Amakihi with debilitating pox lesion on foot. Photo courtesy of U.S. Geological Survey
Mosquito possibly transmitting avian malaria to a bird
Blood smear showing P. relictum infection (stained purple) within red blood cells. Photo courtesy of U.S. Geological Survey
A Bane for Hawai'i's Forest Birds
Hawai’i's native forest birds evolved in isolation over millions of years. This led to one of the most spectacular examples of adaptive radiation the ornithological world has ever seen. Hawai’i's native birds evolved in the absence of many of the threats that are now present on the islands. The evolutionary processes that shaped Hawai’i's avifauna left it extremely vulnerable to disease. With no evolutionarily acquired defenses, two diseases, Avian Malaria and Avian Pox, have spread and are implicated in the extinctions, range contractions and declines of many native Hawaiian forest birds.
It is unclear when or how these diseases first appeared on the islands. It is thought they were first introduced with the importation of non-native bird species, but it may also be that they were present on the islands at a low level with migratory seabirds, shorebirds and waterfowl. However, without an effective vector, these diseases did not pose a threat to Hawai’i's native forest birds. That all changed with the relatively recent introduction of mosquitoes.
Mosquitoes and the Hawaiian Islands
Before humans arrived on the islands, there were no mosquitoes or any other biting or blood sucking insects. By the mid 1800's however, mosquitoes were firmly established throughout the archipelago introduced inadvertently through human activities. Of these, the southern house mosquito (Culex quinquefasciatus) is the greatest threat to Hawaiian birds. Introduced in 1926, it is the primary vector of Avian Malaria and Avian Pox, and is implicated in the decline and extinction of many native forest birds.
Most of Maui's native forest birds exist where mosquitoes do not; high elevation, northeastern slopes of Haleakalā above ~4500 feet. This is because mosquitoes cannot survive the cool climate at these altitudes and the parasite (Avian Malaria) cannot develop in its vector.
Kiwikiu and ‘Akohekohe once occurred throughout low and mid-elevation forests on Maui. They are now extirpated from these habitats and restricted to just a tiny fraction of their historic ranges due to these mosquito-vectored diseases. Although a large amount of suitable habitat still exists below the "mosquito line" it has been rendered almost completely uninhabitable to native birds. Those that wander below this line will most likely become infected and die. With the looming threat of climate change and projected temperature increases, it is likely that the mosquito line will move to higher elevations, further constricting the already limited range of Maui's native forest birds.
Avian Pox (Avipox-virus sp.)
Avian Pox Virus was first documented in forest birds in 1902. Mosquitoes spread this viral parasite, but direct contact between infected birds, contaminated surfaces and contaminated food and water also spread the disease. Symptoms include swollen, often bloody tumor-like lesions on unfeathered parts of a bird's body, such as the feet, legs, eyes, and base of the bill. The virus also causes lesions in the mouth, trachea, esophagus and lungs. Lesions often cause birds to have difficulty with eating, finding food, and flying. Some may even loose limbs. Infected individuals who have not succumbed to the disease may be weak and emaciated making them susceptible to depredation.
Avian Malaria (Plasmodium relictum)
Avian Malaria was first detected in the 1940s and is caused by the unicellular microorganism, Plasmodium relictum. It is spread by mosquitoes. The disease causes birds’ red blood cells to rupture, causing low blood oxygen level. Hawaiian honeycreepers with no immunity to the disease rapidly become anemic and lethargic and die. The disease also causes enlargement of the liver and spleen.
Disease Control and the Future
Both diseases are very difficult to manage. There presently is no feasible treatment or vaccines to control the diseases. 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.
A key control method is to limit the mosquito's ability to breed by eliminating larval mosquito habitat: stagnant pools of water. One major source of larval habitat is the artificial water containers, ditches, puddles, and other places where water might collect in residential and agricultural areas. The other major source of larval habitat is introduced pigs living in native forests. The foraging behavior of pigs is highly destructive; creating wallows, uprooting vegetation, and hollowing out the trunks of tree ferns, which makes habitat for mosquito larvae. Eliminating pigs from the landscape is an essential step in reducing mosquito populations.
Chemical larvicides and adulticides have been very effective in substantially reducing mosquito numbers in agricultural and residential areas, but using these chemicals in the native forests will probably never happen. It is extremely problematic for many reasons and potentially dangerous to non-target native invertebrates in these fragile ecosystems.
Another larval control method has been through the use of biological control (i.e. the intentional introduction of non-native invertebrate and invertebrate predators of mosquitoes). Several species of fish and frogs have been used as biological control agents in the past with limited success. Although these agents may have had some efficacy, many became invasive themselves to the detriment of native species. Recent research has shown that copepods and several bacterial species may be more effective biological control agents with minimal effects on native fauna.
Fortunately, in the last decade we have seen significant advances in mosquito control and technology. These technologies can be grouped into two broad categories; the sterile insect technique (SIT) or the population replacement technique. SIT is when a system is flooded with sterile males that mate with wild females resulting in non-viable eggs. This approach is being applied elsewhere and the initial data look promising. The population replacement approach is when genetically modified mosquitoes are released into the environment. These mosquitoes can be designed to spread specific, beneficial genes throughout the population that may prevent disease development or eliminate a population.
The advantages of these techniques are that they do not involve chemicals, are species-specific, can achieve landscape-scale control, can be self-limiting, and are especially effective against dispersed targets. Neither technique poses any risk to humans. However, unlike SIT, population replacement has not been tested in the wild. In the short term, the focus will be on existing tools that have implementation pathways, such as SIT.
Aside from working on ways to directly control disease and/or mosquitoes, maintaining and protecting high-elevation disease-free habitats is of critical importance. The biggest threat to these refugia may be climate change. Restoring deforested, high elevation lands is extremely important.
There is now evidence that Hawai’i ‘Amakihis are evolving disease resistance. This species is now being found in low elevation areas that have mosquitoes and disease. These low-elevation forests may serve as important grounds for coevolution of native birds with disease. Therefore, the protection of native forests from sea-level to tree line is extremely important for the future survival of native birds.
The continued viability of our native forest birds depends on preservation of their habitat, vector eradication, public outreach and education, vigilance in anticipation of future diseases or population declines, continued research on the native birds and their habitat, and continued laboratory research into innovative disease control methods.