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Title: Differential pathogenicity of five Streptococcus agalactiae isolates of diverse geographic origin in Nile tilapia (Oreochromis niloticus L.)

Author
item Evans, Joyce
item Pasnik, David
item Klesius, Phillip

Submitted to: Aquaculture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2013
Publication Date: 1/28/2014
Citation: Evans, J.J., Pasnik, David J., Klesius, P.H. 2014. Differential pathogenicity of five Streptococcus agalactiae isolates of diverse geographic origin in Nile tilapia (Oreochromis niloticus L.). Aquaculture Research. DOI: 10.1111.are.12393.

Interpretive Summary: Streptococcus agalactiae is a bacterial pathogen of fish and has caused significant mortalities worldwide. The work in this study assessed whether bacterial samples from different geographic locations have different disease-causing capabilities. Nile tilapia fish were separated into groups, and each group was injected with solutions containing S. agalactiae sampled from the USA, Brazil, Honduras, Israel, or Kuwait. The USA and Brazil bacterial samples killed the most fish, followed by the Honduras and Israel samples. The Kuwait samples killed the least number of fish after injection. However, regardless of the geographic sample that they were injected with, some fish in each of the geographic groups became lethargic, stopped eating, developed popeye and cloudy corneas, swam erratically, and developed small skin hemorrhages. Only certain geographic samples caused clearing of the gill cover and distended abdomens after infection. The findings in this study indicate that S. agalactiae from different geographic origins can cause significant mortalities after infection and can have different disease-causing capabilities. Samples from the Americas (USA, Brazil, and Honduras) were more harmful to Nile tilapia than samples from the Middle East/Asia (Israel and Kuwait).

Technical Abstract: Streptococcus agalactiae is an emerging pathogen of fish and has caused significant morbidity amd mortality worldwide. The work in this study assessed whether pathogenic differences exist among isolates from different geographic locations. Nile tilapia (Oreochromis niloticus L.) were administered an intraperitoneal injection of solution containing USDA, Brazil, Hnduras, Israel, or Kuwait S. agalactiae isolate at concentrations ranging from 102 to 107 cfu mL-1. The LD50 values 7 d after challenge were: USA (1.0 x 102 cfu mL-1), Brazil (1.5 x 103 cfu mL-1), Honduras (6.8 x 103 cfu mL-1), Israel (1.0 x 104 cfu mL-1), and Kuwait (7.2 x 105 cfu mL-1). Fish from all groups exhibited lethargy, going off feed, exophthalmia, corneal opacity, erratic swimming, petechiae/hemorrhage, and mortality. Opercular clearing and ascites were only found after infection with certain geographic isolates. The finds in this study indicate that S. agalactiae isolates of different geographic origin can cause significant mortalities after experimental challenge and can have different pathogenic capacities. Isolates form the Americas (USA, Brazil, and Honduras) were more pathogenic to Nile tilapia than isolates from the Middle East/Asia (Israel and Kuwait).