Remembering your A, B, C's: Alzheimer's disease and ABCA1.

The function of ATP binding cassette protein A1 (ABCA1) is central to cholesterol mobilization. Reduced ABCA1 expression or activity is implicated in Alzheimer's disease (AD) and other disorders. Therapeutic approaches to boost ABCA1 activity have yet to be translated successfully to the clinic. The risk factors for AD development and progression, including comorbid disorders such as type 2 diabetes and cardiovascular disease, highlight the intersection of cholesterol transport and inflammation. Upregulation of ABCA1 can positively impact APOE lipidation, insulin sensitivity, peripheral vascular and blood-brain barrier integrity, and anti-inflammatory signaling. Various strategies towards ABCA1-boosting compounds have been described, with a bias toward nuclear hormone receptor (NHR) agonists. These agonists display beneficial preclinical effects; however, important side effects have limited development. In particular, ligands that bind liver X receptor (LXR), the primary NHR that controls ABCA1 expression, have shown positive effects in AD mouse models; however, lipogenesis and unwanted increases in triglyceride production are often observed. The longstanding approach, focusing on LXRß vs. LXRα selectivity, is over-simplistic and has failed. Novel approaches such as phenotypic screening may lead to small molecule NHR modulators that elevate ABCA1 function without inducing lipogenesis and are clinically translatable.

Metabolomic analysis of a selective ABCA1 inducer in obesogenic challenge provides a rationale for therapeutic development.

BACKGROUND: Therapeutic agents with novel mechanisms of action are needed to combat the growing epidemic of type 2 diabetes (T2D) and related metabolic syndromes. Liver X receptor (LXR) agonists possess preclinical efficacy yet produce side effects due to excessive lipogenesis. Anticipating that many beneficial and detrimental effects of LXR agonists are mediated by ABCA1 and SREPB1c expression, respectively, we hypothesized that a phenotypic optimization strategy prioritizing selective ABCA1 induction would identify an efficacious lead compound with an improved side effect profile over existing LXRß agonists. METHODS: We synthesized and characterized a novel small molecule for selective induction of ABCA1 vs. SREBP1c in vitro. This compound was evaluated in both wild-type mice and a high-fat diet (HFD) mouse model of obesity-driven diabetes through functional, biochemical, and metabolomic analysis. FINDINGS: Six weeks of oral administration of our lead compound attenuated weight gain, glucose intolerance, insulin signaling deficits, and adiposity. Global metabolomics revealed suppression of gluconeogenesis, free fatty acids, and pro-inflammatory metabolites. Target identification linked these beneficial effects to selective LXRß agonism and PPAR/RXR antagonism. INTERPRETATION: Our observations in the HFD model, combined with the absence of lipogenesis and neutropenia in WT mice, support this novel approach to therapeutic development for T2D and related conditions.

Discovery of Nonlipogenic ABCA1 Inducing Compounds with Potential in Alzheimer's Disease and Type 2 Diabetes.

Selective liver X receptor (LXR) agonists have been extensively pursued as therapeutics for Alzheimer's disease and related dementia (ADRD) and, for comorbidities such as type 2 diabetes (T2D) and cerebrovascular disease (CVD), disorders with underlying impaired insulin signaling, glucose metabolism, and cholesterol mobilization. The failure of the LXR-focused approach led us to pursue a novel strategy to discover nonlipogenic ATP-binding cassette transporter A1 (ABCA1) inducers (NLAIs): screening for ABCA1-luciferase activation in astrocytoma cells and counterscreening against lipogenic gene upregulation in hepatocarcinoma cells. Beneficial effects of LXRß agonists mediated by ABCA1 include the following: control of cholesterol and phospholipid efflux to lipid-poor apolipoproteins forming beneficial peripheral HDL and HDL-like particles in the brain and attenuation of inflammation. While rare, ABCA1 variants reduce plasma HDL and correlate with an increased risk of ADRD and CVD. In secondary assays, NLAI hits enhanced cholesterol mobilization and positively impacted in vitro biomarkers associated with insulin signaling, inflammatory response, and biogenic properties. In vivo target engagement was demonstrated after oral administration of NLAIs in (i) mice fed a high-fat diet, a model for obesity-linked T2D, (ii) mice administered LPS, and (iii) mice with accelerated oxidative stress. The lack of adverse effects on lipogenesis and positive effects on multiple biomarkers associated with T2D and ADRD supports this novel phenotypic approach to NLAIs as a platform for T2D and ADRD drug discovery.

