African swine fever (ASF) was first described in 1921 by Montgomery, who reported several disease outbreaks of domestic pigs in Kenya since 1910 with a mortality close to 100%. Montgomery recognized the viral nature of the disease, its likely transmission by wild swine which probably acted as virus carriers, and the lack of protection by passive immunization [1]. ASF is a menace to the pig population in the world because there is no vaccine, the virus multiplies in ticks and mutates easily, and different virus isolates can produce diseases with different clinical forms or no disease at all. The control and eradication of ASF require rapid diagnosis, drastic slaughter, and quarantine.

African Swine Fever Virus

Viruses and viruslike particles of eukaryotic algae.

Publisher Summary This chapter focuses on the advances in iridovirus research over the past decade. It summarizes salient features of the iridovirus research studies, and discusses their importance to future work. As knowledge of iridovirus biology increases, so does the understanding of the taxonomic structure of the family. A new genus (Megalocytivirus) has recently been created to accommodate a number of viruses isolated from diseased fish in Southeast Asia, and an established one (Chloriridovirus) is about to be validated following the sequencing of the genome of the mosquito pathogen IIV-3. Genomic sequencing projects have clarified evolutionary relationships between iridoviruses and other families of large, nucleocytoplasmic DNA viruses. Iridoviruses appear situated at the center of a clade of DNA viruses that include the poxviruses, phycodnaviruses,ASFV, ascoviruses, and possibly a new giant virus of amoebae (mimivirus). Not all iridovirus infections result in mortal disease. The recognition that many iridovirus infections are chronic, especially among insects, should stimulate studies on the importance of such illnesses on host fitness. Sublethal effects have been demonstrated in infected insects, and theoretical studies suggest such effects have a major influence on the dynamics of host populations. However, the consequences of chronic disease on the demography (fecundity, fertility, death rate, etc.) of infected amphibian and fish populations are notable by their absence. The ability to detect iridovirus infections has advanced markedly with the adoption of highly sensitive loop-mediated isothermal amplification and multiplex polymerase chain reaction techniques for rapid diagnosis of infected individuals.

A decade of advances in iridovirus research.

Molecular characterization, sequence analysis, and taxonomic position of newly isolated fish iridoviruses.

Complete genome analysis of the mandarin fish infectious spleen and kidney necrosis iridovirus.

We examined the structure of the frog virus 3 (FV 3) genome by using electron microscopic and biochemical techniques. The linear FV 3 DNA molecules (Mr approximately 100 x 10(6) formed circles when partially degraded with bacteriophage lambda 5'-exonuclease and annealed, but not when the annealing was done without prior exonuclease digestion. The results suggest that the DNA molecules contain direct terminal repeats. The repeated region composed about 4% of the genome. Complete denaturation of native FV 3 DNA molecules followed by renaturation produced duplex circles each bearing two single-stranded tails at different points along the circumference. The tails presumably represent the terminal repeats. The formation of duplex circles suggests that the FV 3 genome is circularly permuted. This is further borne out by (i) failure to identify a specific restriction endonuclease fragment containing the label when the molecular ends were radiolabeled by using the polynucleotide kinase procedure, and (ii) similarity in the restriction patterns of virion DNA and large concatemeric replicating viral DNA as revealed by endonucleolytic cleavage of both DNAs with HindIII. From the above data, we conclude that the FV3 genome is both circularly permuted and terminally redundant--unique features for an animal virus.

K G Murti

Papers

The genome of frog virus 3, an animal DNA virus, is circularly permuted and terminally redundant