Statistical correlation between severity of hepatopancreatic parvovirus infection and stunting of farmed black tiger shrimp ( Penaeus monodon)

Abstract

In response to complaints from shrimp farmers, samples were taken to determine whether stunting in cultured Penaeus monodon was related to the presence of known infectious agents. Shrimp were arbitrarily defined as normal at >8 cm length and stunted at <8 cm (base of the rostrum to base of telson) and 10 of each type were collected at harvest from each of four production ponds. Mean length for the normal shrimp was 10 cm (range 8.5–12.3) while that of stunted shrimp was 6 cm (range 2.8–7.7 cm). The shrimp were examined histologically for infections by hepatopancreatic parvovirus (HPV), infectious hypodermal and haematopoietic necrosis virus (IHHNV), monodon baculovirus (MBV), white-spot syndrome virus (WSSV), yellow-head virus (YHV) and the microsporidian, Agmasoma penaei. Infections were found for only HPV, IHHNV and MBV. For all 80 specimens, prevalences were HPV 49% (39), MBV 50% (40) and IHHNV 4% (3). Mean prevalences for HPV in the normal and stunted groups (28±8% and 70±15%, respectively) were not significantly different ( P=0.057). Nor were the mean prevalences of MBV infection (45±9% and 58±15%, respectively; P=0.496). For HPV and MBV, severity of infection was estimated by an ocular grid severity index. Pearson product moment correlation tests on a subgroup of 71 pooled samples showed a negative correlation between HPV severity index and length ( P<0.01) and a positive correlation ( P<0.01) between severity of HPV and severity of MBV infections. In addition, an ANOVA comparison was made for four sub-groups: uninfected (mean length 9.3 cm=17.2 g), MBV infected (mean length 8.3 cm=13.8 g), HPV infected (mean length 6 cm=6.3 g) and dually MBV/HPV infected (mean length 6.5 cm=7.6 g). Results showed that mean lengths of only the HPV and dually infected groups were significantly shorter than mean length of the uninfected group ( P<0.05). The results suggest either that stunted shrimp are more susceptible to HPV infection or that HPV infection contributes to stunting. In our opinion, the similar size distribution for HPV infected and dually HPV/MBV infected shrimp weighs in favour of the latter relationship.