Is endoparasite infection in feral cattle modulated by social structure and behaviors?

Endoparasite infections in ruminant populations are unevenly distributed, with 20% of the group members typically shedding around 80% of the parasites. Those individuals with more shedding may have higher susceptibility or greater exposure to endoparasites. However, how this relates to social behavior and herd structure is largely unknown. This study, on a unique feral crossbreed of Bos taurus and Bos indicus in Hong Kong, aimed to characterize high-shedding individuals within free-roaming herds.

We sampled 95 individuals (36 males, 59 females) from three herds over two consecutive seasons. We collected freshly voided feces from each individual and utilized McMaster's technique to identify and count nematode eggs, cestode eggs and protozoa oocysts. Social interactions were scored using all occurrence sampling, building the social hierarchy using dominance interactions, from which we obtained randomized Elo-scores for individuals. Social network centrality scores were obtained for each individual based on the number of partners and strength of their nodes in allogrooming interactions.

In these three Hong Kong cattle herds, 10.5% to 26.31% (mean=17.17, SD=4.67) of individuals shed 80% of the herd’s endoparasite eggs and oocysts. This was unrelated to the social structure of the group, with neither hierarchies’ steepness (LMM, p=0.51) nor linearity (LMM, p=0.86) influencing the number of high-shedding individuals within a herd. High-shedding individuals were more involved in allogrooming (GLMM, p=0.01), but less in other affiliative interactions (GLMM, p=0.05). Sex (GLMM, p=0.92), dominance rank (GLMM, p=0.48), and centrality within the allogrooming social network (GLMM, p=0.42) did not identify these individuals, indicating that infections are related to social behaviors but not social preferences. Further analysis unearthed that high-shedding individuals performed more allogrooming (GLMM, p=0.001), but they received allogrooming (GLMM, p=0.79) and affiliation (GLMM, p=0.94) similarly to those that shed less.

Our results show a relationship between social behavior and endoparasite infections, regardless of social structure. Across all herds, we found that a small portion of the group members shed most of the endoparasites, and these individuals performed more allogrooming, independent of their position in the social network. While social behaviors reflect endoparasite infection, they were not related to social preferences in these feral cattle. Our results may reflect the higher likeliness of ingesting parasites during allogrooming events, which has been demonstrated in rats and primates, but has been poorly explored in large ruminants. In parallel, highly infected individuals might be performing more allogrooming for their own benefits, such as mineral ingestion or social trading to obtain better access to resources and protection for infection. We suggest that targeted interventions for endoparasite control should consider social behaviors, particularly allogrooming, to identify high-shedding individuals.