Social Immunity and Disease Resistance

Ants present an interesting group for study of disease resistance and how organisms deal with pathogens. As a social organism, colonies can theoretically quickly spread a pathogen due to lifestyles that may include large numbers of individuals living in close contact with one another and their tendency to nest in place (e.g., soil and rotting wood, moist microsites, etc.) that can harbor and support a wide range of pathogens.

What follows are some recent studies that have mainly focused on how ants thwart and protect themselves from potentially harmful infectious agents.

-
 * Bordoni, A., L. Dapporto, I. Tatini, M. Celli, M. Bercigli, S. R. Barrufet, B. Perito, and S. Turillazzi. 2018. Trans-generational immunization in the acrobat ant Crematogaster scutellaris. Biology Letters 14:20170761.

Trans-generational immunization is defined as the transmission of an enhanced resistance to a pathogen from parents to offspring. By using the host–parasite system of the ant Crematogaster scutellaris and the entomopathogenic fungus Metarhizium anisopliae, we describe this phenomenon for the first time in ants. We exposed four groups of hibernating queens to different treatments (i) a non-lethal dose of live conidiospores in Triton, (ii) a dose of heat-killed conidiospores in Triton, (iii) a control Triton solution, and (iv) a naive control. We exposed their first workers to a high dose of conidiospores and measured mortality rates. Workers produced by queens exposed to live conidiospores survived longer than those belonging to the other groups, while exposure to Triton and dead spores had no effect. Starved workers showed a significantly higher mortality. The treatments did not influence queen mortality, nor the number of offspring they produced at the emergence of the first worker, showing no evidence of immunization costs—at least for these parameters in the first year of colony development. We propose that trans-generational immunization represents an important component of social immunity that could affect colony success, particularly during the critical phase of claustral foundation.

-
 * Stroeymeyt, N., A. V. Grasse, A. Crespi, D. P. Mersch, S. Cremer, and L. Keller. 2018. Social network plasticity decreases disease transmission in a eusocial insect. Science 362:941-945.

Animal social networks are shaped by multiple selection pressures, including the need to ensure efficient communication and functioning while simultaneously limiting disease transmission. Social animals could potentially further reduce epidemic risk by altering their social networks in the presence of pathogens, yet there is currently no evidence for such pathogen-triggered responses. We tested this hypothesis experimentally in the ant Lasius niger using a combination of automated tracking, controlled pathogen exposure, transmission quantification, and temporally explicit simulations. Pathogen exposure induced behavioral changes in both exposed ants and their nestmates, which helped contain the disease by reinforcing key transmission-inhibitory properties of the colony’s contact network. This suggests that social network plasticity in response to pathogens is an effective strategy for mitigating the effects of disease in social groups.