Infective Dose in Pulmonary Tuberculosis BY A. L. JACOBS

Dorothy Temple Cross Research Fellow in Tuberculosis, x936-37

NATURE OF THE INFECTIVE AGENT

From time to time during recent years the theory has been advanced that tubercle bacilli are capable of existence in the form of a filtrable virus. The evidence so far offered in favour of this theory has not been adequate to justify its acceptance. The almost universally held view, that the bacilli as observed microscopically constitute the infective agent, is accepted here.

It is also doubtful whether there is any significant variation in the virulence of human type bacilli as inhaled by man. Even if there were such variation it is not easy to see how this fact could be utilized in the attempt to control spread of the disease.

PORTAL OF ENTRY

It is well established that the organism nearly always gains access to the human body in one of two ways:

(i) By inhalation into the lung. (2) By ingestion into the alimentary tract.

It is accepted by most workers that the former of these is by far the more common, especially in regard to pulmonary tuberculosis. In less than 5 per cent of pulmonary primary complexes is the organism found to be of bovine type (Blacklock, i932 ) and in phthisis proper the finding of bovine type bacilli is equally rare. The views of von Behring (I 9o3), later revived by Calmette, as to retrograde lymphatic spread into the lung from the alimentary primary complex, are not now generally accepted. While the possibility of haematogenous infection of the lungs by human type bacilli which have entered the body via the alimentary tract cannot be ignored, the patho- logical evidence indicates that this is uncommon, and it may be said that in the great majority of cases pulmonary tuberculosis is acquired by the inhalation of human type bacilli coughed up by phthisical patients. Measures for the control of the disease by attempts to control the dissemination of the organism are largely based on this assumption.

THE UNIVERSALITY OF TUBERCULOUS INFECTION

It has been clearly proved, both by autopsy and by tuberculin investigations, that in modern civilized communities practically every individual acquires the infection by the time middle life has been reached. We have thus to explain why it is that although everyone becomes infected it is only in a small proportion that any clini- cally recognizable form of the disease develops, and that even in these subjects the severity of the disease varies within wide limits. This may be done in one of two ways:

(i) By postulating variations in individual resistance to the infection, e.g. racial, familial, environmental, nutritional, or age-determined factors.

(2) By postulating variations in the dose of bacilli received by different persons, or by theories of repeated infection.

N o v e m b e r i 9 4 i T U B E R 13 L E 267

Most workers acknowledge the importance of variations in individual resistance, but there is also a widely held view that the dose of bacilli and the occurrence of repeated infections are of primary importance in determining the course of the disease. These views are often put forward as justification for the actual measures employed in the attempt to prevent or restrict the spread of the tubercle bacillus, which may range from the posting of notices urging the public not to spit to attempts at compulsory segregation of phthisical patients. It is at any rate quite clear that these measures cannot be justified by the claim that it is possible entirely to prevent the infection of any significant proportion of the general population. For all practical purposes we can be sure that in mest civilized communities ioo per cent of the population receive the infection at some time in their life. This continues to be so, notwithstanding the very great decline in total tuberculosis mortality (which began long before the existence of any serious attempts to limit the spread of infection).

INFECTIVE DOSE IN PRIMARY INFECTION

In the first place it must be emphasized that the view that man may receive large doses of tubercle bacilli by inhalation under normal conditions is purely hypothetical. In no single case is it ever possible to prove that a given patient has inhaled a large dose or a small one.

The theory that the course of the tuberculous infection in man depends upon the dose of bacilli inhaled rests mainly upon the results of animal inoculation experi- ments, in which it has been shown that increase of the number of organisms injected into the animal increases the gravity of the resulting disease. (It has not been shown that the injection of small doses of organisms fails to cause serious disease. According to Bruno Lange (1932), the iniection of even a single tubercle bacillus into a suscep- tible animal may produce fatal disease.) We have to ask ourselves whether the injection of relatively enormous numbers of tubercle bacilli under the skin of highly susceptible animals corresponds to any form of infection encountered by man in real life.

