The frogs disappeared, then people got sick.  It was not a trivial coincidence.

The frogs disappeared, then people got sick. It was not a trivial coincidence.

Since the start of the global pandemic in 2020, the world has become increasingly aware that the health of our species is closely linked to that of other animals. Today, the conversation mostly focuses on birds and mammals, with amphibians rarely considered – but that can be a dangerous oversight.

A recently published study on frogs and malaria illustrates how human health can be affected by these lovable, if somewhat slimy creatures.

In the 1980s, conservationists in Costa Rica and Panama began to notice a quiet and dramatic decline in amphibian numbers.

Frogs and salamanders in this part of the world were preyed upon by a virulent fungal pathogen (Batrachochytrium dendrobatidis), and they were doing so at such a rapid rate that researchers at the time feared a wave of local extinctions.

Some scientists now claim that this pathogen, called Bd for short, has caused “the greatest recorded loss of biodiversity attributable to disease” ever recorded, being responsible for a significant decline in at least 501 species of amphibians, including 90 extinctions, from Asia to South America.

It’s obviously a massive claim, but amphibians are now considered to be among the most endangered groups of animals on Earth, and the worldwide spread of this fungus and others like it are at least partly to blame.

Frogs and salamanders directly influence the size of mosquito populations because mosquitoes are an essential food source, which means that the number of amphibians could ultimately influence the vectors – living organisms that can transmit infectious pathogens – which spread deadly human diseases.

Using Central America as a case study, researchers have now tried to illustrate how creatures like frogs can ultimately benefit human health.

The results, which were first presented in 2020, have now been peer reviewed and show that amphibian losses due to Bd led to a substantial increase in the incidence of malaria – a mosquito-borne disease infected – first in Costa Rica in the 1980s and 1990s, then again in Panama in the early 2000s as the fungus spread eastward.

To the authors’ knowledge, this is the first causal evidence of amphibian losses impacting human health in the wild.

The study used a multiple regression model to estimate the causal impact of Bd-induced amphibian decline on county-level malaria incidence in Costa Rica and Panama.

By comparing a map of amphibian decline and a map of malaria incidence between 1976 and 2016, the researchers found a clear trend that could be predicted with high accuracy and confidence by their model.

In the eight years since Bd’s substantial amphibian losses, there has been a spike in malaria cases equivalent to approximately one additional case per 1,000 people. This additional case would in all likelihood not have occurred had it not been for the recent mass amphibian die-off.

In a typical malaria epidemic, incidence rates typically peak at around 1.1 to 1.5 cases per thousand people. This means that a loss of amphibians in Central America could have resulted in a 70-90% increase in the number of people falling ill.

“The diagram shows a west-to-east wave spreading from the northwest border of Costa Rica around 1980 to the Panama Canal region by 2010,” the authors write in the paper.

After eight years, however, the estimated effect is suddenly reduced, and the researchers don’t know why.

The authors may suggest that an increase in malaria cases prompts greater use of insecticides, which then causes cases to fall again in accordance with this cycle.

Future studies of other mosquito-borne diseases, such as dengue fever, could help establish the link between the loss of amphibians and a growing threat of mosquito-borne diseases.

The researchers were only able to obtain national data on dengue cases in Panama, not county-level data, but at this lower resolution the results also suggest an increase in dengue following the decline of amphibians.

From 2002 to 2007, the increase in dengue fever cases over the previous eight years was 36%.

“This previously unidentified impact of biodiversity loss illustrates the often hidden human well-being costs of conservation failures,” the authors write.

“If scientists and policymakers fail to consider the ramifications of these past events, they also risk failing to fully motivate protection against new calamities, such as the international spread of an emerging and closely related pathogen. Batrachochytrium salamandrivorans through an incompletely regulated trade in live species,” they add.

As you read this, B. salamandrivorans is going around the world with global trade, and it threatens not only the future of amphibians, but the health of our own species.

As this study reveals, the health of frogs and that of humans often go hand in hand. We are stuck together whether we like it or not.

The study was published in Environmental Research Letters.

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