The Eradication of Bovine Tuberculosis and its Importance to Man and Beast
THE THIRTY-SECOND Stephen Paget Memorial Lecture was delivered by Sir John Ritchie, C.B., Chief Veterinary Officer, Ministry of Agriculture, Fisheries and Food, on Tuesday. 26th November, 1963, in the Physiology Lecture Theatre, University of London, the President of the Society, the Rt. Hon. the Earl of Halsblry, in the chair. The President, introducing the Lecturer, said that Sir John Ritchie was a distinguished civil servant. He had been Chief Veterinary Officer, Ministry of Agriculture. Fisheries and Food, since 1952, and his qualifications as recorded in Who's Who were too many to read out. He was a world authority on the subject of the eradication of Bovine Tuberculosis and its importance to man and beast, the subject of his Lecture.
The Eradication of Bovine Tuberculosis and its Importance to Man and Beast
By SIR JOHN RITCHIE, C.B.
It is a very great honour to be invited to present the Paget Memorial Lecture and I am wry conscious of it. The subject upon which I was as asked to speak is, I believe, one which is of sufficient interest and importance to merit inclusion in the long list of Paget Lectures and I hope I may be able to do it justice.
Forty years ago I recollect hearing in Edinburgh one of my teachers opening a discourse on tuberculosis "If carcinoma stands on the right hand of death tuberculosis stands on the left . . ." What I have to say is a record of one of the angles from which tuberculosis has been nudged from that position.
The disease position before eradication
Tuberculosis in cattle was a very serious problem inGreat Britainfor many years. It caused heavy economic loss to farmers, not only directly because of premature death but because animals had to be slaughtered as clinical cases of tuberculosis or had to be discarded from the herd. An affected animal, even although not obviously clinically affected, was less efficient with consequent lowered milk production, reduced fertility and general unthriftiness. There was the danger of infection to man and indeed it was this public health aspect of the problem more than anything else that eventually created the demand to eradicate tuberculosis from cattle.
There is an important difference between tuberculosis in man and in cattle. In man the majority of primary lesions heal, producing the classical small non-progressive globular lesion of the primary complex. Cattle are most commonly affected by the aerogenous route and subsequently develop a slightly progressive broncho-pneumonia. Reinfection, whether endogenous or exogenous, results in further bronchial spread. Encapsulation is not a common feature of the bovine lesion and any viable organisms at the periphery of an apparently quiescent lesion may be activated by the many stresses to which cows are liable. In cattle the disease tends to be progressive.
Infection was widespread among dairy herds and it was estimated at one time that as many as 40 per cent of the cows in the country would react to tuberculin, that is some 20-25 per cent of all cattle. This incidence has been progressively reduced in the course of the eradication campaign until at the most recent assessment it was recorded that during the first nine months of 1963 only 7 animals in every 10,000 tested had to be removed as reactors.
The dairy herds around large cities were the most heavily infected. Many of them were flying herds which did not breed their own replacement cattle. Cows were maintained so long as the amount of milk they produced was profitable, and adult cows were purchased to replace them. With the high incidence of infection in the country it was inevitable that affected animals were bought in various preclinical stages of the disease. The cows were kept in close contact with each other, often in ill-lit and badly ventilated accommodation and these were the ideal conditions for spread of infection; droplet infection is common and thus the disease spreads very readily among animals housed under the same roof especially in bad conditions. Herds in which heifers are bred and reared as replacements were not so heavily infected. It is unusual for a calf to be born affected with tuberculosis, although a proportion are born in this way if the dam has a tuberculous uterus. In those self-contained herds the young stock are usually reared apart from the adult population of the herd and. thus, the heifer does not come into contact with the adult portion of the herd until she is introduced to the milking byre having produced her first calf. Consequently, the incidence of infection among young stock was often extremely low and in many cases it was not uncommon to find as many as 75 per cent of cows reacting and a complete absence or a very minor rate of reactors among the young stock. Where the milk being fed to the calves was infected, a whole batch of young stock might react.
Self-contained herds and herds of beef-type maintained at free range or, at any rate, managed less intensively, were remarkably healthy even if no specific precautions had been taken in their management.
