For Queen and Colony: How ants stop disease outbreaks

By Sophie Berdugo

Human societies are perfectly structured for transmission, whether it be of information, misinformation, or disease. One such social network structure, so-called ‘small world networks’, consisting of sparse connections between tight clusters of individuals, is optimal for rapid and efficient transmission. The COVID-19 pandemic exemplified this: international travel assisted the spread of the virus worldwide, and interactions between close personal contacts propagated it within communities. Globally, extreme measures have been implemented to stop the spread of COVID-19, including lockdowns and limits on travel.

Unsurprisingly, infectious diseases also pose significant risks across the Animal Kingdom. What is unusual, however, is the lengths some species will go to in the name of public health. Garden ants (Lasius spp.) are particularly adept at preventing epidemics, adopting numerous behavioural and physiological techniques to protect the queen and colony.

Much like the self-isolation we have become accustomed to during the COVID-19 pandemic, ants also adopt self-isolation techniques to stop the spread of fungal infections.

Eusocial insect colonies, including ants’, have social networks consisting of multiple generations of related individuals, and are strictly divided into reproducing and non-reproducing labour roles. This makes colonies epidemic-prone, with the lack of genetic diversity being compressed into dense networks of individuals. This shared genotype is potentially lethal, with the odds being poor for the group if one individual succumbs to a disease. Thus, epidemic prevention a strong evolutionary selection pressure, with any transmission-reducing traits being positively selected for in the population. This evolutionary arms race has been in motion for at least 50 million years (evidenced by ants fossilised in amber), allowing ant colonies to win more and more battles in the ongoing war against infections.

One particularly prevalent disease that garden ants have co-evolved with is the generalist fungal pathogen Metarhizium brunneum (M. brunneum). The life cycle of this pathogen inherently relies on an ant being killed, with spores only being released once the ant has died from the infection (‘sporulation’). Lasius spp. are proactive in their defence to break this cycle, and engage in collective, self-organised behaviours to create a colony-level ‘social immunity’.

Much like the self-isolation we have become accustomed to during the COVID-19 pandemic, ants also adopt self-isolation techniques to stop the spread of fungal infections. Research by Nathalie Streoymeyt and colleagues at the University of Lausanne, Switzerland, demonstrated this using an innovative method. The researchers attached miniature barcodes to ants in 22 Lasius niger colonies to assess their disease-free social networks. Spores from M. brunneum were then experimentally exposed to a random 10% subset of workers in 11 of these colonies, whilst a control solution was introduced in the remaining 11 colonies. After a day of monitoring the networks using the same barcode system, the pathogen load of each individual ant in the diseased colonies was calculated.

The results were ground-breaking, as it was the first study to demonstrate just how malleable colony social networks are in the face of an impending epidemic. Critically, the most valuable individuals in the colony—particularly the queen—were separated from the most at-risk ants (the ‘foragers’ who leave the nest). This happened via processes including ‘task assortativity’, where foragers actively associate with each other more than other workers, and ‘clustering’, which creates groups of infected individuals who have low levels of connectivity to the rest of the colony. These behaviours substantially reduce disease transmissibility, thereby protecting the queen and colony at large.

Garden ants also have powerful physiological techniques to stop the spread of fungal infections. These techniques go a step further than the comparatively tame behavioural adjustments. Indeed, there is a so-called ‘care-kill dichotomy’ when physiological defences are employed, with the relative colony-level threat determining whether the ant is treated or sacrificed for the safety of the queen. The care half of these contrasting public health strategies—termed ‘sanitary care’—involves ‘nurse’ ants administering a formic acid antimicrobial poison to their patients whilst grooming them. This is not too dissimilar to the human experience, with specialised health care professionals administering medicine to sick patients. The kill half, on the other hand, follows the same principle but takes two critical further steps in a processed called ‘destructive disinfection.

Indeed, there is a so-called ‘care-kill dichotomy’ when physiological defences are employed, with the relative colony-level threat determining whether the ant is treated or sacrificed for the safety of the queen.

Dr Christopher Pull, now a Lecturer in the Department of Zoology at the University of Oxford, and colleagues exposed Lasius neglectus pupae to either a low, medium, or high dose of M. brunneum, or a control solution. Using chemical signals (‘chemical sickness cues’) which indicated that a pupa is infected but not currently contagious, the ants processed through a well-choreographed routine. Step one was removing the pupa from its protective cocoon (termed ‘unpacking’). Importantly, the pupae were unpacked earlier if they were exposed to more spores. Although most pupae survive the unpacking process, none are destined to recover. Next, the ants cut into the outer layer of the pupa (the ‘cuticle’) and sprayed it with the disinfecting poison. This spraying application is rare during usual sanitary care but predominates when killing is deemed necessary. Finally, the ants bite the pupa and tear off its limbs, allowing the poison further access to the pathogen in the now-corpse.

The researchers found that this destructive disinfection process significantly reduced sporulation in the pupae, hence preventing the fungus from progressing through its ant-dependent life cycle. It is a brutal and extreme means to stop a potential disease outbreak. This destructive sacrifice is unmatched in humans, with disinfectant sprays and hand sanitisers being the typical use of antimicrobials. This departure highlights the grip social network structure holds over disease prevention, and how strategies must be tailored to the network of the group.

Ants epitomise how successful collective action against external threats can be. By proactively responding to chemical sickness cues, their behavioural and physiological techniques ensure diseases do not reach epidemic status. The response complements the degree of threat to an extent unimaginable in humans. Yet, this social immunity has made disease outbreaks notably rare in ant colonies—in other words, they are currently ahead in the evolutionary arms race. Despite the multitude of differences clearly separating ants and humans, capitalising on plastic social network structures is also beneficial in humanity’s own fights against disease. With the third year of the COVID-19 pandemic now upon us, it is important we learn how to use this plasticity to prevent future pathogens reaching pandemic status at all.

Pull, C. D., Ugelvig, L. V., Wiesenhofer, F., Grasse, A. V., Tragust, S., Schmitt, T., … & Cremer, S. (2018). Destructive disinfection of infected brood prevents systemic disease spread in ant colonies. eLife, 7, e32073.