Alzheimer's disease pathology in APOE transgenic mouse models: The Who, What, When, Where, Why, and How.

The focus on amyloid plaques and neurofibrillary tangles has yielded no Alzheimer's disease (AD) modifying treatments in the past several decades, despite successful studies in preclinical mouse models. This inconsistency has caused a renewed focus on improving the fidelity and reliability of AD mouse models, with disparate views on how this improvement can be accomplished. However, the interactive effects of the universal biological variables of AD, which include age, APOE genotype, and sex, are often overlooked. Age is the greatest risk factor for AD, while the ε4 allele of the human APOE gene, encoding apolipoprotein E, is the greatest genetic risk factor. Sex is the final universal biological variable of AD, as females develop AD at almost twice the rate of males and, importantly, female sex exacerbates the effects of APOE4 on AD risk and rate of cognitive decline. Therefore, this review evaluates the importance of context for understanding the role of APOE in preclinical mouse models. Specifically, we detail how human AD pathology is mirrored in current transgenic mouse models ("What") and describe the critical need for introducing human APOE into these mouse models ("Who"). We next outline different methods for introducing human APOE into mice ("How") and highlight efforts to develop temporally defined and location-specific human apoE expression models ("When" and "Where"). We conclude with the importance of choosing the human APOE mouse model relevant to the question being addressed, using the selection of transgenic models for testing apoE-targeted therapeutics as an example ("Why").

Comparison of polystyrene scintillator fiber array and monolithic polystyrene for neutron imaging and radiography.

The neutron imaging diagnostic at the National Ignition Facility has been operating since 2011 generating neutron images of deuterium-tritium (DT) implosions at peak compression. The current design features a scintillating fiber array, which allows for high imaging resolution to discern small-scale structure within the implosion. In recent years, it has become clear that additional neutron imaging systems need to be constructed in order to provide 3D reconstructions of the DT source and these additional views need to be on a shorter line of sight. As a result, there has been increased effort to identify new image collection techniques that improve upon imaging resolution for these next generation neutron imaging systems, such as monolithic deuterated scintillators. This work details measurements performed at the Weapons Neutron Research Facility at Los Alamos National Laboratory that compares the radiographic abilities of the fiber scintillator with a monolithic scintillator, which may be featured in a future short line of sight neutron imaging systems.

JZL184 is anti-hyperalgesic in a murine model of cisplatin-induced peripheral neuropathy.

Cisplatin has been used effectively to treat a variety of cancers but its use is limited by the development of painful peripheral neuropathy. Because the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG) is anti-hyperalgesic in several preclinical models of chronic pain, the anti-hyperalgesic effect of JZL184, an inhibitor of 2-AG hydrolysis, was tested in a murine model of cisplatin-induced hyperalgesia. Systemic injection of cisplatin (1mg/kg) produced mechanical hyperalgesia when administered daily for 7 days. Daily peripheral administration of a low dose of JZL184 in conjunction with cisplatin blocked the expression of mechanical hyperalgesia. Acute injection of a cannabinoid (CB)-1 but not a CB2 receptor antagonist reversed the anti-hyperalgesic effect of JZL184 indicating that downstream activation of CB1 receptors suppressed the expression of mechanical hyperalgesia. Components of endocannabinoid signaling in plantar hind paw skin and lumbar dorsal root ganglia (DRGs) were altered by treatments with cisplatin and JZL184. Treatment with cisplatin alone reduced levels of 2-AG and AEA in skin and DRGs as well as CB2 receptor protein in skin. Combining treatment of JZL184 with cisplatin increased 2-AG in DRGs compared to cisplatin alone but had no effect on the amount of 2-AG in skin. Evidence that JZL184 decreased the uptake of [(3)H]AEA into primary cultures of DRGs at a concentration that also inhibited the enzyme fatty acid amide hydrolase, in conjunction with data that 2-AG mimicked the effect of JZL184 on [(3)H]AEA uptake support the conclusion that AEA most likely mediates the anti-hyperalgesic effect of JZL184 in this model.

Kinetics of UV(254) inactivation of selected viral pathogens in a static system.

