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Title: Evaluation of the link between gyrodactylosis and streptococcosis of Nile tilapia, Oreochromis niloticus (L.)

Author
item Xu, Dehai
item Shoemaker, Craig
item Klesius, Phillip

Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2006
Publication Date: 3/1/2007
Citation: Xu, D., Shoemaker, C.A., Klesius, P.H. 2007. Evaluation of the link between gyrodactylosis and streptococcosis of Nile tilapia, Oreochromis niloticus (L.). Journal of Fish Diseases. 30: 233-238.

Interpretive Summary: Monogenetic trematodes, such as Gyrodactylus sp. are common fish parasites that cause mechanical injuries on fish epithelium and lead to fish mortality under crowded conditions. Streptococcus iniae is a severe bacterial pathogen and the economic losses caused by this bacterium in aquaculture worldwide are estimated to be over US $100 million annually. In tilapia production facilities, it has been observed that parasite infections probably cause epidermal injuries of the fish and led to increased Streptococcus infection and mortality. No information, however, is available on the association of Gyrodactylus and Streptococcus in tilapia. Results of this study demonstrated that parasitism in tilapia with Gyrodactylus niloticus increased infection and mortality following exposure to Streptococcus iniae. The mechanical injury from the parasite apparently provided a portal of entry for the bacterium. Study results also suggest that the parasite harbored viable bacteria and served as a vector for transmission. The information in this study is important to scientists and fish farmers for a better understanding of concurrent infections (i.e. parasite and bacteria) in cultured fish.

Technical Abstract: Streptococcus iniae and Gyrodactylus niloticus are two common pathogens of cultured Nile tilapia,Oreochromis niloticus. Trials were conducted to study 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. Results showed that death mainly occurred in fish with G. niloticus and challenged with S. iniae (G-S group). The accumulative mortality (42.2%) was significantly higher in G-S group than fish un-infected by the parasite (6.7%), but exposed to S. iniae (N-S group). Bacteriological examination revealed S. iniae from almost all dead or moribund fish challenged with S. iniae. Gyrodactylus not only damaged fish epithelium and provided entry for invasive bacteria but also was found to harbor cells of S. iniae for 24 and 72 h. Streptococcus iniae was isolated from 60% and 40% of G. niloticus collected from fish infected by IP injection or immersion, respectively at 24h post challenge. In summary, parasitism of tilapia with G. niloticus increased mortality following exposure to Streptococcus iniae.