The southern house mosquito (Culex quinquefasciatus) arrived in Hawaii in the 1820s, its larvae likely entering the local water supply when contaminated water casks aboard a sailing vessel were dumped at the port of Maui. Up to that time, mosquitoes had been nonexistent on the islands, and hence native birds had evolved in environments without mosquito-borne disease, leaving them with no innate defense against infection. As a result, the arrival of avian pox (in the 1800s) and avian malaria (in the early 1900s), both of which are transmitted by Culex and presumably reached the islands in nonnative birds transported on ships, facilitated the near decimation of Hawaii’s forest birds.
Avian pox is a viral disease that in addition to being transmitted by mosquitoes, can be passed to uninfected birds through physical contact with infected individuals, contact with contaminated surfaces, or ingestion of contaminated food or water. Infection produces wart-like growths in areas of skin that are unprotected by feathers, such as around the eyes and on the beak, potentially impairing vision or the ability to feed. In “wet” pox, the growths form internally, in the mouth and throat or in the respiratory tract, causing difficulty with swallowing and breathing.
Avian malaria is a protozoal disease that in Hawaii is caused primarily by the species Plasmodium relictum, which is transmitted from infected to uninfected birds in the saliva of Culex mosquitoes. Following infection, the parasites go through two rounds of maturation, first in cells in the spleen and skin and then in macrophages (a type of white blood cell) in a variety of tissues, before invading and destroying red blood cells, which in susceptible birds can result in death from anemia. Birds that survive the acute phase develop chronic infection, in which parasites become encysted in tissues and cycle between dormant and active stages, sometimes causing periodic relapses of disease symptoms. Chronic infection also renders birds immune to reinfection with the same parasite.
Hawaiian honeycreepers, a group of songbirds that serves as a classic example of adaptive radiation (evolution into a wide variety of types, each with a specialized ecological role), have been the most heavily affected by avian pox and malaria. Since the time of captain James Cook’s first European discovery of the islands in 1778, disease and factors such as habitat loss, nonnative predators, and nonnative birds competing for habitat have led to the extinction of about one-third of the more than 55 known species of honeycreepers. These losses, combined with the loss of another third between the time when humans settled the islands (around 300 CE) and 1778, means that today, only 18 or 19 species are still alive. The majority of these are considered endangered or critically endangered.
Some of Hawaii’s native forest birds have found refuge from disease in high-elevation forests, which lie above the survival range of Culex mosquitoes. High-elevation forests, however, offer suboptimal habitat for species that evolved in lowland forests. In addition, climate change, and climate warming in particular, could allow Culex to migrate upslope, forcing the birds to move even further beyond the lowlands.
While the outlook remains bleak for many of Hawaii’s native forest birds, populations of some species have stabilized or are increasing, thanks in part to conservation efforts. Conservation successes include stable or growing populations of the bright orange Akepa (Loxops coccineus), the crimson-colored Apapane (Himatione sanguinea), and the yellow Kauai Amakihi (Hemignathus kauaiensis), all of which are honeycreepers. With increased awareness and improved understanding of species’ needs, and with a little help from birds that have been able to survive acute avian malaria, researchers are confident that other species can be rescued from the edge of extinction as well.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.