Neuropathologic findings of bromethalin toxicosis in the cat

Bromethalin Exposure and Possible Toxicosis in a Bald Eagle (Haliaeetus leucocephalus)

A free-living Bald Eagle (Haliaeetus leucocephalus) displayed acute onset neurologic signs. Postmortem analysis of adipose tissue identified desmethylbromethalin, the active metabolite of bromethalin. Antemortem signs, detection of desmethylbromethalin, and results of other diagnostics support the possibility of secondary bromethalin toxicosis. Investigation of bromethalin's potential risk to wildlife is critically needed.

Active metabolite of the neurotoxic rodenticide bromethalin along with anticoagulant rodenticides detected in birds of prey in the northeastern United States

Little is known about the ecologic fate of the neurotoxic rodenticide bromethalin, which is currently registered for use in the United States, Canada, and other countries including Australia. There is minimal research on bromethalin's potential to cause secondary toxicosis in nontarget wildlife. The aim of this study was to evaluate adipose tissue in four species of birds of prey presented to a wildlife clinic in Massachusetts, USA, for desmethylbromethalin (DMB), the active metabolite of bromethalin. Birds were also screened for anticoagulant rodenticides (ARs) in liver tissue to present a more complete picture of rodenticide exposures in this geographic area and to evaluate the impact of current mitigation measures in place during the time of sampling, 2021-2022. A total of 44 hawks and owls were included; DMB was found in 29.5% of birds and ARs were present in 95.5%. All birds with DMB detections also had residues of ARs. Among birds positive for ARs, 81% had two or more compounds. To the authors' knowledge the data presented here represent the first published monitoring study to document bromethalin/DMB bioaccumulation in obligate carnivores. As DMB is a more potent neurotoxicant than its parent compound, these results are cause for concern and an indication that further monitoring and study of the potential risk of bromethalin to wildlife species is needed. These findings have global implications as increasing concern regarding exposure to and toxicosis from ARs in nontarget wildlife worldwide leads to a search for alternatives and effective mitigation approaches.

Case Report: MRI, Clinical, and Pathological Correlates of Bromethalin Toxicosis in Three Dogs

Bromethalin toxicosis is an increasingly common clinical presentation in dogs that may be fatal depending on the extent of intoxication. Antemortem diagnosis of bromethalin toxicosis was achieved in three dogs by demonstration of the active metabolite desmethylbromethalin in fat or serum. Magnetic resonance imaging (MRI) findings were consistent with a diffuse leukoencephalopathy with restricted diffusion and prominent involvement of the corticospinal motor tracts on T2-weighted and diffusion-weighted sequences. Imaging findings were confirmed in one non-surviving dog at necropsy. Resolution of MRI abnormalities was demonstrated in one surviving dog that was consistent with the associated resolution of clinical signs. Initial findings in these dogs support further investigation of specific MRI patterns in cases of leukoencephalopathy to aid differential diagnosis. While antemortem detection of bromethalin and its metabolites confirms exposure, quantitation may be informative as a prognostic biomarker.

Neuropathology of feral conures with bromethalin toxicosis

Bromethalin is a widely used neurotoxic rodenticide sometimes affecting nontarget wildlife. However, the effects of bromethalin on avian species are largely unknown. Here, we report the neuropathology of 14 feral conures (Psittacara sp.) with bromethalin toxicosis. Clinically, all birds presented with different degrees of paraparesis that sometimes progressed to dysphagia, ataxia, and tetraparesis. Histologically, there was astrogliosis, pallor, and vacuolation of white matter in the brain. This was usually more prominent in the medial longitudinal fasciculus, pons, optic tectum, cerebellar peduncle, and ventral funiculus. In most affected areas, there was loss of oligodendrocytes, and axons had extensive myelin loss or marked intramyelinic edema with splitting of myelin sheaths at the intraperiod line. Conures with bromethalin toxicosis had neuropathological changes similar to those of mammals exposed to bromethalin but with a characteristic distribution, probably related to higher susceptibility to cytotoxic edema in certain regions of the avian brain.

Bromethalin Exposure in a Free-Ranging American Black Bear (Ursus americanus)

A free-living black bear (Ursus americanus) with cerebellar ataxia and occasional unilateral epistaxis was given supportive care. Neurologic signs resolved within 7 d; however, the epistaxis progressed and the bear developed pancytopenia and died. Adipose tissue tested positive for desmethylbromethalin, while liver samples were negative for anticoagulant rodenticides and toxins.

