Data from: Co-infections and environmental conditions drive the distributions of blood parasites in wild birds

Experimental work increasingly suggests that non-random pathogen
associations can affect the spread or severity of disease. Yet due to
difficulties distinguishing and interpreting co-infections, evidence for
the presence and directionality of pathogen co-occurrences in wildlife is
rudimentary. We provide empirical evidence for pathogen co-occurrences by
analysing infection matrices for avian malaria (Haemoproteus and
Plasmodium spp.) and parasitic filarial nematodes (microfilariae) in wild
birds (New Caledonian Zosterops spp.). Using visual and genus-specific
molecular parasite screening, we identified high levels of co-infections
that would have been missed using PCR alone. Avian malaria lineages were
assigned to species level using morphological descriptions. We estimated
parasite co-occurrence probabilities, while accounting for environmental
predictors, in a hierarchical multivariate logistic regression.
Co-infections occurred in 36% of infected birds. We identified both
positively and negatively correlated parasite co-occurrence probabilities
when accounting for host, habitat and island effects. Two of three
pairwise avian malaria co-occurrences were strongly negative, despite each
malaria parasite occurring across all islands and habitats. Birds with
microfilariae had elevated heterophil to lymphocyte ratios and were all
co-infected with avian malaria, consistent with evidence that host immune
modulation by parasitic nematodes facilitates malaria co-infections.
Importantly, co-occurrence patterns with microfilariae varied in direction
among avian malaria species; two malaria parasites correlated positively
but a third correlated negatively with microfilariae. We show that
wildlife co-infections are frequent, possibly affecting infection rates
through competition or facilitation. We argue that combining multiple
diagnostic screening methods with multivariate logistic regression offers
a platform to disentangle impacts of environmental factors and parasite
co-occurrences on wildlife disease.