Avian malaria and invasion success: Contrasting infection patterns in two introduced Eurasian sparrows in North America (Passer domesticus and Passer montanus)

Biological invasions present a unique context to investigate how parasites influence host establishment and spread. We compared whether the more invasive house sparrow ( Passer domesticus , HOSP) showed lower haemosporidian ( Plasmodium ) infection probability, predicted relative parasitemia, and lineage diversity than the less invasive Eurasian tree sparrow ( Passer montanus , ETSP), based on previous research that showed species differences in immune responses and pathogen prevalence that may influence invasion success. We sampled 306 sparrows across eight sites in Missouri and Illinois (2019–2022), assessing Plasmodium prevalence, relative parasitemia (via qPCR), and lineage diversity. Contrary to expectations, the more invasive HOSPs had higher Plasmodium prevalence (24.1%, 32/133; 95% CI = 17.6% - 32.0%) compared to the less invasive ETSP (11.6%, 20/173; 95% CI = 7.6% - 17.2%). Model-averaged estimates for both the full dataset and sensitivity analysis restricted to sites with balanced sampling identified host species as the strongest predictor of infection probability with higher odds of infection in HOSPs. Relative parasitemia was also higher in HOSPs, with model-averaged estimates indicating nearly two-fold higher odds of greater relative parasitemia compared to ETSPs. Pathogen lineage richness was similar between species, with three shared lineages and one unique lineage per host. One lineage (PHPAT01) had not previously been reported in ETSPs. Comparison with previous studies suggests a decline in Plasmodium prevalence among ETSP over the past two decades. No infections were detected at the northern range edge of ETSPs, where they may experience reduced parasite pressure. These results challenge the expectation of increased susceptibility in the less invasive host and instead show shifting host-parasite dynamics in introduced populations. Our findings highlight the importance of paired studies of more and less invasive species and the need to examine how pathogen susceptibility changes across different stages of the invasion process and range expansion. • More invasive House Sparrows showed higher Plasmodium prevalence than less invasive Eurasian Tree Sparrows • Relative Plasmodium parasitemia was higher in more invasive House Sparrows • Five Plasmodium lineages were detected, including first detection of lineage PHPAT01 in Eurasian Tree Sparrows • No Plasmodium infections were detected at the northern expanding range edge of Eurasian Tree Sparrows