The neurology and enterology of equine grass sickness: a review of basic mechanisms

Lipofuscin accumulates in ganglionic neurons in chronic equine dysautonomia

Lipofuscin is a complex mixture of highly oxidized, cross-linked macromolecules that accumulates in neurons with age and some neurodegenerative diseases. Equine dysautonomia (ED) is a polyneuropathy that mainly affects autonomic and enteric nervous systems, resulting in alimentary tract dysfunction. Our main aim was to determine whether neuronal lipofuscin increased with increasing duration of ED. We investigated the prevalence of lipofuscin in cranial cervical ganglia of horses with acute (AED), subacute (SED), and chronic ED (CED), young controls (of similar age to ED cases), and aged controls (n = 8 per group). We used Schmorl stain for histologic detection of lipofuscin and assessed its accumulation in neurons using image analysis software. The percentage of neurons positive for lipofuscin increased with age in individual groups and all groups combined (p < 0.001). There were fewer positive neurons in AED and SED compared to aged controls (p < 0.001) and more in CED than AED cases (p = 0.042) and young controls (p = 0.012). We found a strong positive correlation between percentage positive neurons and percentage positive area of the neuron containing lipofuscin for combined groups (p < 0.001). Although neuronal lipofuscin increased in cranial cervical ganglion in CED cases, it remains to be determined whether this is a cause or consequence of neuronal degeneration.

Cultured dissociated primary dorsal root ganglion neurons from adult horses enable study of axonal transport

Neurological disorders are prevalent in horses, but their study is challenging due to anatomic constraints and the large body size; very few host‐specific in vitro models have been established to study these types of diseases, particularly from adult donor tissue. Here we report the generation of primary neuronal dorsal root ganglia (DRG) cultures from adult horses: the mixed, dissociated cultures, containing neurons and glial cells, remained viable for at least 90 days. Similar to DRG neurons in vivo, cultured neurons varied in size, and they developed long neurites. The mitochondrial movement was detected in cultured cells and was significantly slower in glial cells compared to DRG‐derived neurons. In addition, mitochondria were more elongated in glial cells than those in neurons. Our culture model will be a useful tool to study the contribution of axonal transport defects to specific neurodegenerative diseases in horses as well as comparative studies aimed at evaluating species‐specific differences in axonal transport and survival.

Optimisation and validation of immunohistochemical axonal markers for morphological and functional characterisation of equine peripheral nerves

BACKGROUND

Horses are affected by various peripheral nerve disorders but defining their aetiology and pathophysiology is hampered by limited understanding of associated morphological and pathological changes and involvement of specific axonal types.

OBJECTIVES

To investigate the hypothesis that selected antibody markers, used in conjunction with various tissue processing methods, would enable identification of axons with different functional modalities within a range of equine peripheral nerves.

STUDY DESIGN

Optimisation and validation study.

METHODS

A range of antibodies were evaluated immunohistochemically via fluorescence confocal microscopy in cadaver equine nerve samples of primary motor, mixed or primary sensory functions (recurrent laryngeal, phrenic and plantar digital) within formalin-fixed paraffin-embedded (FFPE) and formalin-fixed frozen (FFF) tissues subjected to different antigen retrieval protocols.

RESULTS

Immunohistochemistry of FFPE-derived nerve samples with selected antibodies and specific antigen retrieval methods enabled identification of myelinated and unmyelinated axons, cholinergic, sympathetic and peptidergic axons. The recurrent laryngeal and phrenic nerves are composed of myelinated cholinergic (motor), myelinated sensory fibres, unmyelinated adrenergic (sympathetic) axons and unmyelinated peptidergic (sensory) axons. In contrast, as expected, the plantar digital nerve had no myelinated motor fibres being mainly composed of myelinated sensory fibres, unmyelinated sympathetic and unmyelinated peptidergic sensory axons.

MAIN LIMITATION

Attempts specifically to label parasympathetic fibres were unsuccessful in any nerve examined in both FFPE and FFF tissues.

CONCLUSIONS

A panel of antibody markers can be used to reveal morphological and functional properties of equine nerves. Future work should enable better characterisation of morphological changes in equine neuropathies at various stages of disease development.

