Disease Research

Understanding Disease and Mosquito Impacts

Avian Disease

Hawaiʻi’s native forest birds evolved in isolation over millions of years, resulting in an extraordinary example of adaptive radiation. These birds developed without the many threats that are now present on the islands. Alongside devastating impacts from introduced rats, mongoose, and feral cats, Hawai‘i’s native birds also face a unique and deadly threat: non-native avian diseases spread by invasive mosquitoes. Lacking natural defenses, two diseases — Avian Malaria and Avian Pox — have caused extinctions and extreme population declines among native forest birds.

While avian malaria is common and often mild in other parts of the world, this disease has devastated Hawai‘i’s native honeycreeper populations across the islands. Controlling mosquitoes is essential to preventing the spread of disease and giving Hawaiʻi’s honeycreepers a chance at recovery.

We work to give Hawaiʻi’s honeycreepers a chance at survival. Mosquito control is essential to prevent the spread of disease.

Mosquitoes and the Hawaiian Islands

There are no native mosquitoes in the Hawaiian Islands. Mosquitoes arrived with whaling ships and other shipping activity in the early 1800s. The southern house mosquito (Culex quinquefasciatus), introduced in 1826, is the primary source of Avian Malaria and Avian Pox that are causing declines and extinctions of Hawaiʻi’s native forest birds.

Most native Hawaiian honeycreepers now only survive in high-elevation native forests. On Maui, these forest areas are largely found on the northeastern slopes of Haleakalā, above 5,000 feet (1,200 m) elevation, where cooler climates limit mosquito populations.

Species like kiwikiu and ‘ākohekohe once thrived in low and mid-elevation forests but are now restricted to a tiny sliver of their historic range due to mosquito-borne diseases. Although some suitable habitat remains at lower elevations, these areas are largely uninhabitable. Birds venturing lower in elevation often become infected with avian malaria and die. Climate change is exacerbating this crisis. As rising temperatures allow mosquitoes and disease to invade higher elevations, this shrinks the already limited range of Maui’s native forest birds.

image of mountain and birds — Visualization of how elevation (influenced by climate) distributes bird and mosquitoes (and mosquito-borne disease) across high elevation forests of Hawai’i.

The Diseases

Avian Malaria is only spread by mosquitoes and is caused by a microorganism called Plasmodium relictum. The disease infects birds’ red blood cells, causing them to rupture which leads to low blood oxygen levels. It also results in enlargement and damage to internal organs such as the liver and spleen. Having no immunity to the disease, Hawaiian honeycreepers quickly become anemic, lethargic, and often die. Individuals that manage to survive the initial stage of infection may remain chronically-infected for life. This weakens their immune system and turns them into persistent reservoirs for further transmission.

Avian Pox Virus was first documented in forest birds in 1902. It is primarily transmitted by mosquitoes but can also spread by direct contact between infected birds, contaminated surfaces, and contaminated food and water. Symptoms include swollen, tumor-like lesions on unfeathered parts of a bird’s body, like the feet, legs, eyes, and base of the bill. Lesions can also occur in the mouth, trachea, esophagus, and lungs. These lesions impact the bird’s ability to eat, forage, and fly. Infected birds that do not die immediately from disease become weak and emaciated, and ultimately struggle to survive.

Disease Control and the Future

Both avian malaria and pox are difficult diseases to manage. Currently, there is no feasible treatment or vaccine. The most important disease control method is to limit mosquito populations, which can be difficult. Control programs need to be innovative, cost-effective, environmentally safe, and sustainable.

The survival of Hawaiʻi’s native forest bird populations depend on minimizing introduced mosquito populations and disease, preservation and restoration of forest habitat, and the elimination of introduced predators.

Amakihi with avian pox on a foot — The most common sign of avian pox is swollen, tumor-like lesions on unfeathered parts of a bird’s body.
This Hawaiʻi ʻamakihi has debilitating avian pox on its foot.

Mosquito Research and Monitoring

Our current mosquito research and monitoring efforts include using traps to capture adult mosquitoes and the eggs that they lay. There are many trapping techniques used to collect samples, each used for different purposes.

To collect eggs:

Ovibasins: Ovibasins are black basins about 3 inches deep, 12 inches wide, and 24 inches long. They are filled with “stinky water,” a mixture of water and fermented hay, and used to collect eggs laid by female mosquitoes. The purpose of the “stinky water” is to attract a certain species of mosquitoes that prefer mucky, stagnant water. Once eggs are laid, our field team collects them by scooping them with a paint brush and placing them in a petri dish.

To collect adult mosquitoes:

BG Pros and Sentinels: These traps are used to collect female and male mosquitoes on the landscape using a combination of CO2 and “stinky lures” – both of which attract mosquitoes. These traps consist of a trap body which holds a fan that creates a constant suction. This suction allows mosquitoes to be captured when they get close enough to the opening of the traps. Once sucked into the trap, the mosquitoes are held in a net bag which we collect each day. These net bags filled with captured mosquitoes are then brought back to the MFBRP lab where our team can identify the species and sex of each mosquito using a microscope.

Vanishing Voices: Saving our Hawaiian Forest Birds

Below is a short documentary about mosquitoes, their impact on native forest birds, and the ongoing efforts to protect Hawaiian honeycreepers.