Equine Monocytic Ehrlichiosis Research Articles

Outbreak of equine Monocytic Ehrlichiosis in criollo horses

its alertness improved (Figure 2). All other parameters remained similar. After definitive diagnosis of EIA the colt and his mother were isolated until a second positive Coggins test was confirmed. The foal was euthanized according to the ethical standards of animal care. Horses entry or exit at the premises were not allowed. All animals underwent serial serological sampling every 15 days for 60 days from the first case. 14 new cases were diagnosed in the same farm without clinical signs, including colt’s mother. All positive animals were euthanized.

Pesquisa de trematódeos digenéticos em Heleobia spp. (Mollusca: Hydrobiidae) em área de ocorrência da Ehrlichiose monocítica equina, no Rio Grande do Sul, Brasil

A ehrlichiose monocítica equina (EME) na região Sul do Rio Grande do Sul tem demonstrado ser importante nas criações de cavalos Crioulos. A enfermidade foi relatada e diagnosticada como causa de diarreia, prejuízos com tratamentos e com a morte de equinos não estabulados, sendo apontada como um fator limitante na criação de equinos em algumas regiões. O modo de transmissão pela via oral, intermediada por trematódeos em ambientes aquáticos, tem sido sustentado. Caracóis dulciaquícolas estão envolvidos como hospedeiros intermediários de trematódeos albergadores de Neorickettsia risticii. Um total de 16.846 caracóis Heleobia foi coletado nos municípios de Arroio Grande, Rio Grande, Palmares do Sul e Santa Vitória do Palmar, 92,2% dos quais foram encontrados nas raízes de aguapés (Eichornea spp.). A frequência de trematódeos presentes nos caracóis variou de 2,3 a 12,8% nas propriedades coletadas. Foram encontrados três tipos de cercárias, morfotipo 1, morfotipo 2 e morfotipo 3, e dois morfotipos de metacercárias nos caracóis. Um total de 357 insetos da ordem Odonata foi coletado, fases de metacercárias foram encontradas no tegumento da subordem Anisoptera com frequência de 5,3%. Mais estudos são necessários para identificar as fases larvais encontradas, bem como para conhecer o hospedeiro definitivo, identificar o parasito adulto e a relação de seu ciclo de vida com a ocorrência da ehrlichiose monocítica equina.

Frequency of Equine Monocytic Ehrlichiosis (EME) in Brazil

From a cross-sectional observational study with convenience samples, 347 blood samples from horses were collected from different physiographic regions, as follows: Santa Catarina Plateau (Santa Catarina State - SC), Médio Paraíba do Sul (São Paulo State - SP and Rio de Janeiro State RJ), Mountainous and Metropolitan regions (Rio de Janeiro State - RJ). Samples were tested for the presence of antibodies (IgG) anti Neorickettsia risticii by indirect immunofluorescence assay (IFA). The frequency obtained in this study corroborates with the ones obtained in the U.S.A., which refers to endemic regions. Fisher's exact test showed significant differences in the number of positive animals between regions, indicating that the probability of an animal becoming infected varies depending on the area. The CI 95% revealed no association between infection and geopolitical space. Moreover, Odds ratio test showed differences of an animal getting infected in different regions. This event could be influenced by the type of treatment used in each area, as the seasonal frequency of injury or even potential vectors. Therefore, there are seropositive animals for N. risticii in the studied areas, suggesting that this agent may be circulating in those regions. Future studies mainly based on molecular analyzes are needed to confirm these serological findings.

Equine Monocytic Ehrlichiosis (EME) in Rio de Janeiro State, Brazil

, a biological agent with affinity for monocytes (Holland et al., 1984; Rikihisa et al., 1985). This is a disease of an enzootic type that presents a mortality rate varying between 5% and 30% (Palmer, 1993). This disease was already described in the United States, Canada, European Union, India, Uruguay and the South of Brazil (Whitlock et al., 1985; Vidor et al., 1988; Dutra et al., 2001). The clinical signs are non-specific, mainly the clinical and sub-clinical forms of the disease (Ristic et al., 1986). The animals may present fever of 38.9oC to 41.7oC, as well as, depression, anorexia, diarrhea from moderate to severe, accompanied of abdominal discomfort, laminitis, toxemia, and dehydration. In pregnant mares, it may cause abortion associated with placentitis and even retained placenta (Knowles et al., 1983; Long et al., 1995). Up to recent times, the vector of transmission of

