The purpose of this article is to summarise briefly the knowledge attained by researches on Contagious Bovine Mastitis. It is not intended to present all the evidence which has been compiled from work conducted in many countries, but to present such facts as have been proved and to indicate the trend of the investigation being made at the Veterinary Research Station, Glenfield.
The word mastitis designates simply an inflammation of the mammary gland. Even if the definition is confined to inflammation of bacterial origin it is obvious that any pathogenic organism which gains entrance to the udder is capable of inducing a mastitis. Therefore, it is not surprising to find that Streptococci, Staphylococci, Corynebacterium pyogenes, Mycobacterium tuberculosis, Actinomyces, Brucella abortus and some members of the Salmonella group have all been recorded as being associated with a pathological condition of the udder. However, the result of investigations conducted in Great Britain, the Continent, U.S.A., and in this country, shows that, in the vast majority of cases, Streptococci are associated with the disease. The other organisms may give rise to sporadic cases which, although at times sufficiently serious to cause the death of the animal, cannot be held to be of a contagious character. In this article, the subject is limited to the disease caused by Streptococci and more particularly to Contagious Bovine Mastitis with which Str. agalactiae is associated.
During recent years a considerable advance has been made in the classification of Streptococci. The workers in Great Britain differentiated the Streptococci associated with Bovine Mastitis into three groups by the use of serological and bio-chemical methods. Of these groups. Group 2 is Str. agalactiae, the organism associated with Contagious Bovine Mastitis.
Subsequently, the haemolytic Streptococci in general were divided by serological methods into Groups designated A, B, C to K but for the purposes of this article only the Groups A, B, C need be considered. These three groups contain the chief Streptococci which are pathogenic to man and animals. Further, comparative studies established that the original Group I of the British workers was identical with the Group B of the haemolytic Streptococci, despite the fact that many members of Group B are non-haemolytic. In summation, it may be stated that Group A. contains the Streptococci mainly responsible for human disease (Scarlet fever, puerperal fever, erysipelas). Group B, i.e., Str. agalactiae is the organism associated with Chronic Bovine Mastitis and Group C. contains a variety of species including Str. equi, the cause of Strangles, Beyond these, there are two furthes groups, Groups II. and III, which are, at times, associated with bovine mastitis.
It is generally and quite justifiably held that the condition with which Str. agalactiae is associated is the only contagious disease entity whereas mastitis associated with any other organism must be considered as a sporadic case and not likely to assume epizootiological proportions. The failure to take into consideration this bacteriological differentiation has detracted from the results of many workers who have attempted to assess the economic loss due to this disease. However, studies conducted over several years in Great Britain and adequately controlled bacteriologically and statistically, have shown that the direct loss of production due to mastitis associated with Str. agalactiae attains a percentage between 10 and 20, varying with the breed of cow concerned. Beyond this serious direct loss it has been established that milk taken from quarters affected with mastitis even when mixed with normal milk, has a deleterious effect on the manufacture of milk products.
Although Str. agalactiae is associated with this condition, its role in the cause of the disease has not yet been proved satisfactorily. It is surprisingly difficult to set up the disease with this organism and it cannot be done in any other manner than inoculating cultures of the organism or milk taken from affected udders by way of the teat duct. It is obvious, however, that such a violent method cannot be considered as proof that the organism causes the disease under natural conditions, because any pathogenic organism would set up a mastitis in similar circumstances. Other methods such as placing infected material in contact with teat orifice either for a long period or on successive occasions have yielded inconsistent results. There has been evidence recorded, however, that impairment of the teat sphincter, either naturally or artificially, permits the reproduction of the disease with comparative ease. Further, it must be pointed out that Str. agalactiae is only slightly pathogenic to laboratory animals.
Therefore, the problem is one in which an organism is associated with a disease entity but cannot easily reproduce that disease and possesses only slightly pathogenic powers in the laboratory. The aims of the work at Glenfield have been firstly to adduce additional evidence that the organism is actually causing the disease, secondly to establish whether any major alteration in the structure and power of the organism occurs on artificial culture, and thirdly to discover in what manner the organism exerts its pathogenic effects.
The fact that Str. agalactiae, as it occurs in this country, can be differentiated into six serological types offered a means whereby epizootiological studies could be made. Accordingly, the examination of seven individual herds was undertaken and every strain of Str. agalactiae isolated was typed serologically. This study has shown that the infecting strain, in the great majority of cases, in any one individual herd is of the same serelogical type. This can be interpreted as additional evidence that the organism is actually causing the disease and is contagious from cow to cow. These results have been supported by a similar investigation conducted recently on 52 herds in Great Britain.
The next stage of the investigation is more difficult to accomplish, but the results to date indicate that the organism does not undergo any gross alteration when cultured artificially. Investigations from many aspects have been conducted and possible capsulation of the organism, its "diffusion factor" and its serological behaviour have been closely studied. The strongest support to the contention that the organism does not undergo any major change on isolation is that antibodies can he demonstrated in the blood serum and milk serum of affected cows, using strains of the organism in its customary laboratory state.
Therefore, there is considerable evidence that Str. agalactiae the active cause of the disease and that, when cultured in the laboratory, it does not differ greatly from the state in which it occurs in the animal body. How then, does the organism excite the disease? This problem is not confined to Contagions Bovine Mastitis, but is one which confronts investigators of every disease caused by an organism which produces no detectable exotoxin. The possible conditions under which the organism might develop its pathogenic powers are, of course, unknown, and there is an enormous field to be explored. Our immediate endeavours are to fractionate the organism in an attempt to liberate a toxic substance.
Until this problem is solved no simple method of control of the disease can be expected. Vaccination, as at present employed, does not prevent infection with the organism but does to some degree ameliorate the severity of the clinical symptoms. Treatment of the udder by external applications can do little else than relieve the pain and discomfort. Infusions of the udder with dye disinfectants is widely practised on the Continent, but is not practical under Australian conditions of farming. Treatment orally with the new drug Sulphonamide, which has been used recently very successfully in the treatment of human streptococcal infections, is uneconomical to apply to dairy cows. The only method of controlling the disease which can be utilised here at present is to detect all members of the herd which are either clinically or latently affected by means of bacteriological examination. Affected animals should then be milked last and should be kept separate, as far as possible, from the normal animals. Much can be done in preventing the spread of the disease by disinfection of the milkers' hands between each cow and by applying the ordinary rules of hygiene to the dairy shed.