Early-life infection dynamics and genomic diversity of adenoviruses in a wild primate (Theropithecus gelada)

In humans, adenoviruses (AdVs) are frequently associated with respiratory illnesses, posing risks to children with developing immune systems and immunocompromised individuals. Outbreaks and epidemics are generally centred in close-contact settings, such as childcare facilities, and transmission occurs through faecal-oral and airborne pathways. AdVs have coevolved across the primate lineage, but very little is known about whether the early-life dynamics in non-human primates mirror those in humans. Here, we leverage longitudinal data collected on a population of geladas (<i>Theropithecus gelada</i>) in the Simien Mountains National Park, Ethiopia, to evaluate AdV dynamics across the gelada lifespan. We identified ten coding-complete AdV genomes representing seven unique simian adenovirus (SAdV) types, four of which are adequately different from the known ones to establish new species. We assessed behavioural and seasonal drivers of SAdV presence and richness across repeated faecal samples from known individuals. Contrary to our expectation that the highest risk would occur after the initiation of play behaviour in infancy (~6 months of age), when peer-to-peer transmission risk is expected to increase, SAdV likelihood was highest in infants under 6 months of age. Risk and richness declined over the lifespan, with very few adults infected, and higher minimum temperatures were weakly but significantly negatively associated with richness. Our results suggest that, unlike in humans, SAdV exposure occurs prior to the initiation of close-contact play behaviours and likely results from the close spatial proximity of conspecifics throughout the dependent period. Like AdVs in humans, SAdVs in geladas maintain low levels in adulthood, with early infections potentially conferring life-long immunity.