Deadly frog fungus now thrives where we thought it couldn’t survive

A fungus devastating frog and toad populations around the globe was believed to be restricted to chilly, mountainous climates – but new research shows it isn’t.

Chytrid fungus, also known as Batrachochytrium dendrobatidis or Bd, is a deadly skin disease that stops the skin of frogs and toads from regulating the movement of water and electrolytes. It has been linked to dramatic declines in amphibian numbers around the world, wiping out 200 of the world’s 7800 species and infecting hundreds more over the past 40 years.

Those living in cooler climates, such as the mountainous regions of Australia, North America, Central America and South America, are hardest hit. Biologists have long believed that the pathogen can’t survive in warm tropical lowlands, as there have been no recorded deaths due to the fungus.

To understand how the infection was spreading, Alison Davis Rabosky at the University of Michigan and her colleagues tested frogs for the pathogen in lowland and elevated sites in Peru’s Amazon rainforest.

Far more prevalent

Between 2016 and 2017, the group collected and swabbed 324 frogs living in two warm, lowland sites and a cooler site situated 850 metres above sea level. In total, their study involved 80 different frog species.

The infection was far more prevalent than the researchers were expecting at the two warmer lowland sites. It affected about 24 to 38 per cent of frogs tested, the variation depending on the time of year they collected samples. About 46 per cent of the frogs at the cooler highland site were affected.

Individual frogs were sometimes found with 100,000 fungal spores. This was particularly surprising, because levels of more than 10,000 spores are considered “critically high”, says Davis Rabosky.

Despite this, infected frogs living in lowland areas didn’t have symptoms common in frogs living at higher elevations, such as red and peeling skin, lethargy and failing to flee from intruders.

Lab studies have shown that the chytrid fungus can’t survive in temperatures over 29°C – but Davis Rabosky and her colleagues found it in frogs living in environments where temperatures are routinely higher than that.

Read more: The frogs bouncing back after almost being wiped out by disease

It isn’t clear whether this is a new outbreak or whether the fungus has been in the lowland areas for many years. If it is a new infection in the region, then the frogs might start becoming sick and dying in the near future, says Davis Rabosky.

Otherwise, there may be something that protects the frogs in lowland regions from succumbing to the effects of the fungus.

Cold, mountainous climates may put more stress on the body, allowing the infection to wreak havoc, or species in those areas may be more susceptible, says co-author Timothy James at the University of Michigan.

Even if the lowland frogs aren’t displaying the obvious signs of infection, such high levels of the fungus would likely have some detrimental effect on their general or reproductive health, says James.

The findings suggest that animals in lowland regions may be spreading the disease, say the authors. Rabosky hopes the research will prompt other frog and toad researchers to take skin swabs of every animal they catch.