Coinfection rates of avian blood parasites increase with latitude in parapatric host species

Animals are frequently coinfected with multiple parasites concurrently, and advances in our sampling of these complex intra-host parasite communities have revealed important eco-logical impacts on their hosts. However, the spatial distributions and environmental determi-nants of parasite coinfection remain infrequently studied. Here, we investigated the drivers of haemosporidian blood parasite coinfection in the Bicknell's thrush (Catharus bicknelli) and grey-cheeked thrush (Catharus minimus), parapatric sister species that occur across a broad latitudinal range in northern North America. Using 298 samples from across the distributions of these species, we found high overall infection (86%) and coinfection (41%) rates within host populations. Coinfection rates within populations were highly variable across sampling sites, ranging from 7 to 75%. Latitude was a more important predictor of coinfection frequency than host species identity, with coinfections becoming more abundant at higher latitudes. The 2 host species exhibited similar parasite faunas, and an analysis of the co-occurrence patterns among haemosporidians showed that host species identity was largely not a factor in structur-ing which parasites were found within coinfections. To our knowledge, this is the first study to illustrate a reverse latitudinal gradient in coinfection frequency in a eukaryotic parasite system. Further work is necessary to determine whether vector ecology or some other factor is the primary proximate driver of this pattern.

Automated summary

Animals often experience concurrent infections by multiple parasites, and studying the interactions between these parasite communities and their hosts has revealed important ecological impacts. However, there is limited research on the spatial distributions and environmental determinants of parasite coinfections. This study investigated the factors driving haemosporidian blood parasite coinfection in two closely related bird species across a broad latitudinal range in North America. The results showed high rates of infection and coinfection, with latitude being a more important predictor of coinfection frequency than host species identity.