Family aggregation analysis shows a possible heritable background of equine grass sickness (dysautonomia) in a Hungarian stud population

Equine grass sickness (also known as dysautonomia) is a life-threatening polyneuropathic disease affecting horses with approx. 80% mortality. Since its first description over a century ago, several factors, such as the phenotype, intestinal microbiome, environment, management and climate, have been supposed to be associated with the increased risk of dysautonomia. In this retrospective study, we examined the possible involvement of genetic factors. Medical and pedigree datasets regarding 1,233 horses with 49 affected animals born during a 23-year period were used in the analysis. Among the descendants of some stallions, the proportion of animals diagnosed with dysautonomia was unexpectedly high. Among males, the odds of dysautonomia were found to be higher, albeit not significantly, than among females. Significant familial clustering (genealogical index of familiality, P = 0.001) was observed among the affected animals. Further subgroups were identified with significant (P < 0.001) aggregation among close relatives using kinship-based methods. Our analysis, along with the slightly higher disease frequency in males, suggests that dysautonomia may have a genetic causal factor with an X-linked recessive inheritance pattern. This is the first study providing ancestry data and suggesting a heritable component in the likely multifactorial aetiology of the disease.

Ganglion Cytology: A Novel Rapid Method for the Diagnosis of Equine Dysautonomia

Equine dysautonomia (grass sickness) is characterized by autonomic neuronal degeneration and is often fatal. As outbreaks occur, rapid diagnosis is essential but confirmation currently requires histological examination. This study evaluated diagnostic accuracy of cytological examination of cranial cervical ganglion (CCG) scrapings for dysautonomia diagnosis. CCG smears from 20 controls and 16 dysautonomia cases were stained with May-Grünwald Giemsa (MGG), hematoxylin and eosin (HE), and cresyl fast violet (CFV), with HE-stained histological sections of CCG as gold standard for diagnosis. Examining all 3 stains together, the sensitivity and specificity were 100%. Occasional individual smears (4/107, 3.7%) were nondiagnostic due to low cellularity, and in a few individual smears the final diagnosis was correct but more tentative (CFV: 5/33 [15.1%], HE: 2/34 [5.9%], and MGG: 4/36 [11.1%]), due to low cellularity or suboptimal cell morphology. CCG cytology was considered reliable for rapid postmortem diagnosis of equine dysautonomia, particularly using MGG.

Equine Dysautonomia

Equine dysautonomia (ED; also known as equine grass sickness) is a neurological disease of unknown cause, which primarily affects grazing adult horses. The clinical signs reflect degeneration of specific neuronal populations, predominantly within the autonomic and enteric nervous systems, with disease severity and prognosis determined by the extent of neuronal loss. This review is primarily focused on the major clinical decision-making processes in relation to ED, namely, (1) clinical diagnosis, (2) selection of appropriate ancillary diagnostic tests, (3) obtaining diagnostic confirmation, (4) selection of treatment candidates, and (5) identifying appropriate criteria for euthanasia.

Idiopathic Fibrosis of the Tunica Muscularis of the Large Intestine in Five Horses with Colic

Histological evidence of fibrosis affecting the outer layer of the large intestinal tunica muscularis was identified in five of 32 horses affected by colic. In three cases, foci of pale eosinophilia and vacuolation of myocytes were observed. These findings are suggestive of a degenerative and fibrotic abnormality in the outer layer of the tunica muscularis of the large intestinal smooth muscle of some horses with colic.

Excitatory and inhibitory enteric innervation of horse lower esophageal sphincter

The lower esophageal sphincter (LES) is a specialized, thickened muscle region with a high resting tone mediated by myogenic and neurogenic mechanisms. During swallowing or belching, the LES undergoes strong inhibitory innervation. In the horse, the LES seems to be organized as a "one-way" structure, enabling only the oral-anal progression of food. We characterized the esophageal and gastric pericardial inhibitory and excitatory intramural neurons immunoreactive (IR) for the enzymes neuronal nitric oxide synthase (nNOS) and choline acetyltransferase. Large percentages of myenteric plexus (MP) and submucosal (SMP) plexus nNOS-IR neurons were observed in the esophagus (72 ± 9 and 69 ± 8 %, respectively) and stomach (57 ± 17 and 45 ± 3 %, respectively). In the esophagus, cholinergic MP and SMP neurons were 29 ± 14 and 65 ± 24 vs. 36 ± 8 and 38 ± 20 % in the stomach, respectively. The high percentage of nitrergic inhibitory motor neurons observed in the caudal esophagus reinforces the role of the enteric nervous system in the horse LES relaxation. These findings might allow an evaluation of whether selective groups of enteric neurons are involved in horse neurological disorders such as megaesophagus, equine dysautonomia, and white lethal foal syndrome.

