Ehrlichia equi infection of horses from Minnesota and Wisconsin: detectionof seroconversion and acute disease investigation

Equine granulocytic ehrlichiosis (EGE) is caused by infection with Ehrlichi a equi. EGE has been reported primarily in northern California, where E equ i is transmitted by the tick Ixodes pacificus. Reports of EGE and the emerg ence of human granulocytic ehrlichia in Minnesota prompted a seroprevalence study of E equi in horses of Minnesota and Wisconsin. Tick (Ixodes scapula ris) endemic areas of Minnesota and Wisconsin were compared to nonendemic r egions of Minnesota. Indirect fluorescent antibody was used to detect the p resence of serum antibodies to E equi. Serum samples from healthy horses, 3 75 samples from I scapularis endemic counties, and 366 samples from nonende mic counties were screened at a 1:40 dilution. Results demonstrated a serop revalence of 17.6% in endemic areas versus 3.8% in nonendemic areas. Ehrlic hial DNA from 2 samples was successfully amplified by polymerase chain reac tion and 919 base pairs were sequenced. The DNA sequence of 1 Minnesota/Wis consin strain differed from the GenBank strain (M73223) of E equi at positi ons 84 and 886 and from the MRK strain of E equi at position 84, and was id entical to the human granulocytic ehrlichiosis (HGE) agent. The 2nd Minneso ta/Wisconsin strain was identical to the Ist with the exception of a substi tution of "A" at position 453 that is not present in E phagocytophila, E eq ui, or HGE agent strain sequences. Based on the results of this study, we c oncluded that E equi is present and causes infection in horses in Minnesota and Wisconsin. The occurrence of infection is higher in lick endemic regio ns.