EXPERIMENT STATION BULLETINS. 637 



that many strains of Bad. abortus possess very low invasive properties. It 

 is possible that those strains may have invaded the tissues shortly after 

 inoculation, and could have been isolated therefrom if cultured at that time. 



It was noted in this study that many guinea pigs showed a marked variable 

 susceptibility toward Bad. abortus. For example, occasionally one pig of a 

 duphcate would show an acute diarrhea and become extremely emaciated in 

 less than five days after inoculation, and succumb shortly after the appearance 

 of symptoms. The anatomical changes noted in such cases were a fibrinous 

 peritonitis or a pleuro-pneumonia. Occasionally inoculated pigs show paraly- 

 sis of the hind parts or enlarged joints. 



The apparent failure of Strains 16 and C (abortus) to produce lesions in 

 guinea pigs in each series of inoculations may be explained from the fact that 

 individual pigs are refractory to Bad. abortus infections. The variable suscept- 

 ibility of guinea pigs to infection renders one inoculation test inconclusive 

 in attempting to classify virulent and non-virulent strains of Bad. abortus 

 by this procedure. 



It is clearly shown in Table III that there was a decrease in the weight of 

 pigs inoculated with strains of Bad. abortus which had not lost their lesion- 

 producing property. There were, however, exceptions to this rule which are 

 difficult to explain. In not a single case was there a decrease in the weight of 

 pigs which showed the absence of lesions. The failure of pigs to lose weight 

 cannot always be accepted as a criterion of non-invasion by Bad. abortus. 



While these studies have not lead to a definite grouping of strains of Bad. 

 abortus as regards lesion production, they have, however, demonstrated con- 

 clusively that there exist many strains which will not produce characteristic 

 lesions in the organs of guinea pigs on inoculation. The lack of this property 

 is undoubtedly not inherent, but results from long cultivation upon an 

 artificial medium. The gradual loss of the lesion-producing property is 

 demonstrated in these studies. The loss of this property may be transitory 

 and it may be restored when the organism is placed in a more natural medium. 

 This point, however, should be given further study before it is fully discussed. 



The foregoing results certainly demonstrate the fallacy of undertaking 

 infection experiments with cultures of Bad. abortus without a knowledge of 

 the present virulence of the strains employed. 



SUMMARY AND CONCLUSIONS 



1. These studies show that a large number of strains of Bad. abortus, 

 after several generations of cultivation on an artificial medium, lose the 

 property of producing lesions in the organs of guinea pigs on inoculation. A 

 majority of the strains isolated became non-virulent within one year after 

 isolation. 



2. The cultural findings agreed with the presence or absence of lesions, 

 that is, when characteristic lesions were found in the organs of the guinea pig 

 Bad. abortus was also present, Bad. abortus was not found in the absence of 

 lesions. 



3. The agglutination and complement fixation antibody indices of the 

 blood sera were low in pigs showing no lesions and high in pigs showing 

 lesions. 



4. There was a marked decrease in weight of the majority of the pigs 

 showing lesions and an increase in weight in those showing no lesions. 



5. The results of this study indicate that there is in existence a large 

 number of non- virulent strains of Bad. abortus. 



