Equine Herpesvirus-1 Myeloencephalopathy.

Although equine herpesvirus myeloencephalopathy (EHM) is a relatively uncommon manifestation of equine herpesvirus-1 (EHV-1) infection, it can cause devastating losses during outbreaks. Antemortem diagnosis of EHM relies mainly on the molecular detection of EHV-1 in nasal secretions and blood. Management of horses affected by EHM is aimed at supportive nursing and nutritional care, at reducing central nervous system inflammation and preventing thromboembolic sequelae. Horses exhibiting sudden and severe neurologic signs consistent with a diagnosis of EHM pose a definite risk to the surrounding horse population. Consequently, early intervention to prevent the spread of infection is required.

The neuropathology of fatal encephalomyelitis in human Borna virus infection.

After many years of controversy, there is now recent and solid evidence that classical Borna disease virus 1 (BoDV-1) can infect humans. On the basis of six brain autopsies, we provide the first systematic overview on BoDV-1 tissue distribution and the lesion pattern in fatal BoDV-1-induced encephalitis. All brains revealed a non-purulent, lymphocytic sclerosing panencephalomyelitis with detection of BoDV-1-typical eosinophilic, spherical intranuclear Joest-Degen inclusion bodies. While the composition of histopathological changes was constant, the inflammatory distribution pattern varied interindividually, affecting predominantly the basal nuclei in two patients, hippocampus in one patient, whereas two patients showed a more diffuse distribution. By immunohistochemistry and RNA in situ hybridization, BoDV-1 was detected in all examined brain tissue samples. Furthermore, infection of the peripheral nervous system was observed. This study aims at raising awareness to human bornavirus encephalitis as differential diagnosis in lymphocytic sclerosing panencephalomyelitis. A higher attention to human BoDV-1 infection by health professionals may likely increase the detection of more cases and foster a clearer picture of the disease.

Viral genes and cellular markers associated with neurological complications during herpesvirus infections.

Despite the importance of neurological disorders associated with herpesviruses, the mechanism by which these viruses influence the central nervous system (CNS) has not been definitively established. Owing to the limitations of studying neuropathogenicity of human herpesviruses in their natural host, many aspects of their pathogenicity and immune response are studied in animal models. Here, we present an important model system that enables studying neuropathogenicity of herpesviruses in the natural host. Equine herpesvirus type 1 (EHV-1) is an alphaherpesvirus that causes a devastating neurological disease (EHV-1 myeloencephalopathy; EHM) in horses. Like other alphaherpesviruses, our understanding of virus neuropathogenicity in the natural host beyond the essential role of viraemia is limited. In particular, information on the role of different viral proteins for virus transfer to the spinal cord endothelium in vivo is lacking. In this study, the contribution of two viral proteins, DNA polymerase (ORF30) and glycoprotein D (gD), to the pathogenicity of EHM was addressed. Furthermore, different cellular immune markers, including alpha-interferon (IFN-α), gamma-interferon (IFN-γ), interleukin-10 (IL-10) and interleukin-1 beta (IL-1ß), were identified to play a role during the course of the disease.

Evaluation of Non-Invasive Sampling Techniques for the Molecular Surveillance of Equid Herpesviruses in Yearling Horses.

BACKGROUND: Equid alphaherpesvirus 1 (EHV-1) is a highly contagious respiratory tract pathogen of horses, and infection may be followed by myeloencephalopathy or abortion. Surveillance and early detection have focused on PCR assays using less tolerated nasal swabs. Here, we assess non-invasive non-contact sampling techniques as surveillance tools in naturally equid gammaherpesvirus 2-shedding horses as surrogates for EHV-1. METHODS: Horses were individually housed for 10 h periods on 2 consecutive days. Sampling included nasal swabs, nostril wipes, environmental swabs, droplet-catching devices, and air sampling. The latter was completed via two strategies: a combined air sample collected while going from horse to horse and a collective air sample collected at a stationary central point for 6 h. Samples were screened through quantitative PCR and digital PCR. RESULTS: Nine horses on day 1 and 11 horses on day 2 were positive for EHV-1; overall, 90.9% of the nostril wipes, 81.8% of the environmental surfaces, and 90.9% of the droplet-catching devices were found to be positive. Quantitative analysis showed that the mean DNA copies detection per cm2 of nostril wipe sampled concentration (4.3 × 105 per day) was significantly (p < 0.05) comparable to that of nasal swabs (3.6 × 105 per day) followed by environmental swabs (4.3 × 105 per day) and droplet catchers (3.5 × 103 per day), respectively. Overall, 100% of the air samples collected were positive on both qPCR and dPCR. In individual air samples, a mean concentration of 1.0 × 104 copies of DNA were detected in per m3 air sampled per day, while in the collective air samples, the mean concentration was 1.1 × 103. CONCLUSIONS: Environmental samples look promising in replacing direct contact sampling. Environmental and air sampling could become efficient surveillance tools at equestrian events; however, it needs threshold calculations for minimum detection levels.

