VARIATION OF SALMONELLA PULLORUM 41 



For convenience in the exposition of the results obtained in this phase of the 

 investigation, the virulence studies concerning the natural host are presented 

 first. 



Pathogenicity for the Natural Host 



A total of 312 day-old chicks and 19 different strains of 6 1 . pullorum (including 

 variants and their parent strains) were employed. The various strains were 

 studied in small lots of chicks. Comparative virulence tests were made with the 

 parent strain and its variant in every lot of chicks except one. The chicks were 

 maintained under the best possible sanitary conditions and were given ample 

 food and water. Necessary heat was provided by improvised means. Every 

 possible precaution was taken to eliminate cross-infection, and proved effective, 

 as will be seen later. The procedure of infecting the chicks was in general the same 

 throughout this investigation. Previous experiments made it seem advisable to 

 select an inoculating dose that would produce positive results with the parent 

 strains. Hence, a dose of 0.4 cc. of a saline suspension of the organism, with a 

 turbidity equal to tube 2 of McFarland's nephelometer scale, was inoculated 

 subcutaneously in the region of the loose skin between the thigh and abdomen. 

 The suspended organisms were prepared from 24-hour agar cultures. Under 

 natural conditions it has been observed that the onset and course of a disease 

 outbreak may be greatly influenced if the resistance of the host is lowered by over- 

 heating, chilling, or improper feeding. In order to further insure the establish- 

 ment of infection, the different lots of chicks were chilled immediately after 

 inoculation. The chilling temperature and the time of exposure were not the same 

 for all groups of chicks. However, the chicks within the same lot were subjected 

 to the same treatment, unless otherwise stated. Clinical manifestations were 

 recorded and all dead chicks were necropsied. S. pullorum cultures were identified 

 by the cultural, tinctorial, and biochemical properties. Two weeks after the 

 inoculation the surviving chicks were destroyed and necropsied. The results 

 obtained are presented in Table 13. 



Brief supplementary remarks may be made for each lot of chicks as follows:— 



Lot I. The chicks in this group were not chilled. No clinical manifestations 

 were observed. A similarity in the degree of virulence between Variant C and its 

 parent Strain I may in part be accounted for by Strain I having been subjected to 

 various treatments since the time of isolation, and having at times exhibited 

 characteristics remote from the typical smooth type. 



Lot II. Ten chicks, five of which were chilled, were inoculated with parent 

 Strain IX. Symptoms were observed in both the chilled and unchilled chicks. 

 Extensive gross lesions were observed in most cases, and a marked local tissue 

 response was seen at the site of inoculation. The latter was absent in the chicks 

 inoculated with variant IX-a. 



Lot III. The same parent strain as in Lot II, and its variant IX-b, which 

 was maintained on liver infusion agar, were employed. Also, a strain (Stellar) 

 which was freshly isolated from naturally infected chicks was included. During 

 the chilling process two chicks were lost from each of the groups inoculated 

 with Strains IX and IX-b. No clinical response was seen in chicks inoculated 

 with the variant, while in the other two groups marked reactions were observed in 

 some chicks. The lesions in the latter two groups were more extensive than in 

 the group inoculated with the variant. The organisms recovered resembled the 

 original inoculated strains. The variant grew profusely and the colonies attained 

 a diameter of one centimeter. 



