Avian Disease

Avian diseases, especially avian malaria and avian pox, are one of the greatest factors leading to the decline of native Hawaiian birds (van Riper and van Riper 1986, Banko et al. 2001, van Riper and Scott 2001). According to Jacobi and Atkinson (1995), a major avian malaria and avian pox epidemic in midelevation forest birds was documented during a National Biological Service study in 1992. Avian malaria is probably the most important factor preventing endemic bird populations from recovering in low elevations (Jarvi et al. 2001). Many introduced birds develop nonfatal infections that can be transmitted to other birds by introduced mosquitoes.

Mosquito-borne diseases are a relatively new problem for Hawaiian birds. Culex quinquefasciatus, the mosquito that serves as a vector of avian malaria and avian pox, was not present in Hawaii until it was unwittingly introduced into Maui in 1826 when sailors dumped water containing larvae from Mexico into a stream while refilling their water dregs (Warner 1968). Before this event, mosquito transmitted diseases were not a threat to the Hawaiian avifauna even though migrating birds may have harbored malaria infections. The Culex mosquito has spread to all of the islands, but it is mainly in low to midelevation areas because this subspecies is typically found below around 600 m above sea level (Warner 1968). Mosquitoes can inhabit areas as high as 1800 m above sea level in some years when conditions are favorable (VanderWerf 2001).

Many native Hawaiian birds are extremely susceptible to malaria and pox because they did not evolve in an environment with strong selective pressure for resistance to these diseases. The iiwi is so susceptible to malaria that in a study where juveniles were infected with a dose equivalent to the bite from a single infected mosquito, 90% died (Jacobi and Atkinson 1995). Introduced birds are found in all forests, but most native birds have been found to be restricted to forests with few or no mosquitoes (Shehata et al. 2001).

Some native species have developed immunogenic and behavioral responses that decrease their vulnerability to diseases (van Riper and van Riper 1986). Shehata et al. (2001) found that a population of the endemic amakihi (Hemignathus virens) living in a lowland forest may have evolved resistance to malaria. In this study, the amakihi individuals were not infected with malaria, but several introduced birds in the same area harbored infections. The amakihi may have been able to evolve a resistance more quickly than many other native birds because they reach sexual maturity in only six months (Shehata et al. 2001).

Even though some native birds have evolved resistance to avian malaria, they are still succeptible by avian pox. Avian pox causes lesions on the legs, feet, faces and internal membranes of birds, and can result in the loss of toes, blindness, emaciation and death (VanderWerf 2001). The pox virus can be spread through arthropod bites or by direct contact between a contaminated surface and broken skin or mucous membrane (VanderWerf 2001). This disease was widespread among endemic Hawaiian birds in the late 1800s (van Riper and Scott 2001). Pox was probably responsible for some of the extinctions that have occurred since then, and it continues to affect birds in Hawaii today (van Riper and Scott 2001). VanderWerf (2001) found that in areas where the prevalence of pox in elepaio was high, the density of elepaio and other bird species was low, which is evidence that pox is limiting the success of native birds in these areas and increasing the risk of local extinction. In this study, the site with the highest prevalence of pox and also malaria was at a lower elevation (1,550 m) where more mosquitoes were found, and the at the highest elevation sites (1,800-1,900 m), pox lesions and mosquitoes were very rare.

Avian pox is not always fatal. VanderWerf (2001) found that elepaio individuals that survived the initial infection of pox did not experience decreased future survival and reproductive success, and they probably became at least somewhat immune to subsequent infections. However, the mortality rate of elepaio with active lesions may be as high as 40% on Oahu (VanderWerf 2001), so the disease is still very detrimental to this native species. New diseases such as encephalitis, which is also transmitted by the Culex mosquito, are a huge threat to native Hawaiian avifauna (Warner 1968), and precautions should be taken to prevent additional diseases from entering the islands.