Haemoproteus Infection in Avian Species
Jaime Weisman, DVM; Bruce E. LeRoy, DVM, PhD; Kenneth S. Latimer, DVM, PhD
Class of 2007 (Weisman) and Department of Pathology (LeRoy, Latimer), University of Georgia College of Veterinary Medicine, Athens, GA 30602-7388
Introduction
Haemoproteus spp. are intracellular, protozoan, hemotropic parasites that infect red blood cells (RBCs) of birds, turtles, and lizards.1,4 Like Plasmodium and Leucocytozoon spp., Haemoproteus spp. belong to the family Haemosporidia. With more than 120 reported species, it is the most commonly encountered blood parasite in birds.6 It is found worldwide and is capable of infecting a variety of birds including gamebirds (Galliformes), waterfowl (Anseriformes), raptors (Accipitriformes, Falconiformes, Strigiformes), pigeons and doves (Columbiformes), and perching birds or songbirds (Passeriformes).
Like other Haemosporidia, Haemoproteus is transmitted by blood sucking insects including mosquitoes, hippoboscid flies (louse flies), and culicoides species (biting midges) (Fig. 1).4, 5 As successful transmission depends on the presence of the vector, infections occur more often in the warmer months of the year. The infective stage is the sporozoite which is present in the salivary glands of the insect vector.4 Once the vector bites a new host, the sporozoites enter the blood stream and invade endothelial cells of blood vessels within various tissues including the lung, liver, and spleen.2, 4 Within the endothelial cells, the sporozoites go through asexual reproduction to become schizonts which then produce numerous merozoites. These merozoites penetrate the erythrocytes and mature into either female gametocytes (macrogametocytes) or male gametocytes (microgametocytes).3 Gametocytes can then be ingested by another blood-sucking insect where they undergo sexual reproduction in the midgut of the insect to produce oocysts. The oocysts rupture and release numerous sporozoites that invade the salivary gland and serve as a focus of subsequent infection for another host once the insect takes its next blood meal.4
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| Figure 1. The complex life cycle of hemosporidian parasites (Reprinted with permission from Field Manual of Wildlife Diseases: General Field Procedures and Diseases of Birds. Friend M, Franson J, Hemosporidiosis, Page 193 (1999). |
Diagnosis
Diagnosis of Haemoproteus infection is generally accomplished by the microscopic examination of a Wright- or Giemsa-stained peripheral blood smear. Organisms may appear similar to Plasmodium, but the pigment within the intraerythrocytic gametocytes is more dispersed. The gametocytes partially encircle the erythrocyte nucleus forming a “halter-shaped” appearance. Haemoproteus gametocytes often occupy over one-half of the erythrocyte cytoplasm with little displacement of the host cell nucleus (Fig. 2).2 In Plasmodium spp. infections (avian malaria), merozoites or ring forms of the organism are usually more apparent within erythrocytes (Fig. 3). The rings may be single or multiple. Microrgametocytes and macrogametocytes also form within erythrocytes in Plasmodium infections but are observed infrequently. Both Haemoproteus and Plasmodium produce an insoluble pigment called hemozoin. This pigment is derived from the digestion of hemoglobin found within the host’s erythrocytes and appears as refractile, yellow to brown granules within the host’s erythrocyte.4
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| Figure 2. Haemoproteus sp. gametocyte within an erythrocyte partially surrounds the cell’s nucleus. Notice the multiple, refractile, golden-brown particles of hemozoin pigment. |
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| Figure 3. Plasmodium sp. infection generally appears as intracellular merozoites, forming one to several rings within infected erythrocytes. Gametocytes (left margin) are seen infrequently. |
Due to the differences in pathogenicity, it is important to distinguish Haemoproteus sp. from Plasmodium and Leucocytozoon spp. Leucocytozoon sp. is most easily distinguished because of its large size and football-like distortion of infected cells (Fig. 4).
