(1) Blackquarter has been known to exist in South Africa ever since the first veterinarians came to this country. At the present time it is widespread, occurring as an enzootic in many parts of the country, and being particularly prevalent in low-lying areas.
(2) Natural infection occurs in young cattle and, to a smaller extent, in sheep. In the latter animals, the disease often makes its appearance on a large scale, infection taking place through wounds in the skin.
(3) It is not a scheduled disease under the Stock Diseases Act, the State departments concerned acting only in an advisory capacity. Control is left to the individual stockowners, who are given full advice concerning the best methods of prevention that are known to science. Losses from the disease can be avoided by timely inoculation of susceptible stock with a reliable vaccine.
(4) Bacterial vaccines are based on the principle that the organisms themselves are largely responsible for the immunizing value of a vaccine. This has now been proved to be wrong, the "washed" organisms having neither pathogenic properties nor immunizing value. To render such a vaccine safe for use, the number of organisms contained in it must always be kept at a minimum, find in doing this the immunizing value (aggressin-content) is often greatly reduced. Attempts on a big scale to find a method of preparation that would give a vaccine which could be considered both safe and efficient have ended in failure.
(5) Efficient anti-sera can be obtained easily by hyperimmunizing sheep with either virulent cultures or germ-free filtrates. Such sera may be used in cases where it is desired to obtain an immediate protection or in conjunction with virulent cultures or bacterial vaccines. Under the conditions prevailing in this country, there has so far been no need to resort to the use of anti-sera.
(6) The writers have never detected any sign of toxin-formation in their black-quarter cultures.
(7) The immunizing principle in black-quarter is a bacterial product which is formed during the active multiplication of the causal organism either in the animal tissues or in suitable artificial media. Owing to the aggressive character which it confers on the organism, this substance has been termed aggressin; when it is produced in the animal body it is called "natural aggressin," and when it is formed in artificial cultures it is usually referred to as "artificial aggressin." The writers have not been able to detect any real differences between these two forms of aggressin and are forced to regard them as identical. Apparently the one produced in the animal body is formed under ideal conditions, and is therefore present with more certainty and in greater abundance; the other is produced artificially, and the degree or intensity of its production must, and undoubtedly does, depend on the nearness to which one is able to approach the artificial and natural conditions of bacterial growth. The efficiency of an artificial aggressin as an immunizing substance depends, therefore, on the perfection to which the artificial cultivation of the organisms can be brought. The following properties possessed by aggressins are of considerable importance:
(a) They are absolutely non-toxic and germ-free.
(b) They have excellent keeping properties; natural aggressin preserved in 60 per cent. glycerine has been kept for two and a quarter years, and artificial aggressin for over a year, without their immunizing properties being lost; natural aggressin resists heating at 80° C. for half an hour, artificial aggressin 95° C. for half an hour, and direct sunlight for 100 hours.
(c) They confer a strong, lasting immunity; vaccinated sheep still showed immunity twelve months after inoculation. Artificial aggressin has a great advantage over natural aggressin, in that it can be prepared much more cheaply and simply.
(8) A well-marked variation in the virulence and immunizing value of black-quarter strains from different localities has been established. The variation in virulence is not of great practical importance, since with an efficient vaccine it is possible to immunize animals against all other strains.
(9) A good immunizing vaccine (filtrate) against malignant oedema can be prepared by following the technique employed by us for the preparation of black-quarter filtrate.
(10) Vaccination against black-quarter affords no protection against infection with malignant oedema and vice versa.
(11) A mixed vaccine for use against both diseases can be obtained by growing the respective organisms separately and then mixing the two filtrates in equal proportions.
(12) During the last few years, black-quarter filtrates (artificial aggressins) have been used extensively in South Africa, the annual issues being well over a quarter million doses; for 1924 the figure was 268,040 doses. The results obtained have been extremely satisfactory, so much so that complaints in regard to the efficacy of the vaccine are practically unknown. Experience has shown that a black-quarter vaccine may be considered satisfactory so long as animal tests show it to be efficient against at least one minimum lethal dose of virus. This was the accepted standard with the first artificial aggressins that were issued, and we have on record cases where with this vaccine farmers were able to stop all mortality from black-quarter on extremely badly infected farms; in fact, in some of these cases repeated vaccination with other preparations had been tried unsuccessfully. We now use vaccines of a higher standard, protection being required against at least two or three minimum lethal doses of virus. Several thousand doses of natural aggressin have also been used with great success; this vaccine has not been used more extensively simply because the more easily prepared and cheaper artificial aggressin has given every satisfaction.