Parasite Interactions Affect Risk of Malaria Infection in Wild Birds

September 16, 2016
Nicola M. Parry, BVSc, MRCVS, MSc, DipACVP

Environmental conditions as well as interactions between different parasites co-infecting wild birds may be important in determining whether the birds will develop a malaria infection.

According to a recent study in wild birds, published in the Journal of Animal Ecology, interactions between different parasites co-infecting the birds may be important in determining whether the birds will develop a malaria infection. Environmental conditions also affect the birds’ risk of malaria infection, the authors add.

“We show that wildlife co-infections are frequent, possibly affecting infection rates through competition or facilitation,” Nicholas J. Clark, PhD, Griffith University, Nathan, Queensland, Australia, and colleagues write.

In mammals, evidence increasingly suggests that non-random interactions between parasites in a host animal can influence the spread and severity of disease. However, in wildlife, similar evidence for this phenomenon is still somewhat lacking.

With this in mind, Dr. Clark and colleagues conducted a study to examine the probability of parasite co-infections, while also accounting for environmental predictors.

The researchers analyzed data from a total of 449 New Caledonian Zosterops spp. of wild birds. They collected blood samples from 275 birds from across the South Pacific archipelago of New Caledonia, and prepared blood smears from 245 of them; next, they analyzed the specimens for evidence of malaria and parasitic worms, performing detailed DNA screening tests to identify different parasite strains. Finally, they combined their findings with published malaria data from an additional 174 of these wild birds.

In a press release on the Griffith University website, Dr. Konstans Wells, one of the study’s authors, discusses how the team used mathematical modeling (multivariate logistic regression) during the course of their study: “We developed probabilistic models to ask whether or not one parasite species impacts the presence of others in the same bird individuals,” he says.

Overall, Dr. Clark and colleagues found that 228 of the 449 birds were infected with hematozoan parasites, including 191 Haemoproteus, 88 Plasmodium, and 41 microfilaria (early developmental stages of filarial nematodes that circulate in the blood) infections; and 82 of these 228 infected birds (36%) had 82 parasite co-infections.

The researchers also found that birds living in certain forest types and on certain islands were more likely to be infected with malaria. For example, Haemoproteus zosteropis and microfilaria infections occurred more commonly in lowland rain forest environments, and Plasmodium spp. infections occurred more commonly in open lowland forest. According to the authors, these environmental infection patterns probably reflect distributions of arthropod vectors for the parasites.

Nevertheless, these infection risks were not independent of each other, and the researchers found that a bird’s risk of infection with one parasite was related to whether the bird also had a high risk of being coinfected by another parasite. According to the authors, birds infected with microfilariae had increased heterophil-to-lymphocyte ratios (two types of white blood cell in circulation—an increased ratio of which can indicate stress and disease susceptibility in birds), as well as malaria co-infection (with Haemoproteus zosteropis or Plasmodium spp.). They note that these findings are consistent with evidence that nematode parasites may modulate the host’s immune system to allow malaria co-infection to occur.

“Our results indicate that interspecific associations are an important but overlooked mechanism influencing wildlife parasite infections,” the authors highlight.

“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,” they conclude.