In districts where the herds are very small it was also usual to find a much lower incidence of infection.
In 1938-39, a tuberculin survey of self-contained dairy herds was carried out and it substantially confirmed these findings. We found 13 per cent reactors among 364,286 cattle tested in 12,300 herds (England14,Wales4,Scotland23 per cent). The survey showed that the incidence varied between localities and was high in the milk sheds of the large cities. The large herds were more heavily infected. In 6,971 herds with no reactors there was an average of 18.23 cattle; in 2,897 herds with less than 10 per cent reactors the average was 29.6, and in 2,432 herds with over 10 per cent reactors there was an average of 50.4 cattle. Since only self-contained herds with little or no history of clinical tuberculosis were selected for survey these results did not accurately reflect the incidence of disease but the survey confirmed that there was a heavy task ahead in eradicating the disease.
The tuberculosis order
Regulations were enacted in 1913 and brought into practice in 1925 to authorize the slaughter of cattle affected with clinical tuberculosis. This was essentially a public health measure, and was primarily to assist in the eradication.
Under this regulation a large number of animals were slaughtered annually. The highest figure was in 1936 when as many as 23,716 animals were slaughtered. Many of the animals were reported to the authorities by the owner or by practising veterinary surgeons; but great efforts were made by local authorities to discover affected animals. They carried out biological tests on samples of the milk of the herd, using guinea pigs. When a positive result was recorded, further enquiry was made on the farm and all the cows examined in order to identify the affected cow. Often it was impossible to identify the tuberculous udder at this stage in spite of the fact that infection must have been present during the period of the biological test—more than four weeks. Further samples from groups of cows had to be taken for biological tests, and eight to ten weeks might elapse before the cow could be identified and slaughtered. Besides this method, which was of some value in removing the animals most dangerous to the public health, routine examinations of dairy cattle were made and a number of clinically affected animals was disclosed. The frequency of inspection varied enormously; in some cities, for example in Edinburgh, the animals were examined every month, whereas in some county areas less than one inspection per annum was made and so it can be seen that material contact was made by these means.
There are serious difficulties in diagnosis of tuberculosis in cattle. Early tuberculous mastitis may pass unnoticed. When there is a gross pulmonary lesion it may be readily identified but small areas of broncho-pneumonia may not be detected by the means open to a veterinary surgeon who is without the advantage of X-ray examinations. Discharges and secretions, of course, may be examined and many workers became extremely skilled in identifying tuberculosis in milk by careful search for the organism and particularly by limiting the area of their search as a result of their study of cell groups in which the organism may be the more readily found. It is clearly evident that it was impossible to control bovine tuberculosis effectively by methods based on the clinical examination of cattle for early disease and even fairly extensive disease in organs other than the lung or udder could not be recognized.
If, therefore, any progress was to be made in the control and eradication of the disease, it was essential that some other means had to be found and a more precise method of identifying the infected animal must be used. The tuberculin test was adapted for this purpose.
Research work which had indicated that intravenous inoculation of B.C.G. would protect calves was put to the test of a field trial starting in 1942. The purpose for which B.C.G. might have been used was to protect young cattle in a heavily infected herd and thus possibly speed up eradication in the herd. The trial did not support the use of B.C.G. for the purpose. There were undesirable sequelae to vaccination in many animals, the tuberculin test which had eventually to be applied was difficult to interpret in vaccinated cattle, and owners objected to the labour entailed in vaccination and found the whole process tedious. There was also the difficulty of providing uninfected environment before vaccination and during the negative phase.
Tuberculosis in other animals
Other farm animals may be affected with tuberculosis. Generally they derive infection from cattle so that the incidence of the disease among them is closely related to the health of the cattle on the same farm.
Goats may be as closely housed as cattle are, but they are more resistant and have never presented a serious problem although no specific- programme has been developed for the control and eradication of the disease from goats. There have been one or two instances where cattle have become infected by close contact with an affected goat, and so it is common practice to tuberculin-test goats which are housed with cattle.
Sheep are almost invariably maintained in the open in this country and the only cases of tuberculosis that were encountered were associated with the feeding of infected cows' milk to orphan lambs.
