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Invertebrate iridescent viruses (Iridoviridae) (click for PDF reprints on iridoviruses)

Invertebrate iridescent viruses (IIVs) are icosahedral particles of 120 � 180 nm in diameter, containing an internal lipid membrane and a double-stranded DNA genome of 130 � 210 kbp, that belong to the family Iridoviridae. The genome is a linear molecule that is circularly permuted (i.e., it appears circular when mapped by restriction endonucleases) and terminally redundant (i.e., the terminal sequences overlap by about 5 � 10%). They mostly infect arthropods, particularly insects, in damp or aquatic habitats worldwide (see complete list of invertebrate hosts: Williams 2008).
 

Iridovirus infected (blue) larva of Aedes aegypti next to a healthy larva.

IIV particles are organized into crystalline arrays. Light reflected from such arrays interferes with incident light, resulting in the characteristic iridescent colours that are the most obvious sign of patent infection. Patent disease is lethal in the larval or pupal stages.

The small IIVs (genus Iridovirus) tend to display colours from violet to turquoise. The large IIVs, that infect mosquito and midge larvae (genus Chloriridovirus), tend to display colours from green to orange or red.

Many IIVs possess an external fringe of fibrils extending from the surface of the capsid. When these fibrils are particularly long, the interparticle distance may be increased resulting in an absence of iridescence. The most studied member is Invertebrate iridescent virus 6 (IIV-6), also known as Chilo iridescent virus.  IIV-6 is the type species of the Iridovirus genus (see map of geographical distribution of recognized and tentative invertebrate iridovirus species).
 

Paracrystalline arrays of iridescent virus particles in a cell of a patently-infected copepod (Gladioferens pectinatus) from the Hopkins River, Australia. Image courtesy of �R.L. Roennfeldt. Do not reuse without consent.

Importantly, covert infections that do not kill the host may be common.

Little is known about the factors that determine the virulence of these viruses, but it is assumed that covert infections open the way to vertical transmission of the virus from parent to offspring.

IIVs are highly infectious by injection but have low infectivity by ingestion.

Horizontal transmission can occur by cannibalism or predation of patently infected individuals, or the virus may even be vectored by nematodes and parasitoid wasps that introduce IIV particles into the host insect during the act of penetration or oviposition.

IIVs appear to be highly sensitive to dehydration and rapidly loose activity in dry environments. In contrast, persistence in moist conditions depends on exposure to solar UV light, high temperatures and microbial activity.

Preliminary studies indicate high genetic heterogeneity in IIV populations. Restriction endonuclease analysis of isolates from Diptera and Lepidoptera suggests that individual insects collected at the same place and time may harbour genetically distinct variants.

Quantification of intraspecific variation in IIV genomes would greatly improve our ability to define IIV species and would represent an initial step towards understanding the intriguing relationship between genotype and phenotype in these little-studied viruses.
 

Covert infections by many IIVs can be detected by PCR techniques or by a simple bioassay using larvae of Galleria mellonella (Lepidoptera: Pyralidae)
 

Larvae of G. mellonella develop a patent infection that is readily distinguishable from the appearance of a healthy insect

My work has focused on:

• Evaluation of the influence of covert infections on the reproductive capacity, size and longevity of the infected host.

• Mechanisms and dynamics of horizontal transmission.

• Persistence of IIVs in the environment

• Quantifying the sensitivity of methods for the detection of covert infections.

• Classification of these viruses based on genomic characteristics.

• IIV diversity (intra- and inter-specific).

Click here for iridovirus publications in PDF format
 

ICTV website on virus classification (Iridoviridae)

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