Is avian malaria playing a role in native bird declines in New Zealand?
Collecting blood via brachial venipuncture from a silvereye for screening for the presence of avian malaria. Image - Clare Cross
Emerging infectious diseases, defined as disease-causing agents that rapidly increase in geographic range, host range, or prevalence, pose a serious threat to the conservation of global biodiversity. Avian malaria is one such disease that may affect New Zealand avifauna, considered to be the most extinction-prone in the world. Avian malaria is caused by protozoan parasites of the genus Plasmodium and is vectored by mosquitoes. In Hawaii, avian malaria is vectored by the exotic mosquito, Culex quinquefasciatus, and has been a major factor in the population declines of native forest bird species, limiting many species to higher elevations.
Of the approximately 3500 species of mosquitoes in the world, 15 species are found in New Zealand, three of which are introduced. Culex quinquefasciatus was introduced to New Zealand over a century ago, but in recent decades has expanded its range from its introduction sites in Northland and Auckland to as far south as Christchurch. New Zealand’s native C. pervigilans is also suspected of playing a role in malaria transmission, complicating our understanding of local transmission dynamics.
The presence of malarial parasites has been confirmed in native New Zealand avifauna and has been linked with the deaths of birds of multiple species, including the mōhua (yellowhead), kererū, great spotted and brown kiwi, New Zealand dotterel, and saddleback. The presence of malarial parasites has also been confirmed in multiple non-native species of birds in New Zealand, including the blackbird, song thrush, house sparrow, and starling, leading to suggestions that these non-natives may be acting as reservoir species from which spillover to native species may occur. Although this mosquito-borne disease has impacted on both captive populations and wild individuals in New Zealand, the impact on wild populations of native birds is still unclear.
During the last 30 years, declines in abundance have been reported for five native bird species in the Nelson Lakes region: bellbird, rifleman, grey warbler, New Zealand tomtit, and tūī. Avian malaria is one possible cause of these declines, since the declines have generally occurred at lower altitudes where there is greater abundance of non-native bird species (potential reservoirs of malaria) and likely also higher mosquito densities.
Dan Tompkins and University of Otago PhD candidate Chris Niebuhr are investigating the causes of these declines at Nelson Lakes National Park. Blood samples were taken from native (n =134) and non-native (n =126) forest birds along an elevational gradient (650 m to 1400 m) during three summers (2012–13 to 2014–15), complemented with mosquito sampling. All samples are currently undergoing testing for the presence of malarial parasites using a Polymerase Chain Reaction assay, with Plasmodium sp. identity being confirmed through DNA sequencing.
Although blood samples are still being processed, initial results show the prevalence of avian malaria in non-native species is 10 times that in native species — 13.5% (126) and 1.5% (134) respectively. Additionally, mosquito surveys show the presence of the native C. pervigilans in the region, but not the exotic C. quinquefasciatus. Also, a malaria-positive song thrush was found as high as 1400 m (tree-line) while mosquitoes were not detected above 1000 m. Thus, pathogen reservoirs exist at higher elevations, with only the mosquito vector lacking for the transmission cycle to be complete. Although the results are still being analysed, any supporting evidence for disease impacts could help appropriate management to be developed and put in place, while a lack of evidence would enable resources to remain focused on the other issues facing our native species (e.g. habitat restoration, pest control, translocations and genetic management).
The study, expected to be completed by the end of the year, will also integrate mathematical modelling with empirical data to better investigate avian malaria transmission dynamics in New Zealand. Dan and Chris are currently adapting a malaria-forest bird epidemiological model, originally developed in Hawaii, to fit the New Zealand situation, incorporating both C. quinquefasciatus and C. pervigilans.. This work has potential for indicating management options beyond avian malaria, to include other mosquito-borne diseases (e.g. West Nile, Ross River, dengue) that could eventually make their way into New Zealand.
This work has been funded by Landcare Research and the University of Otago, Department of Zoology.
Chris Niebuhr (University of Otago)
E: chris.niebuhr@postgrad.otago.ac.nz
The St. Arnaud range above Lake Rotoiti in Nelson Lakes National Park where birds and mosquitoes are being sampled. Image - Chris Niebuhr
A banded bellbird caught in a mist-net. Image - Chris Niebuhr
A carbon dioxide trap used to collect host-seeking mosquitoes. Image - Clare Cross
