Social networks of ants slow down spread of disease – study published in Science – ScienceDaily

The high population density, as well as the frequent and close contact between individuals, contribute to the rapid spread of diseases. To protect their colonies, ants have developed defense mechanisms against the disease, including adaptations to their social organization. Ants do not interact randomly with other members of the colony, but are organized into subgroups according to their age and the tasks they perform. While young workers, called "nurses", occupy the precious brood at the center of the colony, older ants become ants that collect food outside the nest. These forage ants are more exposed to pathogens.

Strengthening the fort

Researchers used a "barcode" system developed in the Keller group to track ant interactions, particularly to observe their behavior as the disease spreads. In a first experiment, they placed digital markers on 2 to 266 garden ants. Infrared cameras took an image of colonies every half-second so that researchers could track and measure each individual's movement and position, as well as their social interactions. Researchers have shown that the subdivision of ants into groups acts prophylactically and reduces the risk of spreading the disease.

10% of worker ants (all foragers) were then exposed to fungal spores that spread easily by contact. Comparison of ant colonies before and after exposure to the pathogen showed that ants quickly detect the presence of fungal spores and modify their behavior to reinforce already existing defenses. "Ants change the way they interact and interact with each other," Cremer says, "the cliques between ants get even stronger, and the contact between the cliques is reduced." The fodder interacts more with the fodder all over the place. colony – animals that are not themselves treated with spores alter their behavior. "Laurent Keller added:" This is the first scientific study demonstrating that an animal society is able to modify its organization to reduce the spread of disease. "

Using a highly sensitive method of qPCR established in the Cremer group, researchers were able to accurately quantify the number of spores that an individual ant carried on its body. QPCR monitors how a targeted DNA molecule is amplified during the so-called polymerase chain reaction. This allows the researchers to draw conclusions about the amount of DNA and, by implication, the amount of fungal spores present at the beginning.

Because ants have changed their interaction, spore transfer patterns have also changed. Only a few people have received a high dose of a pathogen that can cause disease. In addition, more ants received a low dose, which Cremer and his group have previously shown not to cause disease, but rather to protect against future infections – similar to variability in humans. "The pathogen is spread across many shoulders and the ant immune system can very well handle this low level of pathogen, which provides a form of immune memory," Cremer said.

Protect the queen

The analyzes also showed that the colony protects particularly valuable animals. The queen, the only individual to breed, and nurses, young ants still able to provide many hours of work to the colony, received less of the pathogen. "In a colony, not all animals need to be protected – but the most valuable individuals must survive," says Keller.

The researchers also conducted a survival experiment to determine the correlation between pathogen load 24 hours after exposure and death by disease. For Nathalie Stroeymeyt, first author and postdoc of Laurent Keller's group, the correlation was strong: "We calculated a predictive spore load for each ant, according to its interactions with other ants during the 24 hours following the first one. exposure to the pathogen: a high predicted spore load was more likely to die nine days after exposure than ants with a low predicted spore burden. "It sums up:" Mortality was higher in the foragers only at the nurses.And all the queens were still alive at the end of the experiment. "

The way in which ants collectively deal with problems, such as the risk of an epidemic, could give an idea of ​​the general principles of disease dynamics, says Cremer: "Social interactions are the pathways on which diseases spread and define how epidemics spread, to help us better understand epidemiological processes, which can also be applied to other social groups. "