Ehrlichiose monocítica eqüina no Rio Grande do Sul: aspectos clínicos, anátomo-patológicos e epidemiológicos

Descreve-se um surto natural de Ehrlichiose mono-cítica eqüina (EME) ocorrido nos meses de novembro/2001 a fevereiro/2002, em uma propriedade no município de Arroio Grande, localizado à margem oeste da Lagoa Mirim, Rio Grande do Sul. De um total de 50 eqüinos, 13 foram afetados pela enfermidade e seis morreram. A morbidade foi de 26% e a letalidade 46,1%. Os animais acometidos tinham idades entre um e cinco anos e o quadro clínico mais evidente foi o desenvolvimento de diarréia aguda aquosa e profusa. Um dos animais que morreram foi necropsiado e as lesões observadas localizavam-se principalmente no trato intestinal, caracterizando-se por intestino grosso com conteúdo líquido e áreas hemorrágicas difusas na mucosa. Em todo o segmento de intestino delgado observaram-se áreas com mucosa congesta e hemorrágica alternadas com áreas de mucosa normal. Na mucosa de jejuno e íleo observou-se a presença de nódulos contendo secreção purulenta. Os achados histológicos em todo trato intestinal caracterizaram-se pela presença de discreto infiltrado mononuclear restrito a mucosa do órgão, caracterizando uma enterite linfo-histiocitária com predominância de macrófagos. No jejuno e íleo também havia regiões com lesões granulomatosas na lâmina própria, relacionadas a invaginações das criptas das vilosidades intestinais. Para o diagnóstico da infecção por Neorickettsia (Ehrlichia) risticii amostras de sangue de sete eqüinos doentes foram testados pela reação em cadeia da polimerase (PCR), resultando em três amostras positivas (42,8%) com a formação de uma banda de 529pb, típica para N. risticii.

Equine monocytic Ehrlichiosis (Potomac horse fever) in horses in Uruguay and southern Brazil.

A disease named locally as churrío or churrido equino (i.e., equine scours) has occurred for at least 100 years in Uruguay and southern Brazil in farms along both shores of the Merín lake. This report describes cases of churrido equino and provides serologic, pathologic, and DNA-based evidence indicating that the disease is in fact equine monocytic ehrlichiosis (Potomac horse fever). Results of an epidemiological investigation conducted on an endemic farm are also presented. Clinical signs in 12 horses were fever, depression, diarrhea, dehydration, and sometimes colic and distal hind limb edema. Postmortem findings of 3 horses were of acute enterocolitis. Inclusion bodies containing ehrlichial organisms were found in the cytoplasm of macrophages of the large colon of 1 horse. Eleven of the 12 horses were serologically positive to Ehrlichia risticii (indirect fluorescent antibody assay) and, of 3 paired samples, 2 showed seroconversion. Ehrlichia risticii DNA was identified by a nested polymerase chain reaction in peripheral blood of an affected horse. A healthy horse inoculated with peripheral blood from an affected horse developed the disease and antibodies to E. risticii. The disease had a peak incidence in March (summer) and was statistically associated with a marshy ecosystem near the Merín lake, where large numbers of Pomacea spp. (Ampullariidae) snails were found. Incidence density was almost 8 times higher in nonnative horses than in native horses. It was concluded that the previous diarrheic disease of horses known in Uruguay and southern Brazil as churrido equino is equine monocytic ehrlichiosis.

Ehrlichial Diseases

Equine granulocytic and monocytic ehrlichiosis caused by Ehrlichia equi and E. risticii, respectively, are seasonal diseases in horses that occur throughout the United States E. equi is transmitted by lxodes ticks and causes high fever, depression, anorexia, limb edema, petechiation, icterus, ataxia, and stiffness in gait. E. risticii, also known as the agent of Potomac horse fever, causes a febrile illness with a colitis of variable severity. Its occurrence is associated with aquatic habitats. The natural route of transmission is oral, through the ingestion of E. risticii infected trematode stages either free in water or in an intermediate host, such as aquatic animals.

Nested polymerase chain reaction for detection of Ehrlichia risticii genomic DNA in infected horses

A nested polymerase chain reaction was developed for amplifying a 529-bp segment of the 16S ribosomal RNA gene of Ehrlichia risticii from equine buffy coat cells. Confirmation of identity of the amplified bands was accomplished by Southern hybridization and DNA sequencing. The study indicated a detection limit of > 10 copies of the target gene, and specificity for E. risticii as based on a panel of test rickettsiae. Ticks ( Ixodes pacificus) collected in an area of northern California enzootic for equine monocytic ehrlichiosis were found to be negative for E. risticii DNA.

