276 
MALARIA 
mosquito production is not dependent upon 
rainfall in our village areas, as most of the 
larval breeding occurs in lagoons and back¬ 
waters of the Chagres River, which are 
always filled with aquatic vegetation, ex¬ 
cept during infrequent periods when this 
vegetation is washed out by flood-waters. 
Water impoundment behind Madden Dam, 
which was begun Sept. 1934, has exerted 
more influence on mosquito production than 
any other single factor. The gates of this 
dam are sometimes closed for months, and 
during such periods the only water released 
is that which passes through the hydro¬ 
electric plant at the dam. This impound¬ 
ment causes a low water level, and sluggish 
current. Conditions of stagnation are most 
favorable for the growth and spread of 
aquatic vegetation, which at such times 
covers the river near all its small tribu¬ 
taries and in all its many shallow lagoons. 
The production of Anopheles mosquitoes is 
tremendously increased while such condi¬ 
tions exist. 
TABLE II 
Average Monthly Parasite Rates. Treated 
Groups Compared with Untreated 
Control Groups 
Year 
Parasite rate 
Treated 
groups 
Control 
groups 
1930-31 
21.6 
1931-32 
16.8 
17.5 (12 months) 
1932-33 
16.8 
27.5 (12 months) 
1933-34 
12.8 
20.5 (8 months) 
1934-35 
15.8 
22.5 (12 months) 
1935-36 
11.5 
18.5 (12 months) 
1936-37 
10.8 
16.2 (12 months) 
1937-38 
6.6 
16.4 (1 survey) 
1938-39 
8.0 
20.8 (1 survey) 
1939-40 
12.2 
32.7 (4 surveys) 
These control records are from various 
areas on the Madden Dam Highway where 
the migratory population made a very poor 
control group. Many of those who had 
malarial attacks went to Panama City for 
treatment and were not caught in the blood- 
film surveys. 
The cumulative rate is obtained by divid- 
TABLE III 
Malarial Parasite Rates. Adult Rates Com¬ 
pared with Children’s (15 years and under) 
Rates, These Rates are Cumulative 
For the Year. Years 1935-1940 
Adults 
Children 
Total 
Initial survey 1929. No 
treatment or control 
% 
% 
% 
measures* . 
Cum. records Sept. 1935- 
37.2 
56.5 
45.6 
Aug. 1936. All villages 
Cum. records Sept. 1936- 
29.1 
41.5 
35.3 
Aug. 1937. All villages 
Cum. records Sept. 1937- 
30.3 
42.7 
36.5 ‘ 
Aug. 1938. All villages 
Cum. records Sept. 1938- 
21.2 
28.8 
25.0 
Aug. 1939. All villages 
Cum. records Sept. 1939- 
25.0 
30.8 
27.9 
Aug. 1940. All villages 
37.9 
42.0 
39.9 
* The rates found in the initial survey represent 
but one examination, while the cumulative rates 
include 12 monthly surveys, and all positives dis¬ 
covered. This shows very clearly that a single 
survey uncovers only a fraction of the annual 
malaria rate, even when drug control is in use. 
One favorable effect of treatment is shown in the 
decreasing children’s rate relative to the adult rate, 
as treatment progressed. Usually children show a 
parasite rate almost double that of adults. After 
continued treatment, the two rates become more 
nearly equal. This is partially explained by the 
fact that children are more likely to be present at 
surveys, and can be more thoroughly examined and 
treated. 
ing the total number of individuals posi¬ 
tive during the year by the total number 
of persons examined. 
It is seen from the last line in Table IV 
that from 1 to 5 examinations of the same 
individual during the year give average 
annual parasite rates of 19.3 per cent, while 
12 consecutive monthly examinations of the 
same individual increase these rates to 47.6 
per cent. These data mean that approxi¬ 
mately half the population of our area 
had malarial parasites in the blood at least 
once during the year. 
The table also indicates one of the diffi¬ 
culties encountered in surveying and treat¬ 
ing native population groups in the field, 
either in villages or in labor camps. Our 
river supervisor lives in one of the towns, 
