Tadpole die-offs in New Brunswick due to ranavirus, summer 2009

Outbreaks of ranavirus infection were recorded near Oromocto, New Brunswick, beginning in early June 2009.  The outbreaks were noted by researchers conducting amphibian studies in the area and were initially apparent as sudden massive die-offs of tadpoles in up to 9 ponds.  The species involved included Wood frogs (Lithobates sylvaticus, formerly Rana sylvatica), Spring peepers (Pseudacris crucifer) and Green frogs (Lithobates (Rana) clamitans).  Representative individuals from 8 different ponds were collected and kept either frozen or stored in 90 or 70% ethanol for subsequent examination.  The tadpoles, a total of 61 individuals, were then submitted to CCWHC for confirmatory diagnosis.  Microscopic examination revealed what appeared to be inclusion bodies in the cells of the liver.  This change is characteristic of ranavirus infection but it can also be present as an artifact of preservation.  Thus, confirmation of the diagnosis was sought via electron microscopy and a PCR test.  Samples from all ponds, either as whole tadpoles or as sections of livers from the largest specimens, were submitted for confirmation of infection via PCR.  Electron microscopy results were disappointing, perhaps in part due to poor tissue preservation, but PCR results were confirmatory of the presence of high loads of ranavirus in the tadpoles from 5 ponds.

a
Ranavirus is a member of the Iridoviridae, a family of viruses that infects fish and amphibians, sometimes with severe consequences.  Ranavirus is particularly damaging to larval amphibians, or tadpoles, which are very susceptible to infection.  The infection can  kill 98% of the  tadpoles in a pond.   It is thought that the virus remains in the few tadpoles that survive infection and are able to undergo metamorphosis; as adults, they will then carry the infection without obvious signs of disease and may transfer it to the next generation of tadpoles the following year.  (Picture of adult wood frog, right).

a

Although this is the first reported outbreak of ranavirus in New Brunswick, similar outbreaks were documented yearly in Acadia, Maine, from 2000 to 2005, and may have occurred but gone unconfirmed in later years.  Further research into the New Brunswick outbreaks is planned for the summer of 2010.

a
Ranavirus is highly contagious and easily transferred from one pond to the next by the movement of amphibians or other animals and by using contaminated equipment, such as rubber boots or fishing gear. A thorough clean up and disinfection of such equipment will help prevent, or at least delay, further dissemination of the disease.  Field disinfection protocols are available from various online sources and more thorough protocols.

By: María Forzán (Canadian Cooperative Wildlife Health Centre, Charlottetown, PEI), Megan Gahl (University of New Brunswick Saint John, Saint John, NB) and Bruce Pauli (Environment Canada, Ottawa, ON).  Photos provided by M.Gahl (ponds) and M. Forzán (wood frog).