Expanded testing of acute and convalescent samples in this study provided evidence of infection with or exposure to vector borne disease in 24% of southern California dogs who presented for evaluation of clinical signs consistent with immune mediated or vector borne disease. These findings are important as anecdotally, vector borne disease is considered to be rare by clinicians practicing in the region and studies exploring the prevalence of vector borne disease in dogs in this region of the United States are limited. Detection of Ehrlichia and Babesia species DNA increased when PCR was performed more than once and/or on more than one sample. That these organisms can circulate in low numbers and intermittently may explain this finding. Seroconversion did not occur in the Babesia infected dogs, a phenomenon that has been described previously in immunosuppressed patients. Clinicians should consider repeated testing for vector-borne agents in patients with suspected idiopathic immune mediated disease. Infection with a species of Ehrlichia closely resembling E. ewingii is suspected but could not be confirmed in these three patients. This study demonstrates that vector borne disease is relatively common in Southern California dogs presenting with compatible clinical signs. It also demonstrates for veterinarians everywhere that it is important to consider comprehensive testing using both serology and PCR and to consider testing more than once using PCR when Babesia or Ehrlichia infection is suspected in canine patients with clinical signs of immune mediated disease. The results of this study have been submitted for presentation as an abstract at the annual American College of Veterinary Internal Medicine meeting in July 2014.