Diagnosis and Treatment
of Equine Protozoal
Myelitis (EPM)
Thomas J. Divers, DVM
It has been exactly 20 years since the
infectious agent of Equine Protozoal Myelitis
(EPM) Sarcocystis neurona was isolated.
Since that time much progress has been made
but many questions remain!
Objective: Review pertinent findings for
EPM over the past 30 years emphasizing
those with direct relevance to:
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DiagnosisTreatmentPrognosisPreventionTatiane Alves da Paixão et al.
Sarcocystis neurona
Life Cycle
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Etiology
Important for understanding
epidemiology, diagnosis, and
prevention of the disease
Definitive host: one
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S. neurona life cycle can be
completed only in opossums
of the genus Didelphis, which
are the definitive hosts Dubey
Intermediate host: Many
Didelphis virginiana (Virginia
Opossum) painting by Todd Zalewski from Kays
and Wilson's Mammals of North America, ©
Princeton University Press (2002)
Epidemiology and Risk
Factors for the Disease
(Helpful in Diagnosis!!)
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Opossum exposure!!
Racing and show animals higher risk
Breed?
Peak age of clinical disease: 4-6 years
(rarely <1 year)
Infection and clinical disease less
common in winter months
Adverse event in the preceeding 30-60
days
– Shipping -experimental model (Saville
OSU) and personal experience in
yearlings
Geographic distribution
of american opossum 2007
Clinical Signs - Important in Diagnosis!
Mayhew PhD
Cornell
(*Signs,
^Pathology)
OSU
Signs
72%
69%
Asymmetrical
66%
40%- atrophy
14%+
LMN signs
(atrophy)
43%
70%
C1-C6 signs
(ataxia)
T3-L4 signs
67%
gluteals and
shoulder
Texas A&M
77% grey
lesion ^
83% C1- C-6 ^
46% C6-T1
47% T3-L3
10% S3-Cocy
23% lesions
^(7,5 8,12,9,3)
70% lesions
in brain ^
11.6%
6%
21%
Cranial Nerve
nr
Seizure
nr
Diagnosis-EPM is an easy diagnosis to make but
often difficult to confirm ante-mortem!
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Epidemiology
Clinical signs
Ruling out other diseases!
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CCM
Serologic testing:
blood or CSF
Sagittal ratio =
Se and Sp > 89%
(Moore et a.l., AJVR, 94)
Serologic Testing
Is There a Best Test?
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Available Test
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Is there evidence-based
information not to use a
certain test?
* Yes
Is spinal fluid testing
better than blood?
* Yes!
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Western Blot
* IDEXX (Michigan State temporarily)
* Equine Diagnostic Solutions- same
individuals who previously ran EBI
* University of California – Davis
* Neogen
* MSU
SAG1/ELISA
* Antech
* Pathogenes
* Endocrine Technologies – rapid test
IFA*UC Davis
SAG 2 & SAG 4/3 ELISAs* EDS
Laboratory Testing /
WB
Blood
CSF
Sensitivity 8798%
Specificity very
low
Treatment
CSF
Specificity < 60%!
Weak+ or Strong+
* Depends
somewhat on
CSF RBC
contamination
Treatment
Treatment
Sensitivity > 87%
experience suggest
higher
Eliminating weak +
improves sp. but
lowers Se.
44% sp. for neurologic
horses
WB- Concerns
• Not quantitative- how to interpret weak
positive (on blood or CSF)
– Certainly the strong positives increase likelihood of EPM
but can not be ruled out if weak positive!!
• Several laboratories perform WB and
results are not always the same
• RBC contamination on CSF- (turn in anyway)
• A few cases are negative on CSF
and/or Serum
– IgG immunoconversion at 8 days experimental Infection
Indirect Fluoresent Antibody
Testing (IFA- UC Davis)
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Quantitative: there is some modest
relationship between serum Ab level and
likelihood of disease
Also assays for Neospora but this infection is
rare in the Northeast
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Seroprevalence in Kentucky study was 1%
1/15 CSF positive in last 2 years at Cornell
May act like Neospora in cow- maternal
transmission but mostly asyptomatic
Pusterla 2010
IFA testing- Concerns
•Is the probability of disease estimates
accurate ?
