Plasmodium屬的一些絲蟲寄生蟲，是傳染鳥類瘧疾的元兇。受傳染的夏威夷鳥類身上有瘧原蟲(Plasmodium relictum capistranoae)寄生蟲。瘧原蟲(P. relictum)在鳥的紅血球中繁殖。如果寄生蟲夠多，鳥會開始尚失紅血球引起貧血。因為紅血細胞負責運送氧氣到全身，是至關重要的，失去這些細胞能夠導致逐漸疲軟，並最終死亡。瘧疾主要影響雀形目鳥類(棲木鳥)。在夏威夷，這包括大部分的本土食蜜鳥與夏威烏鴉。不同的物種之間疾病易感性不同，例如， iiwi 非常易感染瘧疾，而 apapane 比較易感染瘧疾。夏威夷本土鳥類比引入鳥類更易受疾病影響，而且死亡率較高。這已嚴重影響本地鳥類，瘧原蟲(P. relictum)被指責是夏威夷各種鳥類分佈限制和滅絕的元兇，主要是低海拔的森林鳥類，因為那裡蚊子很多。
最近的證據指出一些本土夏威夷低地森林鳥類，發展出一些對瘧原蟲(P. relictum)的耐受力。例如，在剩餘的低地森林棲息地，Amakihi會再次發芽，儘管他們表現出瘧疾的發病率60-70%。雖然這似乎令人鼓舞，Freed and colleagues(2005)指出，隨著越來越多的普通物種進化出容忍力，也讓疾病的載體增加，這反過來又增加傳播到稀有物種的危險，這些物種很容易受到瘧疾感染。大多數食蜜鳥，特別是瀕臨絕種的，現在只活在 1500 公尺海拔以上的森林中，低溫有效避免蚊子身上的瘧疾。夏威夷島上 1900 公尺的夏威夷森林鳥類的瘧疾流行狀況，十年內加倍。這一增長與蚊子滋生和溫暖的夏季空氣溫度有關。原生鳥類對瘧疾的耐受力正在增加，造成高海拔的瘧疾載體增加，雖然高海拔病媒很少，氣溫很低，寄生蟲無法在病媒內完全發展。Freed和他的同事認為，瘧疾正在較高海拔成為一個新出現的傳染病，鳥類瘧疾的傳播瘧疾可部分歸因於氣候變化和日益增加的溫度。
對於已經和寄生蟲一起演化的鳥類，寄生蟲似乎不會使其致病，往往沒有任何病徵。然而，對於尚未和寄生蟲一起演化的鳥類，它會造成不同的致病程度，也可能造成高死亡率。這些敏感的鳥種可能來自沒有載體的地區，如非常寒冷，乾燥，或多風的環境。這就是為什麼鳥類瘧疾會對企鵝如此致命(經由瘧原蟲 Plasmodium relictum與 P. Elongatum)，1986年在美國愛荷華Blank Park動物園爆發的野生麥哲倫企鵝疾病說明了這一點，請參閱Spheniscus magellanicus在IUCN瀕危物種名錄)(Fix et al. 1988)。這是戶外展覽的企鵝，死亡率最高的原因，年輕和成年企鵝，第一次接觸到病媒體時，如果不加以治療，死亡率高達 50% 或更高(Cranfield et al. Undated)。
Tutuila Is. (American Samoa)
寄生: "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).
寄生: "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).
寄生: Causal agent of avian malaria in house martins Delichon urbica.
Oxfordshire (United Kingdom (UK))
寄生: Causal agent of avian malaria in blue tits Cyanistes caeruleus.
Oahu Is. (United States (USA))
寄生: 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))
寄生: 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))
寄生: 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))
寄生: 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))
寄生: "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))
寄生: 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.