A virus was isolated from dead specimens of catfish Ictalurus melas originating from a pond in w h~c h the whole catfish population had succumbed to an acute infection within a week. Diseased catfish showed clinical signs of oedema and haemorrhages while other fish species present in that pond remained healthy. The virus isolate grew well in Epithelioma papulosum cyprini (EPC), blueglll fry (BF2) and channel catfish ovary [CCO) cells at temperatures ranglng from 15 to 25 'C The vlrus formed tiny round plaques [diameter 400 pm) under agarose overlays and the BF2 cells were the most sensitive for thls assay. The infectivity of the catfish vlrus isolate was inhib~ted by chloroform treatment and its replication was suppressed by 5-iodo-2-deoxyuridine indicating the presence of lipid components and a DNA genome. Hexagonal virions with a diameter of 150 to 160 nm, some of them surrounded by a n external envelope, were seen by electron microscopy in the cytoplasm of infected EPC cells. These virions were morphologically similar to those of the Iridoviridae. Infection trials conducted ln subadult and adult catfish demonstrated the strong pathogenicity of the new virus isolate and hlgh levels of virus were detected in the kidney and spleen, induclng a dramatic necrotlslng spleno-nephritis and subsequently death of most of the fish The infection tnal substantiates the causative role of this virus isolate in the massive catfish mortality observed and consututes the first record of an overt natural viral infection of I. melas.

Common ash is a temperate forest tree with a colonizing behaviour, a discontinuous spatial distribution and a peculiar and poorly known mating system. Microsatellite markers were used to study the genetic structure in natural populations of common ash. Twelve populations located in northeastern France were analysed at five loci. Levels of genetic variability within and among stands were estimated for the seedling and adult stages. As expected for a forest tree, our results reveal high levels of intrapopulation diversity and a low genetic differentiation between stands. However, a general and significant heterozygote deficiency was found, with a mean F(IS) of 0.163 for the seedlings and of 0.292 for the adult trees. The different explanations for such an excess homozygosity are discussed: a nonMendelian inheritance of alleles, the presence of null alleles, a Wahlund effect and assortative mating.

In order to identify Echinococcus multilocularis DNA in fox faeces for epidemiological purposes, we have developed a new method to prepare DNA suitable for PCR amplification. DNA isolation from fox excrement was performed according to a novel procedure involving lysis in KOH, phenol-chloroform extraction and a purification step on a matrix (Prep-A-Gene). The target sequence for amplification was the E. multilocularis U1 snRNA gene. PCR products were indistinguishable for 32 different E. multilocularis isolates and no signal was observed after ethidium bromide staining with DNAs from other tapeworm species, including E. granulosus. The sensitivity of amplification was monitored by the addition of E. multilocularis DNA or eggs to faeces free of E. multilocularis and was estimated to be 1 egg per 4 g of faeces. PCR products were blotted onto nylon membranes and hybridized with an internal oligonucleotide probe in order to confirm the results. Twenty nine faecal samples from foxes shot in Franche-Comté (East France) were tested. Out of 10 samples from foxes in which no E. multilocularis adult worms could be observed after necropsy, 7 were PCR positive, showing that the PCR test is more sensitive than microscopical examination. Out of 19 samples from foxes harbouring E. multilocularis adult worms, 18 were PCR positive. The remaining PCR-negative sample could be due either to the misidentification of the species of adult worm (E. granulosus and E. multilocularis), or to DNA variation between different isolates of E. multilocularis. Further work in the field should be initiated in order to confirm these results.