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Bovine Pleuropneumonia
DISCUSSION
The chief consideration in this investigation has been to determine simple and practical methods
of field diagnosis and control by vaccination. In India, where the cost of cattle is low, there can
be no objection to the destruction of a few clinical cases when the disease is suspected in a herd or
in an area; lesions to be taken as strongly suspicious include a tendency for pneumonia to be un-
ilateral, marked pleurisy with a rapidly clotting exudate and marbling of the pneumonic areas with
variegation of the affected lobules. In all fresh centres, the diagnosis should be clinched by sub-
cutaneous inoculation of exudate or lung mash in two, six to twelve-month old cattle. The resultant
syndrome and lesions are fully diagnostic, a fact of great importance in the field where laboratory
facilities do not exist. Fortunately, in Assam, which in our present knowledge is the only area in
India affected, the disease runs a rapid course and is always fatal. It is not necessary, therefore,
at this juncture, to evolve a method of tracking down carrier animals, without which ordinarily
progress in the control of the disease is hardly possible. If the position changes, serological means
of diagnosis will have to be considered, but it is evident that no great reliance can be placed on the
agglutination test. It is felt, however, that a more positive and direct approach to diagnosis, at
least, in acute or recent cases, can often be achieved by tapping the chest and submitting the exudate
so obtained to biological and cultural test; this should be a first step and, at the same time, the pos-
sibility of obtaining positive cultures from the blood should not be forgotten.
A survey of the literature indicates that general agreement has not been reached on the problem
of immunity. The criterion of success must depend on field results assessed over a sufficiently long
period, and these are not yet available. The use of attenuated culture vaccine is fairly general in
Africa and Australia, but infrequent references in official reports leave an impression that this method
has not, as anticipated, brought the disease under control; rigorous quarantine and control of cattle
movement seem still as necessary as ever in affected territories. The report of the Tanganyika Terri-
tory Veterinary administration [1937] states that no efficient or feasible policy for the eradication
of the disease has yet been formulated. Is it possible that existing methods of vaccination, parti-
cularly with attenuated culture vaccine, are not wholly satisfactory ? There are pointers in this
direction. For example, of six cattle vaccinated with 16th generation Assam culture, two reacted
severely and died whilst the remainder were solidly immune to a relatively large dose of lymph
virus given subcutaneously at the point of the shoulder. Using a somewhat further attenuated
Assam culture, i.e. 20th generation, none of six cattle vaccinated showed any reaction, obviously
because of a greater attenuation of the culture, and on a subsequent immunity test none had the
same level of immunity. The inference seems that some loss of immunizing power follows attenua-
tion of the culture, even within narrow limits, and it would appear that, within the limits of safety,
it is desirable to use a potent culture if a strong degree of immunity is to be established. In the
experience of most workers, the experimental work on immunity has not been capable of exact
analysis, on account of the difficulty in deciding on the type of immunity test that should be employed.
It is generally stated that the subcutaneous inoculation of lymph virus is too drastic a test. The
results recorded in this paper plainly show that, if an animal has been successfully immunized, it
can certainly withstand this test. On this criterion, the degree and durability of immunity con-
ferred by the 20th generation Assam culture is relatively slight compared with that following tail-
tip vaccination with a 5th-8th generation culture which inevitably provokes a fatal reaction if ino-
culated subcutaneously at the point of the shoulder in the same cattle. The same remarks apply
to the use of lymph virus somewhat attenuated by storage at +5ºC. It appears from the results
recorded that either of these methods can be expected to give reliable results in the field, and each
has the advantage of only one vaccination being necessary. Little evidence is as yet available in
India to assess the value of vaccination on these lines, but of approximately 200 cattle vaccinated
tail-tip with virulent 8th generation Assam culture in one of the worst affected areas in Assam,
none had contracted the disease 19 months later. Adapting the original Willem's technique of vac-
cination to a fully virulent culture seems a dicided improvement in vaccination technique. It is
quite clear from the literature that the original technique, using fresh virulent lymph, is unsafe,
although productive of a high order of immunity. It has not been therefore, generally employed,