Case Report: Detection of Shamonda Virus in Two Horses with Neurological Signs in Germany

1. Introduction

Since mid-June 2026, Shamonda virus (SHAV), an Orthobunyavirus belonging to the Simbu serogroup and the species Orthobunyavirus schmallenbergense, has emerged in Europe and has been detected in numerous herds of sick cattle [1,2]. Two distinct events of SHAV incursion were identified: SHAV clade Europe 1 (SHAV-EU1) circulating primarily in France, Switzerland and southern Germany and SHAV clade Europe 2 (SHAV-EU2) circulating mainly in the northwest of Germany [3]. As most other viruses of the Simbu serogroup, SHAV is also transmitted by Culicoides biting midges [1,4]. Since June 2026, mild clinical signs such as fever and diarrhea have been reported in cattle, as well as a marked decrease in milk yield in dairy cows [2]. In addition, SHAV has been detected in sheep, goats, and New World camelids, although the clinical outcome in these species remains unknown [5,6].

Soon after the virus was detected in European cattle, the Universities of Bern and Zurich reported six horses with neurological signs in which SHAV was detected in brain tissues [7]. Similarly, SHAV was detected in a horse with neurological abnormalities in the German federal state of Rhineland-Palatinate [5]. Before these, no cases of SHAV in horses have been reported. However, Shuni virus (SHUV), also a member of the Simbu serogroup, has been associated with neurological disease in both horses and cattle [8,9]. Currently, the data available on postmortem and intravital diagnosis of SHAV in horses is very limited. Scientific reports about the pathohistological changes in horses are missing as well as images of lesions or data on the tissues in which SHAV could be detected. SHAV-specific commercial antibody tests are also not yet available.

Here we report two cases of horses with severe neurological signs in which SHAV was detected, providing further insights into the susceptibility and clinical presentation of SHAV in horses.

2. Materials and Methods

2.1. Case Description

In early September, two horses from different municipalities in southern Germany were submitted for pathological examination to determine the cause of disease. Horse A was a 28-year-old Shetland pony mare that reportedly broke out of the stable the night before, fell into a creek, and was rescued from the creek with visible head injuries. That same evening, a seizure-like episode was observed in the stable, and the pony was euthanized.

Horse B was a 21-year-old mixed-breed pony mare who presented with apathy, teeth chattering, and fever (39 °C). She was treated with Metacam, cortisone, and an antibiotic. Over the next two days, her general condition continued to deteriorate. The pony exhibited balance problems, head pressing and was euthanized three days after the onset of neurological signs. Both horses were kept in open stable housing, and their owners did not observe any neurological signs prior to the onset of the illness described above. No other horses from the two farms have shown neurological signs so far.

2.2. Laboratory Investigations

Pathological-anatomical and histopathological examinations were performed on both horses. For the histopathological examination, all organs were fixed in 10% buffered formalin for at least two days and subsequently embedded in paraffin, processed routinely for histologic examination and stained with hematoxylin and eosin (H.E.). In addition, organs were taken for further examination. For Horse A and B cerebrum, brainstem, and cerebellum were removed and pooled. For Horse B, atlanto-occipital cerebrospinal fluid was collected postmortem and prior to necropsy. In addition, the cerebrum, cerebellum, brainstem, spinal cord, heart, lungs, liver, spleen and kidneys were individually collected, as well as EDTA blood and serum. Following nucleic acid extraction using the IndiMag Pathogen Kit (Indical Bioscience, Leipzig, Germany), the samples were tested using a modified version of the RT-qPCR assay described by Tauscher et al. (Sathu-N3-RT-qPCR) [10] that enables the detection of SHAV, Sathuperi virus and Schmallenberg virus (SBV). To exclude SBV, the only Simbu serogroup virus circulating in Central Europe prior to the emergence of SHAV, the samples were tested by the virotype SBV 2.0 RT-PCR Kit (INDICAL BIOSCIENCE GmbH, Leipzig, Germany) according to manufacturer’s instructions. Both pooled brain samples were additionally tested by a commercial RT-qPCR kit offered for the detection of the SHAV clade circulating in southern Germany, Switzerland and France (SHAV-EU1) (ID Gene™ SHLV Duplex, Innovative Diagnostics, Grabels, France). Other potential pathogens that can cause neurological signs, like equine herpesvirus 1, equine herpesvirus 4, borna disease virus 1, West Nile virus, and tick-borne encephalitis virus were tested as differential diagnosis, by PCR as previously described [11,12,13,14].

