Experimental data were presented which indicated that chloromycetin has a rickettsiostatic effect in embryonated eggs infected with R. tsutsugamushi, R. mooseri, R. rickettsi, R. akari, R. prowazeki and R. burneti. Chemotherapeutic tests in mice or guinea pigs infected with any one of the first four agents listed also gave evidence of a beneficial effect of the drug, although the effect on guinea pigs infected with R. rickettsi was much less striking. Treatment was of value in both infected eggs and animals when therapy was delayed for appreciable periods of time after infection.

Chloromycetin had only a suppressive effect on the growth of R. tsutsugamushi in mice since the infectious agent could be recovered from tissues of apparently healthy mice which were treated for 100 days after infection. Preliminary observations suggested that the drug may under certain circumstances sterilize the tissues or cause complete suppression of growth of organisms in mice inoculated with R. akari or guinea pigs injected with R. rickettsi. Chloromycetin had no direct rickettsiocidal action on R. tsutsugamushi in vitro. Furthermore, mice under treatment could not be shown to differ consistently from normal animals in their response to rickettsial toxins.

Values for the blood and urine levels of chloromycetin encountered in mice given the antibiotic followed the general pattern observed in man but guinea pigs showed low levels after corresponding doses.

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