AIMS: The objective of this study was to estimate UV(254) inactivation constants for four viral pathogens: influenza virus type A, porcine respiratory and reproductive syndrome virus (PRRSV), bovine viral diarrhoea virus (BVDV) and reovirus. METHODS AND RESULTS: Viruses in culture medium were exposed to one of nine doses of UV(254) and then titrated for infectious virus. Analysis showed that viral inactivation by UV(254) was more accurately described by a two-stage inactivation model vs a standard one-stage inactivation model. CONCLUSIONS: The results provided evidence for the existence of two heterogeneous viral subpopulations among the viruses tested, one highly susceptible to UV(254) inactivation and the other more resistant. Importantly, inactivation constants based on the one-stage inactivation model would have underestimated the UV(254) dose required for the inactivation of these viruses under the conditions of the experiment. SIGNIFICANCE AND IMPACT OF THE STUDY: To improve the accuracy of estimates, it is recommended that research involving the inactivation of micro-organisms evaluates inactivation kinetics using both one-stage and two-stage models. These results will be of interest to persons responsible for microbial agents under laboratory or field conditions.

Profiles of matrix metalloproteinase activity in equine tear fluid during corneal healing in 10 horses with ulcerative keratitis.

OBJECTIVE: Levels of tear film matrix metalloproteinases (MMPs) activity are significantly elevated in horses with ulcerative keratitis and contribute to the excessive breakdown of stromal collagen. Changes in the amount of proteolytic activity in horse tear film during corneal healing and stromal remodeling have not yet been reported, but we hypothesize they should decrease. In the present study we analyzed serial tear fluid from horses with ulcerative keratitis to identify any changes in MMP activity during corneal healing and stromal remodeling. PROCEDURES: Samples of tear fluid were obtained from both eyes of 10 horses with ulcerative keratitis on the day of admission (day 1) at the hospital and then at various time points until complete healing of the cornea. Tear film MMP2 and MMP9 activity was determined by quantitative gelatin zymography. In all cases medical treatment included topical applications of equine serum, antibiotics, atropine and systemic administration of anti-inflammatory drugs. Surgical procedures were performed in several cases on day 2 in addition to the medical treatment. RESULTS: The mean total MMP activity (+/- SD) measured in relative standard units (RSU) in the tear fluid of the ulcerated eye (2.44 +/- 1.44) of the 10 horses was significantly higher than the mean in the contralateral eye (0.81 +/- 0.68) (P = 0.006), on the day of admission at the VMTH. The mean MMP activity in these ulcerated eyes significantly decreased (-82.4%) between the first day of admission and the day when the ulcer had completely healed (P = 0.0002). The activity level in the healed eye (0.43 +/- 0.17) was not significantly different to the one in the contralateral eye (0.36 +/- 0.18) on the day of complete corneal healing (P = 0.374). The level of MMP activity in the contralateral eye also decreased from 0.81 +/- 0.68-0.36 +/- 0.18 but this decrease (56%) was not significant (P = 0.069). CONCLUSIONS: Ulcerative keratitis in horses is associated with initially high levels of tear film proteolytic activity that decrease as the ulcers heal. The success of medical and surgical treatment of the corneal ulcers is reflected by the enzyme activity in tears. In horses successful treatment does lead to a rapid reduction in tear film proteolytic activity that corresponded with the improvement in the clinical signs of corneal ulceration. Measurement of MMP activity in the tear film might represent a way to monitor the progression of corneal healing in horses with ulcerative keratitis.

Diagnostic ultrasonography of equine lens and posterior segment abnormalities.

PURPOSE: To define the indications for equine ocular ultrasonography and to provide representative ultrasonographic images of lens and posterior segment diseases. METHODS: Retrospective study. Equine medical records dating from January 1983 to March 2001 were reviewed. All cases that: (1) had a lens and/or posterior segment abnormality; and (2) received a complete ophthalmic examination and ocular ultrasonography were included. RESULTS: Forty-three cases (n = 64 eyes) out of 112 total cases of equine lens and/or posterior segment abnormalities qualified. The following conditions were identified ultrasonographically in order of decreasing frequency: cataracts, vitreal opacities, retinal detachment, lens luxation, endophthalmitis, microphthalmos, choroiditis, lens rupture, lenticonus, buphthalmos and phthisis bulbi. Ultrasonography often enabled diagnoses to be made in the presence of anterior opacities in both surgical (pre and/or postoperatively) and nonsurgical cases. Additional ocular conditions were identified in adults with cataracts more frequently than in foals with cataracts. CONCLUSIONS: Cataracts were the most common lens abnormality identified in horses that received ocular ultrasound examination. Cataracts were commonly found in association with other ocular abnormalities. Ultrasonography was a practical and effective method of evaluating the lens and posterior segment, particularly in cases with anterior opacities. Ultrasonography also provided critical information with regard to the potential for surgical removal of cataracts and was a valuable component of postsurgical follow-up.

