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infection to unimmunised cattle, but vaccinated animals are far less On balance, vaccination infective than those that actually abort. reduces the chance that infection may be transmitted. Vaccination is useful if a policy of total eradication by segregation is imprac- ticable. It can, moreover, be used in a herd until shortly before eradication is started, as the tendency to react to the blood- agglutination test, which is the means of separating infected from healthy animals, usually disappears in vaccinated cattle within a few months. On no account should animals be vaccinated with a live virus, unless the herd is already heavily infected.
(iii) Johne's disease.
41. There is no trustworthy evidence regarding the general incidence of Johne's disease. There is considerable variation in the degree to which different parts of the country suffer from it. In Scotland, for example, it is said to be rare. In some districts, the disease is more widespread than formerly, and is causing considerable anxiety.
42. The disease affects only the intestines and the associated lymphatic glands. It results in emaciation and, finally, in death. But it may be latent in an animal for a considerable time before symptoms appear clinically. Animals may contract the disease at an early age. The organisms responsible for the disease are excreted exclusively with the fæces, and it is assumed that infection takes place exclusively by ingestion. Drinking pools, unless fenced off, are a serious focus of infection, as the result of their pollution by cattle.
43. Diagnosis of the disease before it has reached the stage at which clinical symptoms appear is a matter of difficulty. None of the small laboratory animals are susceptible to Johne's disease; the presence of the responsible organism cannot be reliably distinguished by the complement fixation test; there is as yet no diagnostic agent on the injection of which an infected animal gives a certain and recognisable reaction. Considerable progress has, however, been made towards the production of such an agent, and tests have recently been carried out either with a tuberculin prepared from the avian strain of the tubercle bacillus, or with johnin, a substance prepared in the same manner as tuberculin from the specific organism responsible for the disease. Unfortunately this test has not proved thoroughly reliable in practice. Not only are reactions from time to time obtained from animals which prove on examination to be uninfected (a fault which is not incompatible with the successful use of the test for the elimination of the disease), but also some infected animals, which may or may not have reached the stage at which clinical symptoms are apparent, fail to give positive
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reactions. Attempts have been made in the United States to eliminate this disease from a number of herds by means of tests, but the experience so far gained shows the need for further research in the field. No cure for Johne's disease is known.
(iv) Mastitis.
44. Unlike those of the diseases which we have hitherto discussed, mastitis is not a condition brought about by infection with a particular organism, but is the infection of a particular organ, namely, the udder, by several kinds of bacteria. Among these is to be found the tubercle bacillus. We do not, however, include tubercular mastitis in the discussion upon which we are at the moment engaged.
45. A large part of the milking herd of this country is probably affected with mastitis in some form or other. Research on mastitis has been carried out in recent years at the Research Institute in Animal Pathology of the Royal Veterinary College, London. Of the 1,613 cows constituting the 80 herds examined in the course of this research, 660 cows (or 40.9 per cent.) gave samples of milk showing definite infection of the udder, usually of a latent character. The majority of the herds were examined because the owner had complained of udder disease. But allowing for this, and taking account of the experience of other countries, it appears likely that 30 per cent. of the milking cattle of this country are infected with mastitis.
46. Apart from tuberculous mastitis, the disease may be caused by streptococci (a form which may be either chronic or acute), by staphylococci, or by a diphtheroid organism known as Bact. pyogenes.
47. The chronic form of streptococcal mastitis accounts for over 90 per cent. of the cases among milking cows. It results in the slow deterioration of the udder of the animal affected, and of the quantity and quality of the milk. The more acute form of streptococcal mastitis is less common, but as it attacks younger cows and results in an immediate reduction of the milk yield, it is the cause of severe loss. Chronic infection is generally held to be spread by means of the hands of milkers, or by the cups of milking machines. The method by which the more acute form is spread is less certain. Infection with staphylococci is associated with a gangrenous form of mastitis which usually ends fatally. Infection with Bact. pyogenes is responsible for so-called summer mastitis, a severe suppurative form which is prone to attack dry cows and virgin heifers in summer months. Infection is said to be spread by flies.
48. Outbreaks of acute infection are generally dealt with by segregation of the uninfected members of the herd and by the use of vaccines. The latter treatment is of uncertain efficacy. Many cases of mastitis cannot be diagnosed by clinical means, but are
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