Avian malaria can be caused by many different species of plasmodium, a genus of single-celled parasitic protozoa. The species that cause malarial disease in birds differ to those that affect humans. The best studied species is P. relictum; the life-cycle of this pathogen is likely to be shared by other species of avian malaria parasites. Two hosts are required for completion of the plamodium life-cycle; the sexual stages occur in the gut of mosquitoes (including Culex and Anopheles spp.) and the asexual stages of the parasite, which can cause disease or remain dormant, are produced in avian hosts that have been infected via a mosquito bite.
Many species of bird, particularly passerines (songbirds) are susceptible to infection with avian malaria parasites, however, the extent of disease varies significantly depending on the causative strain of the parasite and the bird species. In countries where avian malarial parasites are found, the native bird species often appear to have co-evolved with the parasites so that infection does not necessarily lead to disease. Recent findings from post-mortem examination of blackbirds in Austria reported malaria-associated organ damage in 15% of specimens that was presumed to have caused the death of these birds. The researchers believe that this relatively low incidence of avian malaria-associated death in wild birds is not indicative of a threat to wild bird populations. More importantly, native bird species often act as a reservoir of infection which, if transmitted by the bite of an infected mosquito to a non-native species, may cause severe disease. For example, avian malarial parasites are not found in Antarctica, therefore, penguins have not co-evolved with this parasite. This means that infection of penguins in zoological collections in the rest of the world can be fatal.
Huijben S, Schaftenaar W, Wijsman A, Paaijmans K, Takken W (2007) Avian malaria in Europe: an emerging infectious disease? In: Takken W, Knols BGJ (eds) Emerging pests and vector-borne diseases in Europe. Wageningen Academic Publisher, Wageningen, pp 59–74
Dinhopl, N., Nedorost, N., Mostegl, M.M. et al. Parasitol Res (2015) 114: 1455.
Damage to red blood cells can lead to anemia and weakness. Damage to white blood cells and the immune organs such as the spleen can result in impaired immunity making infected birds vulnerable to secondary infections. Both mechanisms of disease as well as parasite-related organ damage can contribute to death.
Birds become infected when they are bitten by an infected mosquito. Mosquitoes, in turn pick up the parasite when they ingest the blood from infected birds. The parasite reproduces in the gut of the mosquito and the infectious stages (sporozoites) migrate to the salivary glands so that they are injected when the insect feeds. In the bird the parasite can enter a wide range of cells, undergoing different forms of development. Importantly, both red and white blood cells can be infected and are taken up by mosquitoes in their blood-meal, completing the life-cycle.
With the exception of Antarctica, avian malaria parasites are found throughout the world.