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Figure 4. Erythrocyte or leukocyte infected with Leukocytozoon sp. (top center) is characterized by marked cellular enlargement and a football-like appearance with pointed ends. Gametocytes of Haemoproteus sp. are smaller and partially surround the nucleus in three other erythrocytes. |
Organism |
Characteristic Appearance on Stained Blood Smear |
Haemoproteus |
1. Gametocytes are only present within erythrocytes
2. Gametocytes have a “halter-shaped” appearance with little displacement of the host nucleus.
3. Schizonts are not seen on peripheral blood smears
4. Multiple, refractile, yellow to brown pigment granules (hemozoin) are present within the erythrocytes. |
Plasmodium |
1. Merozoites, schizonts or gametocytes can be seen within erythrocytes that may displace the host nucleus
- Merozoites have a “signet-ring” appearance due to a large vacuole that forces the parasite’s nucleus to one pole.
- Schizonts are round to oval inclusions that contain the deeply staining merozoites.
- Gametocytes have a “halter-shaped” appearance similar to Haemoproteus sp. but the refractile, yellow to brown pigment granules are more confined.
2. Parasites also may be found within other cells including thrombocytes, leukocytes, and endothelial cells. |
Leucocytozoon |
1. May infect erythrocytes or leukocytes.
2. Gametocytes cause marked enlargement and distortion of the infected cell producing a football-like appearance.
3. Pigment granules are not present. |
Pathogenesis and Pathology
Infections with most Haemoproteus spp. do not result in significant clinical signs. Experimental infection of turkeys with H. meleagridis resulted in lameness, diarrhea, depression, emaciation, anorexia and occasionally anemia. Muscovey ducks infected with H. nettionis suffered lameness, dyspnea and sudden death. Pigeons infected with H. columbae had enlarged gizzards.6 In other avian species, anemia, anorexia, and depression have been reported occasionally, but Haemoproteus generally is considered non-pathogenic in most avian species.2 Post-mortem findings of infected birds include enlargement of the spleen, liver, and kidneys. These organs also may appear chocolate-brown due to hemozoin deposition.4 Cytologic imprints of these organs may reveal schizont-laden endothelial cells.2 Some species of Haemoproteus will also form large, cyst-like bodies within skeletal muscles that resemble those seen with Sarcocystis spp. infections.4 Treatment and Prevention
| Note:
Treatment of animals should only be performed by a licensed veterinarian.
Veterinarians should consult the current literature and current pharmacological
formularies before initiating any treatment protocol. |
Antimalarial drugs such as chloroquine may be useful in treating Haemoproteus sp. infection; however, medication generally is not recommended parasitic infection usually is non-pathogenic.5 Transmission of Haemoproteus sp. to susceptible birds may be minimized by reducing or eliminating the vector population. This is often achieved by habitat management to decrease vector breeding sites and by killing these insects with judicious application of pesticides that affect larval or adult stages.4 Screening of aviaries may be partially effective in eliminating some vectors such as mosquitoes, but routine screen mesh size may not reliably exclude culicoides species (biting midges) because of their small size (2 mm). Exclusion of biting midges requires a screen mesh size < #16, but this fine mesh size may decrease air flow. Use of ventilation fans to produce air currents in the aviary also may disrupt the feeding cycle of culicoides sp.
References
1. Bowman D: Georgis’ Parasitology for Veterinarians: Eighth Edition. Elsevier Science, St. Louis, MO, 2003.
2. Campbell T: Avian Hematology and Cytology. Iowa State University Press, Ames, 1998.
3. Eldridge B, Edman J: Medical Entomology: A textbook on Public Health and Veterinary Problems Caused by Arthropods. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2000.
4. Friend M, Franson J: Field Manual of Wildlife Diseases: General Field Procedures and Diseases of Birds. 1999. http://www.nwhc.usgs.gov/publications/field_manual/
5. Ritchie B: Avian Medicine: Principles and Application. HBD International Inc, Delray Beach FL, 1999.
6. Swayne D, Fadly A: Diseases of Poultry. Iowa State Press, Ames, Iowa, 2003.
Acknowledgement
Photograph of Wild Turkey (Meleagris gallopavo) by Jaime Weisman
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