ARS | Publication request: Parasitisms in tilapia by Ichthyophthirius enhanced fish susceptility to streptococcosis

Submitted to: American Fishery Society (Fish Health Section) Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 15, 2009
Publication Date: June 8, 2009
Citation: Xu, D., Shoemaker, C.A., Klesius, P.H. 2009. Parasitisms in tilapia by Ichthyophthirius enhanced fish susceptility to streptococcosis. In: Proceedings of the American Fishery Society (Fish Health Section). p. 16.

Technical Abstract: Ichthyophthirius multifiliis Fouquet (Ich) and Streptococcus iniae are two major pathogens of cultured Nile tilapia, Oreochromis niloticus (L). The objective of this study was to determine the influence of parasitism (Ich) on a bacterial infection (S. iniae). Three hundred tilapia were divided into 6 replicated groups and exposed to 0, 10,000 or 20,000 theronts per fish to evaluate the effect of infectious dose on fish susceptibility to S. iniae. To determine the effect of early and late stage trophont development on fish mortality, 270 tilapia were challenged with S. iniae at 4 hour, 2 and 4 days post Ich infestation. Low mortality (less than or equal to 20%) was observed in tilapia exposed to Ich or S. iniae alone. Mortalities increased from 38% in tilapia exposed to Ich at 10,000 theronts/fish to 88% in fish exposed to 20,000 theronts/fish following S. iniae challenge (1 × 107CFU/fish). The median days to death (MDD) was significantly shorter (7 days) in fish exposed to Ich theronts at 20,000 theronts/fish than fish exposed to 10,000 theronts/fish (10 days). A positive correlation (correlation coefficient = 0.83) was noted between tilapia mortality and stage of trophont development at the time of S. iniae challenge. Fish parasitized with late stage trophonts (4day, 2×107 µm3 in volume) suffered higher mortality (47.5%) than fish (10.0%) infested by young trophonts (4 hours, 1.3 × 104 µm3 in volume) after S. iniae challenge. The results of this study demonstrated that both parasitic dose and trophonts size increased mortality of tilapia following S. iniae challenge.