12 



in late summer can hardly be appreciated by those who have 

 not had the experience. The conditions are most favourable 

 for fly transmission, and the circumstantial evidence against 

 flies was so strong as to have left no doubt in the mind of the 

 American Commission that these insects play a large share 

 in disseminating infection, for page 28 of their general 

 statement and conclusions reads thus : ' ' Flies undoubtedly 

 served as carriers of infection." 



An estimate of the fly population and its relation to 

 admissions of enteric fever was made by Ainsworth (1909) 

 in Poona. The observations showed that enteric fever had 

 a very definite season, commencing at the end of the hot, 

 dry weather with the onset of the rains, and reaching a 

 maximum a few weeks after their cessation. The abundance 

 of flies increased also soon after the beginning of the rains, 

 but earlier than the admissions for enteric fever, and 

 speaking generally, the fly curve antedated that for cases 

 of enteric fever by about one month. Taking into account 

 the incubation period for the disease, this is in agreement 

 with the view of a causal relation between fever and flies in 

 Poona. The number of observations is unfortunately small. 



In considering the possible influence of flies in the spread 

 of typhoid fever in a well-sewered city, it must be remembered 

 that the opportunities for flies to pick up the infection are 

 vastly less than under the conditions of a military encamp- 

 ment, or even in rural surroundings. In large cities, no 

 doubt, dejecta and urine from patients may be left available 

 to flies, but the bulk goes promptly into the main drain. The 

 discovery in cities like London and Manchester of the 

 majority of cases of typhoid fever and their removal to 

 hospital before the period when the excreta contain large 

 numbers of bacilli must also greatly reduce the liability to 

 the dissemination of the disease by flies. The typhoid 

 carriers, however, remain. 



Dr. J. Niven (1910) has made observations upon the 

 prevalence of flies in Manchester since 1904. From 12 to 34 

 bell traps were served out to the inhabitants of small 

 dwellings in different quarters of the town, and the catch was 

 collected every few days by his inspector. By these pains- 

 taking observations data have been obtained which, although 

 not extensive, are sufficient to afford information as to the 

 general time relationship between fly prevalence and the 

 incidence of enteric fever and infantile diairhcea respectively. 

 These observations were communicated to the Epidemiological 

 Section of the Koyal Society of Medicine by Dr. Niven in his 

 interesting presidential address in 1910. It must be 

 remembered, however, that the number of flies entrapped in 

 12 or 17 rooms indicates only in a general way the preva- 

 lence of these insects in a large city, so that too much 

 significance must not be attached to other than the main 

 features of the record. 



On pp. 62 to 69 of Dr. Niven's paper are plotted the 