Pigs at one time showed a heavy incidence of tuberculosis from bovine sources and this usually resulted from feeding pigs with milk, or dairy by-products which had not been processed. In the abattoir, several pigs were usually found to be affected because a whole litter of young pigs had been fed from the same source. Pigs are also susceptible to the human type of organism but very few cases occur. Avian infection is, however, not uncommon among pigs but it is non-progressive and usually confined to the lymphatic glands of the throat, which is of some importance in meat inspection. There are very few records of cows acquiring infection from pigs, but pigs may be tuberculin tested to avoid this risk.
Horses are apparently resistant to tuberculosis and the disease is rare among them. However, there were isolated cases accountable to the feeding of cows" milk in the artificial rearing of foals.
A number of cases among dogs and cats was found at one time. They are susceptible to the bovine and human types of the organism. In recent years bovine tuberculosis is rarely found amongst them although it was at one time by far the most common type affecting cats. Dogs usually affected with the human type have often been found to belong to a household where the owner or one of his family or occasionally a neighbour is infected. Tuberculosis may be a problem among animals in zoos. The tuberculin test cannot readily be applied to many of them, and those brought from abroad may already be affected. Infected meat and milk were at one time common sources of infection but this is no longer a problem; and there is less risk now from human sources.
The attested herds scheme
Since 1935, Tuberculosis Attested Herds Schemes have been in force: from 1935 to 1938, applicable only to dairy herds and intended to improve the milk supply. Subsequently a Scheme was applied to all herds and directed towards eradication of the disease from cattle, as a whole.
During these years incentives in the form of bonuses, etc. have been offered to farmers. From 1950 when the Scheme was revised and the Area Eradication Plan was introduced the bonuses have been on the scale of 2d. per gallon of milk sold from the farm for four years following eradication from the herd andId.per gallon for a further two years, or, at the owner's choice, £2 per head of cattle in the herd for four years and £1 per head for two years. The cost of the Scheme up to 1960 was £120-6 millions of which £96-2 millions was for bonuses to farmers, £4-6 for compensation for reactors, and £9-8 millions for tuberculin and tuberculin tests. The Scheme was entirely voluntary up to 1950 when a modicum of compulsion was introduced in the area plan.
Milk, special designations
Before 1935, herds had been tuberculin tested in order that they might be designated to produce certified or tuberculin tested milk. Testing was usually confined to the cows and in the majority of herds no effort was made to eradicate disease but merely to remove reactors at each test. Valuable experience was gained in the use of tuberculin for diagnosis.
Routine of attestation
Before a herd became attested evidence was required that no reactors had been found at two successive tuberculin tests made at the owner's expense. Thereafter the herd was tested officially and. if no reactors were found at the first or at repeated short interval tests, it was declared to be Attested. Rules, designed to keep the herd free of infection, had to be observed.
The routine of tuberculin tests has been that an attested herd was normally tested at an interval of 12 months (in practice 10-15 months) but when reactors were discovered short interval tests were made, at a minimum interval of 60 days, after reactors had been removed and were repeated so long as infection remained. The next test was at an interval of 6 months and thereafter the routine of yearly tests was resumed.
Eradication from a herd
After the first tuberculin test the method of elimination to be followed depended upon the result of the test and had to be considered in the light of the management of the herd and the accommodation available on the farm. Success depends entirely on removal of all reactors at as early a stage as possible, followed by disinfection of the premises. Financial consideration must of course influence the time of removal and method of dealing with reactors.
If there are few reactors it is usually a simple matter to have them removed, to disinfect the sheds they occupied and to repeat tuberculin tests at short intervals of two to three months until no infection remains.
If reactors are too numerous for this straightforward treatment, attempts can be made to segregate the reactors after removing any animals showing clinical signs of disease. Calves born to reactor cows may be moved at the earliest possible moment from the infected to the healthy group, and short interval tests are continued in the healthy section of the herd until all reactors have been removed and a clean herd test has been obtained.
If a group of animals within the herd contained a high proportion of reactors it was good practice to remove the whole of the group.
