Characterization of a new isolate of Ehrlichia platys (Order Rickettsiales) using electron microscopy and polymerase chain reaction.

A mixed-breed pup approximately 3 months old obtained in north central Oklahoma by the Laboratory Animal Resources Unit of Oklahoma State University presented with platelet inclusions. The dog developed severe thrombocytopenia (< 10,000 microliters-1) following the appearance of inclusions. Blood films were monitored daily and when about 75% of platelets had inclusions, samples were collected in EDTA and processed for electron microscopic (EM) studies and polymerase chain reaction (PCR). EM studies on glutaraldehyde-fixed buffy coat revealed rickettsia-like inclusions in numerous platelets. Serologic examination, using Ehrlichia platys antigen, showed high titre suggestive of E. platys infection. PCR primers derived from a highly variable region of the 16S rRNA gene sequence of E. platys were used to specifically amplify that region of the parasite's DNA. Sequencing of the PCR product obtained by general Ehrlichia primers showed one nucleotide difference from the published sequence for E. platys which suggests possible strain variation of this intracellular parasite. Our results indicate that PCR may be a useful tool in the diagnosis of E. platys infection and that, like other Ehrlichia spp., E. platys isolates may vary.