Department of Pathology, Smithsonian National Zoological Park, Washington, D.C., USA
Abstract
Malaria occurs in birds, humans and other primates, and is caused by protozoal parasites in the genus Plasmodium. Sexual reproduction of this parasite occurs in many species of Anopheles, Aedes and Culex mosquitoes resulting in sporozoites that find their way to the insects’ salivary glands and saliva.4 Several sporozoites are injected into the vertebrate host at every blood meal. The parasites pass through two stages of asexual reproduction within the vertebrate host: exoerythrocytic and intraerythrocytic.5 Exoerythrocytic replication within the liver occurs first; the merozoites that are produced eventually break out of the hepatocytes and invade erythrocytes. Once in the cytoplasm of an erythrocyte, the merozoites progress through several morphologic stages. In the ‘ring’ stage, the protozoan contains a centrally located vacuole and a peripheralized, red nucleus. With development into the trophozoite stage, food vacuoles are formed from invagination and pinching off of the host cell’s cytoplasm. Digestion of this meal leaves the characteristic, birefringent pigment associated with fulminant Plasmodium parasitism. As in the exoerythrocytic stage, the parasite replicates and forms merozoites which lyse the host red blood cell and enter other red blood cells.1 Serial infection of erythrocytes may occur indefinitely. Eventually, the intraerythrocytic merozoites develop into sexual gametes that are ingested by a mosquito during a blood meal.5 The incubation time of Plasmodium spp. (i.e., the period between initial infection of the vertebrate host by the sporozoites and the manifestation of disease) in birds has not been extensively studied. However, in humans infected with P. vivax, the incubation time can range between 12–14 days.5
Individual Plasmodium species are not always species specific, but their infectivity and vector preference are usually limited. The human malarial pathogen, P. falciparum executes its sexual cycle only in Anopheles mosquitoes and infects only humans.8 A less selective species, Plasmodium relictum is carried by Culex, Anopheles, Aedes and Culiseta mosquitoes and is infective for many birds, including ducks, doves, and passerines.6
A retrospective survey of the pathology records at the Smithsonian National Zoological Park (SNZP) revealed a high prevalence of malaria in four species of birds within the captive collection. Because the incubation period of Plasmodium is approximately 14 days, only birds that were greater than 14 days old were included in this study. During the period 1975–2000, malaria was diagnosed in 32% (15/47) of Inca terns (Larosterna inca), 22% (2/9) of Patagonian crested ducks (Anas specularioides specularioides), 11% (2/19) of red-breasted geese (Branta ruficollis), and 6.2% (10/161) of common eider ducks (Somateria mollissima dresseri) that died.
Manifestation of disease showed a variably seasonal occurrence. Most of the birds affected with malaria died in the fall and winter with 52% (15/29) presented for necropsy in September and October. A smaller, secondary peak of malaria mortality (7/29 or 24%) occurred in June and July. No deaths from malaria occurred in the spring months of March, April or May.
Birds affected with malaria had either a short clinical course or were found dead with no previous disease manifestations. When clinical signs were evident, they were vague and nonspecific: lethargy and inactivity. The time between onset of clinical signs and death in these cases was commonly no greater than 24 h.
The most significant gross pathologic lesion in birds with malaria at SNZP was enlargement and congestion of the liver and spleen. Intraerythrocytic and/or intrahepatocytic stages of Plasmodium protozoa were readily identified on cytologic impressions of these organs. Many of the birds, especially the eider ducks, were affected concurrently with pulmonary or systemic aspergillosis. These birds had multiple fungal plaques within the airsacs and associated with lungs or liver. Both affected red-breasted geese also had amyloidosis affecting several organs including the liver, spleen and kidneys. These contributing disease factors may have been sequelae to the primary Plasmodium insult.
Histologically, there was periportal lymphocytic and plasmacytic inflammation in the livers. Kupffer cells and macrophages within the livers contained varying amounts of malaria pigment. Malaria pigment was also present within splenic histiocytes and was accompanied by moderate plasmacytosis. Protozoa could occasionally be seen within the red blood cells on histologic sections of various organs. However, tissue fixation and processing for histology causes shrinkage of blood cells, often making it difficult to detect the intraerythrocytic malarial organisms by this method.