Tubercle bacilli are inhaled from the air either in the vehicle of droplets coughed out by phthisical patients or as dust particles formed by the disintegration of dried sputum. There is a large body of experimental work on the mechanism of inhalation infection and a number of important facts have been established. The original studies were made by Chaussd ( 1912-I 916). They have been repeated and extended by Bruno Lange and his co-workers (1924-I93I), who eliminated some sources of possible error which Chaussfi had overlooked. ChaussCs findings have been con- firmed in every important particular by the later workers. It is difficult to believe that the term 'massive infection' would be so widely used if the results of these studies were better known. The main results can be briefly summarized thus:

(I) The only droplets or dust particles that can be inhaled into the lung alveoli are those that have a diameter not greater than about ~o tx*. Larger particles impinge upon the walls of the respiratory passages.

(2) If guinea-pigs be exposed to a spray of droplets containing tubercle bacilli, it is only when the droplets are as small as this that they produce primary

*Chauss~ tbund lhat the inhalable droplets, when received upon a glass slide, had a diameter not greater than i5-2o ix. As Strauss (1926) has pointed out, in th, ' air these droplets would have a spherical form, with a diameter little more than half of lhat which they show on the slide.

~68 TUBERCLE November I94~

foci in the lungs. (When larger droplets are present, however, nasopharyn- geal primary foci may be produced.)

(3) Droplets of diameter not exceeding io t~ will remain floating in the air for several hours, while larger droplets fall to the floor within a minute or two.

(4) Droplets of the inhalable order of minuteness are so small that they cannot contain more than a few tubercle bacilli.

(5) If the spray of small infective droplets be highly concentrated by con- fining it in a small inhalation chamber, numerous scattered primary foci are produced in the lungs of the exposed animals.

(6) Exposure of guinea-pigs to a spray of droplets so arranged that on the average each droplet contains a single tubercle bacillus produces pul- monary primary foci; moreover it was shown that the total number of foci so produced in a given animal Corresponds approximately to the total number of bacilli inhaled by the animal.

(7) The same results were obtained in experiments with the sheep, an animal whose respiratory passages and lungs correspond more nearly in size to those of man, and whose natural resistance to the tubercle bacillus is at least as great as that of man.

From these results it must be concluded that when tubercle bacilli are inhaled by a previously uninfected animal:

(I) Only minute doses of bacilli can reach the alveoli. (2) A primary focus is produced by every such minute dose.

The inhalation of larger groups of bacilli can produce primary foci only in the upper respiratory passages or the bronchial walls. These are. practically never observed in man.

It follows that the only form in which 'massive infection' could occur is the inhalation of large numbers of bacilli, in groups containing very few, to many different

�9 parts of the lungs, and this would result in the production of very numerous primary foci in all parts of the lungs. This is precisely what does happen when experimental animals are artificially exposed to highly concentrated infective sprays (Chauss~, Bruno Lange, op. cit.; Jensen, Bindslev and Holm, i935).

Does anything corresponding to this type o f ' massive infection' occur in man? On the contrary, we know that in man the primary focus is nearly always single, though occasionally two are found and rarely three to six (Ghon, I912 ; Blacklock, 1932). It is reasonable to conclude that the conditions necessary for ' massive infection' do not occur normally, that primary infection in man is an infection produced by a very small dose of bacilli, and that 'massive infection' is a hypothesis which does not appear to be supported by the experimental or by the pathological evidence.

This conclusion is borne out by some experiments conducted in phthisis wards by Wtirtzen (i93I). Nurses, carrying out their ordinary duties in the wards, had attached to their clothing glass slides moistened with glycerin. After three hours' exposure in the ward the slides were examined for tubercle bacilli. Bacilli were found upon about half the slides, usually singly and occasionally in pairs but never in larger groups. This air-borne dissemination of bacilli was sufficient to produce primary infection, as is shown by the fact that of those nurses who were tuberculin- negative when they began their career in the wards, 60 per cent had become positive reactors after the lapse of four months.

November i94I T U B E R C L E 269

T H E O R I E S OF REPEATED I N F E C T I O N

TERMINOI,OGY.---There is no uniformity in the terms used by different writers to express the ideas put forward in these theories. While some use the term ' reinfection' to imply a fresh invasion of a previously infected subject, others use 'superinfection'. It would seem proper to employ the word 'superinfection' to imply a fresh, exogenous infection in a person who still carries in his body living tubercle bacilli from a previous infection, re- serving the term 're-infection' for a fl'esh infection in one whose previous infection has been completely eradicated. The re-activation of an old, quiescent lesion in which living tubercle bacilli are encapsulated can be termed 'recrudescence', while the migration of bacilli fi'om an old lesion to produce a new lesion elsewhere within the body is covered by the term ' metastasis'.

