SEPTEMBER, 1948]
JOHN
B.
POLDING
159
vaccination can be defended on the ground of safety, for, the longer the period between vaccination
and parturition the less the chance of bacterial dissemination at calving. But although the
Americans suggest that the optimum age for immunization is 4 to 8 months, many believe that they
have not proved their case [McEwen 1941]. It is well known, for example, that very young calves
are naturally strongly immune and that this immunity is entirely lost by puberty, and without
experimental proof it is difficult to believe that a weak immunity, conferred by a low-virulent strain,
at 4 to 8 months of age will survive into the first or second calving; it is nonetheless possible, that
by vaccinating between 4 to 8 months, i.e., when the natural immunity of calfhood is beginning to
fade, the animals are caught at the most suitable moment for re-immunization.
A very large scale trial of this system is at present being made in United States. On the whole
the results of first calvings seemed promising, but at the second calving they were less so. In this
connection it has been rightly pointed out that, as heifers are usually kept separate from the adult
herd during their first pregnancy, they are much less exposed to infection than second calvers who
spend a whole pregnancy in contact with infected adults.
In England, McEwen has worked in great detail along parallel lines, and, by artificial passage
has produced from an almost a virulent parent strain (No. 45), two sub-strains [No. 45 (6) and
No. 45(20)] each of rather greater virulence than its predecessor [McEwen 1940]. He has also
developed one or two systems of measuring virulence (p. 146) and has further shown that a strain's
immunizing value is proportional to its virulence. With reason, therefore, he suggests that a vaccine
strain should be of the greatest possible degree of virulence, short of its being virulent enough
to cause generalized infection in cattle; he adds that to achieve and maintain this degree, in
practice, is not at all easy ; McEwen favours the vaccination of heifers and his strain No. 45(20), being
rough, does not invoke an agglutination response in the recipient, which is an advantage, as
confusion over blood tests need not arise.
Very many trials have been made of vaccines prepared from either Cotton's or McEwen's strains,
and, whilst the results have not been uniformly favourable, there is an overall indication that some
measure of protection can be obtained from their use.
THE CHARACTER OF THE VACCINES USED IN INDIA
Cotton's strain No. 19, McEwen's Nos. 45, 45(6) and 45(20) together with another similar strain
Huddleson's No. 805 have been brought to this country and have been tested for virulence under
Indian conditions.
As Cotton's and Huddleson's strains were brought out at the commencement of the scheme, they
have been tested more thoroughly than McEwen's which were received much later. Owing to a
shortage of funds and accommodation, it was not possible to conduct controlled tests on these strains
in adult buffaloes, zebu and crossbreds as was manifestly desirable. Experiments have been made,
therefore, in crossbred calves, goats, guinea-pigs and white mice.
Examination of vaccine strains
Test I. Cotton's strain No. 19 and Huddleson's 805, having been in the writer's possession for
a very long period, were examined as to their general characteristics. They were found to be typical
Br. abortus, to be smooth and of standard antigenic sensitivity.
Test II. Strains 19 and 805 were examined for their virulence in guinea-pigs. The guinea-pigs
received 3500 × 10 6 organisms in 1.0 c.c. of saline in the manner shown in Table XXXIV where the
animals' subsequent serum responses are also recorded.
The guinea-pigs were killed on the 35th day and the liver, spleen and kidneys of each animal
were plate-cultured. Brucella organisms did not appear on any of these cultures.
Test III. The pathogenicity of Cotton's No. 19, Huddleson's 805 and a recently-isolated,
CO 2 sensitive, Indian-field-strain, out of a buffalo, was compared in goats. The virulent control