Evaluation of vaccination of horses as a strategy to control equine monocytic ehrlichiosis

Objective To determine whether preferentially vaccinated horses were at risk for exposure to Ehrlichia risticii, whether horses with equine monocytic ehrlichiosis (EME) were likely to have been nonvaccinated, and whether clinical severity and financial costs associated with care and treatment of EME were less for vaccinated horses with EME than for nonvaccinated horses with EME. Design Cross-sectional and case-control studies. Procedure Information on usage of E risticii bacterins to control EME was collected for 2,587 horses located on 511 farms throughout New York. Each horse was tested for serum antibodies directed against E risticii. Data on efficacy of vaccination to reduce the prevalence and clinical severity of EME and monetary losses associated with EME were collected from 68 horses with EME and 132 clinically normal horses. Results A correlation was not detected between the county seropositive proportion and the proportion of horses vaccinated against EME. Among horses diagnosed for EME, median date of diagnosis was not delayed for vaccinated horses, compared with that for nonvaccinated horses. Mean cost per case was not significantly different for nonvaccinated horses, compared with that for vaccinated horses ($1,082 and $1,001, respectively). Vaccination was not associated with a reduction in prevalence or in severity of EME-related clinical signs. Clinical Implications Administering killed E risticii bacterin once a year to control EME in New York appears to have limited success. Among horses in which EME was diagnosed, severity of illness and financial costs attributable to EME were indistinguishable for vaccinated and nonvaccinated horses. (J Am Vet Med Assoc 1996;208: 1285–1289)

Benefit-cost analysis of vaccination of horses as a strategy to control equine monocytic ehrlichiosis

Objective To determine whether horses in New York should be vaccinated against equine monocytic ehrlichiosis (EME). Design Decision-tree analyses of data from a cross-sectional study and a case-control study. Sample Population Horses in New York. Procedure Annual expected monetary loss per horse attributable to EME was calculated for vaccinated and nonvaccinated horses in New York. Because risk of being seropositive was dependent on county in which the horse was located, farm elevation, and use of each horse, decision-tree analyses were stratified by these factors. Results Annual expected monetary loss per horse attributable to EME for horses vaccinated by veterinarians ranged from $21 to $21.83/horse/y; for horses vaccinated by owners ranged from $10 to $10.83/horse/y; and for nonvaccinated horses ranged from $0 to $4.03/horse/y. Assuming 78% of vaccinated horses were protected and mean losses associated with EME included costs for horses that died, annual incidence density at which expected monetary loss for vaccinated horses was equal to that for nonvaccinated horses was 12 cases/1,000 horses/y and 25 cases/1,000 horses/y for horses vaccinated by owners or by veterinarians, respectively. Clinical Implications Annual vaccination minimizes monetary losses attributable to EME only when the annual incidence density exceeds 12 to 25 cases/1,000 horses/y. In New York, expected monetary losses are minimized when horses are not vaccinated because of the low annual incidence density in most regions. (J Am Vet Med Assoc 1996;208: 1290–1294)

Evidence for a high rate of false-positive results with the indirect fluorescent antibody test for Ehrlichia risticii antibody in horses

Objective— The original objective was to determine seroprevalence of Ehrlichia risticii antibody among horses in California. On the basis of the unexpected results of the survey, an investigation into the accuracy and reproducibility of results of the indirect fluorescent antibody ( ifa) test for E risticii was carried out. Design— Prospective, seroprevalence study. Animals— Healthy horses (n = 655) and horses with clinical signs of equine monocytic ehrlichiosis ( eme; n = 514) from various regions of California. Procedure— The ifa test was performed. Results were compared with results of an elisa and with results of western immunoblot analysis. Results— Overall, 104 of 655 (15.9%) healthy horses had evidence of an antibody response. However, 84 of 514 (16.3%) horses with clinical signs of eme also had positive test results, and of the 8 seropositive diseased horses for which paired (acute and convalescent) samples had been submitted, only 1 had a rise in antibody titers between the acute and convalescent samples. Comparison of results for the ifa test, elisa, and western immunoblot analysis revealed a high rate of false-positive results for the ifa test. Subsequent studies suggested that routine vaccination of horses with non-E risticii vaccines may have contributed to the false-positive reactions. Clinical Implications— The data failed to provide conclusive evidence of E risticii infection among California horses. Owing to the high percentage of false-positive test results, caution is advised when using the ifa test to diagnose eme in horses or to determine the necessity for E risticii vaccination.

