Most canine ehrlichiosis cases in the U.S. are attributed to infection with Ehrlichia canis; however, several other Ehrlichia and Anaplasma species are present throughout the country. This study was conducted to determine the predominant species in dogs in the south central states. Researchers tested 143 dogs in northeast Oklahoma and northwest Arkansas for the presence of 5 Ehrlichia and Anaplasma species using both enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) analysis. ELISA analysis conducted with species-specific antibodies found that 44.8% of dogs tested positive for E ewingii, 17.5% for E chaffeensis, and only 1.4% for E canis. ELISA analysis (using an Anaplasma species genus-specific antibody) revealed that 5.6% of dogs had been exposed to Anaplasma species. PCR analysis identified lower numbers of infected dogs in the same species ratios. These data suggested that E ewingii is the primary species infecting dogs in the south central U.S.

Commentary: The SNAP 4Dx Test (idexx.com) is designed using E canis antigens and does not recognize E ewingii antibodies. Therefore, false-negative results may be common in this region. Microscopic examination of blood samples should be conducted in suspected ehrlichiosis cases. This finding also presents a public health concern, as both E ewingii and E chaffeensis are also infectious to humans. Clients living in the south central states should be advised to check themselves and their pets for ticks after spending time outdoors, particularly during the summer months.—Carly Jordan, PhD candidate

Ehrlichia ewingii infection and exposure rates in dogs from the south central United States. Little SE, O’Connor TP, Hempstead J, et al. VET PARASITOL 172:355-360, 2010.