32
Scientific Memoirs by
helminthological museums. The leading fact brought out by my investigation of
Dochmius duodenalisi 1 in the human subject, of Sclerostomum tetracanthum in
horses, and of the present investigation, is that the persistently endemic presence
of the diseases to which these parasites give rise is owing to the circumstance
that all these species are capable of living and breeding freely, indifferently as
entozoa or as free living animals ; and, further, that these free-stage nematodes
possess wonderful powers of resistance to adverse conditions. Their great enemy
is drought. It is true that they are capable of passively resisting it ; but they
can only oppose it by retiring to a sort of condition of suspended vitality, and
cannot flourish and multiply as they do when favoured by damp surroundings.
The free-stage " rhabditis " of Dochmius duodenalis cannot, for example,
resist prolonged exposure to the full heat of the sun, but they can remain dried
up, but still alive, in moderate shade for an indefinite period ; and it is probable
that the same is the case with the free-stage generations of allied species, though
I have no direct observations to adduce in their case as I have in the case of the
Dochmius rhabditis.
Periods of drought are ; however, periods of suspended vitality, if not of in-
fectiveness. The chances of infection for the host depend entirely on the oppor-
tunities of multiplication enjoyed by the free generations ; and, in the case of those
of Sclerostomum tetracanthum, recently described, and of those of
Œ s o p h a g o s t o - m u m c o l u m b i a n u m ,o
n t h e c o n s i d e r a t i o n o f w h i c h w e a r e a b o u t t o e n t e r , t h e
opportunities of multiplication may be said to depend entirely upon the character
of the weather ; as not only are they unable to breed except in damp surroundings,
but their supply of suitable food depends on the same conditions.
Few climates can offer these conditions in greater perfection than that of
Assam. The protracted period of the rains, lasting practically from March to
November, is immediately succeeded by a cold season characterized by heavy fogs,
which, commencing long before dawn, last often well on towards noon, and which
so thoroughly saturate the soil and herbage that a stranger would imagine that
a heavy shower had fallen during the night. Practically February and a portion
of March are the only months in which, ordinarily, the soil has a chance of
drying, and this period is quite insufficient to do much harm to the free-genera-
tion nematodes, save in situations where the soil is absolutely open and bare,
which, in such a climate, are naturally few and far between. Given then the
presence, in such a climate, of a parasite inimical to the life or well-being of any
animal, and it may be taken as certain that that animal will be unable to thrive
there.
Thus, though my deductions, in the present case, are founded on an isolated
observation, I feel convinced that further investigation will show that the reason
of the difficulty of sheep-keeping in Assam may be considered as solved.
The disease is moreover to be met with, to a less virulent extent, in the
Punjab.
1 Report of an investigation into the causes of the diseases known in Assam as Kala-azar and Beri-Beri. —
Assam Secretariat Press, 1890.