Lack of susceptibility of Ehrlichia canis to imidocarb dipropionate in vitro.

In vitro antimicrobial susceptibility testing was used to compare the efficacy of imidocarb dipropionate and doxycycline on the growth of Ehrlichia canis in DH82 cell cultures. Over a 9-day period there were no significant differences (p < 0.01) in the growth of E. canis in untreated control wells and those to which imidocarb dipropionate was added at 1.2, 2.4, 4.8 or 12 micrograms/ml for the 1st 3 days. Average infection rates rose from 50 to 55% on day 0 to 100% on day 5 or 6. Doxycycline at 1 microgram/ml had residual or rickettsiocidal activity against E. canis with the average percentages of DH82 cells infected declining from 51 to 24% while the organism was exposed to the drug (3 days) and from 21 to 2% in the 6 days following removal of the drug from the cell culture medium.