A study of human malaria oocysts as an aid to species diagnosis

Abstract

1. (1.) It is suggested that fuller and more accurate information would be obtained when malaria surveys are being carried out if both sporozoite and oocysts rates were known. 2. (2.) Experiments repeated by us on many occasions show that the minimum time required for the completion of the parasitic cycle in the mosquito varies considerably among the species. P. vivax has the shortest cycle and P. malarie and P. ovale the longest, with P. falciparum a day or two longer than P. vivax. 3. (3.) Any lowering of the temperature, even for a few hours during each 24-hour period of the incubation period, retards the growth of oocysts and in consequence may delay the completion of the cycle by several days. 4. (4.) It is considered that all species of human malaria oocysts can be diagnosed with some degree of certainty, at least to the stage of development before the dividing nuclei obscure most of the pigment. 5. (5.) Our descriptions of the pigment in the oocysts of human malaria refer to infections in Anopheles maculipennis atroparvus. The mosquitoes were kept in the same insectarium and at the same constant temperature and humidity throughout the incubation period. 6. (6.) Day by day descriptions during the growth of the oocysts of all four species of human plasmodia are given. The colour of the pigment, the number of grains of pigment, its arrangement and degree of coarseness are of value as an aid to diagnosis. 7. (7.) The measurements of the oocysts given in the table were arrived at after dissecting the mid-gut in Locke's fluid and adding a drop of fixative (Bles) just before flattening the gut with a cover-slip. 8. (8.) In some species of avian plasmodia the pigment in oocysts is very coarse, especially in P. gallinaceum in which it is coarser than in any species of human Plasmodium. 9. (9.) At least in one human species of Plasmodium, P. vivax, the pigment is indistinguishable in colour, number of grains and its arrangement, from at least one species of simian Plasmodium, P. cynomolgi. If there are areas where the distribution of these two species overlap, the percentage of mosquitoes infected, arrived at either by gut or salivary gland dissections, may be misleading if the work is being carried out to ascertain the oocysts or sporozoite rate for P. vivax only. 10. (10.) Some drugs, though not gametocytocidal, may disarrange or even displace the pigment of gametocytes. In areas where mass drug prophylaxis and treatment is being carried out, atypical oocysts may be found. Pigmentless oocysts are difficult to identify and species diagnosis is impossible. 11. (11.) In a young oocyst the number of grains of pigment, its colour and arrangement in the cyst, resemble closely the pigment as it is seen in the female gametocyte in an erythrocyte. It is therefore suggested that unstained, or very lightly stained, gametocytes of most species of human plasmodia will help in species diagnosis.