Taylorella equigenitalis in Icelandic intact males compared with other horse breeds using natural cover.

BACKGROUND: Contagious equine metritis (CEM) is caused by Taylorella equigenitalis. It is a venereal disease that is detected in some breeds more than others and can cause temporary infertility with substantial costs for regular testing, sanitation and retesting. There was a perceived increase in T. equigenitalis-positive cases in Icelandic intact males where natural cover is common. OBJECTIVES: We aimed to investigate the prevalence of T. equigenitalis in Icelandic intact males and compare to draught horse and Haflinger intact males. We hypothesised that prevalence of T. equigenitalis is higher in Icelandic compared with draught and Haflinger intact males. STUDY DESIGN: Cross sectional. METHODS: Swabs from 76 Icelandic, 35 Haflinger, and 51 draught horse intact males were collected on 38 different farms and analysed by qPCR. Animals were further stratified into active breeding and non-breeding animals and age groups (1.5-7.0 and 8.0-26.0 years). Fisher's exact tests and mixed effect logistic regression with 'farm' as random effect were used to estimate differences in odds for T. equigenitalis-positive test results. RESULTS: The overall prevalence of T. equigenitalis in included intact males was 16.7% (27/162). The odds for T. equigenitalis-positive intact males were significantly higher in Icelandic compared with draught and Haflinger intact males (Odds ratio [OR] = 6.42, 95% confidence interval (CI) = 1.43-28.8, p = 0.02). Odds for T. equigenitalis-positive intact males were significantly lower in active breeding compared with non-breeding animals (OR = 0.09, 95% CI = 0.01-0.54, p = 0.009). Age had no significant influence on test results. MAIN LIMITATIONS: Convenience sampling with regional restrictions to Southern Germany and Austria, small sample size. CONCLUSIONS: Significantly higher odds for T. equigenitalis-positive intact males were found within Icelandic over draught and Haflinger and within non-breeding animals compared with active breeding animals. Findings suggest that non-breeding animals could be a reservoir for T. equigenitalis. Testing for CEM should therefore be routinely performed in Icelandic horses prior to breeding and investigations into epidemiology and reservoirs on affected farms should be initiated.

Updated ACVIM consensus statement on equine herpesvirus-1.

Equine herpesvirus-1 (EHV-1) is a highly prevalent and frequently pathogenic infection of equids. The most serious clinical consequences of infection are abortion and equine herpesvirus myeloencephalopathy (EHM). The previous consensus statement was published in 2009 and considered pathogenesis, strain variation, epidemiology, diagnostic testing, vaccination, outbreak prevention and control, and treatment. A recent survey of American College of Veterinary Internal Medicine large animal diplomates identified the need for a revision to this original consensus statement. This updated consensus statement is underpinned by 4 systematic reviews that addressed key questions concerning vaccination, pharmaceutical treatment, pathogenesis, and diagnostic testing. Evidence for successful vaccination against, or effective treatment of EHV-1 infection was limited, and improvements in experimental design and reporting of results are needed in future studies of this important disease. This consensus statement also updates the topics considered previously in 2009.

Pharmacologic interventions for the treatment of equine herpesvirus-1 in domesticated horses: A systematic review.

