J. B. POLDING 33
The fundamental facts governing the use of this test are probably as
follows. Shortly after its invasion by Brucella, the host develops a
response which is detectable by the agglutination test. Unfortunately this
response is often inconstant in degree, time of appearance and duration.
This inconstancy is probably governed by the time of ingress of infection in
relation to pregnancy, and by the amount of hold the infection is able to gain
over the host. Experimental records have enabled reactors to be divided
into two classes, viz. ' constant reactors' and ' inconstant reactors'. The
' ' constant reactor ' responds within a short period after infection by producing
in its serum a high agglutinin titre and this persists for several years subject
to a slow over-all decline. The ' inconstant reactor' responds with a low
fluctuating serum titre, which may appear, disappear and reappear at irregular
intervals for several months after infection. Experimental observations
engender the belief that in an average outbreak of Brucellosis 50—60 per cent
of infected animals will be' constant' and 40—50 per cent will be ' inconstant
reactors '. It follows that, if all infected animals are tested once at random,
all the ' constant', but only a few of the ' inconstant reactors ', will be dis-
covered ; or in other words a single random test will have an accuracy of not
more than 60 per cent. In the same way, a single random test of an infected
individual animal will only reveal infection in 60 out of 100 cases. On the
other hand if it is possible to test all infected animals once weekly during
the year succeeding infection, then during one or other of these tests almost
all' inconstant reactors ' will be in a reacting phase and consequently almost
100 per cent of infected animals will be detected.
However, it is seldom possible to test herds more often than once in six
months, if as often, and it follows that some 40 per cent of infectious animals
are escaping diagnosis. For this reason, any system of eradication based
on infrequent tests is doomed to failure. There is experience with well-
regulated European herds which in spite of being tested every six months,
with vigorous disposal of reactors, still show fresh reactors at succeeding tests.
Such systems of testing will minimize but rarely eradicate infection
and when, as is often the case, they cause great inconvenience, they seem of
little practical value, especially if equally good results can be obtained in other
ways.
The value of the agglutination test is, therefore, limited and its chief
use is to diagnose Brucellosis as the cause of an outbreak of abortion and
to give some idea of its extent. Consequently, when an abortion enzootic
occurs, all or at least a representative number of the herd should be tested and
if the result confirms the diagnosis of Brucella infection, the appropriate steps
for its control can be inaugurated. This, apart from experimental and other
special work such as vaccination, appears to be the beginning and end of the
usefulness of the test.
Control of Brucellosis in India
From what has been said above, one method of control of Brucellosis in
India becomes at once apparent, viz. a system based on the practice of hygiene.
Such a method is fully explained in Appendix VI and in the same appendix
reasons are given for the discontinuance of systems based on test and slaughter
test and disposal, and test and segregation.