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A Tenth Year's Observations on Malaria in Panama, with Reference to the Occurrence of Variations in the Parasite Index, during Continued Treatment with Atabrine and Plasmochine

1. The present report is the tenth in a series of observations on malaria in the villages of the mid-basin of the Chagres River in Panama. In addition to the usual breeding-places along the river bank and lagoons, which are normally present, additional breeding-places were formed in the area owing to the impounding of water behind Madden Dam, with consequent lowering of the water-level near the villages, causing an increase in Anopheles mosquito production.

2. The past year has been one of low rainfall in the Madden Dam area, in which our villages are situated. The total was 88.12 inches at Madden Dam, the normal rainfall being approximately 100 inches. As in previous years, there has been no correlation of monthly rainfall with malaria parasite rate. However, the malaria parasite rate does increase with increased mosquito production, which follows conditions of low water and sluggish current resulting from closing the gates of Madden Dam.

3. We have continued to treat the population of New San Juan with quinine sulphate, 15 grains a day for five days, followed with plasmochin simplex 0.01 gram twice a day for five days. The five villages of Santa Rosa, Guayabalito, Gatuncillo, Las Guacas, and Agua Clara, have been treated with atabrine 0.1 three times a day for five days, followed with plasmochin simplex 0.01 gram twice a day for five days. The personnel in the field consists of six native girls and a native supervisor, who administer the treatment. All these are non-medical personnel. Our medical staff takes the blood-films, and pays four visits a month to the villages.

4. Our observations have been interrupted in many instances during the past year, owing to the migration of several families to the Madden Highway, where better schools are available, and where easy and cheap transportation by highway to Panama City is available. The demand for labor owing to the building of a third set of locks in the Panama Canal, and the increase in military forces of the U. S. Army, has increased greatly, and has reduced the population of our villages, as many of the men have obtained work as laborers, and the women have entered domestic service.

5. The stable population of the five Chagres River villages numbers 281 persons, who were examined from six to twelve times during the year. Of these, 156 were positive for malaria one or more times, a rate of 55.5 per cent. This group was treated with atabrine and plasmochin. The stable population of New San Juan, examined from six to twelve times during the year, numbers 197. Of these, a total of 113 were positive for malaria one or more times, giving a rate of 57.3 per cent.
The Madden Highway schools, a control group, were surveyed four times during the year. Among 430 children and teachers, there were 203 persons positive for malaria, giving a rate of 47.2 per cent. After each of these four surveys, the list of names of those positive was furnished, and quinine provided for voluntary use without supervision.
Rio Pescado, a village of 129 people, situated on an arm of Gatun Lake where the Pescado river enters, is an isolated town without medical care, or sanitation. These 129 people were surveyed in August, 1940, and 84 were found positive for malaria, a rate of 65.1 per cent. The location of this village corresponds closely to that of the Chagres River villages, which in 1929, before our studies began, had a total parasite rate of 62.5 per cent.
The average monthly parasite rates for the treated and control groups and their yearly cumulative rates, are as follows: Five Chagres River villages, 11.5 and 55.5 per cent; New San Juan, 12.7 and 57.3 per cent; Madden Highway schools, 32.7 and 47.2 per cent; Rio Pescado, 65.1 per cent (one survey).
These figures indicate that more than half of the population of the treated villages were positive for malaria one or more times during the year. The average monthly rates show well the difference between the treated villages and the untreated control areas.

6. The relative incidence of new infections, and relapses, is still a debated question. There are three facts that support our opinion that most of our positive malaria cases are due to relapse, rather than to newly acquired infections. These are (1) The infant rate for the year for initial infections is 2.5 per cent. (2) During the past year, parasite rates in children and in adults were almost equal in our drug-treated towns, while in uncontrolled regions the children usually show double the rate of the adults. (3) Our positive blood-films are chiefly obtained from the same individuals from the same house or family. During the year, 129 persons were surveyed from 10 to 12 times, who were found to have malaria. Of these 129 individuals, 58 were found positive only once, or 44.9 per cent. Of these same 129 individuals, 71 persons were positive from two to seven times during the year, or 55.0 per cent. We believe that nearly all these latter cases represent relapses. Individuals of this second group perpetuate the disease in their neighborhood, and repeated treatment seems never to eradicate the parasites completely, although good clinical results are obtained by treatment.