Furthermore, the serum of Horse B was tested for the presence of antibodies using the N-protein based ID Screen Schmallenberg virus Competition Multi-species ELISA Kit (Innovative Diagnostics, Grabels, France), which enables the detection of antibodies against several members of the Simbu serogroup. Photometric measurements were performed using a Tecan Sunrise microplate reader, and samples with an S/N% value of ≤ 40% were considered positive, according to the manufacturer’s instructions.

To obtain additional data, the available results from samples taken since early August from cattle exhibiting clinical signs typical of SHAV, such as fever, diarrhea or decreased milk production were analyzed in order to determine a connection between SHAV in cattle and the two cases in horses.

3. Results

3.1. Autopsy

Horse A was obese, and Horse B was in good body condition. Both horses exhibited severe, acute, multifocal skin erosion on the skull with subcutaneous hematomas. The skull vault was intact and unremarkable in both animals. In Horse A, the brain was macroscopically unremarkable, and the spinal cord was not examined. In Horse B, acute, multifocal, petechial hemorrhages were found in the area of the brainstem. Other parts of the brain were macroscopically unremarkable. Acute, extensive unilateral hemorrhages primarily in the substantia grisea were found in the initial segment of the thoracic spinal cord (Figure 1a and b).

In both horses, the stomach was empty, and the intestinal tract contained only a small amount of intestinal content. Furthermore, no macroscopically abnormal changes were found in the other organs.

3.2. Histopathological Examination

Histopathological examination of brain stem of both horses revealed multifocal, severe, subacute lymphohistiocytic perivasculitis with expansion of the Virchow-Robin spaces by many lymphocytes and few (Horse B) to many (Horse A) macrophages that multifocally extend into the surrounding parenchyma (Figure 1c and d). Oligofocal, mild to moderate, subacute hemorrhages were found adjacent to the blood vessels with partial resorption by macrophages (Horse A). Multifocal some lymphocytic infiltrates were detectable in the vessel wall accompanied by edema of the surrounding tissue as well as oligofocal, perivascular accumulation of homogeneous eosinophilic proteinaceous material, consistent with plasma protein extravasation (Figure 1b). Multifocally, there were glial nodules with focal aggregation of glial cells, with overall increased numbers of glial cells and neuronophagia (Figure 1c and d). In horse A, a moderate number of plasma cells were also present in the inflammatory infiltrates. Similar but less severe lesions were observed in the cerebrum (Figure 1e).

In the cerebellum of both horses, multifocal, mild- to moderate lymphohistiocytic perivasculitis, partially extending in the parenchyma, were observed. In addition, multifocal (Horse B) to diffuse (Horse A), mild (Horse B) to moderate (Horse A) subacute lymphohistiocytic meningitis was detected in the cerebellum (Figure 1e). In Horse A, multifocal, mild, subacute lymphoplasmahistiocytic meningitis was also observed in the cerebral cortex. The spinal cord was examined only in Horse B, and changes could be detected in all examined segments (cervical, thoracic, and lumbar), consistent with a multifocal, mild acute to subacute lymphohistiocytic vascular and perivascular infiltration, with massive extravasation of erythrocytes, particularly in the areas where hemorrhages were observed macroscopically (Figure 1b). In these areas, vessels with mild acute lymphohistiocytic infiltrates but no hemorrhages were also identified, as well as hemorrhages without adjacent vessels affected by lymphohistiocytic infiltrates. In addition, oligofocal glial nodules in the cervical segment of the spinal cord were detected. Similar, albeit less pronounced, inflammatory lesions were also found in the lumbar segment.