BACKGROUND: Equine herpes virus type 1 (EHV-1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death. REVIEW QUESTION: Does pharmacological therapy decrease either the incidence or severity of disease or infection caused by EHV-1 in domesticated horses? METHODS: A systematic review was preformed searching AGRICOLA, CAB Abstracts, Cochrane, PubMed, Web of Science, and WHO Global Health Index Medicus Regional Databases to identify articles published before February 15, 2021. Selection criteria were original research reports published in peer reviewed journals, and studies investigating in vivo use of therapeutic agents for prevention or treatment of EHV-1 in horses. Outcomes assessed included measures related to clinical outcomes that reflect symptomatic EHV-1 infection or virus infection. We evaluated risk of bias and performed a GRADE evaluation of the quality of evidence for interventions. RESULTS: A total of 7009 unique studies were identified, of which 9 met the inclusion criteria. Two studies evaluated valacyclovir or small interfering RNAs, and single studies evaluated the use of a Parapoxvirus ovis-based immunomodulator, human alpha interferon, an herbal supplement, a cytosine analog, and heparin. The level of evidence ranged between randomized controlled studies and observational trials. The risk of bias was moderate to high and sample sizes were small. Most studies reported either no benefit or minimal efficacy of the intervention tested. CONCLUSIONS AND CLINICAL IMPORTANCE: Our review indicates minimal or limited benefit either as a prophylactic or post-exposure treatment for any of the studied interventions in the mitigation of EHV-1-associated disease outcome.

Experimental infection with equine herpesvirus type 1 (EHV-1) induces chorioretinal lesions.

Equine herpesvirus myeloencephalitis (EHM) remains one of the most devastating manifestations of equine herpesvirus type 1 (EHV-1) infection but our understanding of its pathogenesis remains rudimentary, partly because of a lack of adequate experimental models. EHV-1 infection of the ocular vasculature may offer an alternative model as EHV-1-induced chorioretinopathy appears to occur in a significant number of horses, and the pathogenesis of EHM and ocular EHV-1 may be similar. To investigate the potential of ocular EHV-1 as a model for EHM, and to determine the frequency of ocular EHV-1, our goal was to study: (1) Dissemination of virus following acute infection, (2) Development and frequency of ocular lesions following infection, and (3) Utility of a GFP-expressing virus for localization of the virus in vivo. Viral antigen could be detected following acute infection in ocular tissues and the central nervous system (experiment 1). Furthermore, EHV-1 infection resulted in multifocal choroidal lesions in 90% (experiment 2) and 50% (experiment 3) of experimentally infected horses, however ocular lesions did not appear in vivo until between 3 weeks and 3 months post-infection. Taken together, the timing of the appearance of lesions and their ophthalmoscopic features suggest that their pathogenesis may involve ischemic injury to the chorioretina following viremic delivery of virus to the eye, mirroring the vascular events that result in EHM. In summary, we show that the frequency of ocular EHV-1 is 50-90% following experimental infection making this model attractive for testing future vaccines or therapeutics in an immunologically relevant age group.

Viremia and nasal shedding for the diagnosis of equine herpesvirus-1 infection in domesticated horses.

BACKGROUND: Equine herpesvirus type 1 (EHV-1) infection is associated with upper respiratory disease, EHM, abortions, and neonatal death. RESEARCH QUESTIONS: Are nasal secretions a more sensitive biological sample compared to blood for the detection of EHV-1 infection? How long is EHV-1 detectable after primary infection by PCR? METHODS: MedLine and Web of Science searches identified original peer-reviewed reports evaluating nasal shedding and viremia using virus isolation methods or PCR published in English before October 9, 2023. RESULTS: Sixty experimental and 20 observational studies met inclusion criteria. EHV-1 detection frequency by qPCR in nasal secretions and blood from naturally-infected horses with fever and respiratory signs were 15% and 9%, respectively; qPCR detection rates in nasal secretions and blood from horses with suspected EHM were 94% and 70%, respectively. In experimental studies the sensitivity of qPCR matched or exceeded that seen for virus isolation from either nasal secretions or blood. Detection of nasal shedding typically occurred within 2 days after EHV-1 inoculation with a detection period of 3 to 7 days. Viremia lasted 2 to 7 days and was usually detected ≥1 days after positive identification of EHV-1 in nasal secretions. Nasal shedding and viremia decreased over time and remained detectable in some horses for several weeks after inoculation. CONCLUSIONS AND CLINICAL IMPORTANCE: Under experimental conditions, blood and nasal secretions have similar sensitivity for the detection of EHV-1 when horses are sampled on multiple consecutive days. In contrast, in observational studies detection of EHV-1 in nasal secretions was consistently more successful.

Relationship between equine herpesvirus-1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review.

