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Oesophagostomum radiatum
THE PARASITISM
The nodules
From the foregoing account, it follows that the infective larvæ cause
nodules in the sub-mucosa of the ileum on their entry into the wall, live inside
them for some days and retrace their steps to the intestinal cavity as different
larvæ. The nodule so produced is a hard globular structure visible in most
cases even from the outside. Its size ranges from that of a pin-head to that
of a pea, and growth to the latter size is very rapid.
A section of a nodule on the 2nd day is shown in Plate X, fig. 2, where
below the mucosa, is formed a cyst enclosing the larva. The cyst is shown
under higher magnification in Plate X, fig. 3, with the larva cut in three
places. The structure of the cyst wall has been discussed by Fourie [1936] and
Spindler [1933]. This cyst-wall stains well with Eosin and does not reveal any
cellular structure, nor is there any striation to indicate that it was formed from
the old skin of the larva [Fourie, 1936]. Spindler [1933] argues that it may be
formed from a coagulum due to a liquefying process during the necrosis caused
by the larvæ. Fourie's objection to it, that the appearance is not that of a
coagulated mass but of a bundle of fine thread-like structures, is supported
in the case of the nodules in cattle also. The nature of the cyst-wall therefore
remains undetermined. In nodules which are more than three days old, a
light-green cheesy substance is obtained on opening them and the fourth-stage
larvæ are usually detected inside them. As a result of the severe necrosis
undergone by the tissues, a process of caseation takes place in the centre of the
nodule and this region is noticed in sections of older nodules as a central, densely
staining mass of cells (Plate X, fig. 4). These nodules are also known to
calcify after many days. Calf No. 2 which had an infection of 1500 larvæ
on 12 September 1939, revealed only four nodules in the ileum on 14
May 1940, so that the rest of the nodules may be said to have disappeared in
that period of eight months, leaving no macroscopic traces of their existence.
It has been consistently recorded that in cattle the nodules are caused in
the ileum of the host. While it is now realized to be the case in mild infections,
in heavy infestations other regions of the alimentary canal are also attacked.
Larvæ as many as 3000 and 4000 were administered to calves Nos. 3 and 10
respectively in a single drench and nodules were also present in the caecum and
a portion of the colon. The locus of parasitization may therefore extend
to the posterior portions also if the dosage is large and in a single instalment,
for in calf No. 4 which had about 10,200 larvæ in many instalments nodules
were however restricted only to the ileum.
Longevity of adults
Oesophagostomum columbianum according to Veglia [1928] can live in the
intestines of sheep for 20-21 months or even longer. The maximum duration
of the infestation by O. radiatum is unknown, as in no case did the host survive
so long. Calf No. 2 received a dose of larvæ on 12 September 1939 and
till its death on 14 May 1940 (8 months) eggs of the worm were being expelled.
Calf No. 8 had an infection of the worm at the time of its purchase on 23
April 1940 and gave evidences of it for the 6 months it lived after that. Calf