7. The species of malaria parasites found during the year were as follows: P. falciparum, 72 per cent. P. vivax, 22 per cent. P. malariae, 6 per cent. Most of the P. malariae positives were found in the Madden Highway surveys. These rates are very similar to the records obtained in former years. Nearly all local hospital records show a greater percentage of P. vivax than of P. falciparum. This may be owing to the fact that P. vivax causes somewhat more severe subjective symptoms, so that persons with vivax infections are more likely to seek treatment.

8. The percentage of crescent carriers in the five Chagres River villages and in New San Juan was the same, 45.1 per cent. The rate for Madden Highway was 54.8 per cent, and for Rio Pescado was 38.9 per cent. These figures seem to indicate that plasmochin produced little result in eliminating crescents, but it should be remembered that many more crescent carriers were revealed by the more frequent surveys in the treated towns, than by the fewer surveys in the control areas.
Among the 286 individual crescent carriers, only 24 were found that could be considered suitable material for mosquito infection experiments. The Chagres River towns had 9 such carriers, New San Juan 4, Madden Highway 10, and Rio Pescado 1.

9. Heavy infections, of the "hospital intensity type," were 22.2 per cent in the Chagres River towns, 24.2 per cent in New San Juan; 10.3 per cent in the Madden Highway control group, and 21.4 per cent in the Rio Pescado control group. We know of no deaths from malaria among the permanent population of our villages during the past year. Extremely few individuals were sent to hospitals for treatment, and these only because drugs given by mouth could not be retained.

10. During the year, 40 infants in the Chagres River villages and New San Juan, and 7 in Rio Pescado were examined. These examinations were made during the first 12 months of life. The initial infection rate so obtained affords some idea of the number of new infections in the community during the year. Only 1 infant, living in New San Juan, was found positive. This child had a P. falciparum infection at the age of 8 months. The rate indicated, 2.5 per cent, probably underestimates the true incidence of new infections in the treated towns during the year. In Rio Pescado, a control group, 3 out of 7 infants examined were positive, a rate of 43 per cent. One of these had a vivax infection at 4 months; two others had falciparum infections at 8 months and ten months respectively. The high infant rate in Rio Pescado indicates the epidemic conditions existing there at the time of our survey.

11. The indidence of malaria by age-groups for all regions surveyed shows that the highest rates are found in the two age-groups of 5 to 10 years, (49.3 per cent) and 10 to 20 years (51.5 per cent). This distribution has been noted in a number of our previous annual reports. It indicates that young adults, who form a large proportion of personnel in labor forces, need attention if their efficiency is to be kept at a high level. A second point of interest is that individuals even 60 years of age or more, who have spent their lives in endemic areas, and have racial tolerance, cannot be considered altogether immune to malaria infection.

12. Ten years of experience in attempting to control malaria in a number of endemic areas in Panama, by making monthly parasite surveys and treating the positives with drugs, leaves us with the impression that it is quite impossible to eradicate malaria parasites by these means, or to reduce them to the point where transmission is very greatly reduced. Drug-control methods cannot prevent an epidemic when unusually great numbers of Anopheles mosquitoes are present. However, we have been successful in reducing almost to the vanishing point the cases of severe clinical malaria.

13. We are of the opinion that quinine and atabrine are equally effective as antimalaria drugs. A course of atabrine is easier to administer than an equivalent course of quinine sulphate, and is preferred by our people. Atabrine, is of course, more costly than quinine sulphate. We do not believe that the use of plasmochin simplex has played a very important part in our results. This drug cannot be given often enough, or in large enough doses, to accomplish its purpose. Toxic symptoms from its use are far too common, in our experience. We can report this year, as in the past, that we have had no trouble in using atabrine in treating malaria. All who received this drug tolerated it very well.

14. Based on our experience, we believe that non-medical personnel can carry out reasonably good drug-control measures, without the use of frequent blood-film surveys, provided such personnel is supervised by a qualified physician who is interested in tropical and industrial medicine. Weekly visits by the physician to villages of labor camps are necessary, not only to assist in the anti-malaria campaign, but to discover and to treat those found ill from other causes. Expenditures incident to such a drug-control program are economically justifiable, and will result in an increased labor efficiency commensurable with the financial outlay required.

Received October 17, 1941.