Evaluation of fetal infection and abortion in pregnant ponies experimentally infected with Ehrlichia risticii

SUMMARY Fetal infectivity of Ehrlichia risticii was investigated in 19 ponies that were E risticii negative on the basis of results of an indirect fluorescent antibody (ifa) test. Thirteen pregnant ponies were infected by iv administration of E risticii between 90 and 180 days of gestation. Six pregnant ponies served as noninfected controls. Each infected pony had clinical signs of equine monocytic ehrlichiosis, was confirmed to be ehrlichemic, and developed an ifa titer to E risticii. Two infected ponies became recumbent, were unresponsive to supportive care, and were euthanatized. After recovery from clinical illness, the remaining ponies were observed throughout gestation for reproductive abnormalities. On abortion, each fetus was necropsied and tissue specimens from the liver, bone marrow, spleen, colon, and mesenteric lymph nodes were inoculated into canine monocyte cell cultures. Six infected ponies aborted at a mean 217 days of gestation, which was between postinoculation days 65 and 111. Five fetuses were recovered for evaluation, and E risticii was isolated from 4 of them. All 5 fetuses recovered had similar histologic findings, including enterocolitis, periportal hepatitis, and lymphoid hyperplasia with necrosis of the mesenteric lymph nodes and spleen. All 5 fetuses tested negative for IgG to E risticii, although 3 had low IgM titer to E risticii. The remaining 5 infected ponies had normal parturition. Presuckle ifa titer to E risticii was measured in 4 of the term foals, and results for 3 were positive. Two foals from infected ponies were monitored for 6 months and daily gain in body weight was comparable to that of a control foal. None of the control ponies became ill or seroconverted during the clinical illness phase, and none aborted throughout gestation. Two control ponies seroconverted to E risticii 6 weeks before parturition. Results of this study indicate that E risticii is a primary abortifacient under experimental conditions.

Estimates of the annual incidence density and financial losses of equine monocytic ehrlichiosis in New York state

Very little is known about the incidence density of equine monocytic ehrlichiosis (EME) in New York state. Obtaining a valid estimate of the state-wide incidence density would provide a measure for how common EME is in the New York state equine population, provide a historical benchmark of the incidence density for future comparisons, and aid in the estimation of the financial losses associated with EME. In this study we used four sets of state-wide data to obtain four estimates of the annual incidence density of EME in New York state. Two methods were used to calculate the incidence density: the average incidence rate of EME over a period of time, and transforming the point prevalence of exposure to Ehrlichia risticii into an incidence density. Given the incidence density, an estimate of the annual state-wide financial losses associated with this disease was calculated. Three of the four estimated ranges of the incidence density fell within the range of 0.44–1.73 per 1000 horse-years at risk. Therefore, given that there were approximately 182 000 horses in New York state in 1988, among which 92.7% (168 714) were susceptible at one time, we would expect 74–292 cases of EME per year under steady-state conditions. Since the mean cost (direct and indirect) associated with a case of EME in New York state was estimated at $1030.00, the annual state-wide financial losses associated with clinical EME for equine owners was $76 220–300 760.

Geographical variation of seropositivity to Ehrlichia risticii (equine monocytic ehrlichiosis) of horses in New York state.

A total of 2,579 serum samples from horses in New York state during 1985-1986 were examined for seropositivity to Ehrlichia risticii using the indirect fluorescent antibody technique. Cluster analysis statistical technique was used to group counties according to their estimated EME-disease rate (seropositive proportion of sampled horses). Counties were clustered into 4 groups of different EME-disease rates, representing high (86% seropositive), medium (66% seropositive), medium-low (47% seropositive) and low (6% seropositive) risk regions. The logistic regression statistical technique was used to examine whether the geographic variation of E. risticii seropositivity is due to other EME-risk factors. Risk factors known to be associated with EME (age, breed, sex, month of sample submission) have not contributed to the apparent geographical variation of the disease in New York state. This lack of an association between equine host factors or temporal factors with the geographical variation of EME suggests that conditions necessary for establishment of EME endemic foci involves other factors. The county equine population density and presence of a major equine racetrack were not associated with the geographical variation of the disease in New York state. The high risk regions for EME were located predominately along river valleys, but an explanation of why EME activity varies geographically remains speculative.