Connective tissue growth factor in tear film of the horse: detection, identification and origin.

BACKGROUND: Healing of corneal ulcers in horses is often associated with profound corneal stromal fibrosis and scar formation resulting in visual impairment. Connective tissue growth factor (CTGF) is a fibrogenic cytokine involved in wound healing and scarring. The purpose of this study was to determine whether CTGF was present in the tear fluid of normal horse eyes and the eyes of horses with corneal ulcers in order to evaluate the role of CTGF in corneal wound healing and corneal scar formation. METHODS: Tear fluid samples were collected from 65 eyes of 44 horses; 32 samples from normal eyes, 21 samples from eyes with corneal ulceration, and 12 samples from the unaffected contralateral eyes of horses with ulcers. CTGF levels in the tears were determined by enzyme immunoassay using goat IgG against human CTGF. Antigenetic similarity of human and horse CTGF was established in a bio-equivalence assay. The identity of horse CTGF was confirmed by western blot. Lacrimal and nictitating membrane glands were investigated by immunohistochemistry in the attempt to clarify the origin of tear fluid CTGF. RESULTS: CTGF was detected in tear film of 23 normal unaffected eyes (72%) and 8 normal contralateral eyes (67%), with the mean CTGF levels (+/- SEM) being 51.5+/-19.2 and 13.4+/-3.9 ng/ml respectively. CTGF was found in 8 eyes with corneal ulcers (38%) with the mean CTGF concentration of 26.3+/-14.8 ng/ml. Western blot identified the protein detected as CTGF. The identification of CTGF in lacrimal glands suggests a major role of these glands in the presence of CTGF in tears. CONCLUSIONS: CTGF is present in horse tear fluid and derives, at least partly, from the lacrimal gland. Equine CTGF has strong antigenic similarity with human CTGF. Corneal disease leads to a decrease of CTGF concentrations in tears. The possible role of CTGF in the healing process of ocular surface requires further investigation.

Enrofloxacin-associated retinal degeneration in cats.

OBJECTIVE: The objective of this study was to evaluate the possible relationship between the administration of parenteral and/or oral [corrected] enrofloxacin and the onset of acute retinal degeneration in cats. The animals studied included 17 cats that received systemic enrofloxacin and developed retinal degeneration soon thereafter. PROCEDURES: In this retrospective clinical study, cats that received parenteral and/or oral [corrected] enrofloxacin and developed acute blindness were identified. Parameters recorded included breed, age, sex, enrofloxacin dosage (daily dose and number of days administered), medical condition for which the antibiotic had been prescribed, ophthalmic signs, examination results, and the visual outcome. Fundus photographs were obtained in seven cats, and electroretinography was performed in five cats. Histopathology was performed on two eyes from one cat (case 1) that received enrofloxacin 5 months previously and developed retinal degeneration. RESULTS: All cats were the domestic shorthair breed; seven were females (one neutered) and ten were males (seven castrated). Ages ranged from 3 to 16 years old (mean +/- SD; 8.8 +/- 4.6 years). The medical disorders for which enrofloxacin was administered ranged from lymphoma and pancreatitis to otitis and dermatitis, and eight cats had urinary diseases. The daily and total dosage of enrofloxacin and number of days of administration were also highly variable. Presenting clinical signs were most often mydriasis and acute blindness. All cats had diffuse retinal degeneration as evidenced by increased tapetal reflectivity and retinal vascular attenuation. Absence of recordable electroretinographic responses suggested diffuse and extensive outer retinal disease. Vision returned in a few cats, but the retinal degeneration persisted or even progressed. Histopathology of two eyes revealed primarily outer retinal degeneration, with diffuse loss of the outer nuclear and photoreceptor layers, and hypertrophy and proliferation of the retinal pigment epithelium. CONCLUSION: Parenteral and/or oral [corrected] enrofloxacin is potentially retinotoxic in some cats, and may result in acute and diffuse retinal degeneration. Blindness often results, but some cats may regain vision. Practitioners should adhere closely to the manufacturer's current enrofloxacin dosage recommendation (5 mg/kg q 24 h), and continue clinical observations for this drug toxicity in cats.

Immunoconversion against Sarcocystis neurona in normal and dexamethasone-treated horses challenged with S. neurona sporocysts.

