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Dosages of doxycycline were equilibrated based on previously determined peak plasma levels in mice [oral, 2.4 µg (ml plasma)–1; sustained release, 1.9 µg (ml plasma)–1] determined 8 hours after inoculation.

In challenge experiments where five Borrelia burgdorferi-infected and five A. phagocytophilum-infected nymphs were used per mouse, only 20% and 3% of mice were protected from B. burgdorferi and A. phagocytophilum infection, respectively, using oral doxycycline.

In contrast, 100% of mice receiving sustained-release doxycycline were protected from A. phagocytophilum infection, as indicated by real-time PCR of blood samples, quantitative PCR and culture isolation of spleen samples, and protected against B. burgdorferi infection as demonstrated by culture of ear, heart and bladder. Although 15–40 copies of A. phagocytophilum could be amplified from the spleens of mice treated with sustained-release doxycycline, no viable A. phagocytophilum from these spleens could be cultured in HL-60 cells.

In contrast, 7/10 mice receiving oral doxycycline were PCR- and culture-positive for A. phagocytophilum, with copy numbers ranging from 800 to 10 000 within the spleen, as determined by quantitative PCR. Other correlates with A. phagocytophilum infection included a significant difference in spleen mass (mean of 110 mg for sustained-release treatment versus a mean of 230 mg for oral treatment) and the number of splenic lymphoid nodules (mean of 8 for sustained-release treatment versus mean of 12.5 for oral doxycycline) as determined by histopathology.

These studies indicate that a single injection of a sustained-release formulation antibiotic may offer a viable prophylactic treatment option for multiple infectious agents in patients presenting with tick bites.

Source: Journal of Medical Microbiology, April 2008. 57 (2008), 463-468; PMID: 18349366 by Zeidner NS, Massung RF, Dolan MC, Dadey E, Gabitzsch E, Dietrich G, Levin ML. Division of Vector-Borne Infectious Diseases, CDC, Fort Collins, Colorado; Division of Viral and Rickettsial Diseases, CDC, Atlanta, Georgia; QLT Laboratories, Fort Collins, Colorado, USA. [E-mail: Naz2@cdc.gov]