1. Farr's 63 healthy districts are, as a whole, representative of the stationary districts of England and Wales, i.e. those districts in which growth or decline of industry has, on the whole, been absent and in which mortality rates are consequently free from the influence of industrialisation, though not necessarily unaffected by urbanisation.

2. These healthy districts maintained, until 1901–10, the advantage they held over England and Wales as regards their relative death rates in 1851–60.

3. From 1851–60 to 1901–10 the death rate of the healthy districts remained roughly constant at 76 per cent, of the death rate of England and Wales. Both improved their total death rate to the extent of 30 per cent, of what it was in 1851–60, the rates for this decennium being—England and Wales 21·17, healthy districts 16·13 per 1000.

4. The population of the healthy districts has been unfavourably constituted for a low death rate throughout the period.

5. That the periods during which the most improvement was made in lowering the death rate—not only in the healthy districts, but in England and Wales as a whole—were 1881—90 and 1901–25.

6. By 1921–5 the position of the healthy districts had become a little less favourable, their death rate having risen from 76 to 86 per cent, of that of England and Wales. It is quite possible, however, that this is due, not so much to a falling-off in the rate of improvement in the healthy districts, as to an exceptional increased improvement in backward, very unhealthy districts.

7. For the following diseases, the healthy districts show improvement at faster rates, of varying degrees, than England and Wales as a whole, during the period 1851–1925. Measles, scarlet fever, whooping cough, diphtheria, pulmonary tuberculosis, and respiratory diseases.

On the other hand, the data relating to diarrhoea seems to indicate less improvement, though inherent deficiencies in the data make this a matter open to doubt.

The death rate from cancer, which has increased considerably in the whole country during the last forty years, appears to have increased at a slightly faster rate in the healthy districts.

But, as discussed in the report, the untrustworthiness of the data, relating to comparisons of individual disease death rates over long periods of time, make it essential to regard the points enumerated in conclusion 7 rather in the manner of interesting possibilities than as proven facts.

8. When the 141 administrative areas, which in 1920–5 corresponded to Dr Farr's original healthy districts, were classified as to whether the majority of their occupied persons worked in other districts or in their own district, the standardised death rates found were as follows:

I. Districts in which 50 per cent, of occupied persons work in other districts. Death rate = 10·55 per 1000.

II. Districts in which more than 50 per cent, of occupied persons work within the district. Death rate = 9·90 per 1000.

When the second of these classes is sub-divided according to whether 50–75 per cent, or over 75 per cent, of occupied persons work within the district, the death rates are:

(a) 50–75 per cent, of occupied persons working within the district. Death rate = 9·69 per 1000.

(b) 75–100 per cent, of occupied persons working within the district. Death rate = 10·14 per 1000.

When the same class is sub-divided according to whether the majority of workers are engaged in non-agricultural or agricultural pursuits, the death rates in each sub-class are:

(a) Mainly non-agricultural. Death rate = 9·85 per 1000.

(b) Mainly agricultural. Death rate = 9·98 per 1000.

The maintenance of a constant supply of healthy guinea-pigs is an important part of the work of laboratories which are engaged in the production of diphtheria and tetanus antitoxin. The subacute infective processes to which malnutrition predisposes enhance the effect of the test dose of toxin, with the result that irregular deaths among the animals under test render difficult the titration of the antitoxin. For this reason it is desirable that breeding stocks should be kept in suitable animal houses under the best hygienic conditions possible. This policy has been followed in the Serum Department of the Institute for many years; there have been comparatively few introductions of stock from outside sources, and within recent years special attention has been devoted to diet.

1. African pouched rats (Cricetomys gambianus) are very suitable animals for plague experiments on account of their tolerance of captivity and their extreme susceptibility. After inoculation with small doses of plague culture of ordinary virulence they always die.

2. Broth-grown vaccine was much more efficient as a prophylactic than agar-grown vaccine. When two doses were given a survival rate of 56 per cent, was obtained with the former and 25 per cent, with the latter.