Equine protozoal myeloencephalitis is a common neurologic disease of horses in the Americas usually caused by Sarcocystis neurona. To date, the disease has not been induced in horses using characterized sporocysts from Didelphis virginiana, the definitive host. S. neurona sporocysts from 15 naturally infected opossums were fed to horses seronegative for antibodies against S. neurona. Eight horses were given 5x10(5) sporocysts daily for 7 days. Horses were examined for abnormal clinical signs, and blood and cerebrospinal fluid were harvested at intervals for 90 days after the first day of challenge and analyzed both qualitatively (western blot) and quantitatively (anti-17kDa) for anti-S. neurona IgG. Four of the challenged horses were given dexamethasone (0.1mg/kg orally once daily) for the duration of the experiment. All challenged horses immunoconverted against S. neurona in blood within 32 days of challenge and in CSF within 61 days. There was a trend (P = 0.057) for horses given dexamethasone to immunoconvert earlier than horses that were not immunosuppressed. Anti-17kDa was detected in the CSF of all challenged horses by day 61. This response was statistically greater at day 32 in horses given dexamethasone. Control horses remained seronegative throughout the period in which all challenged horses converted. One control horse immunoconverted in blood at day 75 and in CSF at day 89. Signs of neurologic disease were mild to equivocal in challenged horses. Horses given dexamethasone had more severe signs of limb weakness than did horses not given dexamethasone; however, we could not determine whether these signs were due to spinal cord disease or to effects of systemic illness. At necropsy, mild-moderate multifocal gliosis and neurophagia were found histologically in the spinal cords of 7/8 challenged horses. No organisms were seen either in routinely processed sections or by immunohistochemistry. Although neurologic disease comparable to naturally occurring equine protozoal myeloencephalitis (EPM) was not produced, we had clear evidence of an immune response to challenge both systemically and in the CNS. Broad immunosuppression with dexamethasone did not increase the severity of histologic changes in the CNS of challenged horses. Future work must focus on defining the factors that govern progression of inapparent S. neurona infection to EPM.

Mechanical and thermal hypersensitivity develops following kainate lesion of the ventral posterior lateral thalamus in rats.

Damage to the ventral-posterior lateral nucleus (VPL) of the thalamus or its afferent pathways can produce moderate to severe on-going pain and pain in response to normally innocuous stimuli (allodynia) and hypersensitivity to mildly noxious stimuli (hyperalgesia). The present study measured the responses to mechanical and thermal stimuli before and 2, 8, 24 and 48 h after a kainate-induced lesion of the VPL in male rats. Compared with control animals, hypersensitivity to mechanical stimulation of the hindpaw was apparent by 24 h post-lesion. At 48 h, the frequency of mechanical response increased from a baseline response frequency of 17+/-4.7 to 46+/-11.6%. Thermal withdrawal latencies 48 h after the lesion decreased from a baseline latency of 9.9+/-1.8 to 5.3+/-0.4 s. It is concluded that a neurotoxic lesion of the VPL results in a delayed onset of mechanical and thermal hyperalgesia. This study suggests a potential model for studying the basic mechanisms and potential therapies for central pain syndrome.

Disease of the equine posterior segment.

The appearance of the equine fundus is reviewed from the perspective of differentiating normal variations from disease, and the descriptions have been updated to include recently published ocular fundic abnormalities. Most pathological lesions are identified near the optic nerve head, and typically involve depigmentation or hyperpigmentation. Depending upon configuration and appearance, linear pigmented bands may reflect the course of the vortex veins, the transition from tapetal to nontapetal fundus, or indicate chorioretinitis or equine motor neuron disease. Choroidal vasculature is readily apparent in color-dilute (subalbinotic) horses and must be differentiated from hemorrhage. Retinal hemorrhages in foals are common and may occur independently to hypoxic ischemic encephalopathy. Retinal cysts may signal more significant disease in the eye such as anterior segment dysgenesis. Prominence of gray or tan-colored material on or near the optic nerve head may represent traumatic optic neuropathy, benign optic neuropathy, proliferative optic neuropathy or actual neoplasia.

Rose bengal positive epithelial microerosions as a manifestation of equine keratomycosis.

