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The details are given of the epidemiology ofS. thompsoninfection in a flock of pullets which survived an outbreak of this disease when chicks, and also of a flock of adult birds suspected of being carriers of the organism. The results of blood testing and bacteriological examinations of cloacal swabs, random faecal samples and eggs from both flocks indicated that:(1) Many chicks which survived an outbreak ofS. thompsoncontinued to carry the organism for some months without showing any symptoms. In most cases the organism was harboured in the intestines and was excreted intermittently in the faeces. On one occasionS. thompsoninfected the gall bladder, and the bacilli were excreted in the faeces for at least 18 months after the outbreak had occurred.(2) The common method of egg infection was by the contamination of the shell with infected faeces. Under conditions of incubation the bacilli penetrated the egg shell and infected the yolk. Although there was little penetration of the organism under storage conditions, the bacilli on infected shells remained viable for at least 21 days.(3) The common methods of spreading infection in a hatchery were from:(a) The contact of infected and non-infected egg shells.(b) The handling of eggs before and during incubation.(c) The contact of egg shells with infected incubators.(d) The ingestion and inhalation of infected fluff and incubator debris at hatching time.(e) The ingestion of food and water contaminated with infected faeces from survivor chicks.For the control ofS. thompsoninfection in poultry, the following procedures have been recommended:(1) The production of agglutinins by carrier birds was not a reliable indication of infection. In known infected flocks, however, the detection of carriers by blood testing and by the examination of cloacal swabs is of value. For such a test an alcoholized antigen is preferable to a heat-treated broth antigen, and a titre of 1/20 or more should be regarded as an indication that a bird is infected.(2) Only clean eggs should be used for hatching. Dirty egg shells should be cleaned by scraping or brushing and not by wiping with a damp cloth.(3) Fertile eggs should be stored in a cool, dry atmosphere for as short a period as possible before incubation.(4) Eggs should be fumigated in the incubator with formaldehyde vapour, not later than 24 hr. after they have been set.We wish to record our appreciation of the assistance received from Mrs E. Sidery in the bacteriological examination of eggs and the preparation of agglutination tests.

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