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

Authors: Lafrentz, Benjamin; LOZANO, CARLOS - Akvaforsk Genetic Center As; Shoemaker, Craig; Garcia, Julio; Xu, Dehai; LOVOLL, MARIE - Veso Vikan; RYE, MORTEN - Akvaforsk Genetic Center As

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2016
Publication Date: 3/11/2016
Citation: Lafrentz, B.R., Lozano, C.A., Shoemaker, C.A., Garcia, J.C., Xu, D., Lovoll, M., Rye, M. 2016. Controlled challenge experiment demonstrates substantial additive genetic variation in resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus iniae. Aquaculture. 458:134-139.

Interpretive Summary: Intensification of tilapia production has resulted in disease outbreaks that negatively affect commercial fish farmers. One bacterial pathogen that commonly causes losses in tilapia production is Streptococcus iniae. Control and prevention of S. iniae can be difficult and requires an integrated fish health management approach consisting of management practices, use of antibiotics, and vaccination. Selective breeding for resistance to disease is a complimentary strategy to improve health and performance in tilapia. This study was performed to determine whether resistance of tilapia to S. iniae is a heritable trait. A total of 143 families of tilapia (on average 9 fish per family) were infected with S. iniae, and the percent of fish that died for each family was determined. Two different genetic models were used to analyze the data and the results of both models revealed that resistance to S. iniae is moderately heritable indicating that is should be possible to develop an improved line of tilapia that are more resistant to disease. The long term goal of this research is to provide fish farmers with a more resistant stock of tilapia as an additional management tool for reducing production losses due to Streptococcus spp.

Technical 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 × 108 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.