D. A. MUNRO 195
danger and the writer therefore used only pigeon-pox virus in the experiments to be presently
described.
According to Picard [1931], fowl-pox virus, by passaging through pigeons, may be attenuated
and brought to a constant degree of virulence, so as to be suitable for inoculation with safety into
fowls. Evidence produced more recently by Donatien and Lestoquard [1941], however, shows that
the virus never reaches a fixed stage, but that it progressively loses its virulence, until the latter
disappears altogether. An inquiry into the question of constancy of virulence of the virus was con-
sidered by the writer hardly profitable to undertake, for, apart from the results reported by the last
two named workers, a biological standardization of the virus to ensure constancy did not, at least,
in theory, seem possible. In fact, Goodpasture and Anderson [1940] have shown that immunity
against fowl-pox is a function of the humeral antibodies and not, even partially of the epithelial
cells, so that, it may be inferred that the virulence varies with the quantity of antibodies elaborated
by individual birds. Moreover, it would seem open to question whether, in practice a state of
constancy in virulence is really desireable if, as has been postulated by Basset [1935], the immunity
produced is directly proportional to the pathogenicity of the vaccine, for this may imply that, for
conferring a strong immunity, the titre of the virus has to be adjusted in relation to the general
condition of the bird so that the reaction may not progress to a fatal one.
MATERIALS AND METHOD
The skin of the abdomen of a sheep was shaved and thoroughly cleansed with warm water and
soap and dried. A one per cent suspension of sheep-pox virus in normal saline solution was now
rubbed into the superficial epithelial layer of the skin which had been lightly scarified. After four
or five days, material from the pustules was collected aseptically by scraping in a receptacle and was
then desiccated in vacuo over phosphorus pentoxide or calcium chloride. The process of drying was
carried out in the cold as high temperatures have a deleterious effect on the keeping quality of the
vaccine. The material was now weighed and ground to a powder in a sterile mortar. It was then
stored both in the form of wet suspension and in the form of dried powder at two temperatures, viz.,
42ºF. and 98.6ºF. The dry vaccine was prepared as follows before use. The material was rubbed
to a smooth paste in a sterile mortar by the addition of a small quantity of 50 per cent glycerine
saline. Sufficiently more of this diluent was now added to make the full quantity, which was then
filtered through sterile muslin to remove extraneous matter and used the same day; 0.3 gm. of the
dried vaccine suspended in the diluent was found sufficient for one hundred animals. A total of
110 animals were used in the present experiments and of these, 56 were sheep and 54 goats. The
vaccinated animals were duly tested for immunity after an interval of 14 days. Controls were kept
in each case.
In the case of pigeons, the feather follicles of both legs were plucked, and immediately there-
after a one per cent suspension of pigeon-pox virus in normal saline, was rubbed on to the denuded
follicles. The crusts were collected from the 15th to the 21st day and the subsequent procedure was
similar to that adopted in the preparation of sheep and goat vaccines as described above. The
vaccinated birds were tested for immunity after 21 days by inoculating them with the specific virus.
Controls were kept in each case.
EFFECT OF VACCINATION
Vaccination when carried out with a good sheep or goat vaccine usually resulted in a crop of
vesicles at the site of inoculation resulting in a benign form of the disease. In a few sensitive patients
the injection was followed by a period of dullness, loss of appetite, and fever, but this seldom lasted
for more than a few days and the animal returned to normal after a short period of complete rest.
The usual experience was, however, that no disturbance to health occurred in the animals that receive
the protective inoculation. In some cases, when testing out brews of freshly prepared vaccine, the
results were not as expected, the animal scarcely showing any reaction whatsoever. These animals
when tested later with a potent virus failed to show any specific reaction, proving thereby that they
were immune. The results are summarised in Table I.
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