3. A vaccine composed of carbolised spleen pulp from animals, which had died of acute plague, was rather more efficient than broth vaccine, giving a survival rate of 75 per cent. But it is not likely to be of practical value, as preparation is difficult, there are undesirable local effects and its superiority over broth vaccine is not very great. Its efficiency is attributed to the fact that the organisms which it contains are a true “body strain.” The method of preparation is similar to that of Hindle's yellow fever prophylactic.

4. A number of experiments are described in which virulence was abolished or reduced by passage through immune or partially immune rats. This occurred only when the passage animal showed a considerable degree of immunity, as indicated by the length of the survival period after the test infection. Cultures made from the abscess at the site of infection were less often attenuated than those from the liver of an immunised animal, but this was observed only in the case of large well-defined abscesses. It is suggested that a firm abscess wall protects organisms inside it against the action of antibodies appearing in the general circulation.

A culture attenuated by passage through an immunised rat was restored to normal virulence by successive passages through normal rats, but the process was slower than in the case of cultures attenuated by long cultivation, five passages being required in the former case and only one in the latter.

5. Cultures used in the later work, unlike those used in the earlier work, lost virulence rapidly when kept at room temperature. Experiments and observations are described which suggest that old strains, even when subjected to rat passage, are less stable as regards virulence than those recently isolated from human epidemic cases, and that the high temperature of the Gold Coast has an adverse effect upon virulence.

6. Attempts to isolate stable variants of B. pestis were not successful. Colonies vary much in form. Three types of colony are mentioned, but all intermediate grades were observed. Certain old avirulent strains had a greater tendency to produce large irregular colonies than recent strains, but the relation was so loose that the form of colony gave little indication of the degree of virulence. In old avirulent strains the bacilli tend to become longer and filaments of great length may be present. Salt stability tends to become less as a strain grows old and avirulent, but the change is not very marked and is apt to be masked by individual fluctuations.

Agglutinins and precipitins. Cultures of S. aureus isolated from infections in man were used for the inoculations. It is well known that rabbits are very susceptible to this organism, and previous inoculation with vaccines, attenuated cultures, or with carefully graduated doses of the live coccus does not confer adequate protective immunity, although the animal's serum contains immune bodies, such as agglutinins and precipitins, to a high titre.

1. The virulence of a given strain of B. aertrycke was tested on 54 occasions during the 3-year period, October 1926–October 1929. Each test was made by inoculating intra-peritoneally about 100 viable organisms into 20 parti-coloured mice, usually of 17–23 grm. in weight, drawn at random from the general laboratory stock. The average number of specific deaths caused by this procedure was 12·71 ± 3·39, but very marked deviations were noticeable, varying from a minimum of 5 to a maximum of 20. These deviations occurred without any observable regularity, and appeared to have no definite seasonal relationship. Evidence is brought to suggest that they were chiefly determined not by variations in the dosage or in the virulence of the strain, but by fluctuations in the resistance of the mice.

2. The meaning of the term “resistance” is discussed, and it is suggested that the term “fluctuating immunity” should be used to denote those variations in resistance which occur from time to time, and which are due to alterations in the physiological behaviour of the animal dependent on changing environmental conditions. This fluctuating immunity should be distinguished from innate, inherited, orgenetic immunity on the one hand and from acquired immunity on the other.

3. It is pointed out that the existence of fluctuations from time to time in the susceptibility of animals to infection renders fruitless any attempt to obtain an absolute measure of virulence. The most that can be done is to compare the relative virulence of two or more strains injected simultaneously.

The spores of B. anthracis are well known to be very resistant, and able to survive for long periods, especially when dry. Some of the older workers have quoted instances of their long survival, but none seem to have ascertained the limit of their survival under definite conditions.

A study of the agglutinin content of the sera of individuals who have suffered from attacks of Sonne dysentery has shown that in the majority of cases specific agglutinins are produced.

