Some asymptomatic dogs have persistent E. canis serologic titers for more than 9 months after treatment. In the majority of these cases, however, the PCR technique has been unable to detect ehrlichial DNA.

For these dogs with persistent titers, recommendations are to perform PCR: if positive, retreat the dog with another 2-3 week course of doxycycline. If negative, monitor the titer periodically and redo PCR if it fails to decline. Some of these dogs may maintain high titers because of re-exposure, but remain asymptomatic because they are immune.

About 30% of dogs exposed to Ehrlichia spp. have positive serologic titers to Bartonella spp. Consider Bartonella infection in dogs with persistent illness not responding completely to treatment for E. canis. In these cases, Bartonella may be spreading by the brown dog tick, Rhipicephalus sanguineus.

We thank Drs. William Hardy and Ed Breitschwerdt for contributing the latest information on these important topics.