The ultimate assessment of the efficiency of malaria control is by the number of cases of malaria contracted in the controlled area, but a method that gives a more immediate and definite information is required because of the:—

(i) Delay before the necessary information is obtained. For example, abandonment of larval control will not affect the number of malaria cases for more than a month;

(ii) Shifting populations. People will come into the area and develop malaria acquired outside and conversely people having acquired malaria inside the area will go outside before developing it;

(iii) Difficulty of distinguishing between relapses and newly acquired infections. The number of relapses has no connection with the efficiency of the control;

(iv) Lack of cases of malaria despite infection in a predominantly immune community in which the non-immune persons are protected by suppressive drugs, mepacrine and quinine. This is a common condition in very malarious areas.