The goldfish in garden ponds may be pretty, but they’re bad news for frogs. Amphibians are under attack from ranaviruses, a group of deadly pathogens that cause ulcers and hemorrhaging. Now researchers have found that more frogs tend to die from ranavirus infections in ponds with exotic fish.

Ranaviruses have killed amphibians around the world, and as many as 200 animals may die in a single event. The viruses also infect fish and reptiles. Scientists suspect that imported goldfish and bullfrogs carried a ranavirus into the UK, where the pathogen has devastated common frog populations.

But researchers know little about what aids or suppresses the disease. To investigate, a team studied reports submitted by UK residents from 1992 to 2000. These citizen scientists provided information about common frog deaths in their garden ponds, including the disease symptoms and the number of dead frogs. The reports also detailed whether the ponds contained fish (usually goldfish or koi) and whether the owners had used herbicides, slug pellets, or other chemicals.

In one analysis, the researchers focused on 702 die-offs from May to September, when the disease is the most common, in which the frogs showed the tell-tale signs of ulcers or hemorrhaging. The percentage of frogs that died in each event was higher in ponds containing fish, the team reports in PLOS ONE. Die-offs also tended to be worse in ponds that were deeper or treated with herbicides or slug pellets.

The fish’s presence might make frogs forage less or produce more stress hormones, the researchers say. Those changes could in turn weaken the frogs’ immune system. Chemicals used for garden maintenance also could affect immune response, or they might indirectly create more favorable conditions for the virus to replicate. So garden owners may need to choose between stocking their ponds with pretty ornamental fish — or creating a more frog-friendly environment. — Roberta Kwok | 4 June 2015

Source: North, A.C. et al. 2015. Anthropogenic and ecological drivers of amphibian disease (Ranavirosis). PLOS ONE doi: 10.1371/journal.pone.0127037.

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