ABSTRACTS
239
membrane after 380 days, although a gradual loss in its infectivity was noticed. In
other fluids the virus completely lost its potency between 93 and 297 days.
Vaccines prepared from egg propagated virus were tested for efficiency and im-
munity. Pigeons vaccinated with a single dose of one per cent saline broth suspen-
sion of 3-5 days egg membrane virus by the intradermal, intraperitoneal or intravenous
routes showed a fairly durable immunity. The subcutaneous route proved less effec-
tive. As an immunising agent for fowls the pigeon virus applied over feather follicles
conferred immunity lasting for about six months. In those birds which were given a
second dose of either pigeon or fowl pox virus intravenously or intraperitoneally a last-
ing immunity could be imparted. [J. A. I.]
I—An estimate of a suitable infective dose of Br. abortus for immunisation
tests on cattle. MCEWEN, A. D., PRIESTLEY, F. W. AND PATERSON,
J. D. (1939). J. Comp. Path. Ther. 52, 116
II—The demonstration of immunity in cattle vaccinated with a non-virulent
strain of Br. abortus. MCEWEN, A. D. (1939). J. Comp. Path. Ther.
52,129
C ONSIDERABLE controversy has centred round the question of the utility of vacci
nation against Br. abortus using strains of reduced virulence.
In these two articles the authors attempt to show that most of the confusion can
be removed by adopting a standardized technique in carrying out experiments designed
to test the potency of Br. abortus vaccines. The measures advocated are :
(i) The use of a dried preparation of Br. abortus to ensure constancy in virulence.
Br. abortus could be preserved in vacuo over P 2 O 5 for long periods of time without loss
of virulence or CO 2 sensitivity. For use, the dried material is to be subcultured on liver
infusion agar. The authors used the second subculture after 48 hours of growth, which
they emulsified with normal saline to the required densities.
(ii) The use, after titration, of a carefully standardized test dose which is large
enough to infect a very large percentage (nearly 100%) of unvaccinated cattle, but not
too massive to overwhelm the state of immunity of vaccinated cattle.
(iii) The use of sufficient number of animals to meet the requirements of statistical
accuracy, particularly if a less drastic test using a smaller test dose is required to assess
the resistance of animals possessing a relatively low grade immunity. According to
the authors, this can be achieved by spreading the work over a period of time and so
including a sufficient number of animals as to eliminate incorrect interpretation owing
to chance error. Since Br. abortus could be preserved over indefinite periods of time
without the occurrence of changes of virulence or of antigenic structure, the same infec-
tive dose or doses could be used at different times, and the results obtained would be
capable of being correlated.
In the first experiment which deals primarily with titration, in normal cows, for
an optimal test dose, certain collateral observations were made, such as how, with changes
in dosage, fluctuations appeared in infections and its sequelae. These included observa-
tions of the proportion of animals which became infected and the proportion which aborted
or calved prematurely ; on the severity of placental lesions, frequency of positive cul-
tures, and the rapidity of appearance of agglutinins in the blood serum. There was an
indication of a threshold dosage below which no effect could be felt. With progressive
increase in dosage, there was a corresponding increase in all the results indicated. An
infective dosage was reached with which nearly 100 per cent of cows became infected
of which a very high percentage aborted or calved prematurely. The authors consider
dosage at this level very suitable for employment for demonstrating the immunity of
vaccinated cattle. Further increase over this dosage—even a 100-fold increase—made
no difference in the percentage of infections and abortions in untreated cows, although,
as has been shown in the second article the larger infective dose caused the more severe
infection of the vaccinated animals.
In the experiments recorded in the second article a batch of cows vaccinated with
a non-virulent strain of Br. abortus was subjected to test for immunity at two dosage
levels—half, comprising five cows, at the dosage determined to be suitable by the first
experiment and the other half of five at 100-times this dosage. In the first instance