Diagnosis of Equine Protozoal Myeloencephalitis Using Indirect Fluorescent Antibody Testing and Enzyme-Linked Immunosorbent Assay Titer Ratios for Sarcocystis neurona and Neospora hughesi

The aim of this study was to compare two serologic tests used to support a diagnosis of equine protozoal myeloencephalitis (EPM). Serum and cerebrospinal fluid (CSF) samples were analyzed for antibodies to Sarcocystis neurona and Neospora hughesi by indirect fluorescent antibody testing (IFAT) and surface antigens of S. neurona and N. hughesi by enzyme-linked immunosorbent assay (ELISA). The samples originated from neurologic horses with confirmed and suspected EPM (nine S. neurona, three N. hughesi), from neurologic horses with confirmed neurologic diseases other than EPM (16 horses) and from healthy horses (10). The IFAT on CSF and ELISA titer ratios showed equal sensitivity in diagnosing EPM caused by S. neurona. The ELISA titer ratios showed slightly greater specificity in diagnosing EPM than the IFAT on CSF. Overall agreement between the IFAT on CSF and ELISA titer ratio was 90.9%. The IFAT on CSF and ELISA serum/CSF ratio are indicated to help support a laboratory diagnosis of EPM.     

Seroprevalence of Neospora,Toxoplasma gondii and Sarcocystis neurona antibodies in horses from Jeju island,South Korea

Parasite-specific antibody responses to Neospora spp. and Toxoplasma gondii, antigens were detected using the indirect fluorescent antibody test (IFAT) and immunoblot analysis in a korean equine population located on Jeju island, South Korea (126 degrees 12' E and 33 degrees 34' N). For comparison, a naturally infected Neospora hughesi horse and an experimentally inoculated T. gondii equid (pony) were used. In addition, all samples were tested for antibodies to Sarcocystis neurona by immunoblot analysis. A total of 191 serum samples from clinically normal horses were evaluated. Only 2% (4 out of 191) and 2.6% (5 out of 191) of the samples had showed reactivity at 1:100 using the IFAT for Neospora spp. and T. gondii, respectively. For T. gondii, two samples matched the antigen banding pattern of the positive control by immunoblot analysis. No sample was positive for N. hughesi by immunoblot analysis in this study. Overall, there was a 1% seroprevalence for T. gondii antibodies in the horses tested based on immunoblot analysis. The seroprevalence for S. neurona and N. hughesi antibodies was 0%. We concluded that these horses are either not routinely exposed to these parasites or antibody titers are not sufficiently elevated to be detectable. It is most likely the former explanation since Jeju island equine farms are isolated from the main land, and the horses were all less than 3 years of age. This na?ve population of horses could be useful when evaluating S. neurona serodiagnostic tests or evaluating potential S. neurona vaccines since exposure risks to S. neurona and closely related parasites are negligible.     

Risk of postnatal exposure to Sarcocystis neurona and Neospora hughesi in horses

OBJECTIVE: To estimate risk of exposure and age at first exposure to Sarcocystis neurona and Neospora hughesi and time to maternal antibody decay in foals. ANIMALS: 484 Thoroughbred and Warmblood foals from 4 farms in California. PROCEDURE: Serum was collected before and after colostrum ingestion and at 3-month intervals thereafter. Samples were tested by use of the indirect fluorescent antibody test; cutoff titers were > or = 40 and > or = 160 for S neurona and N hughesi, respectively. RESULTS: Risk of exposure to S neurona and N hughesi during the study were 8.2% and 3.1%, respectively. Annual rate of exposure was 3.1% for S neurona and 1.7% for N hughesi. There was a significant difference in the risk of exposure to S neurona among farms but not in the risk of exposure to N hughesi. Median age at first exposure was 1.2 years for S neurona and 0.8 years for N hughesi. Highest prevalence of antibodies against S neurona and N hughesi was 6% and 2.1 %, respectively, at a mean age of 1.7 and 1.4 years, respectively. Median time to maternal antibody decay was 96 days for S neurona and 91 days for N hughesi. There were no clinical cases of equine protozoal myeloenchaphlitis (EPM). CONCLUSIONS AND CLINICAL RELEVANCE: Exposure to S neurona and N hughesi was low in foals between birth and 2.5 years of age. Maternally acquired antibodies may cause false-positive results for 3 or 4 months after birth, and EPM was a rare clinical disease in horses < or = 2.5 years of age.     

