MUKTESAR, FOR THE YEAR ENDING THE 31ST MARCH, 1925
37
Although the use of antigen of low virulence proved to suffice for the treatment of
anti-serum producers, it was thought to be very desirable to study the changes undergone
by the hæmorrhagic septicæmia organism outside the body of the natural host, in culture
media and in other hosts. It was also thought that the study might throw some light,
of comparative significance, upon the nature of the degradation believed to have been
observed outside the natural host in the case of the rinderpest virus, as figured
briefly in last year's Report. It has been stated elsewhere that it had been a practice in
the laboratory to pass the hæmorrhagic septicæmia strain from time to time through
rabbits, on the assumption that this procedure assured the retention of virulence for the
natural host and further maintained the virus in a good state of preservation as regards
its antigenic properties. To obtain information concerning the actual changes in patho-
genic properties developed outside the natural, bovine host, a strain of organism which
had just been exalted by passage through cattle to a very high pitch of virulence for these
animals was subcultured on alternate days in plain broth at (i) PH 7.4 and (ii) PH 6.0.
In the former medium the conditions were most favourable for growth, and hence it
was believed that the pathogenicity of the organism would be maintained unimpaired for
a long time. On the other hand, conditions were barely sufficient for propagation in the
second medium, and hence it was thought that considerable changes would take place.
The subcultured strains, i.e., the one propagated at PH 7.4, and the one at.PH 6.0, were
then to be tested at the 10th, 20th, 40th, 80th, 160th generations, and at corresponding
intervals subsequently if considered necessary. The test at each stage comprised passage
through 10 rabbits with intermediate cultures, and the culture obtained was tested.on
cattle after the 3rd and the 10th rabbit passages. Originally, the virus was pathogenic
for bulls at a dose rate of 1/10 c.c. of broth culture. In the tests on bulls the animals
were given doses of 1/10 c.c., 1 c.c., 10 c.c., 100 c.c., and 1000 c.c. if required.
With the PH 6.0 strain, virulence for bovines was found to have dropped to a very
great extent from the 10th subculture onwards. Three passages through rabbits brought
the culture back to its original virulence until the. 80th subculture. Subsequently,
with the 160th subculture, repeated passage through rabbits restored virulence only parti-
ally.
With the PH 7.4. strain, virulence for bovines was maintained until the 40th genera-
tion, at what seemed to be about its original titre. At the 80th subculture, however, the
strain failed to kill bulls even in massive doses. Large doses at this stage were necessary
initially to produce morbid affection in rabbits and after 13 passages through rabbits,
1000 c.c. proved lethal for bulls, but 100 c.c. was harmless. At the 160th generation
massive doses failed to produce any harmful effect either on cattle or rabbits.
The sudden precipitation of pathogenicity in the virus maintained in the favourable
artificial culture medium, as compared with its more sustained character at a rather
low level in the unfavourable artificial medium,was contrary to what had been anticipated.
The information may not be without significance in other pasteurelloses, notably in
human plague, in which disease condition a rodent acts as the reservoir, for it would
appear that provided a pasteurella has not propagated beyond a certain stage in the
outside world and especially if propagation has been restricted by multiplication in
an unfavourable medium it can become exalted for what may be termed its natural host
by passage through the body of another host,—a rodent, in the above experiments.
However, the information obtained is intended to serve as a basis for the elucidation
of a number of practical problems awaiting solution in connection with hæmorrhagic
septicæmia itself.
(c) Anthrax. Researches were commenced towards the close of the year upon (i) the
efficacy of vaccines applied in Indian conditions; and (ii) means of producing
economically an anti-serum of high potency. Tests upon a spore vaccine manu-
factured by a commercial firm in Australia showed that it was, as claimed by
the vendors, very effective in protecting sheep against subsequent virulent
infection, but in horses the dose recommended produced rather violent, although
not fatal, reactions. This vaccine was then recommended for application to the
hounds of the Bombay Hunt in which considerable fatality from anthrax had
occurred during the hunting seasons in past years, apparently owing to the
hounds having been hunted over country strewn with the carcases of animals that
had succumbed naturally to the disease. Prior to recommending the adoption of vaccina-
tion, the vaccine was tested carefully on dogs at the laboratory and it was found that a
dose equal to that employed on sheep could be tolerated safely. There was apparently
no further mortality from anthrax among the hounds during the hunting season after
vaccination. Tests upon a considerable number of dogs at the laboratory failed to
demonstrate the acquisition of immunity following upon the vaccination, by the system
of testing employed. It was found that dogs received at the laboratory possessed a very
high degree of natural resistance, and it was difficult to produce fatal infection even by