BACKGROUND: Equine herpes virus type 1 (EHV-1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death. OBJECTIVE: To determine if there is an association between the level and duration of EHV-1 viremia and either abortion or equine herpesvirus myeloencephalopathy (EHM) in domesticated horses? METHODS: A systematic review was performed searching numerous databases to identify peer reviewed reports that evaluated viremia and EHM, or viremia and abortion published before January 19, 2021. Randomized controlled trials and observational studies were assessed for risk of bias or publication quality. RESULTS: A total of 189 unique studies were identified, of which 34 met the inclusion criteria. Thirty studies evaluated viremia and neurologic outcomes including 4 observational studies. Eight experimental studies examined viremia and abortion, which used the Ab4 and OH03 virus strains or recombinant Ab4 derivatives. Incidence rates for both EHM and abortion in experimental studies varied among the studies as did the level of evidence. Viremia was generally detectable before the onset of either EHM or abortion. Risk of bias was generally low to moderate, sample sizes were small, and multiple studies reported negative outcome data. CONCLUSIONS AND CLINICAL IMPORTANCE: The results of this study support that viremia is regularly present before EHM or abortion occurs. However, no inferences could be made about the relationship between the occurrence of either neurological signs or abortion and the magnitude or duration of viremia.

Vaccination for the prevention of equine herpesvirus-1 disease in domesticated horses: A systematic review and meta-analysis.

BACKGROUND: Equine herpes virus type 1 (EHV-1) infection in horses is associated with respiratory and neurologic disease, abortion, and neonatal death. HYPOTHESIS: Vaccines decrease the occurrence of clinical disease in EHV-1-infected horses. METHODS: A systematic review was performed searching multiple databases to identify relevant studies. Selection criteria were original peer-reviewed research reports that investigated the in vivo use of vaccines for the prevention of disease caused by EHV-1 in domesticated horses. Main outcomes of interest included pyrexia, abortion, neurologic disease, viremia, and nasal shedding. We evaluated risk of bias, conducted exploratory meta-analyses of incidence data for the main outcomes, and performed a GRADE evaluation of the quality of evidence for each vaccine subtype. RESULTS: A total of 1018 unique studies were identified, of which 35 met the inclusion criteria. Experimental studies accounted for 31/35 studies, with the remainder being observational studies. Eight vaccine subclasses were identified including commercial (modified-live, inactivated, mixed) and experimental (modified-live, inactivated, deletion mutant, DNA, recombinant). Risk of bias was generally moderate, often because of underreporting of research methods, and sample sizes were small leading to imprecision in the estimate of the effect size. Several studies reported either no benefit or minimal vaccine efficacy for the primary outcomes of interest. Meta-analyses revealed significant heterogeneity was present, and our confidence in the quality of evidence for most outcomes was low to moderate. CONCLUSIONS AND CLINICAL IMPORTANCE: Our review indicates that commercial and experimental vaccines minimally reduce the incidence of clinical disease associated with EHV-1 infection.

Presumed case of "stiff-horse syndrome" caused by decreased gamma-aminobutyric acid (GABA) production in an American Paint mare.

Glutamic acid decarboxylase (GAD) converts glutamic acid into the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Increased serum GAD (auto) antibody concentrations were found in a mare with increased postural musculature tone resulting in stiffness and recumbence. The mare was treated with dexamethasone which resulted in resolution of clinical signs and decreased GAD antibody concentrations.

Evaluation and Utility of Submaximal Stimulation Intensity in Transcranial Magnetic Stimulation in the Standing Horse☆.