In a series of agglutination tests with sera from 138 normal individuals 24 per cent, were found to give a positive reaction in a dilution of 1 in 50 or greater.

It will be convenient to summarise the results of the foregoing observations and the conclusions which appear justified by a comparison of results obtained by the two methods of standardisation under consideration, namely (1) cutaneous, on the human subject, and (2) intracutaneous (intradermal) on the guinea-pig.

1. In the absence of knowledge regarding the essential chemistry of tuberculin, a chemical method of standardisation is excluded.

2. The phenomena of allergic reactivity point to a biological basis for standardisation.

3. The allergic reactions have been studied in the sensitised animal and in the human tuberculised subject.

4. Tuberculins of unknown potency have been compared with a tuberculin of known potency which has been selected as standard.

5. Comparative observations have been made as between the standard and the unknown tuberculin, by (a) making use of different dilutions, and (6) assessing results in animals (sensitised) and in human (tuberculised) subjects.

6. In the sensitised guinea-pig the intracutaneous method has been used, and a definite procedure followed in the determination of results.

7. In the human tuberculised subject the cutaneous method has been used, and similar procedure followed for the determination of results. The method and results are illustrated in the text.

8. The results in the two sets of observations are definite and comparable. Both afford a basis for standardisation.

9. The average probable errors by the two methods are approximately equal, that is, the accuracy of the one method is approximately equal to that of the other.

10. If this be so, a strong plea may be advanced in favour of the human test. Shortly expressed: human (tuberculised) subjects are readily available for observation. The procedure involves little preparation, and the results are easily read with exactness. The tuberculin under test is to be used thereafter in relation to the human subject. This fact enhances the value of the test observations.

If it be objected that in intracutaneous injection the amount of tuberculin introduced is measured more precisely, it may fairly be maintained that the droplet application of tuberculin is limited to a sharply defined area of skin surface and, further, that the clear skin of the human subject allows of more accurate estimation of the diameters of the areas of reaction.

The present enquiry has shown that assays on a variety of human (tuberculised) subjects yield consistent results, and similarly, assays on various animals give consistent results. Yet the animal results are not always contistent with the human results. The explanation of the discrepancy is not very clear. It is not impossible that certain strains of tuberculin act in less degree on the human subject and in greater degree on the animal, and conversely. The occurrence of such differences might be misleading and even involve risk, if standardisation tests were limited to animals without control from observations on the human subject.

If we grant, as the records have shown, that the procedure in relation to the human subject is sound and is innocuous to the human subject of the test, much may be said for the simplicity of the method and for the clarity of results obtained. As the tuberculins under test are destined for use on human subjects—for diagnostic and therapeutic purposes—it would seem reasonable, and probably safer, to base the standardisation of tuberculin (for human purposes) on observations in relation to man.

The results of this investigation may be summarised as follows:

1. The qx or cage-age mortality curve of a herd increases rapidly to a maximum and thereafter descends to an approximately constant level which is much above the level of the qx for normal mice of ages within the range of real, physiological, age of animals living in the herds.

2. At the latest ages under observation the principal factor of mortality is still the specific factor, so that the advantage produced by selective mortality and active immunisation is brought to a standstill far above the zero line. Exposure in a herd under the conditions of these experiments will not produce an ultimate population fully resistant to the specific factor of infection.

3. It is probable that a large proportion, perhaps a majority, of the members of a herd become infected early in herd life, and gradually increase their degree of immunity because variations of environmental conditions, so far as these are measured by the general herd mortality, become less and less influential on the ultimate mortality of the exposed to risk the later the point in time chosen for measurement.

4. Study of the form of the age-mortality curve is still proceeding; at present we cannot offer an adequate mathematical description of it which takes due account of the biological factors requiring attention.

5. Quite provisionally, we attribute the difference in time of the maxima of the qx curves of Pasteurella and aertrycke epidemics to a difference of average interval between infection and death.