Equine grass sickness

Equine grass sickness (EGS; equine dysautonomia) is a polyneuronopathy affecting both the central and the peripheral nervous systems of horses. As the name implies, EGS almost exclusively affects grazing horses, resulting in the development of a characteristic array of clinical signs, most of which can be attributed to neuronal degeneration in the autonomic and enteric nervous systems. Varying disease severities occur, largely determined by the extent of neuronal degeneration in the myenteric and submucous plexuses of the enteric nervous system. Extensive neuronal degeneration, as seen in acute and subacute forms of EGS, results in intestinal dysmotility, the severity of which is incompatible with survival. In comparison, a proportion of chronic forms of EGS, characterised by less severe neuronal degeneration, will survive. Despite extensive research efforts since EGS was first reported over 100 years ago, the precise aetiology remains elusive. This article reviews much of the scientific literature on EGS, covering epidemiology, pathology, diagnosis, treatment and aetiological hypotheses.

Effects of equine grass sickness on sympathetic neurons in prevertebral and paravertebral ganglia

Acute equine grass sickness (EGS) is a fatal disease of horses that is thought to be due to ingestion of a neurotoxic agent causing extensive damage to autonomic neurons. The aim of this study was to compare the effects of EGS on neurons in two sympathetic ganglia, the paravertebral cranial cervical ganglion (CCG) and the prevertebral coeliac/cranial mesenteric ganglion (CG/CMG). Specimens from horses with EGS and controls were obtained post mortem and processed using single and double immunofluorescence labelling for PGP 9.5 and HuC/HuD (pan-neuronal markers), TUNEL and caspase 3 (markers for apoptosis), vasoactive intestinal polypeptide (VIP) and galanin (markers of the cell body response to injury following axotomy or exposure to sympathetic neurotoxins) and tyrosine hydroxylase (TH, marker for noradrenaline synthesis). In control horses, all neurons contained PGP 9.5 and HuC/HuD. There was a significant loss of PGP 9.5 and HuC/HuD expression in samples from horses with EGS that occurred to a greater extent in the CG/CMG than the CCG. The number of caspase 3-positive neurons increased significantly in both ganglia, but TUNEL staining of sympathetic neurons was only significantly increased in the CG/CMG in EGS. No VIP was observed in any ganglia; however, there was a significant increase in galanin-positive neurons in both ganglia in EGS. In the CCG, there was a significant shift towards increased fluorescence intensity for TH, possibly indicating an initial accumulation of TH within the cell body. In contrast, TH fluorescence intensity was significantly reduced in the CG/CMG in EGS correlating with the greater loss of neurons. These results demonstrate that EGS can induce a cell body response that is similar to the response of sympathetic neurons to a chemical neurotoxin. EGS also causes loss of sympathetic neurons, some of which occurs via apoptosis. Changes were more marked in the CG/CMG than the CCG indicating that the prevertebral ganglia were affected earlier than the paravertebral ganglia in the pathological process and had undergone greater neurodegeneration.

Sympathetic innervation of the ileocecal junction in horses

The distribution and chemical phenotypes of sympathetic and dorsal root ganglion (DRG) neurons innervating the equine ileocecal junction (ICJ) were studied by combining retrograde tracing and immunohistochemistry. Immunoreactivity (IR) for tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP), substance P (SP), and neuropeptide Y (NPY) was investigated. Sympathetic neurons projecting to the ICJ were distributed within the celiac (CG), cranial mesenteric (CranMG), and caudal mesenteric (CaudMG) ganglia, as well as in the last ganglia of the thoracic sympathetic chain and in the splanchnic ganglia. In the CG and CranMG 91 +/- 8% and 93 +/- 12% of the neurons innervating the ICJ expressed TH- and DBH-IR, respectively. In the CaudMG 90 +/- 15% and 94 +/- 5% of ICJ innervating neurons were TH- and DBH-IR, respectively. Sympathetic (TH-IR) fibers innervated the myenteric and submucosal ganglia, ileal blood vessels, and the muscle layers. They were more concentrated at the ICJ level and were also seen encircling myenteric plexus (MP) and submucosal plexus (SMP) descending neurons that were retrogradely labeled from the ICJ. Among the few retrogradely labeled DRG neurons, nNOS-, CGRP-, and SP-IR nerve cells were observed. Dense networks of CGRP-, nNOS-, and SP-IR varicosities were seen around retrogradely labeled prevertebral ganglia neurons. The CGRP-IR fibers are probably the endings of neurons projecting from the intestine to the prevertebral ganglia. These findings indicate that this crucial region of the intestinal tract is strongly influenced by the sympathetic system and that sensory information of visceral origin influences the sympathetic control of the ICJ.

