White spot syndrome virus (WSSV) infection and immunity responses in white shrimp (Litopenaeus vannamei) exposed to sublethal levels of metals

To determine if exposure to a sublethal mixture of metals (Cd, Cu, Fe, Mn, Pb and Zn) increases susceptibility to White spot syndrome virus (WSSV) infection, Litopenaeus vannamei juveniles were fed WSSV-infected shrimp tissues after 21 days of exposure to the metal mixture (WS-MM treatment). Other treatments consisted of shrimp not exposed to metals and fed infected tissues (WS), and shrimp fed healthy tissues and exposed (MM) or not exposed to metals (C). The presence of viral DNA and inclusion bodies was detected at 32 hr postinfection (hpi) in the stomach epithelium of shrimp from the WS treatment, and eight hours later in shrimp from the WS-MM treatment, possibly because of an initial negative effect of metals in viral replication. At 40 hpi, the severity of infection represented by the histopathological index increased in both WS and WS-MM treatments, and values were higher in WS-MM than in WS shrimp at the end of the experiment. From 56 hpi to the end of experiment, total hemocyte counts were lower in both WS-MM and WS treatments, and concentrations were particularly low in WS-MM shrimp. Conversely, phenoloxidase activity was higher in the WS-MM treatment from 32 to 56 hpi, suggesting a possible role of the prophenoloxidase (proPO) system in the antiviral defense against WSSV. The presence of heavy metals at sublethal concentrations may increase shrimp susceptibility to WSSV; this is supported by a decrease in circulating hemocytes, an increase in the humoral response, and the development of a higher number of WSSV inclusion bodies.