Ranavirus associated mortality event in frogs in Ontario

Amphibians are uncommon submissions at the CWHC in Ontario, typically because dead amphibians are small and semiaquatic and likely to go unnoticed. However, when amphibians die in large numbers in a mass mortality event, they are much more likely to be spotted and submitted.

Northern Leopard Frogs (Lithobates pipiens)

In mid-September 2019, eight juvenile frogs were found dead or dying around a kettle lake 30 minutes north of Mississauga. The next day 11 juvenile frogs (northern leopard frogs and green frogs) were found dead. Five days later, 24 juvenile northern leopard frogs, one adult northern leopard frog, ten minnows, and one tadpole were found dead in the same lake. Some of these frogs were observed to be lethargic, emaciated, and swimming irregularly prior to death and some displayed redness on the hind legs and on the throat, a nonspecific sign of disease in amphibians.

A handful of these frogs were submitted to the CWHC for postmortem examination. All of the frogs that were tested using quantitative polymerase chain reaction (qPCR) were positive for Frog Iridovirus-3 (FV3), a ranavirus in the family Iridoviridae. On microscopic examination, one of the frogs had mild multifocal hepatic necrosis with intracytoplasmic inclusion bodies, supporting the diagnosis of FV3 infection. In this outbreak, ranavirus infection was only able to be diagnosed in amphibians due to the advanced decomposition of the minnow specimen.

FV3 has been associated with mortality events in amphibians, bony fish and reptiles. In amphibians, it typically affects tadpoles and frogs undergoing metamorphosis most severely. FV3 has been present in southern Ontario since at least 1999, having been found in species such as the spring peeper, green frog, northern leopard frog and eastern newt. In 2018, FV3 infection was diagnosed in snapping turtles and wood turtles in Ontario for the first time. In amphibians, anthropogenic stressors such as agricultural activity and road traffic can increase the prevalence of ranavirus infections.

Although ranaviruses are considered to be a significant threat to ectothermic wildlife populations, a general lack of longitudinal population data and surveillance makes it difficult to predict what effect this outbreak could have on local amphibian and fish populations. Ranaviruses that affect amphibians are listed as notifiable by the World Organization for Animal Health (OIE).

The discovery of the frogs and minnows was made by Credit Valley Conservation (CVC) staff working in the area as part of CVC’s ongoing Natural Heritage Inventory (NHI) program. The NHI program identifies and documents species distribution and natural features across the Credit River Watershed to improve understanding of the composition and significance of natural areas. CVC and their partners use this data to inform and support activities such as land cover mapping, conservation lands management and municipal natural heritage systems development.

Christina McKenzie, DVM

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