Several species of the filarial parasite Plasmodium are the causal organism for avian malaria. Plasmodium relictum capistranoae Russell is the parasite found in infected Hawaiian birds (USDI and USGS 2005). In birds, P. 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 Hawaii, this includes most of the native honeycreepers and the Hawaiian crow. Susceptibility to the disease varies between species, for example, the iiwi is very susceptible to malaria while the 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 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).
Recent evidence indicates that some native Hawaiian lowland forest birds have developed some tolerance to P. relictum. For example, the Amakihi are once again breeding in remaining lowland forest habitat although they show a incidence of malaria (60-70%) (Trouble in Paradise Undated). Although this appears encouraging Freed and colleagues (2005) point out that as more of the common species evolve tolerance they increase reservoirs of the disease, which in turn increases the risk of transmission to rarer species that are vulnerable to avian malaria. Most honeycreepers, especially endangered species, now persist only in forests above 1500m elevation, where cool temperatures prevent effective malaria development in mosquitoes (Freed et al. 2005). The prevalence of malaria in Hawaiian forest birds at 1900m on the island of Hawaii has more than doubled over a decade. This increase is associated with breeding of mosquitoes and warmer summertime air temperatures. Tolerance to malaria in native birds is adding to a reservoir of malaria at upper elevations even while vectors are rare and air temperatures are too low for complete development of the parasite in the vector. Freed and colleagues argue that malaria is becoming an emergent infectious disease at upper elevations and that the spread of avian malaria can be partly attributed to climate change and increasing temperatures.
The parasite does not appear to be pathogenic in birds that have evolved with the parasite, often causing no signs. However, it causes varying degrees of pathology and can cause high mortalities in species of birds that have not evolved with the parasite. These susceptible species may come from areas without the vector, such as very cold, dry, or windy environments. This is why avian malaria is so lethal to penguins (in which it is caused by Plasmodium relictum and P. Elongatum), as illustrated by the 1986 outbreak of the disease in wild-caught Magellanic penguins (see Spheniscus magellanicus in IUCN Red List of Threatened Species) at the Blank Park Zoo in Des Moines, Iowa, USA (Fix et al. 1988). It is the highest cause of mortality in outdoor penguin exhibits and causes 50% or greater mortality in untreated juvenile and adult penguins when first exposed to the vector (Cranfield et al. Undated). For more detailed information of the impacts of P. relictum, click here
Location Specific Impacts:
Tutuila Is. (American Samoa)
Parasitism: "Malaria in American Samoa may be maintained by native mosquito vectors and Plasmodium variants that are not as pathogenic to native Samoan birds. High prevalence (59%) of chronic infections, the relative stability of the native land bird communities, and the presence of mosquito vectors which are considered endemic and capable of transmitting Plasmodium suggest that these parasites may be indigenous to American Samoa. Thus, unlike Hawaii, they may have a long coevolutionary history with their hosts". "More detailed studies of the epidemiology and pathogenicity of these parasites are needed to determine their physiological costs and population level impacts. The unintended introduction of new parasites, variants, or vectors and the impacts of unpredictable environmental stressors could, however, destabilize this system and affect long-term viability of forest bird populations on these islands" (Jarvi et al. 2003).
Parasitism: "At Auckland Zoo, 60% of a captive population of New Zealand dotterel (Charadrius obscurus) were killed by infection with Plasmodium sp. parasites in 1996 (Reed 1997 in Tompkins and Gleeson 2006). At Orana Park, Christchurch, 80% of a group of native mohua (Mohoua ochrocephala) were killed by infection with Plasmodium sp. parasites after being translocated there from the wild in 2003 (Tara Atkinson-Renton, Orana Park, pers. comm. in Tompkins and Gleeson, 2006)" (Tompkins and Gleeson 2006).
Historical surveys revealed no malarial parasites present in wild New Zealand avifauna (Bennett et al. 1993 in Tompkins and Gleeson 2006). However a recent survey of non-native wild bird populations by Tompkins and Gleeson (2006) found that avian malaria (Plasmodium relictum) was present in bird populations at 7 sites throughout New Zealand. They determined a north-to-south gradient in detection. rates which closely match the distribution of the vector Culex quinquefasciatus (although there were some anomalies which require further investigation). This positive association between P. relictum and Cx. quinquefasciatus suggests that this mosquito may be a cause of disease emergence, although there is evidence that other mosquito species can be vectors such as Cx. pervigilans (Holder et al. 1999 in Tompkins and Gleeson 2006) This is of great concern if P. relictum is introduced to populations of native birds with no history of exposure. “A high degree of susceptibility to infection is likely in many species, given the historical absence of such parasites in much of the New Zealand avifauna” (Tompkins and Gleeson 2006).
Parasitism: Causal agent of avian malaria in house martins Delichon urbica.
Oxfordshire (United Kingdom (UK))
Parasitism: Causal agent of avian malaria in blue tits Cyanistes caeruleus.
Oahu Is. (United States (USA))
Parasitism: Avian malaria Plasmodium relictum and avian pox Poxvirus avium were found in all endangered ‘Elepaio Chasiempis sandwichensis ibidis populations on O‘ahu; no parts of the island were free of these mosquito-borne diseases. However Elepaio populations on the island of O’ahu persist at low elevations where mosquito and malaria levels are high which indicates resistance, although it has not been clinically demonstrated (Vanderwerf et al. 2006).
Baltimore (United States (USA))
Parasitism: Captive juvenile African black-footed penguins (Spheniscus demersus) housed in an outdoor enclosure at the Baltimore Zoo have an average 50% mortality from avian malarial (Plasmodium sp.) infection each year (Grim et al. 2004).
Florida (USA) (United States (USA))
Parasitism: Dusek and Forrester (2002) examined fish crows and American crows for blood parasites. Plasmodium relictum was found in fish crows, where I had not previously been reported.
Georgia (USA) (United States (USA))
Parasitism: Presence of Plasmodium relictum was confirmed in populations of the house finch Carpodacus mexicanus. Infections were observed year round in Georgia.
Hawaii (United States (USA))
Parasitism: "The introduction of avian malaria (Plasmodium relictum) to the remote Hawaiian Islands has been implicated in the widespread decline and the possible extinction of many species within the endemic avian radiation of honeycreepers (Warner 1968; van Riper et al. 1986). While mortality in introduced bird species is negligible, mortality in many endemic species can range from 50 to 90% ( Jarvi et al. 2001), possibly reflecting their long isolation (ca 4 Myr; Fleischer & McIntosh 2001) from malarial parasites." (Beadell et al. 2006).
New York (United States (USA))
Parasitism: Presence of Plasmodium relictum was confirmed in populations of the house finch Carpodacus mexicanus. Infections were observed primarily between June and November in New York.