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White's Method for Collection of Infective Larvae
after treatment, in petridishes of standard size
containing the same quantity of culture material
each time and collecting the infective larvae
developing therein, in a fixed quantity of water
in the trench. In the event of the right treatment
being given for any one species, the infective
larvae of that species will collect in the trench of
water in fewer numbers than before such treat-
ment ; but the occurrence of fewer larvae persists
even after repeated culture in those cases partially
treated. If all the worms of the species have
been expelled by the treatment, no infective
larvae of that species must appear in the trench
of water. The sensitiveness of these findings may
be further enhanced and greater value added to the
interpretation of results based on them, if the
routine of fishing out by the Baermann technique
any infective larvae that may be retained within
the culture be adopted.
PRACTICAL APPLICATION
For the specific diagnosis of verminous infesta-
tions, this method is comparatively simpler.
All that one has to do is to transfer the bottom
dish under a low power microscope, and pick out
through the finely drawn capillary end of a
pipette, fitted with a rubber teat, any larva for
study or identification. A few or many of them
may be gathered in a drop or two of water and
lightly warmed over a flame or a trace of iodine
solution added to kill them, before examination
under a cover-slip.
It would appear that the practice of adhering
to a routine in the use of similar-sized petridishes
amount of culture material, quantity of water used
in the trench and the adoption of the refined
culture methods has other far reaching results.
With some experience, one can estimate by the
amount of larvae collected, the exact position of
parasitism, and draw up a comparison amongst
a number of his patients based on clinical
symptoms as well before deciding on the necessity
for treatment ; otherwise it would be very often
unwise and unnecessary to interfere with the host-
parasite tolerance already existing.
The bottom dish can be examined, whenever
necessary, under a dissection microscope, for the
detection of any migrated larvae, after lifting up,
preferably by forceps, the culture dish and placing
it on the inverted top dish. This advantage
permits of the petridish being replaced by a fresh
one carrying water to make account of the larvae
migrating between any two stated periods, or
make easy study of the variations under which
migration is effected under different experimental
conditions of temperature, humidity and light.
A ready suspension of infective larvae without
any contamination, as obtainable in this method,
is invaluable for experimental work in helmintho-
logy ; it is easy to pick out a required number of
infective larvae of any one species.
In the detection of light infestations with any
species of strongyle worms, when no infective
larvae gather in the trench of water by this
culture method, the Baermann technique should
be employed to fish out any larvae that may be
retained in the culture. The complete absence
of any infective larvae, either in the culture or in
the trench of water, is no indication that an
animal is free from strongyle infestations, as
every one of the eggs passed in the dung need
not turn into an infective larva, even under the
most optimum conditions. However, in practice
it very often happens that one fails to detect the
presence of an egg in the direct smears, but finds
the infective larvae on culture, as the amount
of faeces used for a culture is many times more
than that used for direct smears and far in excess,
to cover all losses occurring during the develop-
ment from the egg to the infective larva. The
mere finding of infective larvae in a faecal culture,
in the absence of clinical symptoms, should not
be taken as sufficient indication for undertaking
treatment. Animals in good condition carry a
very heavy infection of worms, sometimes,
without manifesting apparent symptoms of
parasitism. Treatment of these animals is, no
doubt, indicated from the fact that these animals
prove a source of danger to the worm-free com-
panions, as they would be continually soiling
the common grazing ground by the infective larvae
developing in their dung. In these cases culture
of dung would reveal innumerable infective larvae,
for guiding the practitioner to decide on treatment.
It is observed that in Madras, by this culture me-
thod the infective larvae of Strongyloides papillosus
could be trapped in the trench of water in very
large numbers during the winter months-
October to January—when the temperature is
nearer the lowest in the grade range of 24° to
40°C. The increased number of infective larvae
obtained then, should not be taken as a necessary
indication of a heavy infestation, as the environ-
mental condition is a direct factor to stimulate
this phase in the life-history of the genus Strongy-
loides which involves an indirect mode of develop-
ment, with alternation of parasitic and a free-
living phase. According to Faust [1937], 'the
indirect mode of development undoubtedly pre-
cedes the direct mode in the history of the genus.
The latter has been developed for the conserva-
tion of the species under conditions commonly
obtaining in less favourable millieux'. It would
appear then, that active spread of this infection
occurs in Madras during the winter months.
Animals may harbour an infection of Strongy-
loides papillosus even in the summer months