A Seventh Year's Observations on Malaria in Panama 1

Abstract

Summary A seventh year's observations on malaria in an unsanitated area in the Republic of Panama are reported. It is hoped to continue these observations for a period of ten years, at least. We feel that such a period is required in order to obtain sufficient data of worth from which to draw conclusions as to the most practicable method of controlling malaria under our conditions. Such a period of time will also enable us to prove or disprove various theses advanced in previous papers concerning the natural history of malaria, both in treated and untreated groups. The period herein reported upon was a period of low incidence, as was the year preceding. If our thesis concerning the cyclical nature of malaria incidence in Panama is correct, the time should not be far distant when another epidemic outbreak is to be expected. The methods used during the period reported upon were the same as have been in use for some years past. The monthly blood-parasite rates in groups treated with atabrine-plasmochin, quinine-plasmochin, both under adequate supervision, and with quinine sulphate under no supervision, are respectively 7.4, 14.4, and 16.2 per cent. As in past years, the rates showed no correlation with monthly rainfall. The blood-parasite rate, cumulative for 12 months, in those persons examined at every monthly survey, shows that slightly less than half (43.5 per cent) had parasites in the blood during the year (table 5). The same table shows that a rather high annual rate can exist in all age-groups of a relatively tolerant population. The blood-parasite rates during the period are the lowest so far recorded, and show that adequate treatment can reduce this rate to negligible proportions, during an inter-epidemic period. A record of 83 persons examined in 12 consecutive monthly surveys, who were found positive, is given in table 6. The figures in this table indicate the efficacy of supervised treatment. Table 7 presents the same data in a different manner. Table 8 shows the incidence of species of malaria parasite found in the 682 positives discovered during the year. As in previous years, heavy infections were somewhat more numerous in the two treated groups than in the control group. The malaria rate in infants showed that transmission was not active during the period, as only 8 of the 59 infants examined at least once, showed parasites, giving an annual rate of 13.5 per cent. In the 1935–36 period, this rate among 66 infants was 9.1 per cent. Mosquito infection rates were higher in 355 Anopheles albimanus dissected during the year, than during 1932, when 832 A. albimanus were dissected. The rate for 1936–1937 was 1.1 per cent, while for 1932 it was 0.48 per cent. The rate obtained for the year just past was relatively high, but the number of mosquitoes dissected was small. So many variables enter into the problem that the only safe conclusion that may be drawn is that enough human reservoirs (probably transients) escaped treatment, to permit mosquito infection. The immediate results of the treatment work are well summarized as follows (8): Our human parasite-index has fallen tremendously, and the physical condition of the inhabitants is good, but the mosquito-parasite index has not fallen with the human parasite rate.… The general physical condition of the people in the treated areas is improved; from the standpoint of an industrial organization this is a good result.… The disturbing feature of our work is the number of relapses that occur. Apparently we only suppress the malarial infections, and do not eradicate them by any form or length of drug administration. The parasite index for age-groups is the same this year as last. To which may be added that malaria control by means of drugs, without any attempt to control the Anopheline vector, may be a two-edged sword. Excellent results in controlling clinical illness may be obtained during an inter-epidemic period, such as we have just passed through. But we fear that clinical rates will be much higher, and that more serious cases of malaria may occur among the well-treated groups during an epidemic period, than in groups living in the same locality who have had little treatment, and who therefore have retained a certain degree of tolerance. We believe that the ideal method of malaria control is the elimination of Anopheles, but where this is impossible, as under our conditions, immediate treatment of clinical cases is a most useful measure. Such treatment can be given most economically by non-medical personnel in the field. Such personnel should be supervised by a medical officer, who should make weekly inspection visits. By this method, low-grade infections will not be treated, and there will be little interference with the acquirement of natural immunity.