444 Immunization of Animals Against Bacterial Disease
sheep and a goat died from anthrax and four control cows were febrile and
developed large œdematous swellings [Vallery-Radot, 1919]. The methods
of immunization used up to that time were somewhat crude and in some ways
the success of Pasteur's experiment with anthrax is to be regretted. It made it
appear as if the preparation of efficient vaccines for the immunization of
animals against bacterial diseases was too easy. Whereas, if a survey of the
field of bacteriology is made, it must be admitted that, except in certain ins-
tances, immunization has been a disappointment. It is in those diseases in
which immunization is of value that there is available some fundamental
knowledge of the immunological process involved, that is, how the bacteria
produce their ill effects and how the animal host reacts. If this knowledge is
lacking, then in most cases attempts at immunization fail.
OBJECTS OF IMMUNIZATION
The object of immunization is to increase the resistance of animals to a
particular disease in order to reduce the morbidity or mortality from that
disease. It is obvious that, as there are many factors in the production of
disease in the individual and the spread of disease in a herd, then these factors
must be taken note of in its control. The normal habitat of a bacterial parasite
may be the soil, and spores may be formed which remain latent but viable for
years. On the other hand, the bacterium may be easily destroyed and exist
in the tissues of animals only. The portal of entry into the animal body may
also vary ; some bacteria enter by the respiratory tract, some by the intestinal
tract, some through the skin—broken or unbroken. Bacteria are carried from
one host to another by different ways, sometimes by direct contact, sometimes
by indirect association and often through the medium of a vector. After the
organism enters the tissues—which is necessary except on very few occasions—
it may produce its ill effects by toxin production, by massive proliferation and
invasion, or by both methods. It may vary in its virulence and finally the hosts
themselves vary in their resistance which ranges from susceptibility to a high
degree of immunity. It follows, therefore, that immunization may play a more
important part in some diseases than in others, but whatever the circumstances,
one must not assume that a disease can be finally suppressed by immunization.
That is an assumption which is not justified either by clinical observation or
animal experimentation. It is true that in those diseases in which immuniza-
tion is regarded as successful there is a lowering of mortality or morbidity, but
there may be no equivalent lowering of infection. The work of Topley and his
colleagues has shown that in mouse epidemics the lowering of mortality which
accompanies immunization is not associated with an equivalent lowering of
infection. Many immunized survivors of an epidemic are infected and infective
and as such are a potential source of danger to susceptible animals [Greenwood
et al., 1936]. Dudley and his colleagues [1934] have also shown that there may
be a higher proportion of carriers of diphtheria bacilli in a school after specific
immunization than before. This does not mean that artificial immunization
is not a potent factor in the control of bacterial diseases. It is in fact by far the
greatest factor in diseases in which the natural habitat of the organism is other