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Equine Protozoal Myeloencephalitis (view source)
Revision as of 13:18, 15 July 2010
, 13:18, 15 July 2010no edit summary
Postmortem diagnosis is confirmed by demonstration of protozoa in CNS lesions. An immunoblot (Western blot) test for S neurona is used as an aid to antemortem diagnosis. In horses with neurologic signs, demonstration of specific antibody in CSF (by immunoblot) is highly suggestive of EPM. A positive immunoblot test in serum only indicates exposure to S neurona . Conversely, a negative immunoblot result, in either serum or CSF, tends to exclude the diagnosis of EPM. In a few horses with EPM, CSF analysis reveals abnormalities such as mononuclear pleocytosis and increased protein concentration. (merck)
Postmortem diagnosis is confirmed by demonstration of protozoa in CNS lesions. An immunoblot (Western blot) test for S neurona is used as an aid to antemortem diagnosis. In horses with neurologic signs, demonstration of specific antibody in CSF (by immunoblot) is highly suggestive of EPM. A positive immunoblot test in serum only indicates exposure to S neurona . Conversely, a negative immunoblot result, in either serum or CSF, tends to exclude the diagnosis of EPM. In a few horses with EPM, CSF analysis reveals abnormalities such as mononuclear pleocytosis and increased protein concentration. (merck)
Depending on the clinical signs, differential diagnoses may include cervical stenotic myelopathy, trauma, aberrant metazoan parasite migration, equine degenerative myeloencephalopathy, myeloencephalopathy caused by equine herpesvirus 1, equine motor neuron disease, neuritis of the cauda equina, arboviral (Eastern or Western equine, West Nile) encephalomyelitis, rabies, bacterial meningitis, and leukoencephalomalacia. (Merck)
Depending on the clinical signs, differential diagnoses may include cervical stenotic myelopathy, trauma, aberrant metazoan parasite migration, equine degenerative myeloencephalopathy, myeloencephalopathy caused by equine herpesvirus 1, equine motor neuron disease, neuritis of the cauda equina, arboviral (Eastern or Western equine, West Nile) encephalomyelitis, rabies, bacterial meningitis, and leukoencephalomalacia. (Merck)
Equine protozoal myeloencephalitis (EPM) is a commonly
diagnosed, and sometimes devastating neurological disease of
horses in North America. The most common cause is
Sarcocystis neurona, although other protozoal species, such as
Neospora hughesi, have also been identified in the spinal cord
of horses (Dubey et al. 2001). Ante mortem diagnosis is
considered presumptive, as definitive diagnosis requires post
mortem examination and confirmation of S. neurona infection
via microscopic identification, immunohistochemistry, culture,
or polymerase chain reaction (PCR) (Furr et al. 2002). Several
serological tests have been developed to aid in the
presumptive diagnosis of EPM, but test interpretation is
complicated; many horses develop antibodies against S.
neurona in the absence of neurological disease. The oldest
and most well-established test is the Western blot (WB)
(Granstrom et al. 1993). An indirect fluorescent antibody test
(IFAT) later became available (Duarte et al. 2003), and an
enzyme-linked immunosorbent assay test for an S. neurona
surface antigen (SAG-1 ELISA; Ellison et al. 2003) has recently
become commercially available. Evidence to support the use of
these tests will be reviewed.Specialists appear to agree that the basis of EPM diagnosis
should be the presence of compatible neurological signs and
the exclusion of other potential diseases (Furr et al. 2002).
When additional supportive evidence is desired, 3 main types
of testing are commercially available: the WB1,2,3, the IFAT4
and the SAG-1 ELISA5. The SAG-1 ELISA as designed by Ellison
et al. has the least amount of evidence to support its use and
168 Commercial tests to diagnose Sarcocystis neurona infection
an accurate assessment of its sensitivity and specificity has not
appeared in peer-reviewed literature. Although another group
(Hoane et al. 2005) described the SAG-1 ELISA as having the
lowest sensitivity and specificity out of all the SAG ELISAs,
those values were obtained using Hoane’s group’s tests and
not Antech’s SAG-1 ELISA. However, those results do suggest
that critical review of the SAG-1 ELISA is necessary before
recommending its use.
Both the WB and the IFAT appear to have similar
sensitivities, approaching 90%. Estimates of WB specificity
vary from 44–89%, while estimates of IFAT specificity range
from 97–100%. Results vary depending on the sample set and
whether serum or CSF is used. Therefore, available evidence
suggests that either test is appropriate if high sensitivity (and
therefore high negative predictive value) is desired. If high
specificity (and therefore high positive predictive value) is
desired, the IFAT may be the better choice. Furthermore, blood
contamination of CSF appears to have a more detrimental
effect on WB testing than on IFAT testing. However, infection
with S. fayeri may cause false-positive IFAT results without
causing false-positive WB results. The clinical importance of
this cross-reactivity has yet to be elucidated.
Unfortunately, none of the currently available diagnostic
tests are ideal. Development of new serological tests, such as
the SAG ELISAs (Hoane et al. 2005) and an IgM capture ELISA
(Murphy et al. 2006) may further improve practitioners’ ability
to diagnose EPM. However, veterinarians should remain critical
of new assays until their validity has been documented.(EPM 4)
====Differential Diagnoses====
====Differential Diagnoses====