Hunting the eagle killer: A cyanobacterial neurotoxin causes vacuolar myelinopathy

Vacuolar myelinopathy is a fatal neurological disease that was initially discovered during a mysterious mass mortality of bald eagles in Arkansas in the United States. The cause of this wildlife disease has eluded scientists for decades while its occurrence has continued to spread throughout freshwater reservoirs in the southeastern United States. Recent studies have demonstrated that vacuolar myelinopathy is induced by consumption of the epiphytic cyanobacterial species Aetokthonos hydrillicola growing on aquatic vegetation, primarily the invasive Hydrilla verticillata Here, we describe the identification, biosynthetic gene cluster, and biological activity of aetokthonotoxin, a pentabrominated biindole alkaloid that is produced by the cyanobacterium A. hydrillicola We identify this cyanobacterial neurotoxin as the causal agent of vacuolar myelinopathy and discuss environmental factors-especially bromide availability-that promote toxin production.

Retrospective evaluation of the effects and outcome of bromethalin ingestion: 192 Dogs (2010-2016)

OBJECTIVE

To evaluate the frequency of clinical signs, dose ingested, and outcome in a large group of dogs with bromethalin ingestion.

DESIGN

Retrospective cohort study of dogs from 2010 to 2016.

SETTING

Three university teaching hospitals and 1 private practice.

ANIMALS

A total of 192 dogs with bromethalin ingestion.

MEASUREMENTS AND MAIN RESULTS

Total 192 cases were identified, of which 25 dogs developed clinical signs. Five cases initially had severe neurological signs and were euthanized. A sum of 187 dogs survived to discharge. The total ingested dose was recorded in 59 dogs with a median (interquartile range) 0.2 mg/kg (0.28 mg/kg). The remaining 133 dogs had confirmed ingestion reported by owners (witnessed ingestion or colored feces) but the total dose could not be calculated. The median (interquartile range) time to presentation for all dogs was 2 hours (4.8 h). A majority of patients were treated on an outpatient basis (121/192) and 71 of 192 were treated as inpatients with 58 of 71 receiving fluid diuresis. Decontamination was performed in 179 dogs including emesis induction (14), activated charcoal administration (42), and both (123). Emesis was successful in 128 dogs and apomorphine was the most common emetic agent (121). Mild to severe clinical signs at admission were reported in 19 cases including vomiting (6), tremors (5), lethargy (4), ataxia (3), weakness (2), diarrhea (2), collapse (2), and and anorexia (2). One case developed ataxia and tremors within 72 hours of admission.

CONCLUSIONS AND CLINICAL RELEVANCE

Symptoms of bromethalin toxicosis are uncommon, and most ingested doses are well below the reported dose expected to cause clinical signs. In this patient population, prognosis was excellent unless severe clinical signs were noted, which carried a high euthanasia rate. Effects of treatment on outcome could not be evaluated due to the low number of patients that developed clinical signs.

Emerging and Re-emerging Diseases of Selected Avian Species

Emerging infections and re-emerging diseases in birds can be caused by numerous factors and need to be recognized and understood. This article introduces and summarizes author-selected emerging and re-emerging diseases of avian species. These diseases hold significance as they relate to scientific research, disease recognition and identification, avian welfare aspects, and ecosystem health. Some are significant in human health and others affect production medicine. These and many others remain important pathogens of worldwide consequence.

Evidence of bromethalin toxicosis in feral San Francisco "Telegraph Hill" conures

During 2018, four free-ranging conures, from a naturalized flock in San Francisco, presented with a characteristic set of neurologic signs that had been reported in other individuals from this flock. The cause of morbidity or mortality in historic cases has not been identified. From these four subjects, fresh feces were collected during their initial days of hospitalization and submitted to the University of Georgia Infectious Diseases Laboratory and Center for Applied Isotope Studies for bromethalin and desmethyl-bromethalin quantitation. Using High Performance Liquid Chromatography, the laboratory detected bromethalin, a non-anticoagulant, single-dose rodenticide, in fecal samples from three subjects; half of these samples were also positive for desmethyl-bromethalin, bromethalin’s active metabolite. In three subjects that died, the UGA laboratory screened brain and liver samples and found bromethalin in all samples; desmethyl-bromethalin was detected in all but one brain sample, which was below the detection limit. Our findings suggest the conures are more resistant to bromethalin than are other species in which bromethalin has been studied, and/or that the conures may be ingesting the toxin at a sublethal dose. More data is needed to better assess the long-term effects of bromethalin on animals exposed at the subacute/chronic levels, and also to better understand the compartmentalization of bromethalin and desmethyl-bromethalin in a wider variety of species.

