300 
MALARIA 
upward fluctuations under conditions of 
low endemicity are essentially epidemics. 
If opportunities for malarial transmis¬ 
sion are so widespread that all occupants 
of an area, inhabitants and visitors alike, 
may be bitten not once but several times 
during the transmission season by infected 
mosquitoes, it is unlikely that the chance 
visitor or newcomer will pass the transmis¬ 
sion season and escape infection. This con¬ 
dition may occur temporarily, as during a 
severe epidemic, or it may prevail year 
after year, in which event the situation is 
characterized as a hyperendemic. 
In the latter conditions, all newcomers, 
whether immigrants or newborn, will soon 
be inoculated, and presuming they possess 
the maximum susceptibility will soon suffer 
from the active disease. This experience 
will be repeated probably until they 
acquire an immunity to all the prevalent 
local strains of parasites. The newborn 
who survive this initial infection will 
experience a great deal of clinical malaria 
during infancy and childhood, but with 
adolescence clinical attacks will become 
rarer, and as adults they will seldom or 
never experience a clinical attack and may 
not even exhibit any objective signs of 
infection. Under these conditions we find 
a high incidence of infection in children, 
a low incidence in adults. 
As the grades of endemic intensity di¬ 
minish the chance of a susceptible person’s 
Fig. 1 . Typical relationship of parasitic and 
splenic indices with varying endemicity. (After 
8. P. James, “Malaria at Home and Abroad.” By 
permission of John Bale, Sons and Danielson, Ltd.) 
(From Boyd, “Introduction to Malariology,” 1930, 
Harvard TJniv. Press.) 
passing through the transmission season 
without being bitten by an infected anoph- 
eline increase until conditions are reached 
where such an occurrence will be rare or 
unusual. Consequently many, or even 
most, persons may reach adolescence or 
adult life before they experience their 
first infection, if they do at all. Thus, 
as the grades of endemicity diminish, the 
disparity between the incidence in children 
and adults will also diminish, until when 
of moderate or low degrees, the incidence 
of infection at different age periods may be 
very nearly uniform and but a small pro¬ 
portion of the population will possess an 
immunity (Fig. 1). 
Populations living in regions of low 
endemicity and with low levels of herd 
immunity will respond to an intensification 
of transmission by the exhibition of an 
epidemic. Under this circumstance the 
level of incidence at all age periods will 
rise, without presenting a disparity be¬ 
tween children and adults. This appraisal 
may be useful in forecasting the difficulties 
which will require surmounting in order 
to establish effective control. Satisfactory 
results are much easier and more rapidly 
attained where the incidence is a low 
endemic or an epidemic, than where it is 
high or hyperendemic. 
Differentiation of the endemic picture 
requires the computation of specific indices 
for different age groups, including repre¬ 
sentatives of the adult population. In gen¬ 
eral the curve of the specific parasite 
indices will more closely conform to the 
pictures sketched, regardless of race, than 
will the specific splenic indices, because 
negroes are less subject to splenic enlarge¬ 
ment than whites. 
To discover whether there are seasonal 
variations, histories, diagnostic laboratory 
examinations, outpatient records and mor¬ 
bidity reports must be secured through 
continued surveys. It is important to have 
knowledge of seasonal variation, since it 
has a definite value in planning for control 
because seasonal malaria is easier to 
control. 
Attention must be given to the propor- 