Evaluation of formalin-fixed ileum as the optimum method to diagnose equine dysautonomia (grass sickness) in simulated intestinal biopsies

Equine dysautonomia, or grass sickness, is a frequently fatal disease of unknown etiology, manifested as poor gastrointestinal motility and colic as a result of degenerative changes in the autonomic nervous system. Examination of ileal biopsies collected at laparotomy is currently the best antemortem diagnostic method to distinguish equine dysautonomia from colic cases, which can present with similar signs, but their value has not been previously critically evaluated. Using simulated biopsies collected postmortem from 23 cases of equine dysautonomia and 11 of colic, the sensitivity and specificity of 1-cm long, formalin-fixed ileal biopsies was 100% for the diagnosis of equine dysautonomia. There was therefore no advantage to using larger biopsies or examining jejunum either in addition to or instead of ileal biopsies. Furthermore, although cryostat sections of ileum, 1-cm long, had a sensitivity of 100%, the specificity was only 73%, meaning that 27% of cases would have been misclassified, resulting in unnecessary euthanasia. Increasing the size of the cryostat or examining jejunum in addition to ileum cryostat sections did not significantly improve the specificity. Results of the current study indicate that in diagnostic practice, 1-cm long, formalin-fixed biopsies are likely to be the most suitable for accurate diagnosis, despite the slower turnaround time compared with cryostat sections.

The role of quantitative electromyography (EMG) in horses suspected of acute and chronic grass sickness

REASONS FOR PERFORMING THE STUDY

Clinical evidence of motor neuron involvement in equine grass sickness (EGS) has not been reported.

HYPOTHESIS

Quantitative electromyography (EMG) analysis can elucidate subtle changes of the lower motor neuron system present in horses with EGS, performed ante mortem.

METHODS

Fourteen horses diagnosed clinically with acute, subacute or chronic EGS were examined and quantitative EMG performed. Previously published data on healthy horses and horses with proven lower motor neuron disease (LMND) were used as controls. In 8 horses post mortem examination was performed, and in 7 muscle biopsies of the lateral vastus muscle underwent histopathology and morphometry.

RESULTS

Clinical electrophysiological evidence of neuropathy was present in 12 horses. Analysis of data from the first 4 horses resulted in 95% confidence intervals (CI) of nontransformed data for motor unit action potential (MUP) duration in subclavian, triceps and lateral vastus muscle of 11.0-13.7, 14.8-20.3 and 12.2-17.2 msecs, respectively, and for MUP amplitude 291-453, 1026-1892 and 957-1736 microV, respectively. For number of phases the 95% CI was 3.6-4.4, 2.9-3.6 and 2.9-3.4, respectively, and for number of turns 5.0-6.5, 4.3-5.3 and 3.7-4.6, respectively. No changes in duration of insertional activity were measured. Pathological spontaneous activity was observed in all horses. EGS as evidenced by degenerative changes in the autonomic ganglia in combination with minor degenerative changes of the spinal lower motor neurons was observed on post mortem examination in all 8 available autopsies. In muscle biopsies of 4 out of 7 horses changes consistent with slight neurogenic atrophy were found.

CONCLUSIONS AND POTENTIAL RELEVANCE

EMG results demonstrated the presence of a neuropathy of skeletal muscles in all horses suspected to have EGS. The combination of clinical and electrophysiological evidence may aid differential diagnosis of neurogenic disease in cases of weight loss and colic.