Transcranial magnetic stimulation (TMS) has been successfully used in horses to evaluate function and integrity of descending motor pathways in patients affected by neurological gait abnormalities. In preceding studies, lengthening latency times (LT) of cranially evoked limb muscle potentials have been considered a reliable diagnostic parameter. Standardized settings use device output signal intensities of 100%. The aim of this study was to determine the effect of submaximal stimulation intensities (SI) and to determine the minimum coil output necessary to evoke motor unit potentials. As an additional effect, lower stimulation intensities are supposed to decrease sensory irritation of the equine patient. Altogether, 36 neurologically healthy horses underwent TMS under sedation with a dome coil at stimulation intensities varying from 40% to 100% of device output intensity. Motor potentials were recorded by surface electrodes from all four limbs and LT was calculated in milliseconds. To further refine the stimulation settings, cortical motor thresholds (CMT) were assessed in triplets, using IFCN recommendations. The electromyographic recordings were evaluated in 30 horses. Increasing stimulation intensities resulted in significant (P < .05) LT shortening until application of 80% of maximal output intensity. Further increase to maximal SI of 100%, brought up no significant differences (P > .05). Gating effects were excluded as there was no difference of LT upon ascending and descending SI changes (P > .05). CMT revealed a large inter-individual variability amongst horses independent of their body size. There was a strong linearity in between CMT and LT even within submaximal SI ranges (P < .001). The inverse impact of SI on LT may be explained by deeper penetration of the magnetic field, circumvention of interposed neurons and subsequent activation of fast acting motor pathways. However, in warmblood horses a stimulation intensity of 80% coil output already appeared sufficient for reproducible activation of lower motor neurons in all limbs. Furthermore, due to the strong linear correlation of CMT and LT, the tested CMT algorithms may be used to estimate the normal LT on submaximal stimulation for equine myelopathy patients in future.

Epidemiological Aspects of Equid Herpesvirus-Associated Myeloencephalopathy (EHM) Outbreaks.

Equid Herpesvirus Myeloencephalopathy (EHM) is a multifactorial disease following an EHV-1 infection in Equidae. We investigated a total of 589 horses on 13 premises in Europe in search of risk factors for the development of EHM. We found that fever (p < 0.001), increasing age (p = 0.032), and female sex (p = 0.042) were risk factors for EHM in a logistic mixed model. Some breeds had a decreased risk to develop EHM compared to others (Shetland and Welsh ponies; p = 0.017; p = 0.031), and fewer EHV-1-vaccinated horses were affected by EHM compared to unvaccinated horses (p = 0.02). Data evaluation was complex due to high variability between outbreaks with regards to construction and environment; viral characteristics and the virus's transmissibility were affected by operational management. This study confirms earlier suspected host-specific risk factors, and our data support the benefit of high vaccine coverage at high-traffic boarding facilities.

Evaluation of immune responses following infection of ponies with an EHV-1 ORF1/2 deletion mutant.

Equine herpesvirus-1 (EHV-1) infection remains a significant problem despite the widespread use of vaccines. The inability to generate a protective immune response to EHV-1 vaccination or infection is thought to be due to immunomodulatory properties of the virus, and the ORF1 and ORF2 gene products have been hypothesized as potential candidates with immunoregulatory properties. A pony infection study was performed to define immune responses to EHV-1, and to determine if an EHV-1 ORF1/2 deletion mutant (ΔORF1/2) would have different disease and immunoregulatory effects compared to wild type EHV-1 (WT). Infection with either virus led to cytokine responses that coincided with the course of clinical disease, particularly the biphasic pyrexia, which correlates with respiratory disease and viremia, respectively. Similarly, both viruses caused suppression of proliferative T-cell responses on day 7 post infection (pi). The ΔORF1/ORF2 virus caused significantly shorter primary pyrexia and significantly reduced nasal shedding, and an attenuated decrease in PBMC IL-8 as well as increased Tbet responses compared to WT-infected ponies. In conclusion, our findings are (i) that infection of ponies with EHV-1 leads to modulation of immune responses, which are correlated with disease pathogenesis, and (ii) that the ORF1/2 genes are of importance for disease outcome and modulation of cytokine responses.

Equine Arteritis Virus (EAV) Outbreak in a Show Stallion Population.

(1) Background: Equine arteritis virus (EAV) infection causes reproductive losses and systemic vasculitis in susceptible equidae. The intact male becomes the virus' reservoir upon EAV infection, as it causes a chronic-persistent infection of the accessory sex glands. Infected semen is the main source of virus transmission. (2) Here, we describe acute EAV infection and spread in a stallion population after introduction of new members to the group. (3) Conclusions: acute clinical signs, acute phase detection of antigen via (PCR) nasal swabs or (EDTA) blood, and seroconversion support the idea of transmission via seminal fluids into the respiratory tract(s) of others. This outbreak highlights EAV's horizontal transmission via the respiratory tract. This route should be considered in a chronic-persistently infected herd, when seronegative animals are added to the group.

Apparent Breed Predilection for Equid Herpesvirus-1-Associated Myeloencephalopathy (EHM) in a Multiple-Breed Herd.

