ARS | Publication request: Enhanced mortality in Nile tilapia, Oreochromis niloticus following coinfections with ichthyophthiriasis and streptococcosis

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 2009
Publication Date: July 23, 2009
Repository URL: http://hdl.handle.net/10113/43734
Citation: Xu, D., Shoemaker, C.A., Klesius, P.H. 2009. Enhanced mortality in Nile tilapia, Oreochromis niloticus following coinfections with ichthyophthiriasis and streptococcosis. Diseases of Aquatic Organisms. 85:187-192.

Interpretive Summary: Nile tilapia Oreochromis niloticus is a major farm-raised fish and most studies of tilapia diseases have been conducted for a single pathogen (a parasite or a bacterium). This study condition does not fully reflect those in aquaculture systems where tilapia may be concurrently infected by 2 or more pathogens. The parasitic protozoan Ichthyophthirius multifiliis (Ich) and bacterium Streptococcus iniae are two common pathogens of tilapia, which can cause high mortality of cultured fish. Two trials were conducted to determine whether parasitism in tilapia with Ich increased susceptibility and mortality following immersion infection with S. iniae. Results of the current study demonstrated that concomitant exposure with Ich and S. iniae enhanced tilapia mortality. Both parasite load and parasite development size increased susceptibility of tilapia to S. inia infection. The mechanical injury from the parasite apparently provided a portal of entry for the bacterium. The information in this study is important to scientists and fish farmers for a better understanding of coinfection of pathogens in cultured fish. Fish health management methods may be developed for coinfections that will minimize fish losses due to diseases in aquaculture.

Technical Abstract: Ichthyophthirius multifiliis Fouquet (Ich) and Streptococcus iniae are two common pathogens of cultured Nile tilapia, Oreochromis niloticus (L). Two trials were conducted to determine whether parasitism in tilapia with Ich increased mortality following immersion infection with S. iniae. Results of the current study demonstrated that low mortality was observed in tilapia exposed to Ich or S. iniae alone. Tilapia suffered significantly higher mortality when exposed to 20,000 theronts fish-1 and subsequently challenged with S. iniae compared to those exposed to 10,000 or no theronts fish-1. Fish mortalities were highly correlated with theront infectious dose (r = 0.95) and sizes of trophonts in fish at the time of bacterial challenge (r = 0.83). When challenged with Ich theronts and S. iniae on the same day (day 1), fish showed low mortality. Fish mortalities increased from 10.0% to 47.5% when fish were challenged with S. iniae from day 1 to day 4 post Ich infestation. Trophonts in fish skin increased in volume from 13×1000 (13k) µm3 at hour 4 to 20,490k µm3 at day 4 post infestation. Results in the current study demonstrated that both parasite load and parasite size increased susceptibility of tilapia to S. iniae infection.