Immunohistochemical characterization and quantitative analysis of neurons in the myenteric plexus of the equine intestine

The present study was performed on whole-mount preparations to investigate the chemical neuroanatomy of the equine myenteric plexus throughout its distribution in the intestinal wall. The objective was to quantify neurons of the myenteric plexus, especially the predominant cholinergic and nitrergic subpopulations. Furthermore, we investigated the distribution of vasoactive intestinal polypeptide and the calcium-binding protein calretinin. Samples from different defined areas of the small intestine and the flexura pelvina were taken from 15 adult horses. After fixation and preparation of the tissue, immunofluorescence labeling was performed on free floating whole-mounts. Additionally, samples used for neuropeptide staining were incubated with colchicine to reveal the neuropeptide distribution within the neuronal soma. The evaluation was routinely accomplished using confocal laser-scanning microscopy. For quantitative and qualitative analysis, the pan-neuronal marker anti-HuC/D was applied in combination with the detection of the marker enzymes for cholinergic neurons and nitrergic nerve cells. Quantitative data revealed that the cholinergic subpopulation is larger than the nitrergic one in several different locations of the small intestine. On the contrary, the nitrergic neurons outnumber the cholinergic neurons in the flexura pelvina of the large colon. Furthermore, ganglia are more numerous in the small intestine compared with the large colon, but ganglion sizes are bigger in the large colon. However, comparison of the entire population of neurons in the different locations of the gut showed no difference. The present study adds further data on the chemoarchitecture of the myenteric plexus which might facilitate the understanding of several gastrointestinal disorders in the horse.

Fibrosing gastrointestinal leiomyositis as a cause of chronic intestinal pseudo-obstruction in an 8-month-old dog

An 8-month-old, female, mixed-breed dog presented to the Iowa State University Veterinary Teaching Hospital with a 1-month history of vomiting and diarrhea. An exploratory laparotomy was performed revealing markedly distended and fluid-filled small and large intestines that were not obstructed. The clinical condition of the dog did not improve subsequent to exploratory surgery, and it was euthanized. At necropsy, both the small and large intestines were distended (approximately 4 cm in diameter) and fluid-filled, and the wall was thin. The abdominal cavity contained approximately 500 ml of a brownish clear fluid. Microscopic lesions of the intestines were confined to the intestinal tunica muscularis and muscularis mucosae and consisted of locally extensive-to-diffuse replacement of the smooth muscle by fibrous tissue and multifocal infiltration by a moderately dense mononuclear inflammatory infiltrate. A unique finding was the presence of similar microscopic lesions in the tunica muscularis of the urinary bladder and stomach.

Neurodegenerative diseases in domestic animals: A comparative review

Neurodegenerative diseases are characterised by selective damage to specific neurons in the nervous system. Interest in such diseases in humans has resulted in considerable progress in the molecular understanding of these disorders in recent decades. Numerous neurodegenerative diseases have also been described in domestic animals but relatively little molecular work has been reported. In the present review, we have classified neurodegenerative disease according to neuroanatomical criteria. We have established two large groups, based on whether the neuronal cell body or its axon was primarily affected. Conditions such as motor neuron diseases, cerebellar degenerations and neuroaxonal dystrophies are discussed in terms of their clinical and neuropathological features. In the most studied disorders, we also present what is known about underlying pathomechanisms, and compare them with their human counterparts. The purpose of this review is to re-kindle interest in this group of diseases and to encourage veterinary researchers to investigate molecular mechanisms by taking advantage of current diagnostic tools.

Gastrointestinal function regulation by nitrergic efferent nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

Observations on the Interstitial Cells of Cajal and Neurons in a Recovered Case of Equine Dysautonomia (Grass Sickness)

This paper describes the histopathological and immunohistochemical changes in the central, autonomic and enteric nervous systems in a well-documented case of equine dysautonomia (ED), after the animal had recovered without significant residual clinical signs. Evidence of neuronal degeneration, such as neuronal chromatolysis, glial scars or a decrease in density of neurons, was not observed in the midbrain, pons, medulla oblongata or spinal cord, including the nuclei of cranial nerves III, V, VII, X and XII. In addition, no evidence of muscle denervation or re-innervation, such as group atrophy of clusters of cells or grouping of cells of similar size in the triceps muscle, was observed. These findings support the hypothesis that the chromatolytic motor neurons may not progress to cell death in ED. The autonomic ganglia showed evidence of previous neuronal loss. In the myenteric and submucosal plexuses of the ileum there was significant loss of enteric neurons. However, in the ileum, a continuous network of interstitial cells of Cajal (ICC) was observed by means of immunohistochemical techniques. It is possible that recovery without significant residual clinical signs was associated with lack of damage to, or more likely recovery of, the ICC, which have a pacemaker function in the intestinal tract.

