In the course of a survey which has so far covered a total of 652 birds, belonging to thirty-four different species, fifty-four cases of malaria have been found, a number of them mixed infections. The species of plasmodia represented have included the following, in the order of their frequency:
Number of cases
1. Piasmodium praecox
2. Plasmodium cathemerium
3. Plasmodium circumfiezum
4. Plasmodium vaughani
5. Plasmodium elongatum
6. Plasmodium polare
7. Plasmodium ap.?
8. Plasmodium nucleophilum
This and other work indicate that at least the first five of these species are probably cosmopolitan in distribution (assuming that P. tenue and tumbayensis are the same as vaughani, as seems probable).
Some of the species of birds found to harbor malaria plasmodia have apparently never been found so infected before. These include the following:
The results indicate that a considerable amount of host-parasite specificity exists even in bird malaria, at least with regard to certain species of parasites. This seems most marked in the case of P. vaughani which was found common in robins by Novy and MacNeal (1904) many years ago, and has also been found to be so by us. P. polare has also been found so far only in the cliff swallow.
Subinoculation into English sparrows or canaries known to be free from malarial infection was used to supplement microscopic examination in 127 cases. The results showed it to be much superior as a means of diagnosis to anything else. In the case of the song sparrows it approximately trebled the number of positive cases, and it often revealed mixed infections when only one species of parasite had been originally found. It is also the only method by which subpatent infections can be satisfactorily diagnosed according to species.
Since non-migratory birds and young birds show a very low incidence of infection with malaria, it is probable that most malarial infections are acquired in the southern portion of the range of migratory species. It is possible that in the case of P. vaughani the vector occurs only in the south, since young robins and the non-migratory types have never been found infected.
The highest incidence of malarial infection was found in the robins and song sparrows, all the adult robins examined being infected, and about two-thirds of the song sparrows from which subinoculation was done. It seems likely that in some cases at least difference in frequency of infection with malaria can be accounted for on the basis of difference in habit or migratory range. Our results indicate that malaria is probably the commonest disease from which these species suffer, and our experience with malaria-infected canaries in the laboratory suggests that even though they survive the acute stage of the infection their lives may nevertheless be considerably shortened. Leucocytozoan and Haemoproteus infection is also common, but less frequent, but the latter has been found extremely common among chipping sparrows.
It seems likely that as work of this sort is extended malarial infection will be found much commoner among wild birds than it has been thought to be in the past, and that the number of apparently distinct species of avian plasmodia will be increased. The possibility that some of these differences may be due to hybridization in the mosquito should be investigated.
Read by title at the St. Louis meeting of the American Society of Tropical Medicine, November, 1935. The authors wish to acknowledge their debt of gratitude to the American Association for the Advancement of Science for financial assistance.