ABSTRACT: A large-scale epizootic occurred in the Australasian pilchard Sardinops sagax neopilchardus between March and September 1995 over more than 5000 km of the Australian coastline and 500 km of the New Zealand coastline. Affected fish died within a few minutes of clinical signs of respiratory distress and death was associated with hypoxaemia and hypercapnea. Significant lesions were confined to the gills and comprised acute to subacute inflammation followed by bizarre epithelial hypertrophy and hyperplasia. The lesions were initially focal but progressed to become generalised over about 4 d. Pathological changes in affected fish from western Australia, eastern Australia and New Zealand were similar, suggesting a common aetiology. The lesions were unlike those associated with ichthyotoxic algae, siliceous algae, physicochemical factors, fungi, bacteria, dinoflagellates, amoebae, other protozoa and metazoa. A herpesvirus was consistently present in gills of affected fish and absent from unaffected pilchards and is proposed as the aetiological agent. The rate of spread of the mortality front (approximately 30 km d^-1) in relation to the migration rate of pilchards and prevailing currents suggests that a vector was involved. The disease may have been newly introduced into Australian waters.

KEY WORDS: Clupeoidei · Pilchard · Sardinops sagax neopilchardus · Gill diseases · Pathology · Epizootiology · Mortality · Herpesvirus

Published in DAO Vol. 28, No. 1 (1997) on January 16
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1997