Malaria - Birds
Also known as: Bird malaria
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.
Life Cycle of Plasmodium
Infection begins with the bite of an infected mosquito, usually Culex or Aedes species. Sporozoites introduced with the salivary gland secretions are carried to the reticular cells of the bird's spleen. Each sporozoite develops into thousands of merozoites. These merozoites rupture their host cell and invade endothelial cells or other cells of the reticuloendothelial system to complete another cycle of replication. The merozoites then rupture that host cell and enter erythrocytes in the blood stream. This initiates the intra-erythrocytic cycle. Merozoites multiply in the RBC, forming a schizont (shizogony). The schizont will rupture, killing the RBC and releasing the merozoites to infect more RBCs. During schizogony, the parasites feed on the RBC cytoplasm, ingesting haemoglobin. The intra-erythrocytic cycles continue until the host dies or the parasites are suppressed by host immunity. After the initial cycles in erythrocytes, a few merozoites develop into sexual cells (microgametes and macrogametes) with each new cycle. The sexual cells are maintained in the RBC until they are consumed by a mosquito with its blood meal.
In the mosquito: the sex cells are released and fertilization and zygote formation occur. The zygote matures into an elongated mobile cell that crosses the midgut wall. This cell become an oocyst which divides into thousands of spindle-shaped sporozoites. The oocyst then bursts and releases the sporozoites, some of which migrate to the salivary gland where they are injected into a bird during the mosquito’s blood meal.
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 susceptiblity, 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
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