Fatal, virus-associated peripheral neuropathy and retinopathy in farmed Penaeus monodon in eastern Australia.: II.: Outbreak descriptions

Outbreaks of 'peripheral neuropathy and retinopathy' (PNR) occurring during 2 consecutive growout periods (typically October-April) are described for an intensive Penaeus monodon farm in eastern Australia. In the 1998/99 growout period, outbreaks graded minor to severe occurred in 22 of 25 ponds, 12 to 25 wk post-stocking. In the severely affected index pond, harvested 8 wk after outbreak recognition in mid-January, estimated survival for the period late December to harvest was 50%. Minor to moderate losses could be attributed to PNR in the other ponds. Mean survival over the same period for the 14 ponds harvested within 5 wk of outbreak recognition was 93% (83 to 100%); for the 7 ponds harvested 5 to 8 wk after outbreak recognition was 79% (67 to 92%) and for the 3 unaffected ponds was 90% (86 to 95%). Analysis indicated a significantly lower risk (Fisher's exact p = 0.016) of an outbreak in the 2 ponds stocked only with postlarvae from one hatchery (D) versus the 18 ponds stocked only with postlarvae from 3 other hatcheries (A, B and C). In the 1999/2000 growout period, minor to severe PNR outbreaks occurred in all 26 ponds, each stocked with postlarvae from the same hatchery (E), 19 to 21 wk post-stocking. Stocking date in 1999/2000 appeared to influence PNR outbreak severity; for ponds stocked on 2 of the 7 stocking dates versus those stocked on remaining dates, the crude relative risks (CRR) of a severe outbreak, or either a moderate or severe outbreak, were 11.25 (1.55 < CRR < 81.40) and 2.63 (1.30 < CRR < 5.31), respectively. Although inconclusive, study findings are consistent with the hypothesis that 'gill-associated virus' (GAV), the putative causal pathogen identified in a separate pathological study, entered ponds via postlarvae, and that prevalence and/or severity of infection within postlarval batches influenced outbreak severity. The generally high survival in ponds harvested soon after outbreak recognition, together with PNR prevalence of approximately 50% in prawns collected from 4 ponds 7 wk before those ponds were recognised as affected, also suggest that GAV is highly infectious and that PNR has a relatively long incubation period and/or clinical course.