March, 1954]
A.
K.
VARMA
23
It divides again into a number of branches before actually entering the plains of
the State of Bihar and as these branches go down and down they give out numerous
capricious channels, spreading throughout the area, described above. The river
is notorious for changing its course frequently and over-flooding the adjacent areas,
which get submerged under water during the flood season to such an extent that
it presents the appearance of a broad swiftly flowing sea and it becomes almost
impossible to define the main stream of the river. The errancy and fickleness of
its course during the annual flood season has changed the whole face of the country
from a beautiful landscape into a wilderness of sand and swamp, which has been
described vividly by Inglis [1888]*
" Diverging here, re-uniting there; forming a wide bend in one place and
cutting direct through the sandy soil in another; the whole face of
the country is split up into an infinitude of islands and reticulated
everywhere by a network of dry channels and shifting sand-banks."
The Mahanadi, originating from the Darjeeling Hills, flows through the district
of Purnea with its several tributaries, as a wide meandering stream providing in-
numerable small water-pockets, which expand and cover large areas in the rains
and shrink to small ponds or lakes in the dry season. Nourished by melting glaciers
and fattened by the heavy monsoon rain, these rivers come down in spate in the
rainy season. After the flood water recedes, the water-logged areas, numerous and
sometimes extensive, throughout the entire region, act as most suitable habitats for
the molluscan intermediaries of the trematode parasite under study, providing
innumerable foci of infection. Such a change in the topographical features of the
area is brought about yearly during the flood season by shifting of the river-beds.
Thus, many new suitable breeding centres for the snails are being constantly pro-
vided. This is a factor of paramount importance in the distribution and endemicity
of the disease.
Soil
In the lowland areas, where the soil has become thick and heavy by continuous
addition of fine clay carried by drainage from the highlands of quick disintegrating
mountains of Nepal, which consist mostly of shaly naked rocks, there is a tendency
towards water-logging, which is so favourable for the spread and propagation of
the snails. The lime-content of the soil is generally higher in this area (about 20 per
cent calcium oxide), which is so essential for the molluscan shells. The high per-
centage of lime makes it less tenacious and greatly improves its texture, so that
both its texture and retentiveness of moisture are unique. The land thus suffers
from excess of moisture and provide favourable aestivating ground for the snails in
times of drought.
It has further been found by the agriculturists that a large part of the north
Gangetic plain, of which this region forms a part, suffers from phosphate depletion
and is in a dangerously exhausted condition as regards available phosphate. It is
a well known fact that phosphate-deficiency produces 'deficiency diseases' in animals
grazed on natural herbage, which is a common practice in the area. Such a mal-
nutrition may as well predispose the animals to helminthic infections or at least
* Inglis, J. (1888). Tent life in Tiger-land