Neurological syndromes among horses in The Netherlands a 5 year retrospective survey (1999–2004)

The presence of toxins or infectious agents combined with environmental factors in combination with a susceptible host can be the cause for neurological disease in groups of horses. During a 5 year observational period outbreaks of neurological diseases among horses were evaluated. Causes of occurring neurological diseases were equine botulism, lolitrem intoxications, equine herpesvirus type 1-associated myelo(encephalo)pathy, and encephalitis caused by (disseminated) Streptococcus equi subspecies equi infection. This article focuses on the first three syndromes because of their predominant influence on locomotion. The pathogenesis of each disease is presented, followed by a description of a general presentation of the diseases as encountered under Dutch circumstances.

Two cases of equine grass sickness with evidence for soil-borne origin involving botulinum neurotoxin

Botulism is caused by different types of Clostridium botulinum, a soil bacterium. Equine grass sickness (equine dysautonomia) is suspected of being a clinical form of this disease. On a stud where this disease occurred twice within 8 months, grass and soil samples and necropsy specimens of one horse were tested for the presence of bacterial forms and toxin of C. botulinum. Different types and type mixtures (A-E) of C. botulinum and botulinum neurotoxin were found. For the first time, it has been shown that green grass blades contain botulinum toxin. The results support the hypothesis that equine grass sickness is a clinical form of botulism, a soil-borne disease.

Biomarkers of exposure to cyanogens in horses with grass sickness

To test the hypothesis that equine grass sickness may be associated with the ingestion of cyanogenic glycosides from white clover (Trifolium repens), the concentrations of whole blood cyanide, and plasma and urinary thiocyanate, the main metabolite of cyanide, were measured in 12 horses with acute grass sickness and 10 horses with subacute grass sickness, and in 43 control horses, of which 21 were co-grazing with cases of acute grass sickness, 12 grazed pastures where grass sickness had not been reported, and 10 were stabled horses. The healthy horses which grazed with cases of acute grass sickness had higher concentrations of blood cyanide, and plasma and urinary thiocyanate than the other control horses, consistent with an increased exposure to cyanogens. The horses with grass sickness had no evidence of a recent intake of cyanogens, but may have been exposed to increased levels of cyanogens before they became anorexic.

Presence of in vitro electrical activity in the ileum of horses with enteric nervous system pathology: equine dysautonomia (grass sickness)

Equine dysautonomia (grass sickness) is a frequently fatal disease of horses characterised by intestinal stasis. Interstitial cells of Cajal (ICC) are the pacemakers and mediators of neurotransmission in the gastrointestinal tract. Impaired ICC-mediated control of motility has been implicated in intestinal disorders in laboratory mammals, humans and in equine grass sickness. The aim of this study was to compare the in vitro electrical properties of ileum from grass sickness cases with horses free from gastrointestinal disease. Intracellular microelectrode recordings were made from smooth muscle cells in cross-sectional preparations of equine ileum, superfused in vitro. Samples were taken from six horses with grass sickness and from eight horses free from gastrointestinal disease, all euthanised on humane grounds. Ileal tissues were processed for haematoxylin and eosin histology, and c-Kit immunohistochemistry. Membrane potential oscillations were recorded in the ileal preparations from four of the six horses with grass sickness and from all of the normal horses. A waxing and waning pattern of the membrane potential oscillation activity was noted in some cells. In comparison to the normal horses, the membrane potential oscillations in grass sickness horses had a significantly reduced frequency (P = 0.0001) and increased duration (P = 0.0002). Immunohistochemistry revealed the presence but reduction of ICC in grass sickness. Histological assessment of the same tissues used for analysis of the ICC showed the depletion and pathology of the enteric neurons in grass sickness. Therefore, the majority of ileal preparations from grass sickness-affected horses exhibited prominent membrane potential oscillation activity suggesting that, although the neural elements are damaged severely, the ICC-mediated pacemaker function remains intact.