Correlation of MRI with the Neuropathologic Changes in Two Cats with Bromethalin Intoxication

Two cats were presented with multifocal neurological signs. One cat's signs progressed over 2 wk; the other cat progressed over 5 days. Examinations were consistent with a process involving the prosencephalon, vestibular system, and general proprioceptive/upper motor neuron systems. MRI of the brain and cervical spinal cord reveal widespread T2 hyperintensity of the white matter. Affected areas included the cerebrum, cerebral peduncles, corticospinal tracts of the pons and medulla, and the cerebellum. T2 hyperintensity was present in all funiculi of the spinal cord. Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps were consistent with cytotoxic or intramyelinic edema. Differential diagnosis included toxic or metabolic/degenerative leukoencephalopathies. Necropsies revealed widespread spongy degeneration of the central nervous system white matter. Toxicologic assays of liver specimens revealed desmethylbromethalin, a metabolite of bromethalin. Bromethalin is a rodenticide that causes uncoupling of oxidative phosphorylation. Antemortem diagnosis is challenging. DWI and ADC maps were instrumental in narrowing the differential diagnosis and raised the index of suspicion for bromethalin. Bromethalin intoxication should be considered in all animals with a progressive course of multifocal neurologic deficits. MRI, specifically, DWI and ADC maps, may serve as a biomarker of cytotoxic or intramyelinic edema associated with spongiform leukoencephalomyelopathy.

Retrospective evaluation of feline rodenticide exposure and gastrointestinal decontamination: 146 cases (2000-2010)

OBJECTIVE

To determine the prevalence of rodenticide exposure in cats, describe the use of gastrointestinal decontamination (GID) after rodenticide exposure, and examine the efficacy of GID following exposure to anticoagulant rodenticides (ACR).

DESIGN

Retrospective study from 2000-2010.

SETTING

Emergency service of an urban university teaching hospital.

ANIMALS

One hundred forty-six cats presented for rodenticide exposure.

MAIN RESULTS

Annually, the number of cats that were presented for rodenticide exposure averaged 13 of 3,336 (0.39%) and totaled 146 cases over 11 years. Cats that had been exposed to rodenticide were significantly more likely to be young (P < 0.001), sexually intact (P < 0.001), and presented in the fall season (P = 0.002). The majority of cats lived indoors (67.6%). The type of rodenticide involved in the exposure was unknown in 50% (71/142) of cases. Of the known types, ACRs were most common (59/142, 41.5%) followed by cholecalciferol (7/142, 4.9%) and bromethalin (5/142, 3.5%). Gastrointestinal decontamination was attempted in 21/36 (58%) cats with exposure to a known ACR. Emesis was attempted in 17/21 (81%) and charcoal administered in 14/21 (67%) cats that underwent GID. This study did not detect an effect of GID efforts on prothrombin time (PT) prolongation 48 hours after exposure to a known ACR.

CONCLUSIONS

Cats consume rodenticides. Due to the lack of evidence of altered outcome associated with GID in cats exposed to ACRs, a PT should be evaluated 48 hours after first exposure regardless of whether GID is performed. Treatment should be based on the results of the PT. Gastrointestinal decontamination should be performed at the clinician's discretion based on history, risks, calculated toxic dose, low prevalence of ACR toxicosis in cats, general resistance of cats to ACR toxicosis, and treatment options.

Fatal bromethalin intoxication in 3 cats and 2 dogs with minimal or no histologic central nervous system spongiform change

Use of the neurotoxic rodenticide bromethalin has steadily increased since 2011, resulting in an increased incidence of bromethalin intoxications in pets. Presumptive diagnosis of bromethalin toxicosis relies on history of possible rodenticide exposure coupled with compatible neurologic signs or sudden death, and postmortem examination findings that eliminate other causes of death. Diagnosis is confirmed by detecting the metabolite desmethylbromethalin (DMB) in tissues. In experimental models, spongiform change in white matter of the central nervous system (CNS) is the hallmark histologic feature of bromethalin poisoning. We describe fatal bromethalin intoxication in 3 cats and 2 dogs with equivocal or no CNS white matter spongiform change, illustrating that the lesions described in models can be absent in clinical cases of bromethalin intoxication. Cases with history and clinical signs compatible with bromethalin intoxication warrant tissue analysis for DMB even when CNS lesions are not evident.

