New study: Avian malaria widespread throughout Hawaiʻi bird communities

A new study led by a University of Hawaiʻi at Mānoa researcher shows avian malaria can be transmitted by nearly all forest bird species in Hawaiʻi.

The research — published Feb. 10 in Nature Communications — found the scourge killing native Hawaiian birds was present at 63 of 64 sites tested statewide, including areas with very different bird communities.

It also helps explain why the disease is present almost everywhere mosquitoes are found throughout the islands.

Avian malaria is caused by generalist parasite Plasmodium relictum. It is a major driver of population declines and extinctions of native Hawaiian honeycreepers.

“Avian malaria has taken a devastating toll on Hawaiʻi’s native forest birds, and this study shows why the disease has been so difficult to contain,” said mosquito research and control coordinator for Maui Forest Bird Recovery Project Christa Seidl in a release about the new study. “When so many bird species can quietly sustain transmission, it narrows the options for protecting native birds and makes mosquito control not just helpful, but essential.”

Seidl conducted the research as part of her PhD at University of California, Santa Cruz.

Impact, spread of avian malaria

Avian malaria weakens birds by damaging red blood cells, often leading to anemia, organ failure, reduced survival and, in some species, death.

Reports and studies show that mainly because of avian malaria, ʻiʻiwi, or the scarlet honeycreeper, show a 90% mortality rate if infected.

The ʻakikiki, a Hawaiian honeycreeper endemic to Kauaʻi, is now considered extinct in the wild.

Unlike many diseases, where only a few species play a major role in spreading infection, the new study found that most bird species in Hawaiʻi — native and non-native — are at least moderately capable of infecting avian malaria’s primary vector southern house mosquitoes.

Even birds carrying very low levels of the parasite were able to pass the disease to mosquitoes. As a result, many different bird communities can support ongoing malaria transmission.

“We often understandably think first of the birds when we think of avian malaria, but the parasite needs mosquitoes to reproduce and our work highlights just how good it has gotten at infecting them through many different birds,” Seidl said.

The study analyzed blood samples from more than 4,000 birds throughout Kauaʻi, Oʻahu, Maui and Hawaiʻi Island, combining the data with laboratory experiments measuring how easily mosquitoes became infected after feeding on birds.

Researchers found introduced and native birds often had overlapping levels of infectiousness, meaning both groups can contribute to disease spread.

Because individual birds can harbor chronic avian malaria infections for months to many years, researchers also estimated this long period when birds are low to moderately infectious drives most disease transmission.

The broad ability of avian malaria to infect and spread likely explains why the disease is so widespread throughout the state.

Findings in the new study also suggest there are few, if any, mosquito-infested habitats free from transmission risk. To make matters worse, mosquito-free habitats are rapidly disappearing as warming temperatures allow mosquitoes and avian malaria to develop in former refuges.

Seidl and Maui Forest Bird Recovery Project are part of Birds, Not Mosquitoes, a partnership of academic, state, federal, nonprofit and industry partners facilitating mosquito control for Hawaiian bird conservation.

Maui Forest Bird Recovery Project is housed under the Pacific Cooperative Studies Unit in University of Hawaiʻi at Mānoa College of Natural Sciences.

All birds featured were captured and handled in accordance with state/federal permits by trained ornithologists.