The frogs disappeared, then people got sick. This was no harmless coincidence. : ScienceAlert

Since the global pandemic began 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 focuses mainly on birds and mammals, with amphibians rarely considered, but this can be a dangerous oversight.

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

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

Frogs and salamanders in this part of the world were falling prey to 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 argue that this pathogen, called Bd for short, has caused the largest “disease-attributable loss of biodiversity” ever recorded, being responsible for significant declines in at least 501 amphibian species, including 90 extinctions, since from Asia to South America.

This is obviously a massive claim, but amphibians are now considered among the most threatened 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 a key food source, meaning the number of amphibians could influence vectors, living organisms that can transmit infectious pathogens, which they spread deadly human diseases.

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

The findings, which were first presented in 2020, have now been peer-reviewed and show that losses of amphibians caused by Bd led to a substantial increase in the incidence of malaria, a disease transmitted by infected mosquitoes, first in Costa Rica in the 1980s and 1990s, and again in Panama in the early 2000s, when the fungus spread eastward.

According to the authors, this is the first causal evidence of amphibian losses affecting human health in a natural environment.

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

Comparing a map of amphibian decline and a map of malaria incidence between 1976 and 2016, the researchers found a clear pattern that their model could predict with high accuracy and confidence.

In the eight years following substantial amphibian losses to Bd, there was an increase in malaria cases equivalent to about one additional case per 1,000 people. This additional case, in all probability, would not have occurred had it not been for the recent amphibian die-off.

In a typical malaria outbreak, incidence rates usually peak at about 1.1 to 1.5 cases per thousand people. This means that a loss of amphibians in Central America could have led to a 70-90 percent increase in the amount of people getting sick.

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

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

Perhaps, the authors suggest, an increase in malaria cases leads to greater use of insecticides, which then reduces cases again in line with this cycle.

Future studies on other mosquito-borne diseases, such as dengue, could help support the connection between the loss of amphibians and a growing threat of mosquito-borne diseases.

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

From 2002 to 2007, the increase in dengue cases compared to the previous eight years was 36 percent.

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

“If scientists and decision-makers do not consider the ramifications of these past events, they also risk not fully motivating protection against new calamities, such as the international spread of an emerging and closely related pathogen . Batrachochytrium salamandrivorans through the incompletely regulated live species trade,” they add.

As you read this, B. salamanders is taking a ride around the world with global trade and threatens not only the future of amphibians, but the health of our own species.

As the current study reveals, human health and the frog often go hand in hand. We are united whether we like it or not.

The study was published in Environmental research papers.

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