Characterization of a virus obtained from snakeheads Ophicephalus striatus with epizootic ulcerative syndrome (EUS) in the Philippines

Viruses associated with epizootic ulcerative syndrome: an update

Epizootic ulcerative syndrome is one of the most serious infections noticed in the finfish of Asia-Pacific during the last three decades. Different viral agents and a consistent fungus were isolated from the EUS infected fishes from various countries. Rhabdoviruses are by far the most isolated group of viruses followed by birnaviruses. One reovirus and a ranavirus have also been isolated from ulcerated fishes of which the ranavirus was capable of reproducing the clinical signs of the disease. Albeit heterogenic, due to frequent isolation, range of areas from which isolates have been obtained and ability to induce dermal lesions in experimental infectivity studies make rhabdoviruses one of the likely primary aetiological agents that could be triggering the initiation to EUS. However, further investigations may be required to fully establish the role of these viral agents in the induction of EUS. Viruses observed as persistent infections in fishes evidenced by their presence in cell cultures also require further investigation for their likely role in predisposing the fishes to EUS.

Aphanomyces invadans and ulcerative mycosis in estuarine and freshwater fish in Florida

In the spring of 1998, the Florida Fish and Wildlife Research Institute received numerous reports of lesioned or ulcerated fish primarily from the St. Lucie Estuary on the southeast coast of Florida, an area known since the late 1970s for lesions of the ulcerative mycosis (UM) type. From these and archived reports, as well as others received from different areas of Florida, we documented that diseased specimens had randomly distributed skin ulcers (usually reddened or hemorrhagic) with raised irregular margins and, in some cases, deeply penetrating hyphae in the surrounding muscle tissue. Since 1998, 256 fish (comprising 18 species) with ulcerative lesions (from 15 different locations) were confirmed with hyphae in fresh squash preparation or by histological evaluation. Squash preparations revealed nonseptate, sparsely branching, thick-walled hyphae; histological sections revealed mycotic granulomas in the dermis that occasionally penetrated into the skeletal muscle. These pathological characteristics were consistent with UM caused by the oomycete Aphanomyces invadans in Southeast Asia, Japan, Australia, and the United States. For specific identification, six isolates from ulcerated fish were cultured and prepared for molecular characterization using established diagnostic methods. Ribosomal RNA gene sequence analysis identified three isolates as Aphanomyces invadans, one as the oomycete Achlya bisexualis, and two as the ascomycete Phialemonium dimorphosporum. A more extensive survey of 67 ulcerated skin samples from fish collected between 1998 and 2003 was performed using a polymerase chain reaction assay specific for Aphanomyces invadans. Of these, 26 (38.8%) samples from seven fish species and nine collection locations were positive. Confirmation of UM associated with Aphanomyces invadans represents new host records in Florida for the sheepshead Archosargus probatocephalus, striped mullet Mugil cephalus, white mullet Mugil curema, silver perch Bairdiella chrysoura, black drum Pogonias cromis, largemouth bass Micropterus salmoides, and American shad Alosa sapidissima.

Fish rhabdoviruses: molecular epidemiology and evolution

Rhabdoviruses may cause serious diseases in wild and farmed fish. Within the Rhabdoviridae six genera have been established: Ephemerovirus, Cytorhabdovirus, Nucleorhabdovirus, Lyssavirus, Vesiculovirus, and Novirhabdovirus. Viruses that infect fish are official or tentative members of the genera Vesiculovirus and Novirhabdovirus, or are listed as unassigned rhabdoviruses. In this report, we summarize and discuss published and our own unpublished data on the molecular epidemiology and phylogeography of fish rhabdoviruses including intrapopulational differences and subgrouping of fish rhabdoviruses, in particular the species spring viremia of carp virus (SVCV), infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV).

Characterization of snakehead rhabdovirus infection in zebrafish (Danio rerio)

The zebrafish, Danio rerio, has become recognized as a valuable model for the study of development, genetics, and toxicology. Recently, the zebrafish has been recognized as a useful model for infectious disease and immunity. In this study, the pathogenesis and antiviral immune response of zebrafish to experimental snakehead rhabdovirus (SHRV) infection was characterized. Zebrafish 24 h postfertilization to 30 days postfertilization were susceptible to infection by immersion in 10(6) 50% tissue culture infective doses (TCID50) of SHRV/ml, and adult zebrafish were susceptible to infection by intraperitoneal (i.p.) injection of 10(5) TCID50 of SHRV/ml. Mortalities exceeded 40% in infected fish, and clinical presentation of infection included petechial hemorrhaging, redness of the abdomen, and erratic swim behavior. Virus reisolation and reverse transcription-PCR analysis of the viral nucleocapsid gene confirmed the presence of SHRV. Histological sections of moribund embryonic and juvenile fish revealed necrosis of the pharyngeal epithelium and liver, in addition to congestion of the swim bladder by cell debris. Histopathology in adult fish injected i.p. was confined to the site of injection. The antiviral response in zebrafish was monitored by quantitative real-time PCR analysis of zebrafish interferon (IFN) and Mx expression. IFN and Mx levels were elevated in zebrafish exposed to SHRV, although expression and intensity differed with age and route of infection. This study is the first to examine the pathogenesis of SHRV infection in zebrafish. Furthermore, this study is the first to describe experimental infection of zebrafish embryos with a viral pathogen, which will be important for future experiments involving targeted gene disruption and forward genetic screens.