260
Agglutination Test for Brucella Infection in Cattle
Willems found that the index of agglutin-
ability of the English suspension was 400 and
the titre indicating infection will be obtained
by multiplying the index of agglutinability by
the constant of agglutination 400 × 0.07142=
28.56, or roughly 1/30.
Stableforth regarded a positive reaction as
that of 25 per cent agglutination with a serum
dilution of 1/40.
With the standard serum and the English
suspension 25 per cent of agglutination was
obtained with a serum dilution of 1/500.
The titre of infection, therefore, is 500 × 28.56/400 =
1/35.7, which is very close to Stableforth's
standard of 1/40.
Willems points out that this shows that the
infection titre recognized by him is close to
that adopted by Stableforth, but he stated
that it is not possible to forecast the titre of a
serum with mathematical precision.
He tested the method on 11 sera:—
In 3 cases the titre was accurately forecast.
In 6 cases the titre was 10 per cent in error.
In 5 cases the titre was less than 10 per
cent in error.
In 1 case the titre was less than 20 per cent
in error.
In 1 case the titre was less than 50 per
cent in error.
How far such a method would be found of
value it is difficult to say, but Willems' own
test, although limited, shows a considerable
margin of error in his prediction.
It is obvious, however, that it is not possible
to answer the question "What constitutes a
reaction to the agglutination test" until we
know: (1) the exact technique of the test; (2)
the agglutinability of the suspension used.
So far each laboratory appears to have work-
ed out its own system of technique and inter-
pretation on an empirical basis, although in
some cases the titre indicating infection has
been arrived at by reference to the clinical
history of infected animals and subsequent
bacteriological examinations.
Having adopted a standard method of anti-
gen preparation and technique of test and hav-
ing determined the titre which is indicative
of infection, we are still faced with the prob-
lem of those animals whose sera have a titre
somewhat below the accepted diagnostic titre
and yet above that which might be ascribed
to non-specific agglutination. Donham
and
Fitch have investigated this matter. They
point out that the agglutination titre or titres
which are selected as a basis to differentiate
between specific and non-specific agglutinins
are arbitrary separations in a continuous scale
of agglutination reactions extending from low
to high dilutions of serum. They have been
made after extensive studies and experience,
but they are not infallible.
These authors
tested 303 sera from cattle in herds that did
not contain any animals with positive aggluti-
nation titres, with dilutions of 1 in 10 to 1 in
25. Thirty-seven per cent of these showed
agglutination in a dilution of 1/25.
The signi-
ficance of their findings was that we cannot
consider all agglutination reactions in low
dilutions as evidence of
Bang's disease,
although it may be an indication of a rising
titre in a recently infected animal. Donham
and Fitch do not consider this latter possibil-
ity of much importance as it detects only a
minor percentage of animals that should be
regarded as suspicious. In America the titre
indicating infection is regarded as 1-100.
Taking Donham and Fitch's work, it appears
that somewhere between the 1/25 dilution
and the 1/100 dilution, agglutination which
may or may not be the result of Br. abortus
infection will occur. Since the fixation of the
titre of infection is arbitrary, we have to
recognize that some reactions must be regard-
ed as suspicious because we are not justified
in diagnosing them as positive or negative.
Subsequent tests of sera of the animals from
which such sera have been collected will indi-
cate if the titre is rising and thus permit a
definite diagnosis. Kitselman [1936] reviewed
the after-history of 1,000 cows testing suspi-
cious (1/50) to the Bang agglutination test.
After five months 60.1 per cent had become
negative, 23.1 per cent had become positive,
and 16.8 per cent remained unchanged. These
cattle were in herds in which the percentage
of infection at the initial test ranged from 10
to 17.9 per cent. Thus 82.2 per cent of the
suspicious reactors gave a definite negative or
positive reading within five months of the
doubtful test. There are, however, certain
animals of which the serum titre persists at
the one level of suspicion. In our experience
such animals are not infected, although a
careful bacteriological examination of the ani-
mal after slaughter has only been possible in
one or two cases.
In determining the possible danger of these
animals most authorities are of the opinion that
the interpretation of the agglutination results
can only be made satisfactorily in the light of
the disease status of the herd, the breeding