Detection of antibodies to Neorickettsia risticii (formerly Ehrlichia risticii), the causative agent of Potomac horse fever, by indirect fluorescent antibody test. Test performed at 1:20, 1:40, 1:80, 1:160, 1:320, and 1:640 dilutions. Results available same day for samples received in the lab by 11:30 am. Results can be available same day with the addition of a STAT Fee for samples received in the lab after 11:30 and by 1:00 pm. Otherwise the samples will be held until the next scheduled testing day.