title: Co-infections and environmental conditions drive the distributions of blood parasites in wild birds.
abstract: 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.
authors: Environmental Futures Research Institute, School of Environment, Griffith University, Gold Coast, Qld, 4111, Australia. nicholas.j.clark1214@gmail.com.|Natural Environments Program, Queensland Museum, Institute of Biodiversity and Ecosystem Research, P.O. Box 3300, South Brisbane, Qld, 4101, Australia. nicholas.j.clark1214@gmail.com.
authors: Nicholas J
authors: NJ
authors: Clark
authors: Environmental Futures Research Institute, School of Environment, Griffith University, Gold Coast, Qld, 4111, Australia.
authors: Konstans
authors: K
authors: Wells
authors: Institute of Biodiversity and Ecosystem Research at the Bulgarian Academy of Sciences, 2 Gagarin Street, Sofia 1113, Bulgaria.
authors: Dimitar
authors: D
authors: Dimitrov
authors: Environmental Futures Research Institute, School of Environment, Griffith University, Gold Coast, Qld, 4111, Australia.|Department of Zoology, Edward Grey Institute of Field Ornithology, University of Oxford, Oxford, OX1 3PS, UK.
authors: Sonya M
authors: SM
authors: Clegg
mesh_terms: D000818:Animals; D001715:Bird Diseases / Q000453:epidemiology* / Q000469:parasitology; D060085:Coinfection / Q000453:epidemiology / Q000469:parasitology / Q000662:veterinary*; D004777:Environment; D005368:Filariasis / Q000453:epidemiology / Q000469:parasitology / Q000662:veterinary*; D016787:Haemosporida / Q000235:genetics / Q000302:isolation & purification; D008289:Malaria, Avian / Q000453:epidemiology / Q000469:parasitology; D008842:Microfilariae / Q000235:genetics / Q000302:isolation & purification; D009510:New Caledonia; D010961:Plasmodium / Q000235:genetics / Q000302:isolation & purification; D015995:Prevalence; D011529:Protozoan Infections, Animal / Q000453:epidemiology / Q000469:parasitology*; D017422:Sequence Analysis, DNA / Q000662:veterinary; D020308:Songbirds
keywords: 
  Haemoproteus
  ; avian malaria; filarial parasite; heterophil to lymphocyte ratio; immune modulation; parasite co-occurrence