722 SALMONELLA 



situation. As in animals and birds, a proportion of persons who become infected 

 may harbour the organism for a varying length of time without showing any signs 

 of disease. Though certain tyj)es of Salmonella are far commoner in man than 

 others, it is now clear that practically every species so far described is capable 

 of infecting human beings. Even Salm. gallinarum, which has been regarded as 

 restricted to fowls, has been isolated from a human case of disease. It is impossible 

 here to describe in detail the natural pathogenicity of members of the Salmonella 

 group for man and animals, but some information in this respect will be found 

 in the descriptions of the individual species on pp. 726-44. 



Experimental observations on the pathogenicity of Salmonella for laboratory 

 animals have been confined to a relatively few species, such as Salm. typhi, Salm. 

 typhi-murium and Salm. enteritidis. It will be sufficient in this chapter to give 

 a brief description of the effects produced in experimental animals by some of the 

 better known species, and a short discussion of the nature of the toxic substances 

 that these organisms produce. 



Salm. typhi. 



If massive doses of living typhoid bacilli are administered by the mouth to chimpanzees, 

 it is possible to produce a disease that is very similar to typhoid fever in man (Metchnikoflf 

 and Besredka 1911). 



Administration of typhoid bacilli by the mouth to ordinary laboratory animals (rabbit, 

 guinea-pig, rat or mouse) does not give rise to an infection of this type, or usually to any 

 harmful result at aU. 



The intraperitoneal or intravenous injection of living typhoid bacilU in adequate doses 

 induces a fatal infection, and the baciUi can be recovered from the blood and tissues post 

 mortem. The effect of such injections would seem to be in part toxsemic, in part dependent 

 on the multipUcation of the bacteria in the body. With some strains of typhoid bacilli 

 it may be necessary to inject 1,000 milHon hving baciUi or more into the peritoneum of a 

 mouse to produce a fatal result. A low multiple of this dose (5,000-10,000 miUion bacilh) 

 will usually cause a purely toxaemic death when the bacilh have been kQled by heat before 

 inoculation. A highly virulent strain will induce a fatal infection following the intra- 

 peritoneal injection of 50 million baciUi, or even a httle less. But there is no evidence 

 that the typhoid bacillus possesses an abihty to multiply freely in the blood or tissues of 

 small laboratory rodents when injected in small doses, such as would certainly prove fatal 

 in the case of a frankly invasive organism. 



There is nothing characteristic in the findings at necropsy in a mouse, or guinea-pig, 

 that has died as the result of an intraperitoneal injection of fiviug typhoid baciUi. There 

 is, of course, the usual inflammatory reaction in the peritoneum, sometimes associated 

 with subserous haemorrhages ; and the organism which has become generahzed throughout 

 the body may be recovered from the blood or from any of the tissues. FoUowing the 

 intravenous injection into the rabbit of doses of living typhoid baciUi too small to produce 

 a rapidly fatal infection, the baciUi tend to locafize themselves in certain situations, particu- 

 larly in the gall-bladder. But experiments of this type wiU be more conveniently considered 

 in Chapter 69, in relation to the pathogenesis of enteric infections in man. 



Salm. typhi-murium. 



The effects produced by the administration of hving cultures of this organism to the 

 small rodents of the laboratory are entirely different from those produced by the typhoid 

 baciUus. We are dealing with a natural pathogen of these animals, which gives rise in 

 them to a characteristic disease, usuaUy known as mouse typhoid. This disease is pro- 

 duced when hving cultures of Salm. typhi-murium are given by the mouth as weU as when 

 they are administered by subcutaneous or intraperitoneal injection, though the time to 

 death is longer in the former case than in the latter. The organism has a very definite 