• Likely to cross react with experimental
Sarcocystis fayeri? Saville 2004
Actual importance unknown
SAG1 Assay
• Let me provide the data and you decide?
– SnSAG1 was not detected in 7 of 14 S.
neurona. * Howe
– Sensitivity was 17% in one study Johnson
– Sensitivity was 68% in another study- Hoane
– * SnSAG2, SnSAG3, and SnSAG4 are present in all 14 S.
neurona strains tested, although some variance in SnSAG4 was
observed
SAG 2 and SAG 4,3 Assays
• An ELISA assay detecting antibody against those
surface antigens which are most commonly expressed
by strains of S. neurona
• New ELISA SnSAG ELISA Test for EPM diagnosis
developed at the Gluck Center Aug. 1
• Advantages– Should detect antibody against most or all known
strains
– Quantitative and can be used to determine intrathecal
production of antibodies (simple serum:CSF ratio)
• Disadvantages- needs CSF and blood to be provide
information beyond good WB
When to Use Serum Test
(WB or IFA)
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After signs, age, rule outs are
considered and EPM
diagnosis seems reasonable
Areas of low prevalence
Poor performance horses with
signs of other body system
abnormalities, such as
lameness or airway problems
and, therefore, low clinical
probability of EPM
Summary of Testing
•Approx ½ of adult horses in parts of the US have antibody
Clinical case estimate 1.4/1000 horses
•All current testing and recently proposed testing are antibody
(IgG) test
•Therefore, specificity will always be a problem on blood since
there is not a strong correlation between titer and EPM clinical
disease
•Unless intrathecal production of immunoglubulin is proven,
testing will continue to be a problem
-Low concentrations of serum Ig often present in CSF.
(IgG > IgA > IgM) J Neurol Sci
Treatments
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Antiprotozoal therapy
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“Our ability to treat the disease has been as
good as our ability to diagnose the disease
for years”
“Now with potential improvements in
diagnostics we should seek a better
understanding on effective therapy”
Anti-inflammatory, anti-oxidant therapy
Immune modulators?
Most Commonly Used
Antiprotozoal Drugs
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Ponazuril ( FDA approved)
* Pyrimethamine (previously approved with
sulfa) but that product no longer available
* Diclazuril is FDA approved and will
apparently soon be available for clinical use
* Toltrazuril (not
approved)
What do we know about each drug ?
How might they best be used in treating EPM?
Marquis
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-Ponazuril, a metabolite of Toltrazuril
-Proven clinical efficacy in treating EPM
-70% of horses improved 1 grade or more
which is similar to other FDA approved drugs for EPM
-Minimal safety concerns even with higher doses!
-High inter-horse variability in absorption and low
percentage passage into CSF( 5%)
Administer oil – may increase bioavailability?
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Ponazuril
• Static^ at the label dose (5 mg/kg SID)
- CSF concentrations of ponazuril in horses were < 5%* of those found
in serum and at the approved dosage 0.16^ ug/kg was found on day 728. Furr
-In cell culture S. neurona is inhibited by ponazuril
at 0.1-1^* ug/ml
Lindsey
* after 10 days ponazuril exposure some
renewed multiplication occurred when ponazuril removed
*Blood to CSF ratio seems similar for Ponazuril, Diclazruril and Toltrazuril
*Bioavailability also appears similar
*In vitro sensitivity of S. neurona to diclazuril not well established (1ng/ml to
>100ng/ml)- Lindsey and Marsh had conflicting results
^ Pharmacokinetics, in-vitro sensitivity and moderate
relapse rate (9%) have concerned me for years and I
have used a higher loading dose for several years now
Why Use a Higher Dose?
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Due to long T1/2 of ponazuil and other triazine drugs, maximal serum and CSF
concentration may not be reached for at least 7 days (serum= 1ug/ml on day 1;
4-7 ug/ml on day 7) serum to CSF ratio 24-30:1 in normal horses
-therefore, CSFconcentration < 0.1 for the first 1-5 days
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Concentration in CSF is dose dependent- 0.19ug/ml at 5mg/kg and 0.39 at 7.5mg /kg
Furr
Merozoites inhibited *by 70- 95% @ 0.1 ug/ml, 90-98% at 1 ug/ml and 98% @
5ug/ml Lindsay
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How much of a loading dose to quickly reach CSF concentration = to S.S.
