Ranaviruses; a threat to amphibians?

Alex D. Hyatt¹ and Andrew A. Cunningham²

¹ Australian Animal Health Laboratory, CSIRO, P.O. Bag 24, Geelong, Victoria, Australia 3022.
² Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, U.K.

Ranaviruses are a group of viruses belonging to the genus Ranavirus, family Iridoviridae. The ranaviruses identified to date differ in terms of host organisms, pathologenesis, antigenicity and geographic distribution. Ranaviruses have been identified in a range of cold-blooded (poikilothermic) vertebrates from Australia, South-East Asia, South America, Europe and North America. Most of the viruses have caused disease and death in wild animals. In Europe, ranaviruses have been associated with the disease and death of freshwater fish, frogs and toads. In North America disease and death have been associated with fish, tiger salamanders, turtles and frogs. In South-East Asia disease has been associated with tropical fish and commercial fish species and in China ranaviruses and disease have been associated with commercial fish and reptiles (turtles).

In Australia, ranaviruses have been associated with disease and death of fresh water fish but not wild/field amphibians. One ranavirus, however, has been isolated from metamorphosing frogs (Limnodynastes ornatus). The tadpoles appeared healthy when they were taken from the field but showed signs of disease in captivity. Whilst disease in wild amphibians has not been observed, antibodies have been detected widely in cane toads (Bufo marinus). The presence of antibodies indicates the presence of infection throughout the range of these toads. The consequence of the infection is not known. Laboratory studies have shown that both cane toads and native frogs are vulnerable, specifically, the tadpole stages are the most susceptible. Within North America inter-class infections have been documented in the wild from sympatric fish and amphibians. These results provide a clear indication that iridoviruses can infect wild animals belonging to different taxonomic classes of poikilotherms. These data, together with those from laboratory based infection trials, suggest that any vulnerable poikilothermic vertebrate species may serve as a reservoir for ranaviruses. Consequently, the importance of quarantine strategies on our wildlife cannot be under estimated. The recent detection of illegally imported pythons into Queensland from Irian Jaya and the subsequent detection of a new ranavirus demonstrate the importance of quarantine and the potential for release of exotic viruses into our environment.

Whilst we have recognised that ranaviruses are a threat, we are fortunate in that we have developed a range of diagnostic assays that can rapidly and accurately detect them. It is our practice to confirm identification by a range of procedures. This is an important point as there are some rana-like viruses that do not react in the classical diagnostic assays.

Key References:

Zupanovic, Z.; Hyatt, A. D.; Lopez, G. ; Green, B. ; Bartran, G. ; Parkes, H.; Whittington, R. J., and Speare, R. Toads (Bufo marinus) in Australia and Venezuela have antibodies against ranaviruses. Journal of Wildlife Diseases, Submitted for Publication. 1997.
Hyatt AD, Gould AR, Zupanovic Z, Cunningham AA, Hengstberger S, Whittington RJ, Kattenbelt J, Coupar BE (2000). Comparative studies of piscine and amphibian iridoviruses. Arch Virol;145(2):301-31
Hyatt AD, Coupar BEH, Hengstberger SG, Wise T, Gould AR, P Selleck P, Williamson M, Kattenbelt J, Middleton D, Cunningham AA and Lee J (2000). First identification of a ranavirus from the green tree python (Morelia viridis). J. Wildlife Dis. (submitted).