It is well to reiterate that we are dealing with purely hypothetical conceptions. It is never possible to prove in any given case whether a patient has received a super- infection or not. Further, it should be realized that there is no parallel elsewhere in pathology for the theory of superintkction; this conception appears to have been evolved exclusively for the case of tuberculosis. It should also be pointed out that acceptance of the theory is by no means essential in principle in order to explain the observed facts of tuberculosis pathology. Syphilis in the human subject is a disease which has many features analogous to those of tuberculosis. In both diseases there is a local granulomatous lesion, usually benign, at the point of invasion, and a spread of the organism to the adjacent lymphatic glands, followed by a stage of generalization throughout the body via the blood stream. In both diseases the organism remains alive within the body for years, and in both localized destructive lesions may appear in parts remote from the point of invasion after long periods of latency. A varying susceptibility towards the organism is shown by different organs of the body in both diseases. Yet in the case of syphilis it has never been suggested that superinfection is necessary in order to explain these facts. On the contrary, in normal conditions syphilitic superinfection is regarded as impossible (e.g. Colles' law) and it is rarely questioned that the later phases of the disease are to be explained by metastasis.

The experimental basis of the theory of superinfection is Koch's phenomenon (Koch, I89I ). It has been held, though with what justification is not clear, that the relatively acute, rapidly healing, ulcerative lesion produced in the experimental animal at the site of the superinfecting injection corresponds in man to the destructive but localized lesions of cavernous phthisis. This despite the fact that the Koch phenomenon demonstrates the presence in the previously infected animal of a powerful mechanism of defence against small or moderate doses of superinfection, even in an animal which bears a progressive and malignant infection within its body. This is comparable with Colles' law in the case of syphilis. It is only when the dose of super- infecting organisms is greatly increased that it proves rapidly fatal, and this in a way which hardly corresponds to any form of tuberculosis observed in man. Adherents of the superinfection theory have, of course, had to recognize the existence of this increased resistance to fresh invasion by small numbers of bacilli, and they have therefore postulated that in order to produce phthisis in man the superinfection must be 'massive'. The evidence considered above, however, indicates that 'massive infection' does not occur in man, and this evidence applies .just as much to the inhalation of bacilli by a previously infected host as to primary infection.

During the past twenty years many workers have described the radiological appearances of the early lesions ('Frfihinfiltrat', 'Initialherd', etc.) from which

270 a'u ~ E a c r.F. November I94i

cavernous phthisis develops (Assmann, 193 o; Braeuning, 1932; Hedvall and Malmros, i938; Redeker and Walter, I928 ). The view that these foci are produced at the site of newly inhaled bacilli is not supported by their anatomical distribution. The vast majority appear in the apical or sub-apical regions and in this they differ from pulmonary primary foci, which are known to occur directly as a result of inhalation and whose distribution in all parts of the lungs is in accordance with this fact, the preponderance of right middle and lower lobar primary foci being simply explained on physical grounds by the greater bulk of the right lung and the relatively direct course of the right main bronchus.

Finally, if we turn to the results of observations upon the relative incidence of phthisis in tuberculin-negative and tuberculin-positive adults, we find no support for the theory of superinfection. The well known and widely confirmed work of Heimbeck (i936) and Scheel (I935) has shown that many of the cases of phthisis occurring in young adults (nurses and students) follow relatively closely upon primary infection and that the incidence of the disease in the initially tuberculin- positive group is very small.

I f we accept the fact that the tuberculous infection is universal in our population, and if satisfactory evidence as to the occurrence of 'massive infection' and the signi- ficance of superinfection is so wanting, it has to be admitted that there is something irrational in the widespread belief that it is possible to influence the incidence of phthisis by attempts to prevent the spread of infection. Such attempts, it is true, may have a limited value in preventing the occurrence of primary infection at a time when natural resistance is known to be at a low level, for example during infancy. With this exception it appears doubtful whether our efforts to control the dissemina- tion of tubercle bacilli have achieved results of any real value. Indeed, as Scheel has pointed out (i93 I), any success achieved in preventing primary infection in children between the ages of four and ten years, whose resistance (as shown by their very low tuberculosis mortality) appears to be remarkably good, will result in postponing primary infection to the years of puberty and adolescence, in which we have good reason to believe that resistance is lower and environmental factors less favourable.