-estimated at least 4-6 times normal dose
Can we give too much ?
Even at 5ug/ml,ponazuril is static and higher concentrations needed to
eliminate Neospora parasites in cell culture but may be toxic (Darius )
Therefore, cure in treated horses may also depend upon the horse’s
immune system and or additional anti-protozoal therapy in some cases
Additional Drug Therapy
– Pyrimethamine 1-2mg mg/kg
• Cidal !
– S. neurona was inhibited by pyrimethamine at both 1.0 and
0.1 mg/ml in vitro- Lindsay and Marsh
– in vitro demonstrated a clear reduction in the total merozoite
replication (intracellular and extracellular stages) Marsh
– Has similar pharmacokinetics to triazine drugs except better
CSF: serum ratio (25-50%) and may be concentrated in
brain?? Clarke (peak serum concetrations only 0.2ug/ml)
• Necropsy confirmed horses that had been treated with
Pyrimethamine/sulfa (N=15) were less likely to have detectable
S. neurona organisms (18 cord sections) than were 42 nontreated horses- p<0.003 Boy, Divers JAVMA 1990
• May be synergistic with ponazuril and diclazuril ?
– The activity of diclazuril against acute toxoplasmosis in mice
is also augmented when combined with pyrimethamine
Lindsay
Hypothesis
Successful treatment can be improved
and relapse rate lowered by:
- increasing the Ponazuril dose and/or
duration of treatment
- addition of Pyrimethamine
Could this increase “treatment crisis”?
My personal observations have been this is most common when pyrimethamine is used in
the treatment
EPM: Is DMSO the Cure for
Treatment Issues?
• Toltrazuril sulfone administered at 2.2
mg/kg in DMSO had plasma levels of 1218 ug/ml but only 0.13-0.2 ug/ml in CSF!
Dirikolu 2009
Anti-inflammatory,
Anti-oxidant Therapy
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NSAID: decreased inflammation and could prevent
treatment crisis
DMSO: decreased inflammation and could prevent
treatment crisis
Vitamin E: water soluble natural vitamin E (blue dots on
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Corticosteroids ??
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day 14 represent concentration in CSF
Pusterla 2009
Immunotherapy-to recruit a TH1
response and interferon γ
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Levamisole:
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1-2 mg/kg daily for 10 days
Zylexis:
Although in a study of gene transcription
of cytokines in the CNS tissue of 12
horses with confirmed EPM, Th1
cytokines were commonly expressed
but there was a relative lack of Th2
expression Pusterla 2006
Prognosis
• Severity of Disease
• Correct Diagnosis
• Early and Aggressive
Treatment
• All affect prognosis
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Prevention
Decreased exposure to oocyst
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Factors associated with a reduced risk of
developing EPM included protection of feed
from wildlife Saville 2000
Treatment of opossums and intermediate
host?
Intermittent treatment every 5 days. MacKay
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Theoretically Daily treatment may not be required as
drugs are unlikely to kill sporocyst and may prevent
normal immune response ?
Organism in CNS by 7 days
Daily treatment during “risk periods”might be
more effective
The hypothesis for this is that:
1. Certain horses are susceptible to EPM (the
disease) by genetic and/or “stress” factors
2. After ingestion of the sporocyst, 7-10 days are
required before the organism gains entrance to
the CNS (if it ever does) Mansfield, Elitsur
3. The organism is more easily killed prior to
entering the CNS (most likely via monocytes)
Dubey
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Lastly, for previously affected horses that are prone to relapse
and likely have residual organisms in the CNS, sufficient drug
levels may be present in the CNS to prevent proliferation of the
organism?
Prophylactic/Metaphylactic use of
antiprotozoal medications – additional
studies needed in this area
• Possibilities?
– Horses that have relapsed after recommended
therapy has been discontinued
– “Families” or stables / farms with a high confirmed
incidence of the disease
– Prophylactic treatments beginning at time of training,
shipping, etc.
“Running Late” (story of my life)- Bronze Sculpture by Dr. T. Douglas Byars