Effects of blood contamination of cerebrospinal fluid on western blot analysis for detection of antibodies against Sarcocystis neurona and on albumin quotient and immunoglobulin G index in horses.

OBJECTIVE: To determine effects of blood contamination on western blot (WB) analysis of CSF samples for detection of anti-Sarcocystis neurona antibodies, and on CSF albumin and IgG concentrations, albumin quotient (AQ), and IgG index in horses. DESIGN: Prospective in vitro study. SAMPLES: Blood with various degrees of immunoreactivity against S neurona was collected from 12 healthy horses. Cerebrospinal fluid without immunoreactivity against S neurona was harvested from 4 recently euthanatized horses. PROCEDURE: Blood was serially diluted with pooled nonimmunoreactive CSF so that final dilutions corresponded to 10(-3) to 100 microliters of blood/ml CSF, and WB analysis was performed on contaminated CSF samples. Number of RBC, albumin and IgG concentrations, AQ, and IgG index were also determined. RESULTS: Antibodies against S neurona were detected in CSF contaminated with 10(-3) microliters of strongly immunoreactive blood/ml. In CSF samples contaminated with 10 microliters of blood/ml, AQ remained within reference range. Volume of blood required to increase IgG index varied among blood samples and was primarily influenced by serum IgG concentrations. Number of RBC in contaminated samples was correlated with volume of blood added, but not with degree of immunoreactivity detected in contaminated CSF samples. CONCLUSIONS AND CLINICAL RELEVANCE: During collection of CSF from horses, contamination with blood may introduce serum antibodies against S neurona at concentrations sufficient for detection by WB analysis, thus yielding false-positive results. When blood is moderately or strongly immunoreactive, the amount of contaminating albumin may be small enough as to not increase AQ above reference range. In these cases, AQ and IgG index should be interpreted with caution.     

Prevalence of Neospora hughesi and Sarcocystis neurona antibodies in horses from various geographical locations

Parasite-specific antibody responses to Neospora antigens were detected using the immunofluorescent antibody test (IFAT) and immunoblot analysis in select equine populations. For comparison, a naturally infected Neospora hughesi horse and an experimentally inoculated Neospora caninum horse were used. In addition, all samples were tested for antibodies to Sarcocystis neurona by immunoblot analysis. A total of 208 samples was evaluated. The equine populations were derived from five distinct geographic regions. Locations were selected based on distribution of Didelphis virginiana, the native North American opossum which serves as the definitive host for S. neurona. Only 11% of the samples that had positive titers of 1:100 using the IFAT were also positive for antibodies by immunoblot analysis in this study. Overall, there was a 2% seroprevalence for Neospora antibodies in all horses tested based on immunoblot analysis described. The seroprevalence for S. neurona antibodies varied from 0% (New Zealand and Montana) to 54% (Missouri). We concluded that, in testing for antibodies against Neospora antigens using either IFAT or immunoblot analysis, as described, positive results should not be attributed to the presence of antibodies to S. neurona.     

