Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in tropical regions. There are several other species of Plasmodium that infect birds, such as Plasmodium anasum and Plasmodium gallinaceum, but these are of less importance except, in occasional cases, for the poultry industry. The disease is found worldwide, with important exceptions. Usually, it does not kill birds. However, in areas where avian malaria is newly introduced, such as the islands of Hawaii, it can be devastating to birds that have lost resistance over evolutionary time.
Its main vector in Hawaii is the mosquito Culex quinquefasciatus, which was introduced to the Hawaiian islands in 1826. Since then, avian malaria and avian pox together have devastated the native bird population, resulting in many extinctions. Hawaii has more extinct birds than anywhere else in the world; just since the 1980s, 10 unique birds have disappeared.
Virtually every individual of endemic species below 4000 feet in elevation has been eliminated by the disease. These mosquitoes are limited to lower elevations, below 5,000 feet, by cold temperatures that prevent larval development. However, they appear to be slowly gaining a foothold at higher elevations and their range may be expanding upwards. If so, most remaining Hawaiian land birds may become at risk to extinction.
Most of the Hawaii islands have a maximum elevation of less than 5,000 ft, so with the exception of the Big Island and East Maui, native birds may become extinct on every other island if the mosquito is able to occupy higher elevations.
Disease process and epidemiology
Plasmodium relictum reproduces in red blood cells. If the parasite load is sufficiently high, the bird begins losing red blood cells, causing anemia (USDI and USGS 2005). Because red blood cells are critical for moving oxygen about the body, loss of these cells can lead to progressive weakness and, eventually, death (USDI and USGS 2005). Malaria mainly affects birds in the order Passeriformes (perching birds). In Hawaiʻi, this includes most of the native honeycreepers and the Hawaiian crow. Susceptibility to the disease varies between species, for example, the ʻIʻiwi is very susceptible to malaria while the ʻApapane less so (USDI and USGS 2005). Native Hawaiian birds are more susceptible than introduced birds to the disease and exhibit a higher mortality rate (Van Riper et al. 1982; Atkinson et al. 1995). This has serious implications for native bird faunas (SPREP) with P. relictum being blamed for the range restriction and extinctions of a number of bird species in Hawaii, primarily forest birds of low-land forests habitats where the mosquito vector is most common (Warner 1968; Van Riper 1991; USDI and USGS 2005).
The incidence of this disease has nearly tripled in the last 70 years. Notable among the species of birds most heavily affected were House Sparrows, Great Tits, and Blackcaps. Prior to 1990, when global temperatures were cooler than now, less than 10 percent of House Sparrows (Passer domesticus) were infected with malaria. In recent years, however, this figure has increased to nearly 30 percent. Likewise, since 1995, the percent of malaria-infected Great Tits has risen from 3 percent to 15 percent. In 1999, some 4 percent of Blackcaps — a species once unaffected by avian malaria —were infected. For Tawny Owls in the UK, the incidence had risen from two or three percent to 60%.
The main way to control avian malaria is to control mosquito populations. Hunting and removing pigs helps, because wallows from feral pigs and hollowed out logs of the native hapu'u ferns provide dirty standing water where the mosquito breeds (USDI and USGS 2005). Around houses, reducing the number of potential water catchment containers helps reduce the mosquito breeding sites (SPREP Undated). However, in Hawaii attempts to control the mosquitoes by larval habitat reduction and larvicide use have not eliminated the threat.
It may also be possible to find birds that are resistant to malaria, collect eggs and raise young birds for re-introduction into areas where birds are not resistant, giving the species a head-start on spreading resistance. There is evidence for evolution of resistance to avian malaria in two endemic species, Oahu Amakihi and Hawaii Amakihi. If other species can be preserved for long enough, they may evolve resistance as well. One tactic would be to reforest high-elevation areas on the island of Hawai'i, for example above the refuge of Hakalau on land managed by the Department of Hawaiian Homelands. This could give birds more time to adapt before climate change or mosquito evolution bring avian malaria to the last remaining bird populations.