MTIOLOGY 



1369 



With regard to age, it appears that the transitory carriers arc 

 generally young — five to twenty years of age — ^while the chronic 

 carriers are usually old — thirty to forty-five years of age. 



The greatest inf ectivity of a carrier is during the incubation period, 

 and the first weeks of the attack of fever; but chronic carriers are 

 important in producing the endemicity of enteric fever in houses, 

 groups of houses, and streets, as well as in institutions, etc. 



As to the method of infection of the healthy by the carrier, 

 evidence is being produced in favour of food contamination. So 

 far, the evidence produced in favour of milk contamination by 

 carriers is very strong, and it is quite possible that water, dust, 

 fomites, etc., will all in course of time be shown to often obtain 

 their infectivity from carriers. 



The question of the infection of the house-fly (Musca domestica) 

 by means of the bacilli in the faeces of carriers is most important, 

 because the fly may take the bacilli into its crop or alimentary canal, 

 where they not merely increase in amount, but also increase in 

 virulence. Moreover, according to NicoU's work, it is probable that 

 larvae may become infected, and this infection pass to the adult 

 insect. The mouth-part of the fly consists of a proboscis (vide 

 Fig. 469) with oral lobes provided with tubules, so that solid 

 particles and liquids pass up readily. It is a matter of everyday 

 observation that the house-fly apparently tastes everything on 

 which it settles— that is to say, it extends its proboscis and 

 presses on the substance on which it is standing by means of 

 its oral lobes. When this proceeding is carefully examined, it is 

 seen that the fly is regurgitating a little fluid from its crop via the 

 pseudo-tracheae of the oral lobes on to the solid substance, with the 

 view of attempting to dissolve some of the solid substance so that 

 it may be able to pass readily along the minute pseudo-tracheae. 

 This simple act is of the greatest importance pathologically, because 

 the house-fly is an exceedingly foul feeder, and may absorb the liquid 

 from the faeces of a case of enteric fever or from those of a carrier. 



In this way the bacilli enter its crop, only to be extruded on to 

 sugar, bread, milk, meat, the rim of a cup, etc., and in this way to 

 gain entrance to the human victim. Naturally, the presence of 

 carriers increases greatly the opportunity for the infection of flies. 

 But it is not merely by the mouth that the fly can infect food 

 materials and utensils, but also by means of its faeces; for the fly 

 defaecates very frequently, and the typhoid bacilli can pass from 

 its alimentary canal in an enhanced condition of virulence — accord- 

 ing to some authors — on to food, drink, or utensils, and so reach the 

 human host. 



Intestinal carriers are not the sole carriers of the enteric germs, 

 for, though more rarely, the bacilli can escape in the urine of people 

 who have recovered from enteric; these people are called urinary 

 carriers. It is not uncommon to find bacilli in both the faeces and 

 the urine in the case of transitory carriers, but it is much more 

 uncommon to find them in the faeces and the urine in the case of 



