Effect of dose and challenge routes of Vibrio spp. on co-infection with white spot syndrome virus in Penaeus vannamei

This study was conducted to investigate the effect of dose and challenge routes of Vibrio spp. on co-infection with white spot syndrome virus (WSSV) in specific pathogen-free (SPF) Penaeus vannamei shrimp. Juvenile shrimp were first injected with WSSV at a dose of 30 SID50 shrimp− 1 (SID50 = shrimp infectious dose with 50% endpoint) and 24 h later with 103, 104, 105 or 106 CFU shrimp− 1 of V. campbellii. Controls did not die during the experiment, except the ones that received 106 CFU shrimp− 1 (35–65%). In WSSV-inoculated shrimp, the 100% cumulative mortality were reached at 144–360 h post injection (hpi). WSSV-infected shrimp died much faster when injected with at least 104 CFU of V. campbellii with the 100% cumulative mortality reached at 48–96 hpi of virus. The density of V. campbellii in haemolymph of co-infected moribund shrimp collected 6 h after V. campbellii injection was significantly higher than that in shrimp injected with V. campbellii only. There was no difference in the number of WSSV-infected cells between shrimp inoculated with WSSV only, compared to dually inoculated ones. Shrimp which were first injected with WSSV and 24 h (or 48 h) later exposed to 106, 107, or 108 CFU ml− 1 of V. campbellii by immersion did not show any accelerated mortality. When WSSV-infected shrimp were challenged with another Vibrio species, V. harveyi BB120, no accelerated mortality was noted in WSSV-infected shrimp injected with 106 CFU shrimp− 1 of V. harveyi BB120.In conclusion, it can be stated that the synergistic effect between WSSV and Vibrio is influenced by the dose, species and infection route of inoculation of the Vibrio bacteria.