Experiments to determine the role of particle size in the infectivity of anthrax spores are described. Clouds of homogeneous particles were produced. The mortality-dosage curves for guinea-pigs and monkeys are given for clouds of various particle sizes. Data are given on the effect of time in the concentration-time relationship. The results are compared with those recorded by other workers on the relationship of particle size to respiratory retention.Infectivity was highest with single-spore clouds, falling off as particle size increased. Reasons are given for attributing this effect to difference in site of deposition of different-sized particles.

SUMMARYA modification of the Henderson apparatus is described which is mobile and which can produce bacterial or viral aerosol in air whose relative humidity can be chosen in the range 20–95 %. The apparatus forms a basic utility for microthread, rotating drum or other aerosol studies.Much help and advice has been given by Messrs. J. F. H. Peel and D. J. Garriock of the Engineering Section M.R.E. throughout the development of this apparatus. The proportioning valve is a modification of one originally used for a similar purpose by Mr K. R. May. Thanks are due to Messrs. G. J. Harper and A. M. Hood for criticism of the script.

The infectivity of Brucella suis for the guinea-pig by the respiratory route has been studied. Br. Suis was dispersed in airborne particles of various sizes from single organisms to 12μ in diameter, and it was found that the infectivity decreased 600-fold with increasing particle size within this range. It is suggested that this is due to the ability of Br. Suis to multiply rapidly on the surface of the lower reaches of the respiratory tract.

No abstract

The LD50 dose of Past. pestis is much greater when tested by the respiratory route than by subcutaneous challenge. This is probably due to trauma inflicted on the airborne particles.Two forms of plague, both originating in the respiratory tract of the guinea-pig, can develop according to the size of the particle containing Past. pestis presented to the host. Small particles initiate a broncho-pneumonia which leads to septicaemia and death. Large particles establish a septicaemia, and death results more quickly without the development of a pneumonia.Cross-infection to normal animals occurs irregularly when they are exposed to others developing plague by the respiratory route. Such incident is rare when the initially infected animals are exposed to large particles. Cross-infected animals suffer from the disease characteristic of exposure to large particles. Attempts to establish an epizootic by cross-respiratory infection were abortive, probably due, in some measure, to the type of disease developing in first cross-infections.