In malaria there is no leucocytosis, but rather a leucopenia; Billings gives the average of 100 cases as 4323 leucocytes per c. mm., and Da Costa gives his average of 45 cases as 5622.
The presence, therefore, of a leucocytosis will at once enable us to exclude malaria, or at least cause us to look for some complication, or some altogether additional affection.
By the differential count we gain some valuable information, for in malaria there is an increase of the large mononuclears, rising in nearly all cases to over 14 per cent., and in some to as high as 30 per cent.; we have, however, the record of a case where the parasites were found and the large mononuclears were only 7 per cent. There are comparatively few diseases with so high an average percentage of large mononuclears, and so this feature in itself becomes almost diagnostic.
Trypanosomiasis and kala-azar frequently also show the same feature, and so have to be thought of; in addition they show a leucopenia.
Typhoid fever in the Tropics is peculiarly apt to be confounded with malaria, either at its onset or during its course. A full examination of the blood should prevent our falling
