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 Maui's Forest Birds
Maui's native forest birds evolved in isolation on their island home. This insularity has led to one of the most spectacular examples of adaptive radiation the ornithological world has ever seen. Maui's native birds evolved in the absence of many of the threats that mainland species have been coevolving with for millennia. The evolutionary processes that shaped Maui's avifauna have also left it extremely vulnerable to epizootic disease. Because of anthropogenic disturbances, two such diseases quickly spread through our native bird populations. Able to infect a broad range of avian hosts among populations of birds with no evolutionarily acquired defenses, these diseases are implicated in the extinctions, range contractions and precipitous declines of many of Hawaii's native forest birds. The first was avian pox virus, first documented in forest birds in 1902. The second, first detected in the 1940s, was avian malaria. These two diseases remain as major sources of mortality for Maui's native forest birds and are important limiting factors in their abundance and distribution. 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 have always been 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 Hawaii'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 challenge to Hawaiian bird conservation. Introduced in 1926, it is the primary vector of Avian Malaria and Avian Pox, and is implicated in the devastation and extinction of many of our native forest bird populations. Most of Maui's native forest birds exist where mosquitoes do not; high elevation, northeastern slopes of Haleakala above ~4500 feet. This is because the birds have been relegated there because C. quinquefasciatus (the mosquito) cannot survive the cool climate at these altitudes and because the parasite (Avian Malaria or Pox) cannot develop in its vector at these temperatures. Maui Parrotbill and Akohekohe for example once occurred on Maui throughout low and mid-elevation forests but are now completely extirpated from these habitats and restricted to just a tiny fraction of their historic ranges due in large part to these mosquito-vectored diseases. The lower elevational limits of Maui's native forest birds are largely governed by the upper elevational limits of C. quinquefasciatus. Although a large amount of suitable habitat still exists below this "mosquito line" it has been rendered almost completely uninhabitable to native birds; those that wander below this line will almost certainly become infected. It is widely held that the cool, wet, high elevation habitat Maui's native birds now occupy is relatively marginal in its ability to support forest bird populations. With the looming threat of global 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.)
Mosquitoes spreading this viral parasite are the most important means of transmission, but direct contact between infected birds, contaminated surfaces and contaminated food and water also may play a role in the spread of the disease. Symptoms include swollen, often bloody tumor-like lesions on unfeathered parts of a bird's body, namely the feet and legs as well as around the eyes and at the base of the bill. The virus also causes lesions in the mouth, trachea, esophagus and lungs. Lesions often cause great difficulty and infected individuals who have not succumbed to the disease will appear weak and emaciated.
Avian Malaria (Plasmodium relictum)
Thought to have first infected forest birds through the intentional introduction of non-native passerines (perching birds), Avian Malaria is caused by the unicellular microorganism Plasmodium relictum and is vectored by the same species of mosquito as avian pox. Because P. relictum reproduces in red blood cells, eventually causing them to rupture, non-resistant birds with a large load of the parasite rapidly become anemic and lethargic and die of low blood-oxygen levels. The disease also causes enlargement of the liver and spleen.
Disease Control and the Future
Being transmitted by mosquitoes, both diseases are very difficult to manage. Other than for birds in captive situations, there is presently no feasible treatment or vaccines for either of these diseases. However, with more research and advances in technology, treatments, and high-tech control methods could become a reality in the future. To date, the most important disease control methods include direct control of the mosquito vector. The factors that make these diseases so pervasive in the Hawaiian Islands also make vector control very difficult. Control programs need to be innovative, cost-effective, environmentally safe and sustained indefinitely.
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, impoundments and myriad other places where water might collect in residential and agricultural areas. These human-dominated landscapes often abut more natural habitats where native birds live. Mosquito abundance is often several times higher in these areas compared to natural areas and are a major source of mosquitoes in natural forest habitats. The other major source of larval habitat is introduced invasive pigs living in our native forests. The foraging behavior of pigs is highly destructive. Being voracious omnivores with a penchant for digging, creating wallows, uprooting vegetation, and hollowing out the trunks of tree ferns, pigs create a huge amount of habitat for larval mosquitoes. Eliminating pigs from the landscape is an essential step in reducing mosquito populations.
Reducing larval habitat can be costly and labor intensive and on residential and agricultural areas requires a high level of public support, outreach and education. Vector control must be done in a way that substantially reduces mosquito abundance and strives for eradication below a certain threshold level or else the effort and resources will be wasted without any positive impact on native birds. 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 copepeds and several bacterial species may be more effective biological control agents with minimal effects on native fauna.
Finally, much recent research has focused on the genetic engineering of mosquitoes that prevent the development of the malaria parasite within the mosquitoes themselves. These transgenic mosquitoes could be released into the population, mate with non-transgenic mosquitoes and theoretically spread this malaria-inhibiting trait throughout the population. There are many concerns with this approach and this research is still only in its early stages.
Aside from working on ways to directly control malaria, maintaining and protecting their high-elevation disease transmission-free habitats 'refugia' is of critical importance. The biggest threat to these refugia may come with global warming. Because of this, securing deforested and pasture lands above these areas for future reforestation is extremely important. There is now convincing evidence that Amakihi is evolving pox and malaria resistance. In fact, this species is now able to survive and is increasing in low elevation areas near sea-level with abundant mosquitoes and high pox and malaria infection rates. 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.
Clearly, there is no silver bullet for avian diseases in Hawaii. 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.