Chemically Induced Myelinopathies

The diverse, structurally unrelated chemicals that cause toxic myelinopathies have been investigated and can be categorized into two types of primary demyelinators. Some demyelinating chemicals seem to leave intact the myeli-nating cells (oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system), while others damage the myelinating cells as well as the myelin. The significance between the two is that with the myelinating cells still in tact, repair of the myelin sheath can occur. However, if the myelinating cells are destroyed, repair and reversal of the neuropathy may not occur. Histologically, these chemicals produce an edema of the white matter of the brain, and in some cases the peripheral nervous system, that appears spongy by light microscopy. By electron microscopy, vacuoles can be seen in the myelin surrounding axons. These vacuoles are characterized as fluid-filled separations (splitting) of myelin lamellae at the intraperiod line. In some cases these vacuoles can degenerate further to full demyelination, affecting conduction through those axons. Regeneration of the myelin layers can occur, and in some cases occurs at the same time other axons are undergoing toxic demyelination. Several of these chemicals, however, have been shown to increase cerebrospinal fluid pressure in the brain, optic nerve, and spinal cord, and/or intraneuronal pressure in the perineurium surrounding the axons in the peripheral nervous system. This increased pressure has been correlated with decreased conduction capacity through the axon, ischemia to the neuronal tissue from decreased blood flow because of pressure against the blood vessels, and, if unrelieved, permanent axonal damage. Several of these chemicals havebeen shown to inhibit oxidative phosphorylation, while others uncouple oxidative phosphorylation. One chemical appears to inhibit an enzyme critical to cholesterol synthesis, thus destabilizing myelin. Another hypothesis for a mechanism of action may be in the ability of these compounds to alter membrane permeability.

Atypical bromethalin intoxication in a dog: pathologic features and identification of an isomeric breakdown product

Background

Definitive post mortem confirmation of intoxication by the neurotoxic rodenticide bromethalin can be challenging. Brain lesions are not specific and detection of bromethalin and its metabolites are unpredictable due to rapid photodegradation and inconsistent behavior in tissues.

Case presentation

A 2-year-old dog presented with rapid onset of severe muscle tremors and death within hours after a known ingestion of a reportedly low dosage of bromethalin and subsequent decontamination using activated charcoal. Marked meningeal hemorrhages and multifocal myelin sheath vacuolation were observed in the brain. A marked reactive astrocytosis and neuronal hypoxia/necrosis were identified using immunohistochemistry (IHC) for glial fibrillary acidic protein (GFAP) and for neuron specific protein (NeuN). Bromethalin exposure and tissue absorption was confirmed by identification of one of two isomeric 543.7 molecular weight (MW) breakdown products in the patient’s adipose and kidney samples using gas chromatography (GC) combined with tandem quadrupole mass spectrometry (MS/MS).

Conclusions

The severity of clinical signs and subsequent death of this dog was not expected with the low dosage of bromethalin reportedly ingested, and the use of activated charcoal possibly precipitated a hypernatremic status. Meningeal hemorrhages are atypical of bromethalin intoxication, and might have been caused by hyperthermia, secondary to tremors or hypernatremia. Identification of one of two isomeric breakdown products in the adipose tissue and kidney provides an additional molecule to the toxicologic testing regime for bromethalin intoxication.

Bromethalin poisoning in a raccoon (Procyon lotor): diagnostic considerations and relevance to nontarget wildlife

