Malaria - Birds

Also known as: Bird/Avian Malaria

Caused by: Plasmodium spp.

Introduction

Avian malaria is caused by distinct species of a protozoan blood parasite, Plasmodium, that is transmitted by mosquitoes. There are about 30 species of Plasmodium that infect birds worldwide.

Plasmodium can be pathogenic to penguins, domestic poultry, ducks, canaries, falcons, and pigeons, but is most commonly carried asymptomatically by passerine birds. Avian malaria has a worldwide distribution and is endemic to parts of Asia, Africa, Central and South America, and certain Caribbean islands. It is of great economic significance to the poultry industry. Organisms such as P. gallinaceum, P. juxtanucleare and P. durae may cause up to 90% mortality in poultry.

Birds with avian malaria have been used as model systems for studying the pathogenesis and treatment of malaria in humans.

Most avian infections occur through the bite of a mosquito but it is possible for a direct bird-to-bird transmission to occur. Schizogony occurs in the RBC and, therefore, blood-to-blood transfer without the intermediate host can result in infection.

The life cycle of avian malaria is very similar to that seen in infected human beings.

Clinical Signs

Infected birds are weak, depressed and anorexic. Abdominal protrusion due to splenomegaly may be present.

Central nervous signs may be present, and haemolytic anaemia and haemoglobinuria may occur.

Coma and death may occur quickly when the parasite burden is high. However, many birds, especially passerines, do not become ill and play an important role as asymptomatic carriers of the parasite.

Diagnosis

Microscopic examination of a blood smear using Wright's stain is a sensitive method to detect Plasmodium intraerythrocytic trophozoites, schizonts and gametocytes. The trophozoite is a small round to oval structure with a large vacuole that forces the erythrocyte nucleus to one pole. This results in a “signet-ring” appearance. Schizonts are round to oval inclusions in the red cells containing darkly-stained merozoites.

In birds that die peracutely, organisms may be few to sparse in the blood. In these cases, schizonts can be found in capillaries by examining impression smears of brain, lung, liver, and spleen.

PCR has also been used to diagnose Plasmodium. However, this diagnostic test is most often used in a research setting.

Treatment

Affected flocks can be treated with Chloroquine phosphate potentiated with primaquine. Chloroquine can also be added to the drinking water. Grape or orange juice may be needed to override the bitter taste of the medication.

Additional treatments include sulfonamides combined with trimethoprim, pyrimethamine, and chlorguanil. Due to strain differences in susceptibility, different anti-malarial drugs can be tried.

As treatment is often unrewarding and re-infections are common, malaria prevention is favoured by installing screens in chicken houses to prevent contact with the mosquito vectors.

References

Capinera, J. (2008) Encyclopedia of entomology Springer

ISSG database

Campbell T (1998) Avian Hematology and Cytology Iowa State University Press, Ames

Williams RB (2005) Avian Malaria: clinical and chemical pathology of Plasmodium gallinaceum in the domesticated fowl Gallus gallus Avian Pathology 34(1), 29-47