REGINALD
LOVELL
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than the animal body. Blackleg, lamb dysentery and braxy are diseases of
particular areas and the causal bacteria are found in the soil. There is little
hope of exercising adequate control of these diseases by the elimination of in-
fected animals as in tuberculosis, contagious bovine abortion and bacillary
white diarrhœa, and recourse is made to immunization.
The Principles of Immunization depend very largely on the pathogenesis of
a disease and broadly a pathogenic bacterium may attack an animal host by : —
(1) Local invasion of the tissues with the production of a toxin which
spreads throughout the body; the toxin is the actual agent
which produces ill effects.
(2) Widespread invasion in which the lesions produced are associated
with the actual invasion of the organism.
The tetanus bacillus and the diphtheria bacillus are examples of the former
type of bacterium as they produce toxins which are disseminated throughout
the animal body, whereas the lesions in anthrax and tuberculosis are produced
in response to actual invasion of the bacteria. Some bacteria may combine
both methods by massive invasion and toxin production. Certain kinds of
streptococci invade and produce toxins and Cl. septique invades the stomach
wall and forms a toxin in braxy. The method of attack of an organism must
therefore be considered before immunization can be attempted rationally.
Diphtheria in the human subject and tetanus in man and animals can be
controlled effectively by immunization against the respective toxins.
Pathogenic staphylococci produce toxins and these are an important part of
their armamentarium. It is now customary to treat chronic staphylococcal
infections with staphylococcal toxoid whereas at one time attempts were made
to treat them with autogenous vaccines. These vaccines gave dissimilar re-
sults and this should have been expected for it is only recently that the patho-
genesis of staphylococci is being understood. Attempts are now being made to
assess the value of staphylococcal toxoid as a prophylactic in mastitis of sheep
and certain conditions in dogs due to this organism. (For a review of this
subject and its application to animal diseases see Minett, 1937.) It is probably
a similar story regarding C. pyogenes infections in sheep, cattle and pigs.
Quite recently it has been recorded that this bacterium produces a toxin
[Lovell, 1937] and trials are now being made to test the efficacy of a toxoid in
preventing ' summer mastitis ' of cows and certain forms of calf pneumonia.
There are also important points which require consideration in the prepa-
ration of biological products for the immunization of animals against diseases
due to bacteria which invade. Whereas hitherto bacteria were regraded from
the immunological aspect as entities in themselves, they are now considered as
collections of antigents some of which are of more importance than others.
Certain strains of bacteria may be deficient in the important antigen or its pro-
duction may be suppressed by a particular method of cultivation. The con-
verse also holds true whereby some strains have an abundance of a particularly
important antigen. As far as capsulated bacteria are concerned, it is known