Submission of a raccoon (Procyon lotor) for necropsy following exhaustion at a California wildlife care center revealed minimal gross pathologic changes and only mild vacuolar changes in the white matter of the brain. Turquoise granular material was noted in the gastrointestinal tract and was submitted for toxicological testing along with portions of the brain, liver, kidney, and mesenteric and perirenal adipose tissues. Testing of the turquoise material for 7 anticoagulant rodenticides, strychnine, 4-aminopyridine, starlicide, and salts revealed none of these compounds; however, desmethylbromethalin was detected by high-performance liquid chromatography-tandem mass spectrometry. Other tissues were subsequently analyzed; the mesenteric and perirenal adipose tissues contained desmethylbromethalin. Desmethylbromethalin is the active metabolite of bromethalin, uncouples oxidative phosphorylation, and results in cerebral edema. Bromethalin is a rodenticide that is visually indistinguishable from many other rodenticides, making identification of poisonings by appearance alone nearly impossible. Based on the pathological and toxicological findings, a diagnosis of bromethalin toxicosis was established. In cases of wildlife species with unknown deaths or inconsistent clinical signs with normal or minimal histological findings, bromethalin toxicosis should be considered as a differential. Adipose tissue is the tissue of choice and can be easily harvested from a live or deceased animal to help confirm or rule out bromethalin exposure or intoxication.

Clinicopathologic and MRI Characteristics of Presumptive Hypertensive Encephalopathy in Two Cats and Two Dogs

Two dogs and two cats were evaluated for the acute-onset of abnormal mentation, recumbency, and blindness. All cases had systemic hypertension, ranging from 180 mm Hg to 260 mm Hg. MRI of the brain disclosed noncontrast-enhancing, ill-defined, T2-weighted (T2W) hyperintensities in the white matter of the cerebrum in the areas of the frontal, parietal, temporal, and occipital lobes. Lesions were also observed in the caudate nuclei and thalamus (n = 1 in each). Intracranial hemorrhage was observed in one animal. Diffusion-weighted imaging (DWI) was consistent with vasogenic edema in two animals. Retinal lesions were observed in three animals. Hypertension was secondary to renal disease in three animals. A primary underlying disorder was not identified in one animal. Normalization of blood pressure was achieved with amlodipine either alone or in combination with enalapril. In one cat, hypertension spontaneously resolved. In three cases, neurologic improvement occurred within 24–48 hr of normalization of blood pressure. The presumptive diagnosis of hypertensive encephalopathy was supported by the MRI findings and neurologic dysfunction coincident with systemic hypertension in which the neurologic dysfunction improved with treatment of hypertension. The prognosis appears good for the resolution of neurologic deficits with normalization of blood pressure in animals with hypertensive encephalopathy.

Spongiform neurodegenerative disease in a Persian kitten

A congenital encephalopathy with spongiform degeneration and prominent neuronal apoptosis was observed in a 4-month-old Persian male cat with a history of depressed mental status and ataxia. On clinical examination, signs included right head tilt, ventroflexion of the head and neck, and tetraparesis. Histological examination of the central nervous system revealed multifocal, bilateral and symmetrical vacuolar degeneration of the neuropil, mainly involving the cerebellar and vestibular nuclei area, the caudal colliculi, the mesencephalic nuclei, the tegmental area and the deeper layer of the cerebral cortex. Accumulation of phosphorylated neurofilaments was detected in neuronal perikarya of the deep cortical layers, hippocampus and thalamus. Numerous pyknotic and apoptotic neurons were also observed in the cerebral cortex. These neuropathological changes differ from those observed in previous reports of spongiform degeneration of the grey matter in cats and were suggestive of a congenital neurodegenerative disease.

Fatal Bromethalin Poisoning

Bromethalin is a neurotoxin found in some rodenticides. A delusional 21-year-old male presented to a hospital with altered mental status the day after ingesting a bromethalin-based rodenticide. He died 7 days after his self-reported exposure to c. 17 mg bromethalin (equivalent to 0.33 mg bromethalin/kg). His clinicopathologic course was characterized by altered mental status, obtundation, increased cerebrospinal fluid pressure, cerebral edema, death, and diffuse histologic vacuolization of the white matter in the central nervous system seen on microscopic examination at autopsy. The presence of a demethylated form of bromethalin in the patient's liver and brain was confirmed by gas chromatography with mass spectrometry. Clinical signs and lesions observed in this patient are similar to those seen in animals poisoned with bromethalin. This case illustrates the potential for bromethalin ingestion to result in fatal human poisoning.

