Chytridiomycosis in a free-ranging chorus frog in Quebec.
A chorus frog found in Boucherville (Montreal’s south shore) at the end of April presented weakness and incoordination of the pelvic limbs. Erythema (redness) of the ventral portion of the animal was also present (figure 1).
Due to its poor health, the frog was euthanized. A necropsy was performed by one of the veterinarians at the Biodôme de Montréal who submitted the sampled tissues to the CWHC – Quebec regional centre for histological and molecular analyzes. Microscopic examination of the skin revealed the presence of skin changes highly characteristic of chytridiomycosis, including hyperkeratosis and loss of integrity of the epidermis associated with the presence of numerous clusters of fungal spores (figure 2).
Infection with the fungus Batrachochytrium dendrobatidis (Bd), the agent of chytridiomycosis, was confirmed by molecular analysis (PCR). These observations lead us to believe that the clinical signs observed in this frog were caused by this fungal disease.
Chytridiomycosis at a glance
Chytridiomycosis is an infectious disease that affects many species of amphibians around the world. This emerging condition has been associated in recent decades with massive mortalities in several amphibian populations, mainly in Australia and Central America; chytridiomycosis has been identified as the cause of the decline, and even extinction, of over 200 species of amphibians. Globally, the fungus causing chytridiomycosis is recognized as the infectious agent associated with the greatest loss of biodiversity ever documented.
Situation in the province of Quebec
Cases of chytridiomycosis infection in free-ranging amphibians are rare, even though there are indications that the fungus responsible for this disease has been present in amphibians from the province for at least 60 years. While 38% of the chorus frogs sampled between 1993 and 2001 were carriers of this fungus, this case is, to our knowledge, the first documentation of the disease in a free-ranging chorus frog in the province.
Although this recent observation of chytridiomycosis is a reminder that this disease is indeed present in southern Quebec, no mass mortality associated with this condition has been observed to date. It is possible that chorus frogs are relatively resistant to chytridiomycosis or that mortalities simply go undetected. In fact, an experimental infection performed in the laboratory specifically in the boreal chorus frog was associated with an 85% mortality rate in this species. However, the presence of this pathogen in populations of this species in Arizona does not appear to be associated with a decrease in the abundance of the species. Although B. dendrobatidis is likely to cause mortality, a resilient population could likely compensate for the losses associated with this condition and even display growth. On the other hand, the impact of this pathogen on the dynamics of a fragile population remains a threat that should be assessed.
The decline of chorus frogs
Two species of chorus frog are found in Canada: the boreal chorus frog (Pseudacris maculata) and the western chorus frog (P. triseriata). Since these two species are very difficult to differentiate, there is some debate regarding the identity of the species of chorus frog that is present in southern Quebec. Although until recently it was believed that they were primarily western chorus frogs, genetic studies suggest that they are in fact, at least in part, boreal chorus frogs. Regardless of this classification, the chorus frog populations in southern Quebec and eastern Ontario are considered threatened. In Quebec, the species is in decline and it is estimated that 30% of the breeding sites have been lost during the last 10 years in Montérégie.
Urban development, changes in agricultural practices and the expansion of the road network are increasingly fragmenting the habitats essential to the survival of the species. Other elements, such as climate change, which alter hydrological characteristics, invasive alien species, and diseases, such as chytridiomycosis, could also have a negative impact on these populations. A healthy population is a resilient population that can compensate for the losses associated with mortality. The fragility of the chorus frog populations present in southern Quebec could therefore reduce their ability to cope with various stressors such as chytridiomycosis.
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Submitted by: Stéphane Lair CWHC-Québec