A definitive diagnosis may be difficult to achieve in some cases. It is sometimes necessary to treat a horse for PHF based on a veterinarian's suspicion of the animal's clinical signs and history of living in or traveling to endemic areas. Bloodwork often will show a monocytosis, or a rise in a type of white blood cell called monocytes, and a lymphopenia, or decreased numbers of a type of white blood cells called lymphocytes.
A specific diagnosis is made by drawing blood samples and submitting them to a diagnostic laboratory to run serologic tests such as an indirect fluorescent antibody test or an enzyme-linked immunosorbent assay, abbreviated as ELISA, to determine if the horse's immune system has made antibodies against the organism. These tests are not always conclusive, however, since many horses living in commonly affected regions may have high titers, or levels, of antibodies without being infected. Also, vaccinated horses can become infected and not have a high titer or any titer at all.
Rarely, the organism can be seen in monocytes.
