The incubation period of typhoid fever

Abstract

In most cases of typhoid fever the incubation period cannot be determined accurately, as the precise date of infection is unknown. There are, however, cases in the literature in which the time of infection is definitely known and in which, therefore, the distribution of the incubation period can be determined. Certain of these are treated in the present paper. In an epidemic, described by Sawyer,1 caused by the eating of infected spaghetti at a church dinner the distribution of the incubation periods of definite and of doubtful cases was as given in table 1: The definite and doubtful cases do not therefore differ significantly in length of incubation period. In an epidemic reported by Cumming,2 due to the contamination of ice cream by a typhoid carrier, the distribution of incubation periods is given in table 2. The mean of the fatal cases is 5.67 days, and the standard deviation is 0.471, while the mean of the nonfatal cases is 7.2 and the standard deviation is 1.887. By Pearson's goodness of fit test the probability that the fatal and nonfatal cases are random samples of the same population is 0.8. The difference in the means cannot, therefore, be considered significant on the basis of the present small numbers. The mean of the combined cases is 7 ? 0.26 days, and their standard deviation 1.84 dz 0.18 days. Other cases due to infection of food have been reported by Schenck.3 He reports nineteen cases with an incubation period of 12-16 days. Waterman4 reports seventeen cases, 13-22 days. Pixley 5 reports two cases with an incubation period of 2 weeks ; also 3 cases due to infected water with an incubation period of three weeks.