Cytologic preparations of blood smears (antemortem) or impressions of liver and spleen at necropsy were the most ideal methods for confirming the presence of malarial infections. Wright’s or Giemsa stains were used to visualize the intraerythrocytic and intrahepatocytic forms of the protozoa.
Speciation of Plasmodium organisms required examination of the shape of the meront and schizont stages and determining the degree of displacement of the host cell nucleus.6 In some cases at SNZP, fixed blood smears and/or tissue imprints were sent to the International Reference Centre for Avian Haematozoa in Newfoundland for positive identification. Plasmodium cathemerium was detected in one Inca tern and P. relictum was found in an Inca tern and a common eider. Both P. cathemerium and P. relictum are found in the common house sparrow. Plasmodium hegneri, a rare species previously only found in the green-winged teal (Anas crecca) in Taiwan, was found in both Patagonian crested ducks.
Inca terns and Patagonian crested ducks are native to the western coast of South America.3 Inca terns are permanent residents of offshore islands and the rocky coasts of Peru and Chile. Crested ducks are similarly non-migratory and live on the coasts of Chile and Argentina and on the Falkland Islands. Red-breasted geese and common eiders exist in aquatic environments of the Northern Hemisphere.3 Common eider ducks are pelagic birds of the North Atlantic Ocean and nest along the Atlantic North American coast. Red-breasted geese nest near lakes and reservoirs in the Siberian tundra and spend winters near the Black or Caspian Seas.
Mosquitoes require shallow, still, fresh water in a temperate or tropical environment for breeding.7 Cold coastal waters and a tundra climate are not ideal for the insects’ life cycle. Thus, it is unlikely that these birds evolved with the presence of mosquitoes or malarial organisms; this may explain the increased susceptibility of these birds to malaria, compared to birds from more temperate climates. Black-footed penguins (Spheniscus demersus), like the species of birds that we studied, evolved in a marine environment which does not support the mosquito life cycle. Studies have shown that these penguins are highly susceptible to P. relictum and P. elongatum.2
The increased susceptibility to malaria exhibited by these species warrants strenuous efforts on the part of zoos and wild animal parks aimed at mosquito control and detection and treatment of disease. Control programs may include removal of mosquito breeding areas, utilization of mosquito-feeding fish in standing ponds, mosquito trapping, and/or selective use of pesticides. Enclosed exhibits can minimize access of mosquitoes to malaria-susceptible bird species. In areas where mosquito control is inadequate or not feasible, cytologic screening of stained blood smears from susceptible species in summer and fall may identify affected animals. Early detection and treatment with chloroquine and/or pyrimethamine may minimize mortality due to Plasmodium infections.
Literature Cited
1. Chitnis, C.E. and M.J. Blackman. 2000. Host cell invasion by malaria parasites. Parasitol Today. 16(10):411–416.
2. Cranfield M.R., M. Shaw, F. Beall, M. Skjoldager, and D. Ialeggio. 1990. A review and update of avian malaria in the African penguin (Spheniscus demersus). Proceedings American Association of Zoo Veterinarians. pp. 243– 248.
3. Del Hoyo J., A. Elliott, and J. Sargatal. 1996. Handbook of the Birds of the World. Lynx Edicions, Barcelona, Spain. Vol. 1. pp. 584–585, 608, 621; Vol. 3, pp. 667.
4. Ghosh A., M.J. Edwards, and M. Jacobs-Lorena. 2000. The journey of the malaria parasite in the mosquito: hopes for the new century. Parasitol Today. 16(5):196–201.
5. Glynn J.R. 1994. Infecting dose and severity of malaria: a literature review of induced malaria. J Trop Med Hygiene. 97:300–316.
6. Levine N.D. 1985. Apicomplexa: Plasmodium, Haemoproteus, Leucocytozoon, and related protozoa. In: Veterinary Protozoology. Iowa State University Press. pp. 265–290.
7. Rietschel P. 1984. Diptera I: The mosquitoes and their relatives. In: Grzimek H.C.B. (ed.). Grzimek’s Animal Life Encyclopedia. vol. 2. Insects. Van Nostrand Reinhold Co. New York. pp. 473–481.
8. Samuelson J., and F. von Lichtenberg. 1995. Infectious diseases. In: Cotran R.S., V. Kumar, and S.L. Robbins (eds.). Pathologic Basis of Disease. 5th ed. Academic Press. San Diego, CA. pp. 362–363.