Antibody index and specific antibody quotient in horses after intragastric administration of Sarcocystis neurona sporocysts

OBJECTIVE: To investigate the use of a specific antibody index (AI) that relates Sarcocystis neurona-specific IgG quotient (Q(SN)) to total IgG quotient (Q(IgG)) for the detection of the anti-S neurona antibody fraction of CNS origin in CSF samples obtained from horses after intragastric administration of S neurona sporocysts. ANIMALS: 18 adult horses. PROCEDURES: 14 horses underwent intragastric inoculation (day 0) with S neurona sporocysts, and 4 horses remained unchallenged; blood and CSF samples were collected on days - 1 and 84. For purposes of another study, some challenged horses received intermittent administration of ponazuril (20 mg/kg, PO). Sarcocystis neurona-specific IgG concentrations in CSF (SN(CSF)) and plasma (SN(plasma)) were measured via a direct ELISA involving merozoite lysate antigen and reported as ELISA units (EUs; arbitrary units based on a nominal titer for undiluted immune plasma of 100,000 EUs/mL). Total IgG concentrations in CSF (IgG(CSF)) and plasma (IgG(plasma)) were quantified via a sandwich ELISA and a radial immunodiffusion assay, respectively; Q(SN), Q(IgG), and AI were calculated. RESULTS: Following sporocyst challenge, mean +/- SEM SN(CSF) and SN(plasma) increased significantly (from 8.8 +/- 1.0 EUs/mL to 270.0 +/- 112.7 EUs/mL and from 1,737 +/- 245 EUs/mL to 43,169 +/- 13,770 EUs/mL, respectively). Challenge did not affect total IgG concentration, Q(SN), Q(IgG), or AI. CONCLUSIONS AND CLINICAL RELEVANCE: S neurona-specific IgG detected in CSF samples from sporocyst-challenged horses appeared to be extraneural in origin; thus, this experimental challenge may not reliably result in CNS infection. Calculation of a specific AI may have application to the diagnosis of S neurona-associated myeloencephalitis in horses.     

Indirect fluorescent antibody testing of cerebrospinal fluid for diagnosis of equine protozoal myeloencephalitis

OBJECTIVE: To assess the use of CSF testing with an indirect fluorescent antibody test (IFAT) for diagnosis of equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona. SAMPLE POPULATION: Test results of 428 serum and 355 CSF samples from 182 naturally exposed, experimentally infected, or vaccinated horses. PROCEDURE: EPM was diagnosed on the basis of histologic examination of the CNS. Probability distributions were fitted to serum IFAT results in the EPM+ and EPM-horses, and correlation between serum and CSF results was modeled. Pairs of serum-CSF titers were generated by simulation, and titer-specific likelihood ratios and post-test probabilities of EPM at various pretest probability values were estimated. Post-test probabilities were compared for use of a serum-CSF test combination, a serum test only, and a CSF test only. RESULTS: Post-test probabilities of EPM increased as IFAT serum and CSF titers increased. Post-test probability differences for use of a serum-CSF combination and a serum test only were < or = 19% in 95% of simulations. The largest increases occurred when serum titers were from 40 to 160 and pre-test probabilities were from 5% to 60%. In all simulations, the difference between pre- and post-test probabilities was greater for a CSF test only, compared with a serum test only. CONCLUSIONS AND CLINICAL RELEVANCE: CSF testing after a serum test has limited usefulness in the diagnosis of EPM. A CSF test alone might be used when CSF is required for other procedures. Ruling out other causes of neurologic disease reduces the necessity of additional EPM testing.     

Serologic prevalence of Sarcocystis neurona, Toxoplasma gondii, and Neospora caninum in horses in Brazil.

OBJECTIVE: To determine serologic prevalence of Sarcocystis neurona, Toxoplasma gondii, and Neospora caninum in horses in Brazil. DESIGN: Prevalence survey. ANIMALS: 101 Thoroughbreds in Brazil. PROCEDURE: Blood samples were obtained from horses and tested for serum antibodies against S neurona by use of an immunoblot procedure with culture-derived S neurona merozoites as antigen, and for serum antibodies against T gondii and N caninum by use of a modified agglutination test with formalin-preserved tachyzoites and mercaptoethanol. RESULTS: Antibodies against S neurona and T gondii were detected in 36 and 16 of 101 horses, respectively. Cross-reactivity between antibodies against T gondii and S neurona was not detected. Antibodies against N caninum were not detected in any samples. CONCLUSIONS AND CLINICAL RELEVANCE: The high prevalence of antibodies against S neurona detected in clinically normal horses emphasizes the importance of examining CSF for antibodies when establishing a diagnosis of equine protozoal myeloencephalitis.     

