Avian malaria
Encyclopedia
Avian malaria is a parasitic disease
of birds.
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.
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.
s. 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 Hawaii
, this includes most of the native honeycreeper
s and the Hawaiian crow
. Susceptibility
to the disease varies between species, for example, the [[ʻIʻiwi|Iiwi]] is very susceptible to malaria while the [[ʻApapane|Apapane]] 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 fauna
s (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 habitat
s 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 Sparrow
s, Great Tit
s, and Blackcap
s. 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%.
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.
Parasitic disease
A parasitic disease is an infectious disease caused or transmitted by a parasite. Many parasites do not cause diseases. Parasitic diseases can affect practically all living organisms, including plants and mammals...
of birds.
Etiology
Avian malaria is most notably caused by Plasmodium relictum, a protistProtist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
that infects birds in tropical regions. There are several other species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of Plasmodium that infect birds, such as Plasmodium anasum
Plasmodium anasum
Plasmodium anasum is a species of the genus Plasmodium .Like all species in this genus it has both vertebrate and insect hosts. The vertebrate host are birds....
and Plasmodium gallinaceum
Plasmodium gallinaceum
Plasmodium gallinaceum is a species of the genus Plasmodium that causes malaria in poultry.- Description :...
, but these are of less importance except, in occasional cases, for the poultry
Poultry
Poultry are domesticated birds kept by humans for the purpose of producing eggs, meat, and/or feathers. These most typically are members of the superorder Galloanserae , especially the order Galliformes and the family Anatidae , commonly known as "waterfowl"...
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.
Vector
Its main vector in Hawaii is the mosquitoMosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
Culex quinquefasciatus
Culex quinquefasciatus
Culex quinquefasciatus is the vector oflymphatic filariasis caused by the nematode Wuchereria bancroftiin the tropics and sub tropics.-Primary vector of Lymphatic Filariasis in India:...
, which was introduced
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
to the Hawaiian islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
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 & epidemiology
Plasmodium relictum reproduces in red blood cellRed blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s. If the parasite load is sufficiently high, the bird begins losing red blood cells, causing anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...
(USDI and USGS 2005). Because red blood cells are critical for moving oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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 Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
, this includes most of the native honeycreeper
Hawaiian honeycreeper
Hawaiian honeycreepers are small, passerine birds endemic to Hawaii. Some authorities still categorize this group as a family Drepanididae, but in recent years, most authorities consider them a subfamily, Drepanidinae, of Fringillidae, the finch family...
s and the Hawaiian crow
Hawaiian Crow
The Hawaiian Crow or Alalā is a species of bird in the crow family, Corvidae. It is about the size of the Carrion Crow at in length, but with more rounded wings and a much thicker bill. It has soft, brownish-black plumage and long, bristly throat feathers; the feet, legs and bill are black...
. Susceptibility
Susceptible individual
In epidemiology a susceptible individual is a member of a population who is at risk of becoming infected by a disease, or can not take a certain medicine, antibiotic, etc if he or she is exposed to the infectious agent....
to the disease varies between species, for example, the [[ʻIʻiwi|Iiwi]] is very susceptible to malaria while the [[ʻApapane|Apapane]] less so (USDI and USGS 2005). Native Hawaiian birds are more susceptible than introduced birds to the disease and exhibit a higher mortality rate
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
(Van Riper et al. 1982; Atkinson et al. 1995). This has serious implications for native bird fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
s (SPREP) with P. relictum being blamed for the range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...
restriction and extinctions of a number of bird species in Hawaii, primarily forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
birds of low-land forests habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s 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 Sparrow
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...
s, Great Tit
Great Tit
The Great Tit is a passerine bird in the tit family Paridae. It is a widespread and common species throughout Europe, the Middle East, Central and Northern Asia, and parts of North Africa in any sort of woodland. It is generally resident, and most Great Tits do not migrate except in extremely...
s, and Blackcap
Blackcap
The Blackcap is a common and widespread sylviid warbler which breeds throughout temperate Europe, western Asia and northwestern Africa, and winters from northwestern Europe south to tropical Africa...
s. 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%.
Control
The main way to control avian malaria is to control mosquito populations. Hunting and removing pigs helps, because wallows from feralFeral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
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
Larvicide
A larvicide is an insecticide that is specifically targeted against the larval life stage of an insect. Their most common use is against mosquitoes...
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.