Purpose To describe the clinical appearance of corneal epithelial cell microerosions associated with keratomycosis in the horse. METHODS: Retrospective clinical study. RESULTS: Multifocal, punctate, superficial corneal opacities with positive rose bengal retention were noted in six horses with presumed 'viral keratitis'. Faint fluorescein staining was also present in three cases. Equine herpesvirus tissue culture inoculation was negative for a cytopathic effect in three cases. Aspergillus (n = 3), Curvularia (n = 1), and an unidentified fungus (n = 1) were cultured in five horses, and hyphae found on corneal cytology from the sixth. Mixed bacterial infections were present in three eyes. The eyes of two horses with Aspergillus progressed to deep melting corneal ulcers that required surgical therapy. The microerosions remained superficial, but persistent in the other four eyes. Natamycin was utilized topically in all six horses. Transmission electron microscopy from case 6 revealed mucin layer disruption, an intact corneal epithelial cell layer, and fungal attachment to degenerating epithelial cells. The visual outcome was positive in all six horses, although healing was prolonged (48.5 +/- 14.5 days on average in the horses with no surgery; 62 days on average in the two horses that required surgery). CONCLUSIONS: Complete removal or full-thickness penetration of the corneal epithelial cell barrier may not be necessary to allow fungal adherence and initiation of keratomycosis in the horse. Prior to colonization and invasion of the horse cornea, fungi may induce changes in the mucin layer of the tear film that result in or are associated with rose bengal positive microerosions of the superficial corneal epithelium. Horses with painful eyes, and eyes with superficial, multifocal corneal opacities should have their corneas stained with both fluorescein and rose bengal as fungal microerosions may stain weakly, or not at all, with fluorescein, and may thus be mistaken for presumed 'viral keratitis' of the horse.

Posterior lamellar keratoplasty for treatment of deep stromal absesses in nine horses.

OBJECTIVE: To describe and evaluate the use of posterior lamellar keratoplasty as a surgical treatment for deep corneal stromal abscesses in horses. Animals studied Nine horses of various breeds and ages that presented with corneal stromal abscesses located in the posterior one-third of the cornea. Procedure Retrospective medical record study. RESULTS: Nine horses had deep corneal stromal abscesses that were treated with posterior lamellar keratoplasty. Median patient age was 3 years. Six patients were females and three were geldings. Medical therapy alone had been attempted prior to surgery in all nine animals. Corneal abscess culture and histopathology were performed in 8/9 horses. Cultures were positive for an infectious etiology in 4/8 (50%). Histopathology was positive for an infectious etiology in 5/8 (62.5%). Mean surgical time was 71.0 +/- 18.8 min and the average healing time was 23.7 +/- 5.2 days. Visual outcome was positive in 8/9 cases. Conclusion Posterior lamellar keratoplasty is a promising procedure for treatment of deep corneal stromal abscesses in horses. The procedure resulted in considerable shorter surgery time and healing time than had been observed with full-thickness penetrating keratoplasty. Scar formation with this procedure was not significantly different than with penetrating keratoplasty.

Ulcerative keratitis caused by beta-hemolytic Streptococcus equi in 11 horses.

Purpose To describe 11 clinical cases of ulcerative keratitis in horses associated with beta-hemolytic Streptococcus equi in Florida, USA. METHODS: Retrospective clinical study (1996-99). RESULTS: Beta-hemolytic Streptococcus equi was cultured from 11 horses with deep ulcers, descemetoceles or iris prolapse (n = 8), a suture abscess found with a penetrating keratoplasty for a stromal abscess (n = 1), and ulceration that developed following keratectomy/irradiation for corneal squamous cell carcinoma (n = 2). Beta-hemolytic Streptococcus equi subspecies zooepidemicus was found in 10 eyes and subspecies equi in one. Marked signs of uveitis including miosis and hypopyon were present in 8/11 (72.7%) eyes. Keratomalacia was severe in all eyes. The mean diameter of the ulcers associated with beta-hemolytic Streptococcus was 10.2 +/- 6.1 mm. Eight of the eyes required conjunctival flap surgery (four grafts dehisced) and one eye corneal transplantation. Two eyes were treated with medication only. Isolate sensitivity to antibiotics included ampicillin (6/11), bacitracin (11/11), cephalothin (11/11), chloramphenicol (11/11), gentamicin (5/11), polymyxin B (2/11), and tobramycin (1/11). All isolates were resistant to neomycin. The average healing time was 44.7 +/- 26.7 days. The visual outcome was positive in 8/11 eyes, and the globe retained in 9/11 eyes. CONCLUSIONS: Although Gram-positive bacteria predominate in the normal conjunctival microflora of horses throughout the world, Gram-negative bacteria and fungi are more often isolated from equine ulcers. Beta-hemolytic Streptococcus spp. are associated with a very aggressive ulcerative keratitis with the capability to digest conjunctival graft tissue. Clinical signs are pronounced. Aggressive surgical and intensive medical therapy with topical antibiotics and protease inhibitors is indicated.