No relevant histopathological changes were observed in the other organs examined (heart, lungs, liver, kidneys, spleen), except for diffuse, acute, moderate hyperemia and multifocal, acute, mild alveolar edema in the lungs.

3.3. Further Diagnostic Testing

For both horses the pooled brain material tested positive by the Sathu-N3-PCR RT-qPCR but negative by the virotype SBV 2.0 RT-PCR Kit. In Horse B, the cerebrum, cerebellum, brainstem, spinal cord, and cerebrospinal fluid also tested positive by Sathu-N3-PCR RT-qPCR; the results are shown in Table 1.

In addition, the brain pool samples tested positive with the SHAV-specific ID Gene™ SHLV Duplex RT-qPCR with cq values of 23.02 (Horse A) and 24.70 (Horse B), respectively.

The serum of Horse B was additionally tested for antibodies by ELISA (ID Screen^® Schmallenberg Virus Competition, Innovative Diagnostics, Grabels, France) and yielded a positive result with an S/N% value of 7.8%.

3.4. Differential Diagnostics

As part of differential diagnostics, PCR testing ruled out equine herpesvirus 1, equine herpesvirus 4, borna disease virus 1, West Nile virus, and tick-borne encephalitis virus as potential causes of disease in both horses. In addition, bacteriological examination of the brain of horse B revealed no bacterial growth.

3.5. Additional Data Collected from the Study Region

Since the first detection of SHAV in Germany, numerous samples, primarily from cattle, have been submitted for testing from the region where both horses were kept. Based on these samples, we were able to confirm the occurrence of SHAV-positive cases in cattle in the vicinity of the two equine cases described here. This data was used to better estimate the timeline and the possible duration from infection till onset of the clinical signs (see supplementary material).

In southern Germany, the first cattle blood samples in which SHAV was detected were collected in late July [1]. However, given that clinical signs had already been reported before the first confirmed detection of the virus, circulation in the region might began earlier. In the vicinity of the stable of Horse A, SHAV infections in cattle were detected between 29 days and 8 days before the horse was euthanized. In the vicinity of the stable where Horse B was kept, SHAV infections in cattle were detected between 30 days before and 21 days before the horse was euthanized (supplementary material).

4. Discussion

Based on the exclusion of other viral and bacterial pathogens known to cause CNS disorders in horses and the detection of SHAV-EU1 in the affected tissue with high genome loads, SHAV is the most likely cause of the disease in both horses. However, further studies are needed to strengthen the association between the CNS lesions and virus detection, e.g., using RNA in situ hybridization.

In Horse B, extended testing of various tissue samples was performed and showed that SHAV was limited to the central nervous system. The cerebrospinal fluid tested only weakly positive, which is consistent with reports from Switzerland, where the pathogen could not be detected in the cerebrospinal fluid [7]. It should be also noted, that the cerebrospinal fluid was collected postmortem; therefore, contamination resulting from the decomposition of CNS tissue cannot be ruled out. As reported from the cases in Switzerland, SHAV could not be detected in the blood of both horses at the time point of sample collection [7]. This is consistent with other equine neurotropic arboviruses, such as WNV or TBEV, in which severe neurological disorder typically develops after the viremic phase has subsided [15,16,17]. In addition, the closely related SBV induces only a very short viremia of a few days in its target species [18]. A similarly transient viremic phase may also occur during SHAV-infection in horses and could provide a plausible explanation for the diagnostic findings observed in animals with encephalitis.

Furthermore, antibodies were detected in Horse B using the commercially available ID Screen Schmallenberg virus Competition Multi-species ELISA Kit, which enables the detection of antibodies against several members of the Simbu serogroup including SHAV [19]. Still the antibody test can only indicate that the horse has been exposed to a simbuvirus, but cannot confirm whether an infection of the central nervous system is present.

