الفهرس 
introduction
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Abstract
Ten C.ovis isolates which had been isolated from different localities ,were cultivated on nutrient broth containing C.V.at a final concentration of
1:750,000 and incubated at 37C for 24,48,72,96 and
120 hours respec— tively. it was found that all strains had grown after 24 and 48 hours on blood agar, the resultant organism lost some of its characters such as haemolysis on blood agar, colonies become small in size inucoid. in consistency, gram positivity slightly changed and, the pathogenic action o guinea pigs
disappeared.
After 72 hours incubation at ‘7°C on C.V. broth strain No.2 did not grow, while after 96 hours and. 120 hr strain No.1 and 6 did not grow on blood agar plates while other strains grown and had the same characters of 24 hr and 48 hr cultures. Strain No.4 Sids strain was selected and. subjected tolO time passage on C.. broth it was found thatthis strain still viable and had the new characters, it was taken as an attenuated C.ovis strain for testing its power as a homologous attenuated vaccine in protection 0± guinea pigs against infection with virulent C.ovis.