Epizootic mortality in the pilchard Sardinops sagax neopilchardus in Australia and New Zealand in 1995. I. Pathology and epizootiology

Abstract

A large-scale eplzootic occurred In the Austialas~an pllchard S a r d ~n o p s sagax neop ~l c h a r d u s between March and September 1995 ovei more than 5000 km of the Australian coastline and 500 km of the New Zealand coastline Affected fish died wlthln a tew mlnutes of cl~nical slgns of respiratory distress and death was associated wlth hypoxaemla a n d hypercapnea Significant leslons were confined to the gllls and comprised acute to subacute inflammation followed by blzal re epithelia1 hypertrophy and hyperplasia The lesions were initially focal but progressed to become generalised over about 4 d Pathological changes in atfeded fish from xvestern Australia eastern Australia a n d New Zealand were simila~, suggesting a common aetiology The lesions were unllke those associated w ~t h ichthyotoxic algae, s ~l ~c e o u s algae, phys~cochemical factors fungi, bacterla dinoflagellates, amoebde, other protozoa and inetazoa A herpesvlrus was consistently present In gills of affected flsh and absent from unaffected pilchards and IS proposed as the aetiological agent T h e late of spread of the mortal~ty front (approulmately 30 km d ' ) In relation to the migration late of pilchaids a n d plevaillng currents suggests that a vector was involved The dlsease may have been newly introduced lnto Australian wateis