Cultural, molecular, and immunological characterization of the etiologic agent for atypical canine ehrlichiosis

More than 100 cases of canine ehrlichiosis, with three fatalities, were serologically negative by the indirect immunofluorescent antibody (IFA) test with Ehrlichia canis or E. sennetsu antigen but were reactive at titers of 10 to 640 with E. risticii. Ehrlichia-like agents were isolated from three such cases. The agents isolated from those cases were morphologically indistinguishable from each other and from a prototype, E. risticii, the etiologic agent of equine monocytic ehrlichiosis, in terms of growth characteristics and by light or electron microscopy. The patterns of and products from PCR were identical to those of E. risticii. The 16S rRNA sequences were distinct from those of E. canis and E. ewingii but were identical to those of E. risticii. A PCR product corresponding to the 5' half of the 16S rRNA gene was obtained from amplification of DNA from E. risticii and both sources of the atypical canine ehrlichiosis agent but was not obtained from uninfected host cells. The entire sequence of 719 nucleotides was identical for all three sources. The percentages of relatedness of the partial 16S rRNA gene of the atypical canine ehrlichiosis agent to E. risticii, E. sennetsu, E. platys, E. equi, E. phagocytophila, E. canis, E. chaffeensis, and E. ewingii were 100.0, 98.9, 83.7, 83.0, 83.0, 82.2, 81.8, and 81.5, respectively. These data are consistent with the identity of these isolates as E. risticii. The caninotropic characteristics of naturally acquired infections due to E. risticii are herein described for the first time, and the epizootiological implications are discussed in relation to the host range of E. risticii, which may include dogs as reservoirs.

Serological titers of equine monocytic ehrlichiosis associated with gastro-intestinal disorders and serological follow-up on two endemic farms

The purpose of this work was to study the association of positive serological titers to Ehrlichia risticii, the causative agent of equine monocytic ehrlichiosis (EME) with gastro-intestinal disorders in hospitalized horses referred to The Ohio State University College of Veterinary Medicine Teaching Hospital (OSU VMTH). In addition, serological titers for E. risticii were monitored in two horse populations with endemic EME for one season to monitor temporal changes in titers. A statistically significant difference was found between the proportion of the total hospitalized horse population presented with a gastro-intestinal disorder during the study period, and study horses with IFA titers ≥1:80 with these signs ( P<0.05). No such difference was found between the proportion of the total hospital horse population presented with signs of gastro-intestinal disorder, and the study horses with IFA titers of 1:20–1:40 with these signs, suggesting that these titers may not have any clinical significance for EME ( P>0.05) Thirty-eight horses on two farms endemic for EME were tested approximately every 3 weeks, 33 of which were tested serially at least two times. Five of the 38 horses (13.2%) had IFA titers ≥1:80-two that were positive initially and three that seroconverted the study; 15 horses' titers fluctuated between negative (IFA titers <1:20) and exposed titers (1:20 through 1:40); and 18 horses remained negative throughout the study.

Retrospective evaluation of factors associated with the risk of seropositivity to Ehrlichia risticii in horses in New York state

SUMMARY A retrospective study was designed to determine the distribution of equine monocytic ehrlichiosis among the equine population in New York state, and to identify factors associated with risk of disease. Serum samples submitted to the diagnostic laboratory of the university during the period from January 1985 through December 1986 were examined for antibodies to Ehrlichia risticii, using the indirect fluorescent antibody technique. Factors evaluated included geographic origin and date of submission of the sample, and age, breed, and sex of the horse. Logistic regression analysis was used to identify which factors were significantly associated with the risk of seropositivity to E risticii, while simultaneously controlling for other factors. Of the 2,579 tested samples, 1,950 (76%) had positive results. Factors significantly associated with risk of seropositivity to E risticii were: breed of the horse (Thoroughbreds were 3 times more likely to have been exposed to E risticii, compared with non-Standardbred, non-Thoroughbred breeds); sex (female horses were 2.7 times more likely to have been exposed, compared with male horses); age of the horse (the risk of being exposed to E risticii increased with age, peaked at around 12 years, and decreased thereafter); and month of submission (horses tested during November and December had the highest odds of being seropositive [odds ratio = 2.1], and horses tested during March through April were least likely to be seropositive [odds ratio = 0.5], compared with horses tested during January and February).

