COMMON TOADS ARE DECLINING, IS A HERPESVIRUS CONTRIBUTING?

Common Toads have been quietly declining across Flanders for years, without any obvious explanation. In our recent study, we looked at whether a little-known virus, bufonid herpesvirus 1, discovered less than a decade ago, might be part of the story. We found that this virus has spread dramatically over the past 13 years and is now present almost everywhere we looked, though its exact role in the toads' decline is still unclear.

Common Toad, Bufo bufo (© Ellen Blomme)

Why this matters?

Common Toads are one of Europe's most familiar amphibians, but their populations have been slowly shrinking for about 20 years, for reasons that aren't yet fully understood. In 2018, scientists discovered a new virus, bufonid herpesvirus 1 (BfHV1), which causes dark skin lesions in toads and was linked to a mass die-off in Switzerland. Since then it's turned up in the UK, Luxembourg and Germany. Until now, nobody knew how common it was in Belgium, or whether it could be contributing to the toads' disappearance.

Common toad with herpes lesions (© Naomi Terriere)

What we did and found

During the 2024 toad migration, we examined toads from 20 ponds across Flanders, half with shrinking populations, half stable, and also tested toad liver samples collected in 2011 for comparison. We found that:

• Skin lesions consistent with the virus were seen in toads at 19 out of 20 ponds in 2024.

• The virus was present in far more locations in 2024 than in 2011, rising from about 40% to almost 100%.

• Infected toads weren't in worse body condition, and infection wasn't linked to whether a population was declining or stable.

  
  

In other words: this virus has spread rapidly and is now widespread, but on its own it doesn't seem to be making toads visibly sicker or driving the decline in any straightforward way.

What's next?

While we can't pin the toad decline on this virus alone, its rapid spread alongside ongoing population drops means it can't be ruled out either, it may be one of several pressures, including habitat loss and pollution, adding up over time. Continued monitoring will help us understand whether its impact grows in the future.

Want to learn more?

You can read the full scientific study here. This research part of the OSSOBufo project investigating causes of decline of the Common Toad (Bufo bufo). If you want to learn more about this project, you can visit the project page on our website.

We thank the special research fund of Ghent University for funding this research (BOF23/GOA/008) as well as FWO (1105824N), and all the volunteers from Natuurpunt that assisted in the toad patrols. All samples were collected in conformity with national permits and regulations (ANB/BL-FF/V24-00174).