A Novel Spongiform Degeneration of the Grey Matter in the Brain of a Kitten

A young female domestic short-hair cat presented with neurological signs consistent with a multifocal encephalic lesion (depressed mental status, head tilt to the right, cervical ventroflexion, head tremors, tetraparesis, conscious propioceptive deficits in all four limbs and visual deficits). No gross lesions were seen at necropsy. On light microscopical examination lesions were found only in the brain and cervical spinal cord. A generalized vacuolation of the grey matter of the brain was observed. Special staining techniques, immunohistochemistry, lectin affinity histochemistry and ultrastructural studies were performed to characterize the lesion; preservation of the white matter and a reactive astrogliosis were demonstrated. Feline retroviruses and PrPsc were not detected. Ultrastructurally, a dilatation of intracytoplasmic membrane-bounded organelles with membrane disruption and dendritic and somatic swelling was found in astrocytes and neurons. The age of the animal and histological changes suggested a novel, possibly congenital, spongiform degeneration of the brain and cervical spinal cord.

Lipophilic organic compounds in lake sediment and American coot (Fulica americana) tissues, both affected and unaffected by avian vacuolar myelinopathy

Avian vacuolar myelinopathy (AVM) is a disease of unknown etiology, which has been diagnosed in a variety of birds from surface water reservoirs in the southeastern United States. Pathology suggests a natural or anthropogenic compound may be the cause of this disease. With the goal of identifying the toxicant that causes AVM, we qualitatively analyzed sediments and American coot (Fulica americana) tissues from reservoirs that were affected and unaffected by AVM using high-resolution gas chromatographic low-resolution mass spectrometry. Polychlorinated biphenyls (PCBs), octachlorodibenzo-p-dioxin, and biogenic and anthropogenic polycyclic aromatic hydrocarbons (such as retene) were the most abundant compounds in the sediment. Penta- and hexachlorobenzene, oxychlordane, p,p'-DDE, dieldrin, and polychlorinated biphenyls were the most abundant compounds in the avian tissues. None of these compounds were more abundant in the AVM affected sediments and tissues than in the unaffected media. Therefore, it is unlikely that any of these compounds are the cause of this avian disease.

Clinical features of avian vacuolar myelinopathy in American coots

OBJECTIVE

To characterize clinical features of avian vacuolar myelinopathy (AVM) in American coots.

DESIGN

Case-control study.

ANIMALS

26 AVM-affected American coots and 12 unaffected coots.

PROCEDURES

Complete physical, neurologic, hematologic, and plasma biochemical evaluations were performed. Affected coots received supportive care. All coots died or were euthanatized, and AVM status was confirmed via histopathologic findings.

RESULTS

3 severely affected coots were euthanatized immediately after examination. Seventeen affected coots were found dead within 7 days of admission, but 5 affected coots survived > 21 days and had signs of clinical recovery. Abnormal physical examination findings appeared to be related to general debilitation. Ataxia (88%), decreased withdrawal reflexes (88%), proprioceptive deficits (81%), decreased vent responses (69%), beak or tongue weakness (42%), and head tremors (31%), as well as absent pupillary light responses (46%), anisocoria (15%), apparent blindness (4%), nystagmus (4%), and strabismus (4%) were detected. Few gross abnormalities were detected at necropsy, but histologically, all AVM-affected coots had severe vacuolation of white matter of the brain. None of the control coots had vacuolation.

CONCLUSIONS AND CLINICAL RELEVANCE

Although there was considerable variability in form and severity of clinical neurologic abnormalities, clinical signs common in AVM-affected birds were identified. Clinical recovery of some AVM-affected coots can occur when supportive care is administered. Until the etiology is identified, caution should be exercised when rehabilitating and releasing coots thought to be affected by AVM.

Rodenticides

Rodenticides are second only to insecticides in the prevalence of pesticide exposure. Hundreds of rodenticide products currently exist, yet only a handful of them are involved in most toxicoses of companion animals. The most commonly reported toxicoses in the United States are those caused by anticoagulant rodenticides, bromethalin, cholecalciferol, strychnine, and zinc phosphide. The pathophysiologic findings, diagnosis, and treatment of each of these five rodenticides are discussed.

The potential for vigabatrin-induced intramyelinic edema in humans

PURPOSE

Vigabatrin (Sabril, Hoechst Marion Roussel) is an antiepilepsy drug (AED) presently marketed in 64 countries for the treatment of partial and secondarily generalized seizures. Vigabatrin (VGB) is marketed in a subset of these countries for the treatment of infantile spasms. Clinical experience in humans has shown that VGB provides effective seizure control with a wide margin of safety. However, animal toxicity studies raised concern when prolonged administration of VGB was shown to induce intramyelinic edema (IME) in some laboratory animal species.

