Page 1097 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1072 CHAPTER 10

VetBooks.ir the CNS, where the infection may localise in any Differential diagnosis
Given that EPM may mimic almost any other neu-
area, with the exception of the peripheral nerves.
As with all neurological disease, the locations of
extensive.
infection determine the clinical presentation and, rological disease, the list of diagnostic rule-outs is
given the potential multifocal CNS distribution of
the parasite, a wide range of clinical signs may be Diagnosis
evident. It appears that some types of immunosup- A definitive ante-mortem diagnosis cannot be
pression and stress (such as weaning) may alter the achieved by clinicopathological means and most
likelihood of invasion of the CNS and the devel- often a clinical diagnosis of EPM is reached based on
opment of neurological disease after oral ingestion an accumulation of data. Important considerations
of sporocysts. in reaching a diagnosis are relevant history and com-
patible clinical signs, clinical progression, laboratory
Clinical presentation and other diagnostic aids, exclusion of other possible
EPM is renowned for its diversity of clinical signs, causes, and response to treatment. Laboratory and
as a result of lesions of varying size and severity in other diagnostic aids have been divided into three
any part of the CNS. The onset of signs may be categories: (1) positive serology titres for S. neu-
insidious or acute. Spinal cord involvement occurs rona antibodies, which supports the diagnosis; (2)
in most cases, while only 5% of cases are reported negative titres, which tends to exclude the diagno-
to show evidence of brain disease. Other than neu- sis; and (3) tests that support alternative diagnoses
rological signs, there are no other syndromes asso- and thus indirectly exclude a diagnosis of EPM.
ciated with EPM. The disease course is also highly The main problem with CSF antibody detection is
variable, with progression over hours or years pos- that inadvertent contamination of the sample with
sible, or a waxing and waning of signs over extended serum antibodies is common and can produce false-
periods. The prevalence of different neurological positive results. Many researchers suggest analy-
signs varies to some extent between reports, but the sis of CSF and serum for antibodies using surface
trend, in decreasing order, is: spinal cord lesions antigen ELISAs and producing a ratio to determine
(causing ataxia, paresis, proprioceptive deficits and/ active infection. For detection of N. hughesi, there is
or spasticity, usually asymmetrical), muscle atro- an ELISA based on surface antigens and an indirect
phy, CN deficits (from brainstem lesions) and (least fluorescent antibody test using whole tachyzoites.
likely) central/cerebral signs (e.g. seizures). Rarely, Detection of S. neurona antigen by PCR is highly
EPM may be the cause of acute onset tetraparesis suggestive, although positive results are uncom-
and recumbency. Regions of hypalgesia, hyper- mon in affected animals because of the intracellu-
aesthesia and increased sweating can also be seen. lar nature of the organism. However, because of the
Horses may present with urinary incontinence, high sensitivity, a negative result is often considered
associated with damage to the sacral spinal cord more significant, making it highly unlikely that the
segments. Full ‘cauda equina syndrome’ signs have horse has the disease. The poor specificity of serum
also been reported, where there is incontinence and and CSF testing has forced many clinicians to use
paralysis of the bladder, rectum, anus and penis, other criteria, such as a response to treatment, to
and sensory loss to the tail and perineum. allow them to make a retrospective diagnosis. Post-
Where CN deficits are present, there is often mortem analysis may be required for a convincing
associated muscle atrophy (e.g. lingual or mastica- diagnosis (Fig. 10.25).
tory muscles [see Fig. 10.4]). These types of EPM
cases usually have an insidious onset and may appear Management
as a singular syndrome. They do not normally prog- The use of anticoccidial medication is recommended
ress beyond complete and permanent atrophy of the for the treatment of EPM. Traditional treatment
muscle affected. has centred around the use of combinations of