Successful eradication normally followed the removal of the adult cattle and any reacting young stock. Cowsheds were washed down with washing soda and scraped or scrubbed clean before disinfection. Troughs and movable fittings were soaked in disinfectant or sterilised by steam after cleaning. Manure was stacked and used for arable land. It is of interest that it was found sufficient to leave grazings free of cattle for a month after they had been used by reactors. This system is only possible at a stage when sufficient replacement cattle are available, but when this stage was reached it did provide an outlet for tubercle-free animals which were surplus to home requirements and which might otherwise have had to go to infected herds.
There was slow progress in eradication up to the end of 1938 when only 4,644 attested herds had been established; but in 1939 more than 9,000 herds were added. However, during the war the scheme was suspended except for herds already licensed to produce T.T. milk and there was very little progress untila premium of 4d. per gallon was offered by the Ministry of Food to encourage the production of T.T. milk which resulted in 55.045 herds becoming attested by the end of 1950. At this time the area eradication plan was launched with bonuses for attested herds embodying the T.T. premiums and whereas it took 15 years to produce 55.000 herds almost 100.000 were added in the next five years.
The suspension of the scheme during the war was extremely disappointing at the time but in the event it allowed improvements to be made in the tuberculin and an opportunity for development of the comparative test at a time when otherwise it was possible to do no more than hold the ground already gained.
Area eradication plan
The area plan was devised in order to direct the progress of eradication in an orderly was and further to protect those herds which had already become attested from reinfection by removing sources of infection in their neighbourhood. In free areas the interval between routine tests may be increased and there is, of course, no need to control cattle movements in an area in which all the cattle are of attested standard.
Areas were selected in the first place because good progress had been made in them, because they were essentially self-contained, and because market facilities were available. At the early stages particularly, cattle-breeding districts were selected from which supplies of healthy cattle could flow to replace reactors in other areas.
There were three stages in the area plan. First, free-testing followed by eradication and attestation. Free testing was offered for a period of two years in order to exploit voluntary eradication as far as possible. Reactors were disposed of at the owner's expense but when herds had two successive tests without reactors, bonuses were paid. At the end of the two-year period the area became an eradication area, and at this stage movement of stock into the area was controlled and all herds not already under test were subjected to compulsory tests. Those herds tested compulsorily did not qualify for bonuses but the reactors were valued as untested animals, slaughtered, and compensation paid up to a maximum of £100 per animal. After the second compulsory tests were completed and the reactors had been slaughtered the area was declared to be attested.
From 1st October. 1950, when the area plan was introduced, attested areas were established each year until in October, 1960, the whole ofGreat Britainhad been covered.
It is still necessary to test all the cattle at regular intervals. It is intended to increase the interval between tests and. so far, two years is the maximum. This will be increased as the risks of reinfection are progressively reduced. .It may also of course be possible to confine tests in some instances to particular classes of cattle—the cows in dairy herds and the breeding stock in rearing and beef producing herds. Reports of affected animals from abattoirs and knackeries are of great value in identifying reinfection in a herd. For this reason, among others, all cattle are marked by ear tattoo or ear tag so that they may be identified and an infected animal traced to the herd from which it originated.
The first tuberculin test used was the subcutaneous test using old tuberculin. Since it depended on temperature reactions it was not reliable especially in young cattle in which the temperature is usually erratic. Many observations of temperature had to be made, so that it was very time consuming. After work by the M.R.C., reported by Buxton and McNalty, 1928, the double intradermal test was brought into operation and was later used in the comparative test using mammalian and avian tuberculins from 1940 onwards until superseded by the single intradermal comparative test in 1947. The single intradermal test is essentially that used by Moussou and Mantoux. Old Tuberculin was used at first but later heat concentrated synthetic medium tuberculin was introduced which removed the side effects produced by the residual animal protein.
Mammalian tuberculin used in this country is in fact produced from human strains.
In the double intradermal test the first injection was intended to give a sensitizing dose and since the tuberculin was of unknown potency, it may very well be that this expedient was followed because the tuberculin was of low quality.
Lesslie has very well defined the objective of-research on tuberculin which, he says, is "'to obtain a preparation which will give a maximum response in the infected animal at all stages of sensitization and a minimum response in the non-infected or non-specilically sensitized animal: the latter is a property of particular interest in the veterinary field where the allergic capacity of the individual is low and the diagnostic dose of tuberculin must consequently be high".
