Enhanced mortality in Nile tilapia Oreochromis niloticus following coinfections with ichthyophthiriasis and streptococcosis

Ichthyophthirius multifiliis Fouquet (Ich) and Streptococcus iniae are 2 major pathogens of cultured Nile tilapia Oreochromis niloticus (L). Currently there is no information available for the effect of coinfection by Ich and S. iniae on fish. The objective of this study was to determine the effects of parasite load and Ich development size on fish mortality following S. iniae infection. Low mortality (<= 20%) was observed in tilapia exposed to Ich or S. iniae alone. Mortalities increased from 38% in tilapia exposed to Ich at 10000 theronts fish(-1) to 88% in fish at 20000 theronts fish(-1) following S. iniae exposure. The median days to death were significantly fewer (7 d) in fish exposed to Ich at 20000 theronts fish(-1) than fish exposed to 10000 theronts fish(-1) (10 d). A positive correlation (correlation coefficient = 0.83) was noted between tilapia mortality and size of Ich trophonts at the time of S. iniae challenge. Fish parasitized with well-developed trophonts (Day 4, 2 x 10(7) mu m(3) in volume) suffered higher mortality (47.5%) than fish (10.0%) infested by young trophonts (Hour 4, 1.3 x 10(4) mu m(3) in volume) after S. iniae challenge. The results of this study demonstrated that both parasite load and trophont size increased susceptibility and mortality of tilapia to S. iniae infection.