific immune response of genetically different common carp families after an experimental challenge with A. hydrophila. Ten resistant and ten sensitive families were used for the experiment. These were selected out of 96 families, based on the survival data of a previous challenge test. Blood samples were taken from the fish 12 h, one week and 21 days after the challenge. Non-challenged fish from the same families were used as controls. Phagocytic and respiratory burst activities of phagocytic cells and plasma lysozyme activity were determined from these samples. The level of specific antibodies against A. hydrophila and level of natural antibodies were measured from the samples taken on the 28th day. Experimental data of challenged and non-challenged resistant and sensitive families were evaluated and compared to each other.