Ranavirus

This fact sheet has been updated on March 21st, 2020. 

Cause

Ranavirus (family Iridovirdae), is a double stranded DNA virus that can cause severe infections in a number of cold-blooded taxa, including amphibians, reptiles, and fish. There are several different species of Ranavirus that can cause varying levels of disease in the infected animals. Some of the best known ranaviruses include: epizootic hematopoietic necrosis virus and largemouth bass virus (both piscine ranaviruses), frog virus-3 and tadpole edema virus (in frogs), and Ambystoma tigrinum virus in tiger salamanders.

Significance

Ranavirus is believed to be the cause of several recent mass mortality events in amphibian populations across the globe. With a mortality rate of 90%-100%, the disease has the potential to eliminate entire species if not controlled. Scientists in the United States have identified ranavirus to be associated with mortality in over 25 states in 20 different species of turtles and amphibians. Ranavirus outbreaks can affect multiple species at the same time and are often associated with acute die-offs and population declines of wild amphibians.

Ranaviruses are considered a Notifiable Pathogen by the OIE (the World Organization for Animal Health).

Species Affected

Ranaviruses affect amphibians, reptiles, and fish, though susceptibility varies by species. Natural infections have been identified in at least 91 species representing 14 families. The most commonly infected and impacted family is the Ranidae. In the United States, the USGS National Wildlife Health Center has identified ranaviruses in at least 16 species of frogs, 1 species of toad, and 6 species of salamanders, including the true frog, the tree frog, and the mole salamander.

Ranavirus can also severely affect both captive and wild populations of eastern box turtles and true tortoises and has been diagnosed in snakes and lizards. The viruses have been linked to infections and mortality in several species of boney fish, including the pallid sturgeon and the threespine stickleback. Ranaviruses affect these animals at all life stages, though in general, mortality rates are highest if the animal contracts the virus during the larval stage (hatchlings and metamorphs).  

At this time, ranavirus is not known to infect humans or other warmblooded species.

Distribution

Ranaviruses have a truly global distribution; infections and associated moralities have been documented in North and South America, Europe, Asia, Australia and Africa. In the United States, confirmed cases have been documented in the northeast, southeast, midwest, and northwest. There have also been cases in parts of southern Canada.

Transmission

Transmission of ranavirus occurs horizontally, through direct contact, ingestion of the virus, ingestion of infected animals, or exposure to infected soil or water sources. Vertical transmission is thought to occur but remains unverified. Substantial evidence exists for interclass transmission among amphibians, reptiles, and fish during a disease outbreak. Mortality events may be seasonal, based on the susceptibility of the infected hosts and life stage(s) present. Disease and associated mortality may occur in as little as 3 days post-infection, though it may also take up to several weeks.

Ranaviruses are highly persistent and are able to remain infectious in the environment for a period of time, likely surviving for months in water under favorable conditions.

Clinical Signs

Frequently, the first sign of an outbreak of ranavirus is the sudden onset of illness and death in large numbers of amphibians and/or reptiles over a 1-5 day period. Clinical signs in amphibians include mild to severe hemorrhages in the skin, especially near the base of the hind limbs and the vent opening, lethargy, weak or erratic swimming, buoyancy problems, gasping for air, and mild to severe fluid accumulation under the skin of the abdomen or hind legs. At necropsy, there may be fluid accumulation in the body cavity and hemorrhages on the surfaces of the heart, stomach, and liver. Occasionally, the liver or spleen are affected and ulcers in the skin and palate may be observed. In turtles, clinical signs include weakness, swollen eyelids, discharge from the nose and mouth, ulcers on the feet, and dull white or thick yellow plaque on the tongue, palate, pharynx, and esophagus.

Diagnosis

Ranavirus infection and disease can be diagnosed through primary isolation and cell culture of the virus, molecular identification (PCR and sequencing), and light or electron microscopy.  

Treatment

To date, there is no treatment or vaccine for ranavirus.

Management

Since there is no treatment for ranavirus, management is focused on biosecurity, quarantine, and decontamination to prevent the spread of the virus to other animals and new environments. Any infected animals should be quarantined to prevent infection of other animals. Water from captive facilities and home aquariums should be disinfected prior to disposal to prevent the virus from spreading into new water sources. All equipment, surfaces, and storage tanks in captive facilities should be disinfected after use. Wildlife biologists, veterinarians, rehabilitators, and anyone else involved in amphibian or reptilian fieldwork should employ strict biosecurity protocols and clean and disinfect all equipment and clothes before and after working at a field site.

Suggested Reading

Images

Global distribution of known Ranavirus cases (2015).Global distribution of known Ranavirus cases (2015).
Tadpole (American bullfrog, Rana catesbeiana) from Georgia (USA) with a likely Ranavirus infection. Tadpole (American bullfrog, Rana catesbeiana) from Georgia (USA) with a likely Ranavirus infection.
Box turtle with clinical signs of Ranavirus, including swollen eyelids, nasal discharge, and yellow/opaque plaques in the mouth. Box turtle with clinical signs of Ranavirus, including swollen eyelids, nasal discharge, and yellow/opaque plaques in the mouth.