To understand infections affect populations, it’s important to understand the relationship between parasitism and social behavior. Parasites can disrupt social networks in two main ways: by changing the behavior of infected individuals—often leading to decreased activity—and by prompting uninfected individuals to alter their behavior, such as avoiding those showing signs of illness. These behavioral adjustments can shift group dynamics and influence how parasites spread.

In this study, we explored how a parasitic nematode infection influences both individual social interactions and the structure of social networks in sheep. We created three experimental treatments, each replicated across four independent groups: (1) all individuals infected (Parasitised), (2) all individuals uninfected (Non-parasitised), and (3) a mixture of infected and uninfected individuals (Mixed). We tracked social contact patterns using proximity loggers over four stages of infection: prior to infection, the pre-patent period, the patent phase, and after the infection had cleared.

Our findings revealed that infected lambs—whether in fully parasitised or mixed groups—engaged in fewer social contacts after becoming infected. Interestingly, in the mixed groups, uninfected lambs did not reduce their interactions with infected group members, maintaining contact levels similar to those before infection.

These results indicate that parasitic infections can alter social behaviors across an entire group, and that the behavioral responses may depend not only on an individual's infection status but also on the composition and responses of the social group as a whole.