No abstract

No abstract

A cytopathic viral agent was isolated from the pooled viscera of three moribund 0-f juvenile redfin perch (2-5cni-3'5cm), Perca fluviatilis L., collected from Lake Nillahcootie, near Benalla, Victoria, Australia. The fish were collected during investigations into the death of large numbers of juvenile redfin perch in artificial water impoundments in north-east Victoria during November and December 1984. Many fish displayed focal hepatocellular and haematopoietic necrosis.Pooled kidney, liver and spleen samples were homogenized with a chilled mortar and pestle, diluted 1:20 with Hanks' balanced salt solution containing penicillin and streptomycin, and centrifuged for 25 min at 3500g. Supernatant aliquots of 0-2 ml were inoculated into duplicate 5-cm^ drained confluent monolayers of RTG-2 (rainbow trout gonad) cells for 1 h at 25°C. The inoculum was then poured off and replaced with Eagle's minimum essential medium (pH7-4) containing Earle's Salts and 2% foetal calf serum, and the cultures incubated at 15'^C. A cytopathic effect (CPE) was first detected at 10 days post-inoculation and was reproduced by inoculation of supernatant fluid into fresh RTG-2 cultures. The CPE appeared progressively earlier with serial passages until by pass 10 the initial focal rounding of cells was detectable 24 h after inoculation. The CPE took the form of focal rounding followed by puncturing of the cell sheet and the development of plaques by progressive rounding, detachment and degeneration or lysis of the perimeter cells (Fig. 1). The agent was cytopathic for eight fish cell lines or primary cultures tested though the nature and development rate of the CPE, and titres reached, varied (Table 1). The highest titres were achieved using FHM (fathead minnow) cells, with a titre of 10^'T CID5oml attained in 7 days. Treatment of supernatant fluid containing lO' TCIDsoml of virus with 50% (v/v) ether for 60 min reduced the titre to 10^ TCIDsoml.Electron microscopy was performed on pellets produced by freeze-thawing and centrifugation of infected tissue-culture supernatants at 8000^ and 80000 g for 15 min and 90 min respectively. The pellets were resuspended in phosphate-buffered saline and prepared for electron microscopy by the pseudoreplica technique. Grids were negatively stained with 2% or 3% phospho-tungstic acid (PTA) at pH 6-0, 6-8 and 7 2. Particles ranging from 165 nm to 172 nm in diameter (distance between opposite vertices) were

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 KEY WORDS Clupeoldel . Pllchard . Sardinops sagax neopilchardus . Gill dlseases . Pathology .