METHODS

Animal and human data were reviewed with respect to the potential for VGB-induced IME. Surveillance of patients receiving VGB in clinical trials or by prescription has been conducted for >15 years to identify patients developing clinical abnormalities that might be IME related.

RESULTS

The histologic lesions of VGB-induced IME in animals are reliably reproduced and correlate with changes in multimodality evoked potentials (EPs) and magnetic resonance imaging (MRI). Numerous studies of the effects of VGB on EP and MRI in epilepsy patients have demonstrated no clear-cut IME-related changes in these modalities. Additionally, autopsy and surgical brain samples from VGB-treated patients have been scrutinized for potential IME histopathology. In an estimated 350,000 patient-years of VGB exposure (approximately 175,000 patients exposed for 2 years at an average dose of 2 g/day), no definite case of VGB-induced IME has been identified.

CONCLUSIONS

Comprehensive review of a variety of sources of data failed to identify any definite case of IME in humans treated with VGB.

Epizootic vacuolar myelinopathy of the central nervous system of bald eagles (Haliaeetus leucocephalus) and American coots (Fulica americana)

Unprecedented mortality occurred in bald eagles (Haliaeetus leucocephalus) at DeGray Lake, Arkansas, during the winters of 1994-1995 and 1996-1997. The first eagles were found dead during November, soon after arrival from fall migration, and deaths continued into January during both episodes. In total, 29 eagles died at or near DeGray Lake in the winter of 1994-1995 and 26 died in the winter of 1996-1997; no eagle mortality was noted during the same months of the intervening winter or in the earlier history of the lake. During the mortality events, sick eagles were observed overflying perches or colliding with rock walls. Signs of incoordination and limb paresis were also observed in American coots (Fulica americana) during the episodes of eagle mortality, but mortality in coots was minimal. No consistent abnormalities were seen on gross necropsy of either species. No microscopic findings in organs other than the central nervous system (CNS) could explain the cause of death. By light microscopy, all 26 eagles examined and 62/77 (81%) coots had striking, diffuse, spongy degeneration of the white matter of the CNS. Vacuolation occurred in all myelinated CNS tissue, including the cerebellar folia and medulla oblongata, but was most prominent in the optic tectum. In the spinal cord, vacuoles were concentrated near the gray matter, and occasional swollen axons were seen. Vacuoles were uniformly present in optic nerves but were not evident in the retina or peripheral or autonomic nerves. Cellular inflammatory response to the lesion was distinctly lacking. Vacuoles were 8-50 microns in diameter and occurred individually, in clusters, or in rows. In sections stained by luxol fast blue/periodic acid-Schiff stain, the vacuoles were delimited and transected by myelin strands. Transmission electron microscopy revealed intramyelinic vacuoles formed in the myelin sheaths by splitting of one or more myelin lamellae at the intraperiodic line. This lesion is characteristic of toxicity from hexachlorophene, triethyltin, bromethalin, isonicotinic acid hydrazide, and certain exotic plant toxins; however, despite exhaustive testing, no etiology was determined for the DeGray Lake mortality events. This is the first report of vacuolar myelinopathy associated with spontaneous mortality in wild birds.

Spongiform central nervous system myelinopathy in African dwarf goats

A novel spongiform myelinopathy of the central nervous system (CNS) of eleven African dwarf goats was examined by light and electron microscopy. Histological lesions consisted of extensive vacuolation predominantly of the white matter of the diencephalon, midbrain and cerebellar peduncles, as well as of spinal white matter. Ultrastructurally, vacuoles were shown to be intramyelinic, resulting from the splitting of the outer myelin lamellae at the intraperiod line. A few oligodendrocytes showed vacuolar degeneration of cell bodies and processes. Inflammatory reactions were absent. The observed lesions point to an unknown primary damage of oligodendroglia and central myelin. A hereditary background of the disorder is suspected as all investigated dwarf goats were half-brothers or -sisters and partly descended from the mating of adult females with their own sire.

Myelinolysis after correction of hyponatremia in two dogs

Two dogs developed delayed neurological deterioration after rapid correction of severe hyponatremia. Sequential magnetic resonance imaging showed the development of lesions in the thalamus. One dog was necropsied, and the lesions were characterized by myelinolysis with sparing of axons and neurons. The second dog gradually recovered with no detectable neurological deficits. The syndrome seems analogous to central pontine myelinolysis in human beings. Guidelines for correction of hyponatremia to prevent development of myelinolysis are given.