JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1847808): Black-footed penguin (Spheniscus demerus)
HISTORY: Died suddenly
HISTOPATHOLOGIC DESCRIPTION: Lung: There is an overall thickening of the pulmonary interstitium. Diffusely, air capillary and vascular capillary walls are moderately thickened by macrophages, fewer heterophils and lymphocytes, rare plasma cells, and small amounts of edema and cellular debris. Multifocally macrophages and erythrocytes contain intracytoplasmic parasitic schizonts measuring up to 10 um in diameter; schizonts contain up to twenty 1-2 um diameter, round, basophilic merozoites. Other macrophages occasionally contain intracytoplasmic granular to spicular, brown to black, variably birefringent pigment (hemozoin). Diffusely, septa and perivascular areas are expanded with edema.
MORPHOLOGIC DIAGNOSIS: Lung: Pneumonia, interstitial, histiocytic, diffuse, moderate, with intrahistiocytic and intraerythrocytic schizonts, and intrahistiocytic hemozoin, etiology consistent with Plasmodium spp., penguin (Spheniscus demerus), avian.
ETIOLOGIC DIAGNOSIS: Plasmodial pneumonia
CAUSE: Plasmodium sp. (phylum Apicomplexa)
CONDITION: Avian malaria
- Schizogony occurs in blood; gametocytes mature in erythrocytes
- Malaria is caused by parasites of the genus Plasmodium and is transmitted by mosquitoes
- Avian malaria develops in mosquitoes of genera Culex and Aedes, and rarely Anopheles
- Some passerines may serve as asymptomatic carriers
- Outbreaks in North America have been seen in Anseriformes (ducks, geese, and swans), Passeriformes (perching birds; robins, blue jays, and etc.), and Columbiformes (doves and pigeons), zoo kept penguins, zoo kept Chilian flamingos
- Species that have not coevolved with malaria, such as penguins and Hawaiian birds, are generally most susceptible
- Many plasmodia can be introduced into domestic fowl, but few appear to be natural parasites
- gallinaceum, P. juxtanucleare, and P. durae are the most pathogenic for domestic fowl, and may cause up to 90% mortality
- retictum is the species most often reported in penguins and other zoo birds
- Plasmodial parasites feed on hemoglobin, which is taken into the organism by phagocytosis, resulting severe anemia and death
- Malarial pigment (hemozoin) represents the residue of hemoglobin digestion
- Intense and severe anemia and generalized hypoxia may occur in acute gallinaceum malaria.
- Exoerythrocytic stages of gallinaceum may block capillaries in brain, resulting in death due to CNS dysfunction.
- durae produces exoerythrocytic schizonts in the capillaries of the brain.
- An infected female mosquito takes a blood meal, releasing sporozoites into the avian’s peripheral circulation
- The sporozoites invade cells of the reticuloendothelial system where they undergo two stages of exoerythrocytic development to produce shizonts; most commonly this development is in the liver, but they will also develop in other tissues such as the kidney and lungs
- The shizonts produce thousands of merozoites, which are released and infect erythrocytes
- In the erythrocyte, they develop into trophozoites, the feeding or “ring” stage; this stage then develops into a shizont with 8-32 merozoites, which can lyse the red blood cell and reinfect more erythrocytes and continue the erythrocyte stage
- After several cycles of invading erythrocytes, some of the merozoites transform into microgametocytes and macrogametocytes; these gametes are ingested by a mosquito where they further develop and fuse to form oocysts that develop into sporozoites
TYPICAL CLINICAL FINDINGS:
- Signs range from unapparent to severe anemia with hemolysis and death
- Anorexia, depression, vomiting and dyspnea may occur hours or days before death
- Penguins may have seizures
TYPICAL GROSS FINDINGS:
- Varying degrees of hepatomegaly and splenomegaly and brown to black discoloration of liver and spleen
- Pulmonary edema and hydropericardium
- Pallor in severe cases from anemia and possibly brown-tinged skin and mucosa from malarial pigment
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Trophozoites, gametocytes or schizonts in peripheral blood and/or hepatocytes and endothelial cells of lung, liver, spleen, brain
- Malarial pigment in the erythrocyte from digestion of hemoglobin
- Gametocytes occupy less than 50% of the host cell cytoplasm
- Reticuloendothelial hyperplasia, lymphocytic infiltration, and often high numbers of macrophages containing malarial pigment in the lung, liver, kidney and spleen
- durae (turkeys) causes extensive fibrosis in many tissues
- In brain: focal hemorrhages with meronts in endothelial cells
ADDITIONAL DIAGNOSTIC TESTS:
- Giemsa stained smears – for the organism in the erythrocyte; the nucleus stains blue, the cytoplasm stains blue, and the pigments yellow to black
- Chromogenic in-situ hybridization procedure has been developed recently for detection of Plasmodium parasites in paraffin wax-embeddded tissues
Avian Plasmodium species may be differentiated by the number of merozoites formed in the exoerythrocytic and erythrocytic stages, timing of the life cycle, and morphology of the gametocytes
Other intracellular parasites in Avian blood smears
- Leukocytozoon spp: Intracellular round-to-elongate gametocyte that causes the host cell (usually immature erythrocytes) to enlarge and appear to have two nuclei (marginalized host nucleus and parasite nucleus)
- Haemoproteus spp: Within erythrocytes, the gametocyte contains yellow to brown refractile spicules; the gametocyte occupies over 50% of the of the red cell cytoplasm that partially encircles the nucleus
- Atoxoplasma spp: Characteristic sporozoite in the mononuclear leukocyte cytoplasm; it is a pale-staining, round-to-oval intracytoplasmic inclusion that compresses the host cell nucleus into a crescent shape
- Often seen in penguins & flamingo from Chile reported
- Plasmodia infect a variety of avian, reptilian and mammalian species
- Most important as a disease of humans (malaria) and birds (avian malaria)
- Rodents and non-human primates are often used as experimental models
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