Sarcocystis neurona-specific immunoglobulin G in the serum and cerebrospinal fluid of horses administered S neurona vaccine

A vaccine against Sarcocystis neurona, which induces equine protozoal myeloencephalitis (EPM), has received conditional licensure in the United States. A major concern is whether the immunoglobulin G (IgG) response elicited by the vaccine will compromise the use of Western blotting (WB) as a diagnostic tool in vaccinated horses with neurologic disease. Our goals were to determine if vaccination (1) causes seroconversion: (2) causes at least a transient increase in S neurona-specific IgG in the cerebrospinal fluid (CSF); and (3) induces an IgG response that can be differentiated from that induced by natural exposure. Horses included in the study (n = 29) were older than 6 months with no evidence of neurologic disease. The presence or absence of anti-S neurona antibodies in the serum of each horse was determined by WB analysis. Seropositive horses had CSF collected and submitted for cytology, CSF index, and WB analysis. The vaccine was administered to all the horses and boostered 3-4 weeks later. On day 14 after the 2nd administration, serum and CSF were collected and analyzed. Eighty-nine percent (8 of 9) of the initial seronegative horses seroconverted after vaccination, of which 57% (4 of 7) had anti-S neurona IgG in their CSE Eighty percent (16 of 20) of the seropositive horses had an increase in serum S neurona IgG after vaccination. Of the 6 of 20 horses that were initially seropositive/CSF negative, 2 were borderline positive for anti-S neurona IgG in the CSF, 2 tested positive, and 2 were excluded because the CSF sample had been contaminated by blood. There were no WB banding patterns that distinguished samples from horses that seroconverted due to vaccination versus natural exposure. Caution must be used in interpreting WB analysis from neurologic horses that have been recently vaccinated for EPM.     

Development and evaluation of a Sarcocystis neurona-specific IgM capture enzyme-linked immunosorbent assay

Equine protozoal myeloencephalitis (EPM) is a serious neurologic disease of horses caused primarily by the protozoal parasite Sarcocystis neurona. Currently available antemortem diagnostic testing has low specificity. The hypothesis of this study was that serum and cerebrospinal fluid (CSF) of horses experimentally challenged with S neurona would have an increased S neurona-specific IgM (Sn-IgM) concentration after infection, as determined by an IgM capture enzyme linked immunoassay (ELISA). The ELISA was based on the S neurona low molecular weight protein SNUCD-1 antigen and the monoclonal antibody 2G5 labeled with horseradish peroxidase. The test was evaluated using serum and CSF from 12 horses experimentally infected with 1.5 million S neurona sporocysts and 16 horses experimentally infected with varying doses (100 to 100,000) of S neurona sporocysts, for which results of histopathologic examination of the central nervous system were available. For horses challenged with 1.5 million sporocysts, there was a significant increase in serum Sn-IgM concentrations compared with values before infection at weeks 2-6 after inoculation (P < .0001). For horses inoculated with lower doses of S neurona, there were significant increases in serum Sn-IgM concentration at various points in time after inoculation, depending on the challenge dose (P < .01). In addition, there was a significant increase between the CSF Sn-IgM concentrations before and after inoculation (P < .0001). These results support further evaluation of the assay as a diagnostic test during the acute phase of EPM.     