In the 1930's a great deal of research was done under the aegis of the A.R.C. to find a tuberculin which would meet these requirements. It was not at that time fully realized that sensitivity on account of non-specific infections was so important in causing positive reactions to tuberculin. Indeed, sensitivity may be induced by infection with the human, bovine and avian types, M. johnei. so-called skin tuberculosis, the vole bacillus and probably others.
Johne's disease is characterized clinically by enteritis giving rise to profuse diarrhoea and progressive wasting and is invariably fatal if these symptoms have appeared. Infection apparently occurs in early calf hood; there is a long incubation period and many animals may never develop clinical symptoms. In a survey of wastage from dairy herds it was estimated that something under 1 per cent of cows developed clinical Johne's disease, whereas surveys have recovered the organism from 15 per cent of animals passing through an abattoir. Infected animals are sensitive to tuberculin although this is erratic in that sensitivity varies throughout the life of the animal, usually being depressed when clinical symptoms appear. Sensitivity to tuberculin would be expected in a herd with a known history of Johne's disease.
A great deal of research has been done on this disease much of which is relevant to the subject under discussion. The most recent work concerns a vaccine which has been under close experiment and field trial. It appears to be of some value in preventing or postponing clinical disease but it has the disadvantage of producing a marked sensitivity to tuberculin and this has prevented its extensive use so far. Its use in more heavily infected herds is presently under discussion and it may well come to be used for such herds now that the climate of tuberculosis in the national herd is so much improved.
Avian tuberculosis affects all species of birds. In this country outbreaks of disease in flocks of domestic poultry were at one time not uncommon. It has always been rare in turkeys, geese and ducks. The disease occurs under bad conditions of overcrowding, particularly where poultry are kept for years on contaminated ground. In recent times there has been more intensive management of poultry and under the better conditions which prevail in the large modern commercial undertakings tuberculosis has practically disappeared. There is a far quicker turn-over of birds. Broilers go for slaughter at about 10 weeks old and it is uncommon for egg producing farms to keep hens after their second laying season, so that this has also helped to reduce the incidence of tuberculosis. Disease has been found in wild birds of many species—pheasants, wood pigeons, rooks, starlings and sparrows most commonly. There is no evidence that wild birds have infected domestic poultry but rather that the reverse is the rule.
Perhaps the most important aspect of avian tuberculosis nowadays is its effect upon the tuberculin test in cattle. At present there is no suggestion that fewer cattle are now sensitized in this way but it is of course impossible to specify from the test whether an animal has been sensitized, for example, by Johne's disease or avian tuberculosis.
There may however be a reduction in the number in view of the alteration of the disease incidence in domestic poultry and the expected effect of this upon wild birds.
Lesions of so-called skin tuberculosis occur mainly on the lower part of the legs, frequently below the knee and hock, but they are also found on the shoulder and occasionally on the side of the chest or on the flank.
The lesions usually take the form of nodules in the subcutaneous tissues, but they may involve the skin and are occasionally confined to the skin. There are no signs of inflammation, the lesion being firm, cold and non-painful, but larger swelling may be soft and fluctuating.
Smears from the lesions usually show the presence of varying number of short acid-fast organisms. Histologically the lesions show a typical tuberculous structure with a small cell infiltration, and the presence of giant cells.
Infected animals react to the intradermal tuberculin test, but apparently not so markedly as those definitely infected with true bovine type tuberculosis. All attempts to isolate and identify the organisms concerned, either by cultural methods or small-animal inoculation, have so far been unsuccessful.
the application of the tuberculin test
Many tests and field trials were carried out with a range of tuberculins including P.P.D. tuberculin of graduated tuberculo-protein content and tuberculins produced from the bovine type of organism, but all were unsuccessful in producing a sufficiently specific tuberculin.
With the early synthetic medium tuberculins in use in the field it was evident that several tuberculous animals did not react. A more potent tuberculin, based on that produced by the Bureau of Animal Industry inAmerica, was introduced after field trial in limited parts of the country. While this was more successful in eliciting reactions in tuberculous cattle it also produced reactions in many more animals sensitized with other organisms.
