KING INSTITUTE, GUINDY, FOR 1927-28
9
remains until November and December when it becomes a running river by
receiving local drainage and the overflow from the Mopad reservoir. Then it
probably ceases to breed mosquitoes to any important extent until the water again
drops in. January. From this argument we conclude that pending the result of an
inspection in September, the important breeding places are the Maneru river bed,
and the drainage streams and the pit pools all partially dependant on the level of
the sub-soil water whose general height seems to be the one factor most responsible
for malaria.
The importance of the Maneru bed in producing malaria is indicated by the
fact that the unirrigated villages on higher ground north of the river showed as
heavy an infection with malaria as villages in the irrigated Mopad area.
The importance of drainage streams and other similar sources is shown by
the severity of malaria at Kattakindipalle in the irrigated area well away from
the Maneru—the sub-soil water here was practically at the surface of the ground
even in March.
The importance of the Maneru bed and the drainage streams and the general
conditions of the Mopad irrigated area is shown by the fact that the villages of
Inimerla and Nutssupoda on unirrigated land west of Mopad suffer distinctly less
from malaria than does the Mopad area, and this though near Inimerla, is a reser-
voir with a shallow edge that breeds anophelires and which is probably mainly
responsible for the existence of malaria there.
Effect of irrigation.—Irrigation appears to have raised the sub-soil water
level, probably mainly directly by the irrigation of wet crops and possibly partly
by the holding up of water in the reservoir at a higher level than the Mopad area.
The high sub-soil water level produces breeding places, firstly directly as in pits
and probably in the pools and streams of the Maneru, and secondly maintains
breeding places as in drainage streams by delaying the absorption of water into
the soil. Therefore we may say that irrigation or rather wet crop irrigation
is probably mainly responsible for the severity of malaria in Mopad.
The facts alone without a malaria survey in September are almost sufficient
to warrant this conclusion. Thus, before 1921 there were dry crops with well
irrigation, sub-soil water at 25 feet and no complaints of malaria ; since then we
have the reservoir with canal irrigation of wet crops, sub-soil water near the
surface and severe malaria. The irrigation project was started as a famine
protective work with the intention at first of irrigating as much land as possible,
and so of irrigating not wet crops but dry crops only. This decision was changed
for some reason unknown to us probably financial—with the same return for water
the expense of construction is less if the water be spread over a small area than
when over a large. Whatever the reason, it appears that no public health
authority was consulted. We suggest that in all future irrigation works such an
opinion should always be obtained, for being concerned with the effect on health
it is surely as important as the opinion of the Agriculturist concerned with the
supply of food. There is another point about the change of crops. The Mopad
people are and have been for centuries, a dry crop people—as shown by their
staple food still being millets. To let them grow wet crops is almost equivalent
to letting them migrate to a wet crop area. Accordingly it would take a great
many years before they adjusted themselves to the new conditions.
Recommendations.—1. That subject to confirmation of our present finding by
another visit in September, wet crop irrigation in the Mopad area should cease
and only dry crop irrigation he permitted. This will probably result in such a
lowering of the sub-soil water level that most pit pools would probably cease to
exist ; further the drainage streams would be smaller and probably dry up in the
intervals of watering. One uncertain factor is the Maneru river bed, but
probably the Irrigation Department could say whether the lowering of the general
sub-soil water level would lead to drying up.of its pools as we rather expect, or
whether there is an underground stream flowing from the Mopad reservoir and
welling up at intervals into surface pools and streams along the river bed. In
the latter case, an annual rough canalization of the bed is indicated.
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