Risk factors for equine monocytic ehrlichiosis seropositivity in horses in Colorado

A study was conducted in April through August 1989 whereby 318 horses were tested for immunofluorescent antibody (IFA) titers to Ehrlichia risticii. The state of Colorado was divided into four regions and an approximately equal number of horses was tested in each region. Questionnaires were completed for each horse; a multiple logistic regression model was used to identify risk factors for horses being exposed to E. risticii.Of the 318 horses tested, 48 (16%) had IFA titers of 1:40 or greater (range 1:40–1:1280). There was no association between the region of Colorado and seropositivity. Other variables which were not identified as risk factors included gender, transportation to other parts of Colorado, transportation to other states and exposure to other horses. When the variables were examined individually, older horses, Quarter horses, horses housed at a boarding stable, and horses having access to pasture or direct access to a stream, pond or lake were more likely to have positive titers. Horses confined to a stall with a run were less likely to be exposed to E. risticii. All apparent associations disappeared when the factors were tested in the multiple logistic regression model.

Diagnostic application of polymerase chain reaction for detection of Ehrlichia risticii in equine monocytic ehrlichiosis (Potomac horse fever)

Genomic amplification by the polymerase chain reaction (PCR) was used to identify a unique genomic sequence of Ehrlichia risticii directly in DNA isolated from peripheral-blood buffy coat cells of E. risticii-infected horses (Potomac horse fever) and from infected cell cultures. A specific primer pair, selected from a cloned, species-specific, 1-kb DNA fragment of the E. risticii genome as a template, was used for the amplification of the target DNA of 247 bp. The optimal number of 40 PCR cycles, determined by analyzing an amplification profile obtained with a constant Taq polymerase concentration, was used to achieve maximum amplification of the E. risticii DNA segment. Efficient amplification of target DNA was achieved with specimens processed by either the phenol extraction or rapid lysis method. The specificity of the amplified DNA product was confirmed by the proper size (247 bp) and appropriate restriction enzyme cleavage pattern of the amplified target DNA, as well as by the specific hybridization signal obtained by using a PCR-amplified 185-bp internal DNA probe. A 10(5)- to 10(6)-fold amplification of target DNA, which allowed detection of E. risticii from as few as two to three infected cells in culture and from a very small volume of buffy coat cells from infected horses, was achieved. This PCR amplification procedure was found to be highly specific and sensitive for the detection of E. risticii for the study of Potomac horse fever.

Serosurvey of horses with evidence of equine monocytic ehrlichiosis

Summary In August 1986, an extensive serosurvey for prevalence of IgG and IgM antibodies against Ehrlichia risticii, the causative agent of equine monocytic ehrlichiosis (eme), was performed at 2 Ohio racetracks, River Downs (rd) and Beulah Park (bp). Of 840 horses at rd and 574 at bp, 13 and 20%, respectively, were IgG antibody-positive (by indirect fluorescent antibody test results), with antibody titer ranging from 1:20 to 1:10,240. The titer observed at highest frequency at both racetracks was 1:80. A higher proportion of horses was ill at rd (operating during the summer months) than at bp (winter track). Of ill horses, 41% (24/58) at rd and 58% (11/19) at bp were seropositive. At rd, 70% (589/840) of all horses and 95% (102/107) of IgG seropositive horses had been stabled only at rd during the month prior to testing. Analysis of these sera by use of an elisa to detect IgM antibody against E risticii antigen indicated that at rd, 42% (57/137) of the seropositive horses were IgM seropositive. At bp, 17% (20/120) of seropositive horses were IgM seropositive. The larger number of IgM seropositive horses at rd indicates that more horses were recently infected at rd than at bp (P = 0.0001). Therefore, at least half the seropositive horses at rd seemed to have acquired the infection at rd. These serosurvey data also indicate that at bp and rd, 78% (85/109) and 91% (111/122) of IgG seropositive horses, respectively, had subclinical infection. At ≤ 1:40 titer, there was no difference in seropositive rates between healthy and ill horses. However, the higher the titer, the greater the difference in seropositive rates between healthy and ill horses. Thus, the higher the titer, the more reliable the serodiagnosis of eme seems to be. In contrast, sera submitted from 285 horses in Ohio for serologic testing for E risticii had a higher (63%) positivity rate than sera from horses of the racetrack serosurvey. Of 120 horses with clinical signs compatible with eme, 69% were seropositive. Typical clinical signs of eme in seropositive horses were those of enteric disease (72%; 60/83), with laminitis being a secondary sign (12%; 10/83). Results indicate that seroprevalence is much greater than prevalence of clinical disease. Combination of IgG and IgM testing with horse movement history should help to identify the location of actual infection.