A comparative study of the intestinal microbiota of healthy horses and those suffering from equine grass sickness

This study compares quantitatively the microbiota of the gastrointestinal tract of healthy horses with that of horses with equine grass sickness (EGS). Faecal and ileal samples were cultured quantitatively on selective and non-selective media. Confirmed anaerobes were identified to species level. Overall faecal counts gave a ratio of aerobes:anaerobes of approximately 1:1. However, the mean counts in healthy horses of 4.4x10(8) aerobes:3.7x10(8) anaerobes per gram wet weight were different from counts in EGS (means were 10-100-fold higher), with statistically significant differences for the anaerobes (p=0.04). There were 10-100-fold more anaerobic cocci in EGS samples compared to healthy controls. Most of the seven species of anaerobic cocci were found in both healthy horses and EGS. Differences in clostridia isolated between health and disease were notable: fourteen species were isolated from EGS cases, compared to only one (C. bifermentans) in controls. The mean faecal clostridial counts in chronic disease were higher than in controls (10-fold) and in acute EGS (100-fold). In contrast, mean counts for ileal samples from acute cases, showed a 10-fold increase for clostridia compared to 1000-fold reduction in chronic cases (compared to faecal counts). Results indicate an increase in the bacterial numbers in the GI tract of animals with EGS compared to the controls and clostridia are prominent in EGS. Whether the increase in clostridia is the cause of GI stasis or a consequence remains uncertain.

Canine dysautonomia: two clinical cases

Two clinical cases of canine dysautonomia are described. Two young female neutered dogs were presented with clinical signs including vomiting, diarrhoea, faecal tenesmus, dysphagia and urinary retention. Decreased tear production, dry mucous membranes, bilateral Horner's syndrome, decreased anal sphincter tone and gastrointestinal hypomotility were also observed. Presumptive diagnoses of dysautonomia were made based on the clinical presentation and investigations. Postmortem histopathological examination in one of the cases demonstrated marked depletion of neuronal cell bodies in the intestinal myenteric plexuses and parasympathetic ganglia, confirming the diagnosis in this case. Criteria for aiding the antemortem diagnosis of this rare condition based on clinical observations and diagnostic testing are proposed.

Systemic antibodies to Clostridium botulinum type C: do they protect horses from grass sickness (dysautonomia)?

The aetiology of equine grass sickness (EGS) is still unknown. There is increasing evidence that toxicoinfection with Clostridium botulinum type C is involved. Epidemiological evidence shows that resistance to EGS can occur in older horses and those that have been on a particular pasture for longer or have been in prior contact with the disease. This resistance may be in the form of an immune response to the aetiological agent. Levels of systemic antibodies to the surface antigens of C. botulinum type C (using the closely related and safe C. novyi type A as a phenotypic marker) and to the botulinum type C neurotoxin (BoNT/C) were investigated in horses with and without EGS. Horses with grass sickness were found to have significantly lower levels of systemic IgG to both surface antigens and BoNT/C. Horses with low levels of systemic immunity to these antigens may be more susceptible to developing EGS. There were no significant differences in antibody levels between the different categories of EGS, suggesting systemic immunity to C. botulinum type C does not play a significant role in influencing the severity of the disease. However, horses that had been in contact with EGS or that were grazing land where it had occurred frequently in the past had significantly higher antibody levels to these antigens. These horses may have been exposed to subclinical doses of C. botulinum type C and BoNT/C, resulting in the production of a protective immune response against the putative aetiological agent. This finding is of potential significance for the prospect of prevention of EGS by vaccination against C. botulinum type C.

A reduction in interstitial cells of Cajal in horses with equine dysautonomia (grass sickness)

Equine dysautonomia (grass sickness) is a common, frequently fatal disease of horses characterised by dysfunction of the gastrointestinal tract. Interstitial cells of Cajal are the c-Kit-immunoreactive cells responsible for the generation of pacemaker activity in gastrointestinal smooth muscle. Impairment of this pacemaker action has been implicated in several motility disorders in humans and laboratory mammals. The aim of this study was to test the hypothesis that changes in interstitial cells of Cajal may be involved in the pathophysiology of the intestinal dysfunction observed in equine grass sickness. Interstitial cells of Cajal were identified using immunohistochemical labelling with an anti-c-Kit antibody and their density was assessed using a semi-quantitative grading system. Segments of ileum were examined from 24 horses free from gastrointestinal disease and compared to tissues from 28 horses with grass sickness. Segments of the pelvic flexure region of the large colon were examined from 13 horses free from gastrointestinal disease and compared to tissues from 10 horses with grass sickness. In horses with grass sickness, interstitial cells of Cajal were significantly decreased in both the myenteric plexus and circular muscle regions of both ileum and pelvic flexure compared to normal animals. Therefore, it is possible that the decline in interstitial cells of Cajal may be an important factor in the development of intestinal dysmotility observed in grass sickness.

