National Wildlife Health Center

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Avian Malaria

Question Amakihi

What is it?

Avian malaria is caused by a microscopic parasites called a protozoan. The specific name of the protozoan that causes avian malaria is Plasmodium relictum . P. relictum affects only birds. Mosquitoes move the parasite around by biting an infected bird and transmitting it to an uninfected bird. The parasite cannot be transmitted directly from one bird to another and requires the mosquito to reproduce and move from one bird to another. In Hawaii, the mosquito that transmits the parasite is the common house mosquito, Culex quinquefasciatus.

What does it do?

In birds, P. relictum reproduces in red blood cells. If the parasite load is sufficiently high, the bird begins losing red blood cells (anemia). Because red blood cells are critical for moving oxygen about the body, loss of these cells can lead to progressive weakness and, eventually, death.

What does it affect?

Malaria mainly affects birds in the order Passeriformes (perching birds). In Hawaii, that includes most of the native honeycreepers and the Hawaiian crow. However, for reasons yet to be determined, there are species-differences in susceptibility to the disease. For example, the iiwi is very susceptible to malaria, however, the apapane less so.

What are the field signs?

Unusual mortalities of birds are often the only indication that avian malaria is a problem. At necropsy, birds may not show lesions or may have enlarged livers or spleens.

Where and when does it occur?

In Hawaii, avian malaria occurs most often where the house mosquito is present. Extensive and elegant ecological studies done by researchers at the Pacific Islands Ecosystem Science Center have revealed a lot about the ecology of the bird malaria in Hawaii. Because mosquitoes are more numerous at lower elevations, malaria has a higher prevalence in birds there. In fact, it is thought that the malaria is one of the reasons for the disappearance of native forest birds in low-land forests in Hawaii. On a positive note, more recent evidence indicates that some native species of forest birds at low elevation have developed some tolerance to infection with the parasite.

How do you detect it?

Determining whether malaria is the cause of death requires examining blood in the laboratory for the presence of the parasite. Typical samples used for this include frozen serum, frozen blood, or blood smeared on a microscope slide and air-dried.

How do you manage it?

Because of its ecology (mosquito transmission), malaria is very hard to eradicate or manage. To effectively eliminate malaria from a habitat, you need to eliminate the mosquito or prevent the mosquito from feeding on the bird. This is a tall order, particularly in remote areas in the wet forests of Hawaii where wallows from feral pigs and hollowed out logs of the native apuu ferns provide ample areas of standing water where the mosquito breeds.

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Page Last Modified: Jun 20, 2018