These conflicting reports on the efficacy of tetracyclines
for chemosterilization suggest that differences in susceptibility
between isolates may exist. This hypothesis is
supported by the recent identification of two multidrugresistant
efflux pumps in the genome of A. marginale
(Brayton et al., 2005). However, the clinical significance
or activity of these pumps has yet to be examined. If
there are indeed differences in susceptibility between
strains, it would be necessary to know which A. marginale
strain(s) are present in the herd and the efficacy of CTC