Although no clear explanation for this epidemic of inhalation anthrax was discovered, the circumstantial evidence suggested a relationship to a particular batch of goat hair. It is possible that the observed temporal association of the processing of a single hair order from India with the onsets of the epidemic cases was a coincidental one. Nevertheless, an hypothesis which suggests itself is that this particular batch contained an unusual number of highly virulent anthrax spores to which was added during most of the epidemic a virulence-enhancing substance (the scouring detergent). In support of this hypothesis are the following facts: that anthrax organisms were demonstrated in large numbers in this hair order; that the agents recovered from the patients were virulent by the inhalation route for monkeys and guinea pigs; and that the scouring detergent enhanced this virulence. Insufficient numbers of strains have been tested for virulence by this route to characterize the virulence of these agents relative to other strains. The available data were consistent with high virulence. By extrapolation to man of the experimental dose-response curve of one Manchester strain in monkeys (LD50 = 6,000), it can be calculated that a 10-fold increase in the observed concentration of spores might have caused an epidemic if the organisms belonged to the Manchester strain. If enhancement of virulence by the scouring detergent in the Manchester mill is also assumed, then a lower spore concentration might have been dangerous.

The focal question appears to be whether a large dose of anthrax spores of the right size is sufficient to provoke infection, or is an additional factor necessary? The fact that inhalation anthrax occurs so rarely in mills where contaminated materials are handled day after day argues for a precipitating cause of the disease other than the presence of a sufficient quantity of organisms. Yet acute observers of the nineteenth century epidemics such as Spear (1) and Bell (19) were unable to find a common thread to bind their cases together, other than work with raw goat hair and exposure to dust therefrom.

It appears that hair and wool workers exposed to B. anthracis aerosols may inhale hundreds of spores into their alveoli each day without suffering clinical illness. In the presence of an unusually large number of spores, or of a highly virulent strain, however, the minimum lethal dose may be exceeded by the aerosols produced in the manufacturing processes, and an inhalation anthrax epidemic results.

It is to be stressed that this hypothesis, although attractive, is based not on proven facts, but on suggestive data and unproven assumptions. Our speculations are justified only by the rarity of the disease, which renders the elucidation of this epidemiologic problem in the near future unlikely. Important information touching on the subject could be obtained, however, by more frequent air sampling of textile mills to determine the maximum safe concentration of spores, by the development of methods for estimating the concentration of B. anthracis in hair or wool, and by the perfection of serologic methods for determining past infection with B. anthracis. From the point of view of industrial hygiene this conclusion appears to place stress on better ventilation of goat hair mills, or on sterilization of the hair as performed in England (25). The potential civil defense problem posed by anthrax aerosols is also emphasized (26).