Plasmodium spp infect a wide variety of domestic and wild birds in most areas of the world. Infection is often not species specific. More than 40 species of Plasmodium have been reported. P gallinaceum, P juxtanucleare, and P durae are the most pathogenic species found in poultry.
P gallinaceum infects chickens in Asia and Africa and causes low mortality rates in indigenous chickens; however, mortality rates can be as high as 80–90% in commercial birds.
P juxtanucleare infects chickens and turkeys in Asia, Africa, and South America; most infections are mild or subclinical.
P durae infects turkeys and gallinaceous birds other than chickens in Africa; the mortality rate in turkeys can approach 100%.
Clinical malaria has not been reported in domestic poultry in North America. However, indigenous wild turkeys can be infected by at least five different Plasmodium species, and experimental transmission of Plasmodium hermani between wild and domestic turkeys and between domestic turkeys and bobwhite quail has been reported.
The most common plasmodial species that affects wild birds is Plasmodium relictum, which has been found in at least 419 bird species. Subclinical infections in endemic or introduced birds can be transmitted via mosquitoes and cause fatal disease in introduced (eg, zoo birds) or resident (eg, Hawaiian avifauna) birds, respectively.
Passerine birds commonly carry P relictum subclinically. Cold-climate species that are held outside their natural range (eg, penguins, snowy owls, and gyrfalcons in captivity) are particularly susceptible to developing clinical signs of infection and fatal disease.
Invertebrate hosts are ornithophilic mosquitoes—usually Culex, Culiseta, or Aedes spp.
Clinical Findings of Plasmodium Infection in Poultry and Other Birds
Infection with Plasmodium spp can be nonclinical or cause illness with the following clinical signs:
weakness
lethargy
dyspnea
anemia
abdominal distention
hepatomegaly
splenomegaly
right-sided myocardial hypertrophy
ocular hemorrhage
biliverdinuria
death
Death due to infection by Plasmodium spp results from severe anemia or blockage of capillaries in the brain, lungs, or other vital organs by exoerythrocytic meronts in endothelial cells.
Lesions of Plasmodium Infection
The liver and spleen are markedly enlarged and often dark brown.
Diagnosis of Plasmodium Infection in Poultry and Other Birds
Diagnosis of Plasmodium infection is typically based on blood smears.
Cytological evaluation
Histological evaluation
PCR assay
Pigmented parasites, including meronts, are found in both immature and mature RBCs (see gyrfalcon blood image). Infrequently, parasites are found in thrombocytes and WBCs.
Courtesy of Dr. A. J. Van Wettere.
In birds that die rapidly after infection by Plasmodium, organisms can be sparse or absent in blood; however, numerous meronts can be found in capillaries on histological examination or examination of squash or impression smears of brain, lung, liver, and spleen.
Serological methods exist but are not available commercially. Serological tests and PCR assays can detect infection when parasites are too few to be identified in blood smears.
Treatment and Control of Plasmodium Infection in Poultry and Other Birds
Antimalarial drug therapy
Mosquito control for prevention
Treatment of birds infected by Plasmodium spp is variably effective. Persistent parasitemia or relapse can occur during or after treatment. Birds that survive initial infections can relapse with clinical signs.
Preventing exposure to mosquitoes is essential to preventing infection by Plasmodium spp. Appropriate housing, elimination of standing water where mosquitoes can breed, and CO2 mosquito traps can help decrease the mosquito population near birds.
No antimalarial drug is commercially available or approved to treat poultry flocks. However, a mixture of trimethoprim and sulfaquinoxaline in feed has been effective against experimentally induced P gallinaceum malaria in chickens.
An experimental study on the pathogenicity and chemotherapy of P durae suggested that a combination of sulfamonomethoxine and sulfachloropyrazine could be an effective treatment; halofuginone was suggested for chemoprophylaxis in endemic areas.
Chloroquine administered in Leghorn chickens experimentally infected with P juxtanucleare can decrease parasitemia.
For caged birds and penguins, chloroquine and primaquine are administered orally and are followed by scheduled administration of chloroquine. Chloroquine in drinking water has also been used to treat songbirds. Grape or orange juice can disguise chloroquine’s bitterness.
Treatment for Plasmodium infection that includes both primaquine and chloroquine is recommended over chloroquine alone, because only primaquine is active against tissue schizonts. Chloroquine has activity against erythrocytic schizonts and gametocytes. Primaquine also has activity against erythrocytic gametocytes.
For treatment purposes, it should be noted that a 500-mg tablet of chloroquine phosphate contains 300 mg of active chloroquine base, and a 26-mg primaquine phosphate tablet contains 15 mg of active primaquine base.
In raptors, control of Plasmodium infection has been achieved by oral administration of mefloquine. Alternatively, a combination of chloroquine and primaquine can be given orally, followed by scheduled administration of chloroquine. In endemic areas, mefloquine has been used successfully for chemoprophylaxis in large falcons.
Local guidance must be followed in the administration of all medications to food-producing animals, observing appropriate withdrawal times and residue avoidance.
Key Points
Plasmodium species that are pathogenic for poultry occur mainly in Asia, Africa, and South America.
Infection is most commonly subclinical.
Diagnosis is based on examination of blood smears, tissue cytological evaluation, histological examination, and/or PCR assay.
No treatment is approved for poultry, so the disease is controlled by limiting exposure to mosquitoes.
For More Information
McDougald LR, Cervantes HM, Jenkins MC, Hess M, Beckstead R. Protozoal infections. In: Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, associate eds. Diseases of Poultry. 14th ed. Wiley Blackwell; 2020:1192-1254.
Atkinson CT. Avian malaria. In: Atkinson CT, Thomas NJ, Hunter BD, eds. Parasitic Diseases of Wild Birds. Wiley-Blackwell; 2008:35-53.