At this stage experiments had shown that in guinea pigs sensitized with various infections a specificity factor could be developed. It was shown that it would require one unit of human type tuberculin to produce the same response in guinea pigs sensitized with human, bovine and B.C.G. organisms but that 20 to 40 times that amount was necessary to elicit the same response in animals sensitized with avian organisms and Johne's bacilli and as much as 150 times when phlei was used as a sensitizing agent. From this basic experiment in guinea pigs, which was repeated, and variations of it made as further information became available, it seemed possible that a test using two tuberculins might allow of a differentiation between bovine type infection and other, non-specific, infections. In the field avian tuberculosis had been identified at postmortem examination in a number of reactors and Johne's disease seemed to account for reactions in other herds. It was already well known that skin tuberculosis could also sensitize. With all this evidence, a comparative test was applied in a number of tested herds
which had a good disease history. Gradually, as more tests were carried out, a system of interpretation of this test was devised and only minor modifications have subsequently been made.
After a very extensive field trial the single intradermal comparative test was substituted for the double in 1947 and this is the test that is now used throughout all the herds inGreat Britain.
The test depends on the injection of mammalian and avian tuberculin in doses of 0-1 ml. in two separate sites on the side of the neck. The sites have to be carefully selected so that closely comparable results may be obtained. The interpretation of the result in each animal depends on a comparison of the reactions at the two sites and the interpretation is varied depending on the presence or absence of non-specific infection in the herd and on the herd history. With a good herd history and evidence of nonspecific infection animals responding to both tuberculins may be retained in the herd even though there is a marked response to mammalian tuberculin; on the other hand when the standard interpretation points to a significant incidence of bovine type infection the avian test should be disregarded in the whole herd or in the particular group of animals and the interpretation based solely on the response to mammalian tuberculin.
The use of P.P.D. tuberculins ensures that batches of tuberculin are comparable but biological standardization is also practised. The mammalian tuberculin prepared from human type organisms contains 2 mg per c.c. and the avian 0-5 mg. This relation of four to one in the two tuberculins arises from the content of tuberculo-protein in the mammalian and avian tuberculins originally used for the comparative test trials. The interpretation of the test was devised on the earlier tuberculins and the same relationship between the two was therefore maintained when P.P.D. tuberculin was substituted.
Reinfection of attested herds
A brief reference to reinfection in attested herds prepared by my colleagues in Animal Health Division is worth summarizing. An incidence of reactors may be expected for many years to come and in fact there is no assurance that the tubercle bacillus can be completely eradicated. The risk of cross-infection from other species of domestic animals is receding. Having broken the cycle of infection at the bovine stage there is little risk of a reservoir of infection remaining in the pig population for any length of time. Man may continue to constitute a risk to tubercle-free cattle. Several instances of cross-infection of bovine type tubercle from man to cattle have already been noted but it is only when a human contact is proved to be an open case that there is acceptable evidence of this source of infection. There is no knowledge of the true incidence of bovine infection from the human source. Human infection would however not appear to be a major hazard, but records since 1950 show nine herds in which circumstantial evidence indicates that tuberculosis of a bovine type was conveyed by an infected owner or farm worker to cattle. In these 9 herds 245 reactors were found as a result of 35 tests. One herd accounted for 103 of the reactors in two tests (80 and 23) resulting from a cowman who was an open case.
To date the most frequent cause of massive reinfection in attested herds has been the affected cow with depressed sensitivity. The open lung case, if in an extensively managed herd, may not infect many other members of the group but in the dairy byre a chain reaction may occur. In the early stages only a few cowshed contacts may be found positive to the test. Should the infection remain undiagnosed for any length of time however the reactor incidence normally increases and secondary open cases may appear and multiple spread ensue. The test picture was capable on occasion of indicating a possible origin of infection. Batches of reactor calves pointed to infected milk and clinical examination of the milking herd usually detected the affected udder. In adult cattle the primary complex was most frequently found in the respiratory system and. where a high adult incidence obtained, a careful clinical examination of the chest cavity was often rewarding. Major breakdowns in previously clean herds were most commonly due to the affected uterus. Highly infective uterine discharges were responsible for the widespread dissemination of disease and on occasion among all age groups. In the cow-shed the dessicated discharges were responsible for airborne infection, and contamination of utensils and cattle food resulted in primary lesions of the digestive system in batches of cattle which had been husbanded away from the offender. The accidental introduction of disease to a clean herd has been caused by intramammary infusion, obstetrical treatment, subcutaneous injections and the surgical procedures of de-horning and castration. The use of single-dose tubes of intramammary antibiotics, attention to personal hygiene and effective sterilization of instruments should obviate these catastrophes.
