Biological properties of the M-44 strain of Coxiella burneti.

The attenuated M-44 strain of Coxiella burneti was investigated for biological characteristics affecting its use as a live vaccine. After 25 serial passages in guinea pigs, the strain did not develop phase I properties detectable by the microagglutination test. Infectious organisms were not detected in untreated guinea pigs later than 12 weeks after inoculation; however, infection could be reactivated at later times by the production of stress (by pregnancy or drug treatment) in guinea pigs that had recovered. Evidence of reactivated infection was observed in recovered pregnant animals just before parturition at 46 weeks after infection, in methyl prednisolone-treated animals at 56 weeks, and in cyclophosphamide-treated guinea pigs at 96 weeks. A low rate of intercage and intracage infection was recorded.