IN-VITRO EFFECTIVENESS OF AZITHROMYCIN AGAINST DOXYCYCLINE-RESISTANT AND DOXYCYCLINE-SUSCEPTIBLE STRAINS OF RICKETTSIA-TSUTSUGAMUSHI, ETIOLOGIC AGENT OF SCRUB TYPHUS

In an effort to find a potential alternative treatment for scrub typhu s, we evaluated the effectiveness of the standard drug doxycycline and the new macrolide azithromycin against a doxycycline-susceptible stra in (Karp) and a doxycycline-resistant strain (AFSC-4) of Rickettsia ts utsugamushi, The antibiotics were tested in an in vitro assay system i n which infected mouse fibroblast cells (L929) were incubated for 3 da ys in various concentrations of the drugs. Rickettsial growth was eval uated by direct visual counts of rickettsiae in Giemsastained cells or by flow cytometry, Initial tests were conducted at the concentration of each antibiotic considered to be the upper breakpoint for susceptib ility (16 mu g/ml for doxycycline and 8 mu g/ml for azithromycin). Gro wth of both Karp and AFSC-4 was strongly inhibited with both antibioti cs, as measured by visual counts, although the percentage of cells inf ected with AFSC-3 in the presence of doxycycline was three times great er than the percentage of cells infected with Karp but was only 60% as great as the percentage of cells infected with Karp in the presence o f azithromycin, Flow cytometry confirmed that rickettsial growth occur red in the absence of antibiotics, but it failed to detect it in the p resence of high concentrations of either drug, Visual counts of ricket tsial growth at lower concentrations of the antibiotics (0.25 to 0.007 8 mu g/ml) showed that the Karp strain was 16 times more susceptible t hat the AFSC-4 strain to doxycycline. Azithromycin was much more effec tive than doxycycline against AFSC-4, inhibiting rickettsial growth at 0.0156 mu g/ml to levels below that achieved by 0.25 mu g of doxycycl ine per mi, Azithromycin was also more effective than doxycycline agai nst the Karp strain, causing greater reductions in the number of ricke ttsiae per cell at lower concentrations. If in vivo testing confirms t he in vitro effectiveness of azithromycin, it may prove to be the drug of choice for the treatment of scrub typhus in children and pregnant women, who should not take doxycycline, and in patients with refractor y disease from locations where doxycycline-resistant strains of R. tsu tsugamushi have been found, When tested in an in vitro assay system, a zithromycin was more effective than doxycycline against doxycycline-su sceptible and -resistant strains of R. tsutsugamushi.