Breakdowns have been noted most frequently in herds which reached attested status by repeated tuberculin testing and in which the original incidence had been high. In due course the undisclosed reactor either dies without having become infective or it is pin-pointed by its clinical condition or by the positive reactions provoked in contact cattle.
Accuracy of the tuberculin test
The number of animals affected with bovine tuberculosis which fail to react to the test has apparently been reduced to a negligible level but breakdowns do occur and some are accountable to this error. A breakdown in a herd is so called if eight or more animals are removed as reactors at a herd test.
In 1961, the year following the declaration of the whole ofGreat Britainas an attested area, there were 266 such herd tests involving 4,084 reactors. In 1962. there were 152 inEngland, 12 inWalesand II inScotlandinvolving 2,786 reactors. In the first 6 months of 1963, there were 59 inEngland, 3 inWalesand 7 inScotland, involving 1.060 reactors.
But in the other direction there is an increasing error of test. As the number of reactors is reduced so the proportion of them which shows no evidence of tuberculosis at post-mortem examination rises. This varies greatly in localities for the reactor rate usually depends on the length of time the area has been attested. The country rate in 1961 was 0-16 per cent; in 1962 it was 0T1 per cent and for the first nine months of 1963, 007 per cent. ForEnglandthe comparable figures are 0-18, 0T2 and 0-08. In Wales and Scotland, attested a year earlier, the figures for each year from 1960 are 0-23, 0-13, 0-09 and 0-06 in Wales: and 0-07, 005, 0 05 and 004 in Scotland. The oldest attested areas are inScotlandandWales. In one Scottish county no reactors were found in 1962 and in an English county, one of the last to be attested, the figure in 1962 was 0-43 per cent. In some areas as few as 50 per cent of reactors have demonstrable lesions of tuberculosis while in others there is at least 75 per cent accuracy. It has been found that bovine type organisms may be recovered from a proportion (around 8 per cent) of animals showing no lesion but this figure varies widely with the reliability of the post-mortem examination.
It is possible that further detailed study of the tuberculo-protein content of the tuberculins might be rewarding because it has been shown that a reduction of the avian tuberculo-protein below 0-5 mg gives the maximum discrimination in the comparative tuberculin test. There is, however, justifiably a great reluctance to make any alteration which would change the interpretation of the test unless accuracy is to be greatly increased and this seems unlikely.
Summary of effects on cattle
The incidence of reactors has dropped from about 20 per cent in 1930 to about 0-07 per cent for the first nine months of 1963.
The number of animals slaughtered as clinical cases under the Tuberculosis Order has steadily fallen since eradication was commenced: from the peak figure of 23,716 in 1936 when there were 414 attested herds it dropped to 8.266 with 25,355 attested herds and to 1,103 ten years later with 167,757 attested herds. Since 1960, after the declaration of the whole ofGreat Britainas an attested area, the figures are 20 for 1961, 10 for 1962, and 5 for the first 9 months of 1963.
Since 1937 the average milk yield per cow in dairy herds inEnglandandWaleshas risen from 560 gallons to 780 gallons in 1961-62. This is not entirely due to the elimination of tuberculosis for other diseases are under better control; feeding and management have also been improved and there has been a shift, of course, towards the high producing dairy breeds. In the absence of the disease, however, a dairy herd requires fewer young animals to replace wastage among the cows and this has certainly allowed farmers to plan their breeding programmes and to retain only the best animals. In general the herd free of tuberculosis is a much healthier herd with more regular breeding, less digestive disorders and far less pulmonary disease.
