Controlled challenge experiment demonstrates substantial additive genetic variation in resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus iniae

Abstract Streptococcus iniae is an etiologic agent of streptococcal disease in tilapia and is one of several Streptococcus spp. that negatively impact worldwide tilapia production. Methods for the prevention and control of S. iniae include vaccines, management strategies, and antibiotics. A complimentary preventative approach may include selective breeding for disease resistance, but the potential for this is unknown in tilapia. This study was initiated to challenge Nile tilapia ( Oreochromis niloticus ) families for estimation of genetic parameters for resistance to S. iniae . A total of 143 full- and paternal half-sib families (avg. 176 g, sd = 50 g) were divided into two groups with each containing on average 9 fish per family. The challenge was designed with the intent to utilize fish injected with S. iniae (Group 1) as shedder fish to transfer the bacterium to cohabitated fish (Group 2). Tilapia from Group 1 were challenged by intraperitoneal injection with a volume containing 1.15 × 10 8 colony-forming units S. iniae per fish, and then cohabitated with tilapia from Group 2 in a single tank. Accumulated mortality at the end of the experiment was 60% for the fish challenged by injection and 6.4% for fish challenged by cohabitation. The results revealed high variation for mean survival of the families injected with S. iniae (range from 0% to 100%, CV 69%). The estimated heritability of post-challenge survival in Group 1 was 0.42 ± 0.07 on the observed binary scale and 0.58 ± 0.09 on the underlying liability scale, derived from fitting a linear animal model and a sire-dam threshold model, respectively. In summary, substantial additive genetic variation in resistance to S. iniae was observed when fish were challenged by injection, and this suggests promise for genetic improvement of tilapia for resistance to S. iniae through selective breeding. Statement of relevance This research suggests promise for genetic improvement of tilapia for resistance to Streptococcus iniae through selective breeding, thereby providing commercial fish farmers with a more resistant stock of tilapia as an additional management tool for reducing production losses due to Streptococcus spp.