Gill-associated virus and its association with decreased production of Penaeus monodon in Australian prawn farms J Munro, R Callinan and L Owens Microbiology and Immunology, James Cook University, Townsville, Queensland, Australia Abstract Gill-associated virus (GAV) was found to be asso- ciated with decreased prawn, Penaeus monodon, production when prawns from three farms (n = 45 ponds, 1800 prawns) were monitored for GAV over the production season using a graded RT- nPCR. The grading system used was a visualization of either the outer or inner nested PCR products. Prevalence and loading of GAV were associated with disease severity. Ponds with a higher initial prevalence and a larger increase in GAV load over the production period suffered disease outbreaks. Ponds with low initial prevalence of GAV but a larger increase in prevalence and large increase in load over the production period suffered chronic disease with no disease outbreak identified, yet low production. However, the ponds with moderate to low initial prevalence of GAV with a low increase in prevalence and load of GAV over the production period incurred no disease outbreak and compara- tively high production. Ponds with GAV prevalence greater than 75% at 1 month post-stocking should be considered for termination as they have a high probability (95%) of having a disease outbreak. Emergency harvest when a disease outbreak occurs will significantly limit mortality losses. Keywords: gill-associated virus (GAV), Penaeus monodon, yellow head virus (YHV). Introduction Gill-associated virus (GAV) is the type species of the genus Okavirus in the family Roniviridae of the Nidovirales (Mayo 2002). It is the junior synonym of lymphoid organ virus (LOV), which was reported in 1995 (Spann, Vickers & Lester 1995). The reported prevalence of GAV infection in Penaeus monodon within Australia approaches 100% (Walker, Cowley, Spann, Hodgson, Hall & Withyachumnarnkul 2001). GAV has been reported to be highly virulent to P. monodon (Spann, Cowley, Walker & Lester 1997). The pathogenicity of GAV was determined by the inoculation of filtered homogenates of lymphoid organ, gills and whole cephalothoraces from P. monodon that were positive for GAV, resulting in mortality from 7 to 8 days post-inoculation (Spann et al. 1997). However, at the time of the pathogenicity trial, there were at least four other viruses commonly infecting Australian farmed P. monodon, including monodon baculovirus (Doubrovsky, Paynter, Sambhi, Atherton & Lester 1998), lymphoid parvo-like virus(Owens, De Beer & Smith 1991), spawner-isolated mortality virus (Fraser & Owens 1996) and Mourilyan virus (Cowley, McCulloch, Spann, Cadogan & Walker 2005), which may have influenced the mortality of the P. monodon in that trial. A similar infection trial was repeated by De La Vega, Degnan, Hall, Cowley & Wilson (2004), and the prawns were again inoculated with filtered prawn homogenate con- taining GAV, not purified virus. They reported that 100% (15/15) of the GAV-inoculated prawns died compared to 40% (2/5) of the uninoculated controls. There was a significant increase (P = 0.010) in GAV load for both the infected prawns and the control prawns. However, the GAV increase was significantly higher (P = 0.047) in the GAV-injected prawns than in the control prawns. Again, no other viruses were tested for in that study or referred to as potential pathogens. Nevertheless, Journal of Fish Diseases 2011, 34, 13–20 doi:10.1111/j.1365-2761.2010.01209.x Correspondence J Munro, School of Animal and Veterinary Sciences, G11 Corridor Block, Roseworthy Campus, University of Adelaide, Roseworthy, SA 5371, Australia (e-mail: James.Munro@adelaide.edu.au) 13 Ó 2010 Blackwell Publishing Ltd