Although the condemnation of meat was never an important item in the account against tuberculosis a few figures may be of interest. For example inEdinburghabattoir 11-15 per cent of cattle were affected in 1935—in 1962 0.37 per cent. The comparable figures for pigs were 5.18 and 0.96. The figures for 1962 in proportion to 1936 in several abattoirs were as follows.
The public health aspect
Until comparatively recently in the more advanced countries of the world, tuberculosis was responsible for the death of more individuals between the ages of 15 and 45 than was any other disease. Infection by the bovine type has always been much more frequent in children than in adults, and as the custom of feeding raw milk to infants was an accepted practice in Great Britain until comparatively recently, human tuberculosis of bovine origin was more common in Great Britain than in any other country in the world.
Using data obtained from surveys, Savage, in 1929, calculated that 5 per cent of all deaths at all ages due to tuberculosis was caused by bovine infection, but that in children under 5 years the proportion was 30 per cent.
Reports of the Ministry of Health (EnglandandWales) in the pre-war years 1931-39 show an average mortality per annum of 4.972 from non-pulmonary forms of tuberculosis. The figure for 1960 is 330.
That this was due in no small measure to the eradication of tuberculosis from our national dairy herd will not be argued, although obviously the extension of pasteurization made a major contribution but only in the urban areas. Rural districts get no benefit from pasteurization and indeed may be subjected to still greater hazard because there is no dilution factor—in the extreme example a family may be supplied from the milk of one cow, which may be particularly dangerous.
In addition, those people who came closely into contact with cattle were at risk of acquiring pulmonary tuberculosis. Very little typing of tubercle bacilli has been done in this country, but from what has been done and from Sigurdson's work in Denmark, it is clear that a proportion of the cases of pulmonary tuberculosis in man is caused by the bovine organism. It is accepted that this is capable of setting up in man tuberculosis as severe as that caused by the human type. Much of the recorded work was done inScotlandandScandinavia, and Black-lock has suggested that, because of this, these countries have been credited with a higher percentage of bovine infection than elsewhere. InDenmarkin a comprehensive survey in the 1930's 5.3 per cent of pulmonary tuberculosis was associated with the bovine type of organism. In this country the highest incidence has been recorded in north-east Scotland and the high incidence in the 1935 series was attributed to the fact that people in the north of Scotland drink more milk than in the rest of the country, whereas in the later series a study of the employment of the patients indicates possible contact with cattle in some instances. Aerogenous infection may readily occur in a cowshed.
I have not given many examples of the detailed figures showing the reduction in the incidence of bovine type tuberculosis in man. With the great advance in the prevention through B.C.G. vaccination and otherwise, the better diagnosis and ascertainment of disease, and with modern treatment of tuberculosis in man, the situation to-day cannot be compared with the position even in the immediate post-war years. Figures cannot begin to convey the change in the situation from the earlier tale of misery whether it arose through disfigurement from cervical gland infection, the crippling effects of bone and joint disease ranging from ankylosis of one joint to the hunchback now so rarely seen, or stemmed from long spells in sanitoria undergoing surgical and other treatment.
It can be claimed that bovine tuberculosis is now a rare disease of cattle inBritainand that the infection is increasingly rare among other domestic animals. Man who was at one time a victim not only from infection acquired in childhood from infected milk but through contact with affected cattle now enjoys protection in many ways besides those afforded through eradication of the disease from cattle. He has unfortunately taken up a different role in the epidemiology of tuberculosis in cattle because of his ability to re-infect healthy animals.
This has been a great co-operative effort. Eradication was founded on a sound appreciation of the epidemiology of tuberculosis in cattle and plans were adjusted accordingly; research played a basic part in all the work, for without a good practical tuberculin test success would have been impossible, and there was a constant need for the typing of organisms in the laboratory; veterinary surgeons in the public service had to carry out much of the work in the field and with their administrative colleagues they had to organize it; veterinary surgeons in private practice bore the greatest burden of tuberculin testing; and lastly the British farming organizations showed an enlightened attitude and the individual farmer was usually helpful in spite of the difficulties which he had sometimes to face.
It has been a great privilege to play some part in the eradication of bovine tuberculosis, and this evening it has also been a privilege to give
Last edited: 20 May 2015 14:32