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Acute phase proteins (APPs) can be very helpful in detection of disease. These proteins will increase in serum level very quickly in the face of infection or inflammation. That’s why they are typically great markers for disease processes as well as response to therapy. However, they do not work for every disorder.
A recent study looked at two acute phase proteins – Serum Amyloid A (SAA) and C-Reactive Protein (CRP) – and how they were associated with Equine Protozoal Myeloencephalitis. (We often use SAA on the farm to determine if an infectious process is present and to help us direct treatment.) One of the issues we have with EPM is that many horses will have a positive blood test, however they do not have clinical disease. By offering other tests that can help us determine infection vs non-infection, we can help narrow down a diagnosis.

The researchers looked at 25 horses with post-mortem diagnoses. 10 of these cases had EPM confirmed, the others were a variety of neurologic diagnoses such as cervical stenotic myelopathy, Lyme neuroborreliosis and 3 other disorders. Both CSF and blood were tested for CRP and SAA.
What they found was surprising. All EPM cases had normal SAA levels and only one had an elevated CRP. CSF levels were low in all cases and the levels of both were not any different from those horses with non-infectious causes of neurologic disease.
It does not appear that using SAA or CRP as an aid to EPM diagnosis is useful.