Studies on the experimental induction of ptosis in horses

The precise appearance of ptosis due to lesions at different sites was investigated in experimental ponies. The angles of the eyelashes to the head was used as an objective measurement of ptosis after local anaesthesia of the sympathetic trunk or the palpebral nerve and the administration of an ocular alpha agonist or antagonist. It was shown that ptosis is not an inevitable consequence of palpebral nerve pathology, that ocular alpha antagonists can induce ptosis, and that alpha agonist eyedrops have an inconsistent effect on the equine pupil, but are consistent at reversing ptosis induced by sympathetic denervation in unsedated horses.

Investigation of the susceptibility of equine autonomic neuronal cell lines, clonally derived from the same paravertebral ganglion, to toxic plasma from equine dysautonomia (grass sickness) cases

In the autonomic nervous system (ANS) of equine grass sickness (GS) cases, some neurones show abnormal changes while neighbouring neurones are unaffected. To test whether noradrenergic neurones showed variable susceptibility to the GS toxin in culture, clonally-derived populations isolated from the same fetal thoracic sympathetic chain ganglion were challenged with plasma from GS cases previously shown to induce ANS damage when injected into normal horses. During the early stages of exposure to toxic plasma, cells within a clonal population showed variable susceptibility ranging from no obvious effect to characteristic patterns of pathology. However, after 3 days of exposure to toxic plasma all cells were killed. Dose response analysis on selected clonal populations showed no significant difference in TD(50) values.

Properties of herbage in relation to equine dysautonomia: biochemical composition and antioxidant and prooxidant actions

To investigate the etiology of equine dysautonomia (ED), a degenerative polyneuropathy affecting grazing horses, the biochemical composition and antioxidant/prooxidant activities of aqueous extracts of plants collected from ED pastures were determined. Plants collected immediately after an outbreak of ED had reduced antioxidant and weak prooxidant activities when compared with control plants (plants collected from ED pastures out of ED season and control plants from ED pastures that were grown under favorable conditions). ED plants also had significantly increased concentrations of fructose and low molecular weight phenolic compounds, significantly more of one amino acid zone (probably valine), significantly less tartaric acid, and a nonsignificant decrease in ascorbic acid content when compared with control plants from ED pastures that were grown under favorable conditions. These findings suggest that ED plants may be under oxidative stress, possibly due to chilling, drought, or fungal colonization. However, experimental drought and chilling of plants did not reproduce the biochemical alterations identified in ED plants. It is possible that the altered biochemical content of ingested plants may contribute, directly or indirectly, to the development of ED in grazing horses.

The association of Clostridium botulinum type C with equine grass sickness: a toxicoinfection?

The cause of grass sickness, an equine dysautonomia, is unknown. The disease usually results in death. Gastrointestinal (GI) dysfunction is a common clinical manifestation in all forms of the disease. It is generally thought that equine grass sickness (EGS) is caused by an ingested or enterically produced neurotoxin which is absorbed through the GI tract. Clostridium botulinum was first implicated as a causative agent when it was isolated from the GI tract of a horse with EGS in 1919. The aim of the present study was to investigate the hypothesis that EGS results from toxicoinfection with C. botulinum type C: growth of the bacterium in the GI tract with production of toxin (BoNT/C). Ileum contents and faeces from horses with EGS were investigated for BoNT/C, and indirectly for the presence of C. botulinum type C, and compared with control samples from horses without EGS. BoNT/C was detected directly by ELISA in the ileum of 45% (13/29) of horses with EGS compared to 4% (1/28) of controls, and in the faeces of 44% (20/45) of horses with EGS compared to 4% (3/77) of controls. Levels of up to 10 Mlg toxin/g wet weight of gut contents were observed. The one control horse with detectable toxin in the ileum had been clinically diagnosed as having acute EGS, but this was not confirmed by histopathology. The organism was detected indirectly by assaying for BoNT/C by ELISA after enrichment in culture medium. C. botulinum type C was shown to be present in 48% (14/29) of ileum samples and 44% (20/45) of faecal samples from horses with EGS, compared with 7% (2/27) of ileum samples and 8% (6/72) of faecal samples from controls. These results support the hypothesis that EGS results from a C. botulinum type C toxicoinfection.