Before SHAV has been detected in Europe in 2026, there had been no reports of infections in horses, even though SHAV had been circulating in other regions of the world for decades [20]. When comparing the clinical course of the cases described here with other reports of SHAV in horses in Europe, we find that both horses in our case report exhibited clinically noticeable neurological signs although these manifested differently in both horses. Similarly, the horses examined in Switzerland exhibited balance disturbances and drowsiness, as described for Horse B [7]. In addition, hypersensitivity to touch and other stimuli was reported, which is consistent with the seizure episodes observed in Horse A [7]. However, apart from the press releases from the Universities of Bern and Zurich regarding horses with neurological abnormalities, as well as the report of a case in Rhineland-Palatinate, Germany, there are currently no known reports of SHAV causing neurological signs in horses [5,7]. Nevertheless, another related virus belonging to the Simbu serogroup, namely SHUV, has been reported to cause neurological signs in cattle and horses [8,9]. The clinical signs described from affected horses from South Africa included fever, ataxia, and episodes of muscle spasms, and are thus partially consistent with the clinical signs of SHAV infection in horses [7,8].

The reported SHUV-induced neurological cases in cattle involved young animals (5 and 16 months old) in which no abnormalities were detected during macroscopic examination of the brain [9], which is in contrast to the hemorrhages observed in the brainstem and thoracic segment of the spinal cord of horse B. However, histological examination revealed perivascular cuffs in the brainstem and cerebrum, with neutrophil involvement in both animals [9]. In addition, cerebral hemorrhages were observed histologically in one cattle [9]. Perivascular cuffing and hemorrhages were also found in these brain regions in our horse cases; however, they were significantly more pronounced in the brainstem and exhibited a different cellular composition of the inflammation (lymphohistiocytic). Furthermore, in one of the cattle, infiltrates were found in the cerebellar meninges, similar to those observed in our horses, though with a different composition of inflammatory cells. In addition, gliosis was found in both cattle, one cattle exhibited cerebral neuronal necrosis with neuronophagia as well as aggregates of lymphocytes and neutrophils. The histologic lesions described for SHUV in cattle with neurological signs thus resembles the changes we observed in terms of the localization of the lesions; however, the severity and cellular composition of the inflammation differ. In an experimental study of cattle infected with SHUV showing no clinical neurological signs, the animals developed non-suppurative meningoencephalitis with gliosis, which resembles the changes observed in our horses in regard to lesion and cellular composition [21].

Another factor that must not be overlooked is that both horses exhibited erosions on the head indicating trauma, likely related to the neurological signs described. Therefore, in the differential diagnosis—particularly for the pronounced hemorrhages in the spinal cord with only mild perivascular lymphohistiocytic infiltration—a traumatic etiology with resorptive inflammation must also be considered. Nevertheless, perivascular infiltrates without hemorrhages were also found in that area, which argues against a purely traumatic etiology.

An important open question is whether SHAV infection may also cause neurological disease in other susceptible species, such as cattle or sheep, or whether milder neurological manifestations occur that currently remain unrecognized, similarly to what was observed in infection experiments with SHUV in cattle. Although the cattle did not show any neurological signs, four of the six infected cattle developed a non-suppurative encephalitis [21]. It will therefore be essential to closely monitor such cases, to develop and validate improved serological assays for the retrospective detection of SHAV-specific antibodies, and to perform experimental infection studies in ruminants to better define the clinical spectrum, tissue tropism, and pathogenesis of SHAV infection.

5. Conclusions

We report on two cases in which SHAV could be detected in the CNS tissue of horses, associated with severe neurological signs and histopathological changes. The detection of SHAV in the cerebrospinal fluid is only conclusive following in vivo sampling and in positive cases. The viral load is very low, as seen in Horse B, or not detectable at all, as seen in the horses in Switzerland [7], making ante-mortem diagnosis based on cerebrospinal fluid a highly challenging process. Postmortem, CNS material is very suitable for diagnostic purposes. A high viral genome load and clearly visible pathohistological changes can be found particularly in the brainstem.

Overall, the prevalence of SHAV in horses should be further investigated. It is imperative to investigate the frequency with which equines become infected and the probability that they will subsequently develop neurological disorders and under which circumstances infections remain asymptomatic.