Abstract
In an effort to find a potential alternative treatment for scrub typhus, we evaluated the effectiveness of the standard drug doxycycline and the new macrolide azithromycin against a doxycycline-susceptible strain (Karp) and a doxycycline-resistant strain (AFSC-4) of Rickettsia tsutsugamushi. The antibiotics were tested in an in vitro assay system in which infected mouse fibroblast cells (L929) were incubated for 3 days in various concentrations of the drugs. Rickettsial growth was evaluated 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 susceptibility (16 micrograms/ml for doxycycline and 8 micrograms/ml for azithromycin). Growth of both Karp and AFSC-4 was strongly inhibited with both antibiotics, as measured by visual counts, although the percentage of cells infected with AFSC-4 in the presence of doxycycline was three times greater 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 of azithromycin. Flow cytometry confirmed that rickettsial growth occurred in the absence of antibiotics, but it failed to detect it in the presence of high concentrations of either drug. Visual counts of rickettsial growth at lower concentrations of the antibiotics (0.25 to 0.0078 microgram/ml) showed that the Karp strain was 16 times more susceptible that the AFSC-4 strain to doxycycline. Azithromycin was much more effective than doxycycline against AFSC-4, inhibiting rickettsial growth at 0.0156 microgram/ml to levels below that achieved by 0.25 microgram of doxycycline per ml. Azithromycin was also more effective than doxycycline against the Karp strain, causing greater reductions in the number of rickettsiae per cell at lower concentrations. If in vivo testing confirms the 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 refractory disease from locations where doxycycline-resistant strains of R. tsutsugamushi have been found. When tested in an in vitro assay system, azithromycin was more effective than doxycycline against doxycycline-susceptible and -resistant strains of R. tsutsugamushi.
Full Text
The Full Text of this article is available as a PDF (262.5 KB).
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Barker L. F. Determination of antibiotic susceptibility of Rickettsiae and Chlamydiae in BS-C-1 cell cultures. Antimicrob Agents Chemother (Bethesda) 1968;8:425–428. [PubMed] [Google Scholar]
- Berman S. J., Irving G. S., Kundin W. D., Gunning J. J., Watten R. H. Epidemiology of the acute fevers of unknown origin in South Vietnam: effect of laboratory support upon clinical diagnosis. Am J Trop Med Hyg. 1973 Nov;22(6):796–801. doi: 10.4269/ajtmh.1973.22.796. [DOI] [PubMed] [Google Scholar]
- Kawamura A., Jr, Murata M., Osono M., Nogami S., Shirasaka A., Tanaka H., Sudo K., Suzuki K., Miyairi T., Kijima H. Studies on inapparent infection of tsutsugamuschi disease in Izu Shichito Islands: seroepidemiology and demonstration of an avirulent Rickettsia strain for mice. Jpn J Exp Med. 1980 Apr;50(2):91–105. [PubMed] [Google Scholar]
- McClain J. B., Joshi B., Rice R. Chloramphenicol, gentamicin, and ciprofloxacin against murine scrub typhus. Antimicrob Agents Chemother. 1988 Feb;32(2):285–286. doi: 10.1128/aac.32.2.285. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Oaks S. C., Jr, Osterman J. V., Hetrick F. M. Plaque assay and cloning of scrub typhus rickettsiae in irradiated L-929 cells. J Clin Microbiol. 1977 Jul;6(1):76–80. doi: 10.1128/jcm.6.1.76-80.1977. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Pecorari D., Bellone F. Antibiotici in gravidanza. G Clin Med. 1990 Mar;71(3):163–170. [PubMed] [Google Scholar]
- Raoult D. Antibiotic susceptibility of rickettsia and treatment of rickettsioses. Eur J Epidemiol. 1989 Dec;5(4):432–435. doi: 10.1007/BF00140135. [DOI] [PubMed] [Google Scholar]
- Raoult D., Drancourt M. Antimicrobial therapy of rickettsial diseases. Antimicrob Agents Chemother. 1991 Dec;35(12):2457–2462. doi: 10.1128/aac.35.12.2457. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Schentag J. J., Ballow C. H. Tissue-directed pharmacokinetics. Am J Med. 1991 Sep 12;91(3A):5S–11S. doi: 10.1016/0002-9343(91)90394-d. [DOI] [PubMed] [Google Scholar]
- Smadel J. E., Jackson E. B. Chloromycetin, an Antibiotic With Chemotherapeutic Activity in Experimental Rickettsial and Viral Infections. Science. 1947 Oct 31;106(2757):418–419. doi: 10.1126/science.106.2757.418. [DOI] [PubMed] [Google Scholar]
- Smadel J. E., Woodward T. E., Ley H. L., Jr, Philip C. B., Traub R., Lewthwaite B., Savoor S. R. Chloromycetin in the Treatment of Scrub Typhus. Science. 1948 Aug 13;108(2798):160–161. doi: 10.1126/science.108.2798.160. [DOI] [PubMed] [Google Scholar]
- Strickman D., Tanskul P., Eamsila C., Kelly D. J. Prevalence of antibodies to rickettsiae in the human population of suburban Bangkok. Am J Trop Med Hyg. 1994 Aug;51(2):149–153. doi: 10.4269/ajtmh.1994.51.149. [DOI] [PubMed] [Google Scholar]
- Traub R., Wisseman C. L., Jr The ecology of chigger-borne rickettsiosis (scrub typhus). J Med Entomol. 1974 Jul 15;11(3):237–303. doi: 10.1093/jmedent/11.3.237. [DOI] [PubMed] [Google Scholar]
- Weiss E., Coolbaugh J. C., Williams J. C. Separation of viable Rickettsia typhi from yolk sac and L cell host components by renografin density gradient centrifugation. Appl Microbiol. 1975 Sep;30(3):456–463. doi: 10.1128/am.30.3.456-463.1975. [DOI] [PMC free article] [PubMed] [Google Scholar]