Iridoviridae

• “Iridoviridae” derived from Greek/Roman
goddess of the rainbow “Iris”.
• This is due to the iridescence that can be
observed in heavily infected insects.

Iridoviridae
First found in 1954 in the larvae of a crane fly
Most likely found world wide

Large, 120-300 nm in diameter
Linear ds DNA
150 to 280 kbp
Can have an envelope, acquired by
budding through the host’s membrane.
Viral Genome and Structure

Iridoviridae
• Found in invertebrate and non-mamalian
vertebrate hosts.
• 4 genera
Ranavirus----amphibians & reptiles
Lymphocystivirus------fresh and marine fish
Iridovirus----invertebrates (including barnacles)
Choloridovirus----insects

Genus Vernacular name Type species
Iridovirus Small iridescent insect virus Chilo iridescent virus (IV6)
Chloriridovirus Large iridescent insect viruses Mosquito iridescent virus (IV3)
Lymphocystivirus Lymphocystis disease virus Lymphocystivirus type 1 (LCDV-1)
Ranavirus Frog virus Frog Virus 3 (FV3)
Iridoviridae Diseases

Virus Structure
Virion composed of three
concentric domains
1. Outer proteinaceous
icosahedral capsid
(Common feature of all genera, makes up
~45% of total virion protein)
2. Intermediate lipid membrane
3. Central core

Viral Replication
1. Virus enters host via endocytosis
and uncoating occurs
2. Viral DNA transported to the cell nucleus
and transcription is initiated by the host’s
RNA polymerase II
3. Parental DNA used to produce genome and greater
than genome length DNA that is used as a template
4. Progeny DNA transported to the cytoplasm where
large concatamers are formed
5.Concatemers then packaged into virions and
exit host by budding or cell lysis

Iridoviridae Replication

Disease Presentation
Chronic or benign skin infections (lesions, ulcers),
enlarged cells of organs. Problematic aesthetically.
In insects: iridescent patches
In fish: swim bladder expands, loss of equilibrium
In amphibians: has been implicated in mass die offs

Pathogenesis
Little is known about the pathogenesis of
iridoviruses. The pathogenesis is, however,
temperature dependent and iridoviruses are
thus confined to cold-blooded hosts. In a
lethal infection by insect iridoviruses the fat
bodies and haemocytes are the initial sites of
replication, this leading to a systemic
infection. Insects become flaccid and
iridescent 7-10 days post-infection although
death may take 3 weeks or longer.

Pathogenesis
Virus is very stable and can survive outside the host
It can tolerate a wide range of pH (4-12) and is able to
overwinter in the bottom of ponds.
Could be transmitted through water, on feathers or
beaks of birds, nets, or through cannibalism.

Possible Implications
Insects: Apiculture, Biocontrol
In insects, virus has been isolated
from black flies, Japanese beetles,
corn earworms, rice stem borers,
locusts, mosquitoes, and honey bees

Possible Implications
Vertebrates: Aquaculture, Animal
Husbandry, Zoos, Ecological
In fish: systemic disease found in Gouramis (pet fish),
salmonids (salmon & trout), catfish, Large Mouth Bass
In reptiles: Box turtles, Gopher Tortoise, Green Tree Python
In amphibians: Bullfrog, Edible Frog, Leopard Frog, Tiger
Salamander, Red Spotted Newt Eft.
*Because of similar characteristics in disease among fish,
amphibians, and reptiles, one of these species may act as a
reservoir host or amplifying host to others in the same
environment.

Release
August 8, 2000
USGS Diagnoses Causes of Many U.S. Amphibian Die-Offs
U. S. Geological Survey (USGS) scientists are making headway in unraveling clues
to the causes of massive die-offs of frogs and other amphibians. The agency
announced today that a little-understood, emerging iridovirus disease associated
with large die-offs of frogs and salamanders in the Midwest and the East has caused
another recent die-off, in North Dakota.
USGS wildlife pathologist D. Earl Green said an iridovirus infection is the culprit in
most of the deaths of U.S. western tiger salamanders at the U.S. Fish and Wildlife
Service's Cottonwood Lake Study Area near Jamestown, North Dakota.
"The U.S. Geological Survey is leading the government's efforts to help determine
why amphibians are disappearing," said Interior Secretary Bruce Babbitt. "This is a
crisis that has attracted worldwide concern. It requires timely, aggressive research. It
is no exaggeration to say that USGS research on these die-offs has global
implications."

Amphibian Decline
• Ranavirus
• World wide decline of frogs, salamanders and other
amphibians
• Amphibians hypothesized to be “sentinels of
environmental degradation” because of characteristics of
their biology and physiology, such as permeable eggs, skin
and gills and complex life-cycles, which make them more
susceptible to these effects.
• North American species under threat is the Tiger
Salamander Ambystoma tigrinum.

African Swine Fever
• Originally grouped as an
Iridovirus now has its own
Family: Asfarviridae from
African Swine Fever And
Related Viruses
• Spread mechanically and
with a tick (Ornithodoros
spp.) vector
• In severe disease can cause
up to 100% mortality in
pigs.