Speculation upon 'massive infection' and superinfection tends to divert attention from the many factors affecting individual resistance which might be more amenable to our control. The war of I914-I918 brought a great increase in tuberculosis mortality in every one of the countries engaged in it or affected by its blockade. There are already indications that this story is to be repeated in the present war and it is to be hoped that ingenuity will not be wasted in further elaborations of the already tortuous theories of infection in order to make them fit the facts. A far more promising line of attack upon the problem would be an improvement of diet and conditions of life among the adolescent, while the possibilities of active immunization by the B.C.G. vaccine deserve more attention than they have hitherto received in this country.

SUMMARY

(I) A brief consideration is given to some of the available evidence as to the mechanism of infection in tuberculosis.

(2) It is concluded that the conception 'massive infection' as applied to man has very little evidential support.

November ~94~ TUBERGLE 27I

(3) There is no satisfactory evidence that repeated exogenous infections are of any significance in determining the course of the disease.

I am considerably indebted to the admirable review of this subject published by R. H6yer Dahl (1933).

REFERENCES

Assman, H. (193 o) Ergen. Tuberk. forsch., i, 115. v. Behring, E. (i9o3) (Abstr.) Brit. Med. J., 11, 993. Blacklock, J. W. S. (I932) Med. Res. C. Spec. Rep., No. 172. Braeuning, H. (1932) Z- Tuberk., LXtV, 416. Chaussfi, P. (I91 @ C. R. Acad. Sci., CLV, 486. Chauss~, P. (i9i3a) Ibid., CLVl, 638. Chauss~, P. (I913b) Idid., CLVl, 954. Chauss6, P. (I913c) Idid., CLVI, 1485. Chauss~, P. (I913d) l~iJ., CLVn, 862. Chauss6, P. (I914a) Ibid, CLVm, 134. Chauss6, P. (I914b) Ann. Inst. Pasteur, xxvi11, 6o8. Chauss6, P. (I914c) Ibid., xxvI11, 72o. Chaussd, P. (1914d) Ibid., xxvm, 771. Chauss6, P. (i916) Ibid., xxx, 613. Dahl, R. H6yer (1933) Nord. Med. Tidskr., vI, 14o 9. Ghon, A. (1912) "Der primfire Lungenherd bei der Tuberkulose der Kinder', Berlin. Hedvall, E., and Malmros, H. (I938) Tuberkulosebibliothek, No. 68, Leipzig. Heimbeck, J. (I936) Tubercle, xvIn, 9oi. Jensen, K. A., Bindslev, G., and Holm, J. (I935) Acta tuberc, scand., IX, 27. Koch, R. (I89I) Dtsch. reed. Wschr., XVlI, 3. Lange, B. (I924) Z. Hyg. Infektkr., cnI, I. Lange, B., and Keschischian, K. H. (I924) Ibid., cIH, 569 . Lange, B., and Keschischian, K. H. (1925) Ibid., cIv, 256. Lange, B., and Nowoselsky, W. (1925) Ibid., cIv, 286. Lange, B. (i926a) Ibid., CVl, 1. Lange, B. (i926b) Z. Tuberk., XLVl, 455. Lange, B. (i93i) Ibid., LxL 44, 97 and i77. Lange, B. (1932) Beit. Klin. Tuberk., LXXXl, 235. Lange, B. (1934) Dtsch. reed. Wschr., LX, I97. Opie, E. L., and McPhedran, F. M. (1932) Arch. Intern. Med., L, 945- Redeker, F., and Walter, O. (I928) 'Entstehung und Entwicklung der Lungen-

schwindsucht des Erwachsenen', Leipzig. Scheel, O. (I931) Nord. reed. tidskr., nl, 633 , 644 and 658. Scheel, O. (I935) Ibid., Ix, 48I. Strauss, W. (I925) Z. Hyg. Infektkr., cv. 416. Wfirtzen, C. H. (1931) Hosp. tidende, LXXIV, IOI.