Immunoconversion against Sarcocystis neurona in normal and dexamethasone-treated horses challenged with S. neurona sporocysts

Equine protozoal myeloencephalitis is a common neurologic disease of horses in the Americas usually caused by Sarcocystis neurona. To date, the disease has not been induced in horses using characterized sporocysts from Didelphis virginiana, the definitive host. S. neurona sporocysts from 15 naturally infected opossums were fed to horses seronegative for antibodies against S. neurona. Eight horses were given 5x10(5) sporocysts daily for 7 days. Horses were examined for abnormal clinical signs, and blood and cerebrospinal fluid were harvested at intervals for 90 days after the first day of challenge and analyzed both qualitatively (western blot) and quantitatively (anti-17kDa) for anti-S. neurona IgG. Four of the challenged horses were given dexamethasone (0.1mg/kg orally once daily) for the duration of the experiment. All challenged horses immunoconverted against S. neurona in blood within 32 days of challenge and in CSF within 61 days. There was a trend (P = 0.057) for horses given dexamethasone to immunoconvert earlier than horses that were not immunosuppressed. Anti-17kDa was detected in the CSF of all challenged horses by day 61. This response was statistically greater at day 32 in horses given dexamethasone. Control horses remained seronegative throughout the period in which all challenged horses converted. One control horse immunoconverted in blood at day 75 and in CSF at day 89. Signs of neurologic disease were mild to equivocal in challenged horses. Horses given dexamethasone had more severe signs of limb weakness than did horses not given dexamethasone; however, we could not determine whether these signs were due to spinal cord disease or to effects of systemic illness. At necropsy, mild-moderate multifocal gliosis and neurophagia were found histologically in the spinal cords of 7/8 challenged horses. No organisms were seen either in routinely processed sections or by immunohistochemistry. Although neurologic disease comparable to naturally occurring equine protozoal myeloencephalitis (EPM) was not produced, we had clear evidence of an immune response to challenge both systemically and in the CNS. Broad immunosuppression with dexamethasone did not increase the severity of histologic changes in the CNS of challenged horses. Future work must focus on defining the factors that govern progression of inapparent S. neurona infection to EPM.     

Inoculation of Sarcocystis neurona merozoites into the central nervous system of horses

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome in horses from the Americas and is usually caused by infection with the apicomplexan parasite, Sarcocystis neurona. A horse model of EPM is needed to test the efficacy of chemotherapeutic agents and potential vaccines. Five horses that were negative for antibodies to S. neurona in their serum and cerebrospinal fluid (CSF) were injected in the subarachnoid space with living merozoites of the SN2 isolate of S. neurona. None of the horses developed clinical disease or died over a 132-day observation period. All five horses developed antibodies to S. neurona in their CSF and serum 3-4 weeks after injection. Two of the horses were examined at necropsy and no parasite induced lesions were observed in their tissues and no parasites were recovered from portions of their spinal cords inoculated on to cell cultures. Results of this study demonstrate that merozoites of the SN2 isolate of S. neurona will induce seroconversion but not clinical disease when inoculated directly into the CSF of nonimmune horses.     

Effect of intermittent oral administration of ponazuril on experimental Sarcocystis neurona infection of horses

OBJECTIVE: To evaluate the effect of intermittent oral administration of ponazuril on immunoconversion against Sarcocystis neurona in horses inoculated intragastrically with S neurona sporocysts. ANIMALS: 20 healthy horses that were seronegative for S neurona-specific IgG. PROCEDURES: 5 control horses were neither inoculated with sporocysts nor treated. Other horses (5 horses/group) each received 612,500 S neurona sporocysts via nasogastric tube (day 0) and were not treated or were administered ponazuril (20 mg/kg, PO) every 7 days (beginning on day 5) or every 14 days (beginning on day 12) for 12 weeks. Blood and CSF samples were collected on day - 1 and then every 14 days after challenge for western blot assessment of immunoconversion. Clinical signs of equine protozoal myeloencephalitis (EPM) were monitored, and tissues were examined histologically after euthanasia. Results: Sera from all challenged horses yielded positive western blot results within 56 days. Immunoconversion in CSF was detected in only 2 of 5 horses that were treated weekly; all other challenged horses immunoconverted within 84 days. Weekly administration of ponazuril significantly reduced the antibody response against the S neurona 17-kd antigen in CSF. Neurologic signs consistent with EPM did not develop in any group; likewise, histologic examination of CNS tissue did not reveal protozoa or consistent degenerative or inflammatory changes. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of ponazuril every 7 days, but not every 14 days, significantly decreased intrathecal anti-S neurona antibody responses in horses inoculated with S neurona sporocysts. Protocols involving intermittent administration of ponazuril may have application in prevention of EPM.     

