40 March 2012 red mouth disease, rainbow trout fry syndrome, and viral haemorrhagic septicaemia (Henryon et al. 2002, 2005); Atlantic salmon against furunculosis, bacterial kidney disease, cold water vibriosis, and infectious salmon anaemia (Standal and Gjerde 1987, Gjedrem and Gjoen 1995, Odegard et al. 2007); Atlantic cod against vibriosis (Kettunen and Fjalestad 2006); rohu against aeromoniasis (Mahapatra et al. 2008); and Pacific white shrimp against white spot syndrome virus (Fjalestad et al. 1997, Argue et al. 2002, Gitterle et al. 2005). Here we highlight the achievements made with respect to ongoing selection programs to prevent aeromoniasis in fish and the constraints and future of these programs. Ongoing Selection Programs There are three major culture species for which selection programs for A. hydrophila resistance have been reported. Common Carp The Research Institute for Fisheries, Aquaculture and Irrigation (HAKI) at Szarvas, Hungary, is a unique live gene bank of common carp Cyprinus carpio comprising of 17 foreign and 20 Hungarian strains (Bakos and Gorda 2001). Initially Jeney et al. (2009) studied the stress response in two genetically distinct carp strains (wild Duna, D and inbred Szarvas, 22) by measuring cortisol level. Males of high (H) and low (L) stress response from both lines were crossed with inbred Szarvas 22 females, producing four lines of fishes (22 × 22 H, 22 × 22 L, 22 × D H, 22 × D L). These four lines, along with the initial D and 22 strains, were vaccinated and immune responses were studied in vaccinated and control groups by measuring plasma antibody titer. The wild carp D and crossbred 22 × D with high and low stress responses were most sensitive to the challenge test, with 80 percent mortality in response to A. hydrophila infection. The inbred line 22 and the crossbred 22 × 22 L were more resistant, with 50 percent mortality rate and higher antibody titer against A. hydrophila. Later the group performed a diallele cross using four genetically distinct common carp strains (inbred minor line Szarvas, 15; scaly noble carp Tata, T; Hungarian wild carp Duna, D; and an East Asian wild carp Amur, A) differing in their origin and breeding history, and produced 96 families. The challenge test with A. hydrophila showed no significant correlation of pond survival with resistance, and heritability was also low (Odegard et al. 2010). The wild strains (Duna and Amur) indicated better heterosis for pond survival and growth rate compared to the two farmed strains (Tata and Szarvas 15). Survival in ponds of Szarvas 15 was even lower than that of previous generations from greater inbreeding. The heritability estimate for growth with pond survival was also high, indicating a successful selective breeding for growth in common carp (Nielsen et al. 2010). The challenge test to A. hydrophila indicated that families belonging to the domesticated strains Tata and Szarvas 15 were more resistant, while the wild Duna and Amur strains were most susceptible (Jeney et al. 2011). From the challenge results ten most-resistant and ten most-sensitive families were selected and immune responses were studied from challenged and their respective control groups separately (Ardo et al. 2010). Results indicated some correlations in phagocytic, lysozyme activities, and level of specific antibody titer between resistant and sensitive families, but no relation between the two could be established in non-infected or control groups. The selection program continues. Rohu A selective breeding program on rohu Labeo rohita, the major contributor to freshwater aquaculture production in India, was carried out through Indo-Norwegian collaboration. Initially the goal of the selection program was increased growth (Reddy et al. 2002). Later, genetic variation was studied for different immunological parameters and their association with survival against aeromoniasis in 13 full-sib families (Sahoo et al. 2004). The specific immune response (such as haemagglutination titer) and nonspecific immunity levels (such as lysozyme activity and natural haemolysin titer) were not significantly different between resistant and susceptible groups. However, aeromoniasis resistance was positively correlated with bactericidal activity of serum. A. hydrophila challenge of these 13 full-sib families resulted in mortalities ranging from 0 to 100 percent. These preliminary findings open up the scope for selection against aeromoniasis based on wide differences in survival among full-sib families. Furthermore, genetic variation studied by taking different full-sib families of two year classes (2003 and 2004) after A. hydrophila challenge indicated that mean body weight of 2003 year-class fish that survived the challenge test was less than those that died during the test. However, contradictory results were found for 2004 year-class fish due to inconsistent challenge survival results in duplicate tanks (Mahapatra et al. 2008). In another study, seven immune system parameters were investigated in 64 full-sib families of the 2003 and 2004 year classes and correlations with the challenge survival data were obtained. The data indicated absence of correlation of survival to aeromoniasis with serum myeloperoxidase, superoxide and lysozyme activities, negative correlations with bacterial agglutination titer, haemolysin titer and haemagglutination titer, and a significant positive correlation with ceruloplasmin level among families (Sahoo et al. 2008). In first-generation lines generated from 15 higher-ranked (resistant) and ten lower-ranked (susceptible) full-sib families, challenge testing resulted in differences in immunological response and differential expression of a few immune-related genes, in addition to a very high selection response (i.e., 58 percent greater survival in challenge test in the resistant line over the susceptible line) (Sahoo et al. In press). Few immune parameters measured showed significantly greater response in the resistant line compared to the susceptible line, such as respiratory burst activity of blood phagocytes, serum myeloperoxidase activity and ceruloplasmin level. However, the reverse was the case in the level of blood glucose and serum natural haemolysin
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