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.

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

Mosquitoes and the Hawaiian Islands

There are no native mosquitoes in the Hawaiian Islands. Mosquitoes arrived with whaling ships in the early 1800s. The southern house mosquito (Culex quinquefasciatus), introduced in 1826, is the primary vector of Avian Malaria and Avian Pox, driving declines and extinctions of many 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, it is largely uninhabitable; birds venturing lower in elevation are likely to become infected 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 was first detected in Hawaiʻi in the 1940s. It is caused by a microorganism called Plasmodium relictum and is only spread by mosquitoes. The disease infects birds’ red blood cells, causing them to rupture and leading to low blood oxygen levels. It also results in enlargement and damage to internal organs such as the liver and spleen. Hawaiian honeycreepers, with no immunity to the disease, quickly become anemic, lethargic, and often die. Individuals which 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.

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.

Disease Control and the Future

Both avian malaria and pox are difficult diseases to manage. There presently is no feasible treatment or vaccines. The most important disease control methods currently are 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 depends on the preservation and restoration of their habitat, the elimination of introduced predators, and suppression of introduced mosquito populations and disease.

Examples of mosquito traps: Modified Centers for Disease Control and Prevention (CDC) light traps with CO2 cylinder (left) and Reiter-Cummings Modified Gravid Trap (right).

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.