152
Contagious Abortion of Cattle, Sheep and Goats
[VOL. XVIII, PART III
for, either of these fluids may be positive when the other is negative. A final important use of the
quick-test is in vaccination work ; here, it is usually desirable to vaccinate Brucella negative animals
only and, if tests are made by the tube method, the animals must be twice-handled, once for bleeding
and once several days later for vaccination. A better method is to secure the animal and take
5.0 cc, of blood into a bottle containing a few drops of a 20 per cent solution of sodium citrate. The
mixture is shaken and the whole-blood tested immediately. The animals which have not yet been
released, may now be vaccinated, or not, according to the result. Using whole-blood in place of serum,
the quick-test is rather more difficult to read and, unless the operator has great experience, some
sort of reading box, such as described by Huddleson [1939], should be employed. But failing this
the plate should at least be screened from the direct rays of the sun, which, coming from above,
obscure the reading, for it is light from beneath the plate that is required if the flocculation is to be
detected amongst the turbid blood.
The accuracy of the agglutination test
Different authorities have placed the accuracy of the serum agglutination test, variously between
65 and 95 per cent, and it is fairly well established that perhaps one-third of all Brucella infected animals
are transitory reactors, if indeed they react at all. In Indian herds, it has generally been found
that about 70 per cent of animals, aborting during a true epidemic, are Brucella positive, whilst,
in one very severe primary epidemic, 95 per cent was recorded. When, however, a minor epidemic
is prolonged into an endemic, such a high ratio is not always seen, e.g., of 38 crossbred cows aborting
during the period 1935 to 1940 and tested early in 1941, but 14 (36 per cent), were Brucella positive.
In such cases, it is clear that very few temporary reactors have been picked out by the test and that
the reactions of a number of permanent reactors has subsided. Nevertheless, this does not entirely
account for the scarcity of reactors amongst aborters, for, of the above animals nine aborted during
1940 and of them, but two (22 per cent), were positive. It seems probable, therefore, that at the
end of an endemic period, or perhaps in some secondary epidemics, the presence of permanent reactors
is greatly diminished.
If whole herds, and not simply aborting animals, are considered, it appears that, when abortions
are frequent, the incidence of reactors reflects to some extent the abortion rate, but when abortions
are rare this relationship fails. In the early stages of sharp epidemics, when the abortion rate is in
the order of 12 per cent to 15 per cent of the herd, the total percentage reactors in the herd will be
from 45 to 60 and a number of animals are found to be infected that have not yet aborted. During
a typical minor endemics, on the other hand, this rate is lower and was found, over an average of
seven herds, to be 35 per cent. This last figure agrees exactly with that found by Doyle and Bechett
[1936] in English cattle.
The diagnostic titre of the aggiutination use has been much debated. In India, for example,
it has been suggested that many indigenous cattle possess a week natural non-specific agglutination
response to Brucella antigens, and that allowance should be made for this in diagnosis. But such
a concept seems hardly tenable, for, in herds in which abortions do not occur, seldom a single animals
can be found with even a doubtful reaction, whilst, in the comparatively disease-free districts of
northwestern India, almost all the village animals that have been tested were strictly negative (p.117).
On the other hand, in herds where abortions prevail, titres are usually quite high (1/320-1/2560)
and sometimes very high (1/20,000), no matter the particular breed of cattle that is concerned. It
is rarely difficult, therefore, to confirm an outbreak of Brucellosis by means of agglutination, but
the diagnosis of infection in a single animal is only really informative when the test is positive. Opera-
tors should, therefore, view a single suspect animal in conjunction with the herd, as a whole, taking
into consideration the magnitude and duration of an epidemic, the influence of intercurrent disease
and all relevant data such as clinical symptoms, susceptibility, and epidemiology. To assist in
interpreting the agglutination test, a list of commonly accepted diagnostic standards has been
published. [Stableforth 1936].