Prevalence of Sarcocystis neurona and Neospora spp. infection in horses from Brazil based on presence of serum antibodies to parasite surface antigen

Sera from 961 horses from Brazil were tested for antibodies against the major surface antigens SnSAG4 and NhSAG1 to determine the seroprevalence of Sarcocystis neurona and Neospora hughesi, respectively. Antibodies against SnSAG4 were detected in 669 (69.6%) of the horses, while antibodies against NhSAG1 were detected in only 24 (2.5%) of the horses. These serologic results suggest that there is a high concentration of S. neurona in the environment of Brazil, which results in marked exposure of horses to this parasite. Additionally, the data further confirm that infection with Neospora spp. is relatively uncommon in horses.     

Immune response to Sarcocystis neurona infection in naturally infected horses with equine protozoal myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is one of the most common neurologic diseases of horses in the United States. The primary etiologic agent is Sarcocystis neurona. Currently, there is limited knowledge regarding the protective or pathophysiologic immune response to S. neurona infection or the subsequent development of EPM. The objectives of this study were to determine whether S. neurona infected horses with clinical signs of EPM had altered or suppressed immune responses compared to neurologically normal horses and if blood sample storage would influence these findings. Twenty clinically normal horses and 22 horses with EPM, diagnosed by the presence of S. neurona specific antibodies in the serum and/or cerebrospinal (CSF) and clinical signs, were evaluated for differences in the immune cell subsets and function. Our results demonstrated that naturally infected horses had significantly (P<0.05) higher percentages of CD4 T-lymphocytes and neutrophils (PMN) in separated peripheral blood leukocytes than clinically normal horses. Leukocytes from naturally infected EPM horses had significantly lower proliferation responses, as measured by thymidine incorporation, to a non-antigen specific mitogen than did clinically normal horses (P<0.05). Currently, studies are in progress to determine the role of CD4 T cells in disease and protection against S. neurona in horses, as well as to determine the mechanism associated with suppressed in vitro proliferation responses. Finally, overnight storage of blood samples appears to alter T lymphocyte phenotypes and viability among leukocytes.     

Prevalence of antibodies to Sarcocystis neurona, Toxoplasma gondii and Neospora caninum in horses from Argentina.

Sera from 76 horses from Argentina were examined for antibodies to Sarcocystis neurona, Toxoplasma gondii and Neospora caninum. Antibodies to S. neurona were found in 27 (35.5%) of 76 horses using immunoblots with culture derived merozoites as antigen. Antibodies to T. gondii were found in 10 (13.1%) of 76 horses by using the modified agglutination test with formalin-fixed tachyzoites and mercaptoethanol; titers were 1:25 (two horses), 1:50 (six horses), 1:100 (two horses), and 1:200 (one horse). Antibodies to N. caninum were not found in any of the 76 horses by the use of N. caninum agglutination test. This is the first report of S. neurona infection in horses in Argentina.     

Reactivity against Sarcocystis neurona and Neospora by serum antibodies in healthy French horses from two farms with previous equine protozoal myeloencephalitis-like cases

Sarcocystis neurona is considered a leading cause of equine protozoal myeloencephalitis (EPM), a common infectious neurological disease in horses in the Americas. EPM-like cases associated with S. neurona peptide reactive antibodies in Western blots were recently described in Normandy, France. In this report, antibodies reacting with S. neurona merozoites were detected using an agglutination assay at titers ranging from 50 to 500 in sera from 18/50 healthy horses from two farms with a previous EPM-like case. Higher values were found in older animals. Four out of six horses which traveled or stayed in the US exhibited titers over 50, a higher figure than in the group which did not travel out of France or stayed in an other European country. No correlation was found between anti-S. neurona and anti-Neospora sp. antibody titers. Data prompt further study of significance of anti-S. neurona antibodies in clinically healthy or diseased European horses, and identification of putative immunizing parasite(s) and their host(s).     

