20 STUDIES ON NASAL SCHISTOSOMIASIS [Vol. XXIV, Part I,
to which they remained mostly attached below the surface of water. In some of
the rice fields they were found in clusters near the inlets of water and the villagers
reported that during the inflow of water into different plots these snails move against
the current and assemble together.
In areas lying over routes (4) and (5) neither Limnaea nor Planorbis were found
and the predominating type was Vivipara bengalensis. This accounts for the
absence of the disease there; and whatever few cases were found in some localities
of this area may be imported ones from the surrounding infected areas.
A collection of both the types of snails, supposed to carry the infection, was
made from different localities and some specimens, totalling over 100, were tested
for natural infection by ordinary test-tube method. They were later dissected
out for cercarial stages; but the results in both the cases were negative. Some
snails, however, showed natural infection with some leptocercous distome cercariae.
Most of the snails in the collection appeared to be young ones, while some older
ones, when being tested in tubes, were found to lay eggs.
Thus, the investigations to find out the potential molluscan vector of the disease
in the area proved inconclusive. But in view of the most wide distribution of
Limnaea luteola throughout the infected zone, which corresponds with the intensity
of the disease, the writer feels empirically inclined to establish some epidemiological
correlation between this snail species and the causal trematode parasite under study
and to conclude that this is a potential intermediate vector of the disease in this
area. Rao [1933] also found in Madras that the rate of infection was higher in
Limnaea luteola than in Indoplanorbis exustus. Experimental infection could not
be undertaken in India due to lack of laboratory facilities and unfortunately none
of the samples of infective material arrived alive in London to pursue this.
Other snails collected from this area were, on identification, found to be Vivipara
bengalensis, which on account of their gill-breathing habit, lived in somewhat deeper
water at the bottom. They were of various sizes and plenty in number, particularly
in areas lying on routes (4) and (5). Some Melania variabilis were also collected
from some marshes in the district of Patna, particularly about the Bakhtiarpur-
Bihar road in the pools. These were found to favour muddy habitats. Scarcity
or uneven distribution of these species suggests that they are not concerned in the
transmission of the disease. Routine examination of some of these snails for larval
stages of trematodes yielded negative results.
CONTROL
As this disease is clearly a great menace to the area, it is desirable that the
preventive measures against the malady should be taken with a clear appreciation
of the factors and conditions under which the disease is contracted and propagated.
Unfortunately, although the parasitic nature of the disease has been established, no
measures have been taken so far to control it. The strictly limited distribution of
the disease in parts of Shahabad, Patna and Gaya districts, as shown in the map
of Bihar (Fig. 1), suggests that the bionomics of the snail vectors are the critical
influence in the endemicity of the infection, which is favoured by the physical and
topographical features of the area, discussed earlier for the animal-snail contact.