[LUM#21] Tracking down viruses: a painstaking task

[LUM Magazine, Podcast] - Published on April 3, 2024 in Science-Society

When we think of the inhabitants of tropical forests, we think of jaguars, leopards, and monkeys of all kinds. Or birds and reptiles, such as toucans and chameleons. But there are tiny, tiny inhabitants that live there by the millions. These are viruses, a growing source of concern in terms of human health, which virologists Eric Leroy and Philippe Roumagnac are studying closely thanks to ants.

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COVID-19, Ebola, HIV. But also hepatitis, measles, and mumps. These infectious diseases have one thing in common: they all originate in animals, as do three-quarters of epidemics that affect humans. "These are what we call zoonoses. The viruses responsible for these diseases are originally hosted by animals and have been passed on to humans, " explains Eric Leroy, virologist at the Infectious Diseases and Vectors: Ecology, Genetics, Evolution and Control (Mivegec) laboratory.¹.

While these viruses may originate in farm animals, they most often come from wild animals, mainly those living in tropical forests. "This is partly because these areas are the richest in biodiversity, and partly because certain socio-cultural customs increase contact with wildlife, which facilitates transmission to humans," explains Eric Leroy.

Public health issue

Scrutinizing these viruses hidden in forests has therefore become a public health issue. "We need to be able to monitor animal reservoirs upstream before the viruses are transmitted to humans, " explains the virologist. But how can we learn more about the tiny inhabitants of these vast and impenetrable forests?

Eric Leroy, who knows tropical forests well having lived and worked for many years in Gabon and Congo, and his colleague Philippe Roumagnac, a virologist at the Montpellier Plant Health Institute (Phim), had an idea. Since it is impossible to capture, collect, and analyze all the animals and plants that inhabit forests in order to examine their viruses, why not let others take care of this valuable sampling? These others form an army of tiny insects unlike any other: army ants, or magnan ants.

These carnivores attack anything that falls within reach of their mandibles: from arthropods and other invertebrates to small animals such as birds, reptiles, and small mammals. "They are known for their spectacular raids, carried out in columns of millions of individuals stretching for tens of meters," says the virologist. Not picky eaters, these nomads will also consume the carcasses of large animals if the opportunity arises, hence their nickname "the garbage cans of Africa."

"We therefore hypothesized that these magnan ants would be likely to absorb and accumulate viruses hosted by the organisms they consume, whether plants, invertebrates, or vertebrates. By analyzing genetic material from the ants, the researchers hope to identify the viruses with which they have come into contact and thus gain a better understanding of what they call the virosphere of tropical forests.

Pilot study

To test this hypothesis, Eric Leroy and his colleagues conducted a pilot study. They captured 209 ants from around 30 colonies in northeastern Gabon and analyzed each insect using a viral metagenomics approach. "We collect all the genetic material, DNA and RNA, present in a sample—in this case, the ant—to analyze the portion corresponding to the total viral genome, that of the ant, but also the genetic material from the animals or plants it has consumed," explains Eric Leroy.

The researchers detected an exceptional number of genomic sequences, 46,000 of which were similar to sequences from viruses, bacteria, plants, invertebrates, and vertebrates. "It's a real treasure trove, " enthuses Eric Leroy. A treasure trove that also holds a few surprises : "Only half of these sequences corresponded to recognized viral genera, which means that the forest ecosystems traversed by ants probably contain a large number of as yet unknown viruses."

Sentinel ants

While researchers will conduct larger-scale studies, these preliminary results already prove that the idea is a good one. "Not only would this method provide a better understanding of the virosphere and map the presence of viruses in a given area, but we could also consider setting up a system to monitor these zoonotic viruses before they are transmitted to humans," explains the researcher, who is considering, for example, collecting army ants on a monthly basis in specific locations to identify the viruses present in those areas.

"If problematic viruses reach a critical threshold, measures could be put in place," Eric Leroy imagines. These six-legged sentinels would thus participate in early virus surveillance, making it possible to predict, anticipate, and prevent the emergence of diseases and epidemics "to which we are still too often mere spectators, stunned and powerless, " concludes the researcher.