Utilization of stress in the development of an equine model for equine protozoal myeloencephalitis

Neurologic disease in horses caused by Sarcocystis neurona is difficult to diagnose, treat, or prevent, due to the lack of knowledge about the pathogenesis of the disease. This in turn is confounded by the lack of a reliable equine model of equine protozoal myeloencephalitis (EPM). Epidemiologic studies have implicated stress as a risk factor for this disease, thus, the role of transport stress was evaluated for incorporation into an equine model for EPM. Sporocysts from feral opossums were bioassayed in interferon-gamma gene knockout (KO) mice to determine minimum number of viable S. neurona sporocysts in the inoculum. A minimum of 80,000 viable S. neurona sporocysts were fed to each of the nine horses. A total of 12 S. neurona antibody negative horses were divided into four groups (1-4). Three horses (group 1) were fed sporocysts on the day of arrival at the study site, three horses were fed sporocysts 14 days after acclimatization (group 2), three horses were given sporocysts and dexamethasone 14 days after acclimatization (group 3) and three horses were controls (group 4). All horses fed sporocysts in the study developed antibodies to S. neurona in serum and cerebrospinal fluid (CSF) and developed clinical signs of neurologic disease. The most severe clinical signs were in horses in group 1 subjected to transport stress. The least severe neurologic signs were in horses treated with dexamethasone (group 3). Clinical signs improved in four horses from two treatment groups by the time of euthanasia (group 1, day 44; group 3, day 47). Post-mortem examinations, and tissues that were collected for light microscopy, immunohistochemistry, tissue cultures, and bioassay in KO mice, revealed no direct evidence of S. neurona infection. However, there were lesions compatible with S. neurona infection in horses. The results of this investigation suggest that stress can play a role in the pathogenesis of EPM. There is also evidence to suggest that horses in nature may clear the organism routinely, which may explain the relatively high number of normal horses with CSF antibodies to S. neurona compared to the prevalence of EPM.     

Effect of daily administration of pyrantel tartrate in preventing infection in horses experimentally challenged with Sarcocystis neurona

OBJECTIVE: To determine whether daily administration of pyrantel tartrate can prevent infection in horses experimentally challenged with Sarcocystis neurona. ANIMALS: 24 mixed-breed specific-pathogen-free weanling horses, 10 adult horses, 1 opossum, and 6 mice. PROCEDURE: Sarcocystis neurona-na?ve weanling horses were randomly allocated to 2 groups. Group A received pyrantel tartrate at the labeled dose, and group B received a nonmedicated pellet. Both groups were orally inoculated with 100 sporocysts/d for 28 days, 500 sporocysts/d for 28 days, and 1000 sporocysts/d for 56 days. Blood samples were collected weekly, and CSF was collected monthly. Ten seronegative adult horses were monitored as untreated, uninfected control animals. All serum and CSF samples were tested by use of western blot tests to detect antibodies against S. neurona. At the end of the study, the number of seropositive and CSF-positive horses in groups A and B were compared by use of the Fisher exact test. Time to seroconversion on the basis of treatment groups and sex of horses was compared in 2 univariable Cox proportional hazards models. RESULTS: After 134 days of sporocyst inoculation, no significant differences were found between groups A and B for results of western blot tests of serum or CSF There were no significant differences in number of days to seroconversion on the basis of treatment groups or sex of horses. The control horses remained seronegative. CONCLUSIONS AND CLINICAL RELEVANCE: Daily administration of pyrantel tartrate at the current labeled dose does not prevent S. neurona infection in horses.