Comparison of virulence for guinea pigs and mice of different Coxiella burnetii phase I strains.

Virulence of four Coxiella burnettii phase I strains associated with acute (Nine Mile and Luga strains) and chronic (S and Priscilla strains) forms of Q fever, respectively, for outbred guinea pigs and inbred BALB/c mice was compared. In guinea pigs exposed to infectious aerosol, virulence of the strains Nine Mile, Luga and S was similar, because they all caused lethal infection and high febrile reaction (FR). By contrast, exposure to similar doses of the strain Priscilla resulted in the non-lethal infection manifested only by low, dispersed and protracted FR during a 20 day-period of observation. Lower virulence of the strain Priscilla than of the strains Nine Mile and S was observed also in guinea pigs infected intraperitoneally (ip). In mice, comparable degree of multiplication in the spleen on day 6 post ip infection was observed with the strains Nine Mile, Luga and S, but not with the strain Priscilla, namely when lower infectious doses were used. Whereas the highest doses (10(7) EID50) of the strains Nine Mile and S were lethal for mice during the second week post infection (p.i.), the strains Luga and Priscilla did not kill mice with 10-times lower (10(6) EID50) doses. At the same time agglutination antibody response detected 4 weeks p.i. was very similar in guinea pigs with all strains under study and in mice it did not differ markedly, reflecting the infectious dose used.