ARS | Publication request: Are Tilapia Infected with Gyrodactylus More Susceptible to Streptococcus?

Submitted to: American Fisheries Society Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: April 4, 2007
Publication Date: June 4, 2007
Citation: Xu, D., Shoemaker, C.A., Klesius, P.H. 2007. Are Tilapia Infected with Gyrodactylus More Susceptible to Streptococcus?. American Fisheries Society Annual Meeting Proceedings. page 9.

Technical Abstract: Streptococcus iniae and Gyrodactylus niloticus are two common pathogens of cultured Nile tilapia, Oreochromis niloticus. We studied concurrent infection of tilapia by G. niloticus and S. iniae and evaluated whether parasitism in tilapia with Gyrodactylus increased susceptibility and mortality following immersion infection with S. iniae. A total of 200 tilapia infected with G. niloticus and 200 fish without parasite burden were divided into 4 groups and received the following treatments: 1) G. niloticus infected and challenged with S. iniae (G-S), 2) no parasite infection and challenged with S. iniae (N-S), 3) G. niloticus infected and no S. iniae (G-N) and 4) no G. niloticus and no S. iniae (N-N). Results showed that death mainly occurred in fish parasitized by G. niloticus and challenged with S. iniae (G-S group). The accumulative mortality (42.2%) was significantly higher in the G-S group than fish not infected by the parasite, but exposed to S. iniae (6.7%). Bacteriological examination revealed S. iniae from 92% of dead or moribund fish challenged with S. iniae. In another trial to determine if the parasite can potentially harbor the bacterial pathogen, 105 tilapia infected by Gyrodactylus were divided into 3 groups and fish in each group received one of following treatments: 1) S. iniae IP injected, 2) S. iniae immersion exposed, and 3) non-exposed control. Gyrodactylus was collected from fish 24 and 72 h post exposure and S. iniae was isolated and identified from parasite samples. The result showed that S. iniae was isolated from 60% and 40% of G. niloticus collected from fish infected by IP injection or immersion, respectively at 24 h post challenge. The present study confirms that parasitism of tilapia by G. niloticus increases host mortality following exposure to the bacterial pathogen S. iniae. Gyrodactylus not only damaged fish epithelium and provided entry for invasive bacteria but also was found to harbor viable cells of S. iniae.