Infectious Disease

Emerging infectious diseases have become a major threat to amphibians and reptiles in the new millennium. Several diseases that pose a risk to Canadian species include Snake Fungal Disease (SFD), Chytridiomycosis (Chytrid Fungus; Bd), Salamander Chytridiomycosis (Salamander Chytrid Fungus; Bsal) and ranavirus.

SFD is caused by the fungus, Ophidiomyces ophiodiicola. First documented in 2006, SFD has caused mortality and population declines in several snake species throughout the eastern United States and was first confirmed in Canada in 2015 from an Eastern Foxsnake in Point Pelee, Ontario. In 2016, SFD was confirmed at an additional two sites in southwestern Ontario, and the fungus O. ophiodiicola has been confirmed in snakes from several other locations in southern and central Ontario, although it is not known if the fungus was causing clinical symptoms of the disease in those specimens. Skin lesions and subcutaneous swelling are primary symptoms in pit-vipers like rattlesnakes, while SFD more often affects the respiratory system in non-venomous species. A proactive approach to preventing the spread of SFD and managing outbreaks is necessary to combat this new threat.

Bsal is a microscopic fungal pathogen of Asian origin that recently surfaced in the Netherlands and Belgium, where it caused a 96 per cent decline in European Fire Salamander populations in only a few years. Bsal causes skin lesions and variable secondary infections, with up to 100 per cent mortality in susceptible species. Bsal has not been detected in North America, but lab studies indicate at least two Canadian salamander species are vulnerable. If accidentally introduced to North America, experts warn that Bsal could trigger a salamander mass-extinction. Release of infected salamanders imported for the pet trade, or the improper disposal of terrarium/aquarium water in which they were kept, is the most likely route of introduction. A Bsal threat assessment by the Canadian Wildlife Health Cooperative stresses the importance of preventing introduction to Canada because it may be impossible to eradicate once present, and other disease-control options are untested, have limited effectiveness, or aren’t feasible. CWHC and other national and international experts have called for an immediate ban on international trade in amphibians until a coordinated screening program is in place. The Pet Industry Joint Advisory Council (PIJAC) has implemented a voluntary ban on the import of Paddle-tail Newts and Fire-bellied Newts (two carriers of Bsal) in the U.S. and Canada. The U.S. Fish and Wildlife Service has banned the trade on 201 salamander species under the Lacey Act, and a temporary trade ban is being considered in Canada.

Fears around Bsal rest on what has already occurred with a related fungus. In the past two decades, Batrachochytrium dendrobatidis (Bd) or “chytrid”, has cut a deadly swath through the world‘s frogs. Populations and species have disappeared at alarming rates—even in pristine and protected wilderness areas. The toll began with the mid-’80s extinction of the Golden Toad (Bufo periglenes) of Costa Rica’s Monteverde cloud forest, which vanished within a year, and two species of Rheobatrachus, Australia’s Gastric-brooding Frogs (the young developed in the mother’s stomach), lost from the undisturbed rainforests of Queensland. Statistics from the epidemic are staggering. As of 2008—the internationally declared Year of the Frog—120 amphibian species had fully disappeared since 1980, and some 30-50 per cent of all frog species were declining. Ecosystem-level extinctions in Central America comprised a wave of disaster moving south at about 28 km/year. Dozens of tropical frog species disappeared or were displaced from high elevations. A multi-country conservation initiative in the Andes with genus Atelopus showed that of 110 species, one third had disappeared and the rest were at risk. Similarly, in the South American genus Craugaster, only 1 of 38 species remained. Amphibians’ intrinsic vulnerability to the fungus is exacerbated by other factors including climate change. For example, hot and dry conditions make it difficult for frogs to re-hydrate, and subsequent water stress causes them to huddle together where chytrid spores are easily spread. As with all of ecology, the chytrid epidemic likely doesn’t have a single cause—nor so other emerging infectious diseases like ranavirus and SFD.

The fungus has been found throughout much of Canada. Which, if any, amphibians are most at risk is unknown, but already threatened and endangered species would top the list—Rocky Mountain Tailed Frog, Red-legged Frog, Leopard Frog (in BC), Oregon Spotted Frog, Great Basin Spadefoot Toad, Western Toad, and Fowler’s Toad. American bullfrogs—invasive around the world (see below) appear immune and may be carriers of the fungus. On Vancouver Island, Bd has been detected on about 50 per cent of individual invasive bullfrogs.

Ranaviruses are DNA (as opposed to RNA) viruses of the family Iridoviridae that cause a sudden onset of severe illness with gruesome symptoms (e.g., massive hemorrhaging, fluid accumulation under the skin and bloating), followed by death within hours or days. Amphibian populations can experience mortality rates as high as 90-100%, particularly the larvae, and the virus can decimate entire populations in a matter of days. An emerging worldwide threat going back 10-15 years, the range of ranaviruses is expanding and susceptible species include fish, turtles and amphibians. It is transmitted through ingestion of infected organic material, water and substrates, as well as skin injuries and by blood-feeding parasites like mosquitoes. Ranaviruses can also be detrimental to turtle populations; mortality was greater than 80 per cent in a study with captive Red-eared Sliders, and several wild populations of Eastern Box Turtle have been virtually wiped out by the virus. One ranavirus strain—RV3—has been shown to jump classes, from fishes to turtles to frogs and back to fish.

In response to the growing threat that disease poses to Canada’s native amphibians and reptiles, the Canadian Herpetofauna Health Working Group was formed in 2015 to facilitate inter-agency information sharing and provide advice to government on herpetofauna health issues in Canada. The Canadian Wildlife Health Cooperative (CWHC) monitors wildlife disease in Canada and is an excellent resource for information about amphibian and reptile disease in Canada, including fact sheets about all of the diseases discussed in this section.

Infectious Disease

Further Reading