A tremendous amount of work has been done on the bird malarias. The avian species of Plasmodium lend themselves well to experimentation, and, until the discovery of P. berghei in rodents in 1948, birds were the only experimental animals in which malaria could be conveniently studied. All the drug screening for antimalarials in World War II was carried out in birds (Wiselogle, 1946).
About 40 species of Plasmodium have been described from birds, but only 14 or 15 are accepted as valid (Hewitt, 1940; Bray, 1957; Laird and Lari, 1958). Many wild birds are commonly infected. The most complete general review of bird malaria is that of Hewitt (1940), altho it is now somewhat out of date. Herman (1944) and Coatney and Roudabush (1949) have given catalogs and host-indices of the species of Plasmodium in birds. Levine and Hanson (1953) tabulated reports of Plasmodium from waterfowl, and Levine and Kantor (1959) did the same for birds of the order Columbiformes. Other more recent general papers are those of Becker (1958), Bray (1957), Herman et al. (1954), Huff (1954) and Wolfson (1941).
Bird malaria is not of great veterinary importance, but it may occasionally cause losses, especially in pigeons. Most of the species are not strongly host specific and can infect several species of birds. Most laboratory studies have been carried out with Plasmodium cathemerium and P. relictum in the canary, P. gallinaceum in the chicken and P. lophurae in the duck.
The avian species of Plasmodium, fall into 2 groups, depending upon whether their gametocytes are round or elongate. Among those with round gametocytes are P. cathemerium, P. relictum and P. gallinaceum. Among those with elongate gametocytes are P. circumflexum, P. nucleophilum, P. rouxi, P. elongatum, P. hexamerium, P. vaughani and P. polare. P. lophurae is somewhat different; its gametocytes are elongate at first but continue to grow and come to fill up the whole host cell except for the nucleus.
Diagnosis: Bird malaria can be diagnosed by finding and identifying the protozoa in stained blood smears. If schizonts or merozoites are present, it is easy to differentiate Plasmodium from Haemoproteus, since these stages are not found in the peripheral blood in the latter. However, if elongate gametocytes alone are found, differentiation is usually not possible.
Treatment: The bird malarias respond to treatment with quinacrine, chloroquine and other antimalarial drugs. Indeed, these were discovered by screening against bird malarias. Chloroquine at the rate of 5 mg per kg, chlorguanide at 7.5 mg per kg and pyrimethamine at 0.3 mg per kg protect chickens against P. gallinaceum infections. However, as a practical matter, treatment is usually hardly worthwhile, and preventive measures are recommended instead.
Prevention and Control: Since bird malaria is carried by mosquitoes, prevention depends upon mosquito control. Residual spraying of poultry houses with insecticides such as DDT or lindane should be effective. Birds can also be raised in screened quarters where mosquitoes cannot get to them.