The Spread of Bacterial Infection. The Problem of Herd-Immunity

Abstract

(With 9 Charts.) IN our previous studies on the spread of bacterial infection among mice, the resistance of the host has not been especiallv investigated; although the importance of this factor has been repeatedly referred to in discussing the experimental results obtained.The recent reports on experimental epidemiology from the Rockefeller Institute contain many references to this aspect of the question, and, in particular, the careful and important investigation reported by Webster (1922) affords ample evidence of the possibility of conferring some degree of immunity on mice by the administration of dead or living suspensions of bacteria of the B. enteritidis group, whether by feeding or by inoculation.The experiments recorded in the present report were undertaken in order to study any differences, which might occur in the mode of spread of enteric infection among mice, when the immunity of the population at risk was varied by including, among the normal and presumablv susceptible mice, a varying proportion of artificially immunised animals.Consideration of the results obtained during the past five years, both in experiments which have formed the subject of reports and in many others not yet recorded, has led us to believe that the question of immunity as an attribute of a herd should be studied as a separate problem, closely related to, but in many ways distinct from, the problem of the immunity of an individual host. Experiment 1. For the purpose of this, and of the next experiment, a vaccine was prepared from a strain of B. enteritidis (Aertrycke). This was killed by heat, standardised to contain 4000 million bacteria per c.c., and preserved by the addition of 05 per cent. phenol.In the present experiment 127 mice were given an intraperitoneal injection of 0125 c.c. (500 million bacteria). Eight days later there were 119 survivors, and each of these was inoculated intraperitoneally with 0-25 c.c. of vaccine (1000 million bacteria).The main experiment was started 21 days after the second injection. Four of the immunised mice were killed on this day, and their sera were tested

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