Equid herpesvirus type 1 (EHV-1) causes several outbreaks of abortion and/or equid herpesvirus-associated myeloencephalopathy (EHM) worldwide each year. EHM is of great concern, as permanent neurological gait anomalies can leave a horse unfit for future use. The study assesses the risk factors associated with the occurrence of EHM. During an unmitigated outbreak, 141 adult horses/ponies of several distinct breeds were evaluated-using multiple Bayesian logistic regression calculating the odds ratios for breed, age, and sex. In total, 33 of the 141 horses showed signs of EHM. Fjord horses and warmblood horses were overrepresented among those developing EHM. The pony breeds, Welsh and Shetland ponies, were underrepresented. In addition, age and sex were not associated with the risk for EHM. The main limitation was that it was a retrospective analysis with some flaws of documentation. It can be concluded that breed was a significant risk factor for developing EHM during this outbreak.

Tick-Borne Encephalitis Virus (TBEV) Infection in Two Horses.

A final diagnosis in a horse with clinical signs of encephalopathy can be challenging despite the use of extensive diagnostics. Clinical signs are often not pathognomonic and need to be interpreted in combination with (specific) laboratory results and epidemiological data of the geographical region of the origin of the case(s). Here we describe the diagnostic pathway of tick-borne encephalitis virus infection in two horses using established molecular diagnostic methods and a novel in situ hybridization technique to differentiate between regionally important/emerging diseases for central Europe: (i) hepatoencephalopathy, (ii) Borna disease virus, and (iii) West Nile virus infections.

Environmental persistence of equid herpesvirus type-1.

BACKGROUND: Equid herpesvirus type 1 (EHV-1) is ubiquitous in equine populations causing respiratory disease, and complications including late-term abortion and neurological disease. Eradication of EHV-1 from housing environments that typically contain unsealed wood and porous bedding materials can be challenging. However, consideration should be given to take advantage of the viral envelope's susceptibility to environmental conditions. OBJECTIVE: To determine environmental persistence of EHV-1 on materials and in environmental conditions commonly found in equine facilities. We hypothesised that environmental conditions and materials would limit environmental persistence of EHV-1 in horse housing environments. STUDY DESIGN: Experimental study. METHODS: Standard inoculum of EHV-1 strain OH03 was applied to leather, polyester-cotton fabric, two bedding materials (pinewood shavings and wheat straw) and polystyrene (plastic), and placed under three different environmental conditions (4°C, indoors and outdoors). Virus titration and quantitative PCR (qPCR) were performed at six time points between 0 and 48 hours and the number of plaque-forming units (PFUs) was determined. RESULTS: Viable EHV-1 was recovered up to 48 hours from all material-environmental condition combinations, with persistence decreasing over time. In general, outdoor environment had the greatest impact, irrespective of material tested, followed by indoor environment and 4°C. On average, wood shavings had the greatest impact on persistence, followed by leather, straw, fabric and polystyrene. MAIN LIMITATIONS: The inoculum used in this study was not in a milieu consistent with nasal secretions. As such, virus particles may have been more sensitive to the materials and/or environmental conditions evaluated. CONCLUSIONS: Environmental factors had variable effects on environmental persistence. Although there were significant reductions in PFUs within the first 3 hours, irrespective of environment-material evaluated, viable virus was still recovered at 48 hours likely representing a transmission risk. Barrier precautions should be used to prevent spread of EHV-1 from unrecognised environmental reservoirs.

Urogenital Hypoplasia and X Chromosome Monosomy in a Draft Horse Filly.

A 5-month-old draft horse filly was presented with incontinence and severe perivulvar dermatitis, which developed during the previous 2 months. Left-sided ectopic ureter entering in the caudal vaginal lumen, signs of cervix hypoplasia, and urine accumulation in the uterus were found during initial vaginal endoscopy. Therefore, a left ureter-nephrectomy was conducted under general anesthesia. Additionally, a cytogenetic examination was performed, which showed a XO monosomy with a 63,X/64,XX mosaic. This is the first case report presenting a chromosome abnormality in a draft horse filly combined with a left-sided ectopic ureter. Cytogenetic evaluation is recommended in any female horse with developmental abnormalities of the cervix, uterus, ovaries, or with irregularities of estrus.