Equine projects update, September 2005

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  • 326 Journal of Equine Veterinary Science July 2006

    Current Research

    INVESTIGATING THE ROLE OF IMPAIRED GLUCOSE UPTAKE IN LAMINITIS

    August 2004May 2007 Assoc. Prof. Martin Sillence, School of Agriculture,

    Charles Sturt University

    Laminitis (founder) is the most serious disease ofthe equine foot and when it strikes, it can be heart-breaking for owners and causes great stress and pain tothe horse.

    Past and current research has mainly focused on is-sues associated with laminitis caused by dietary factorssuch as carbohydrate overload and overfeeding.Laminitis also can be caused by metabolic problemssuch as equine metabolic syndrome (obesity and insulinresistance), Cushings syndrome (overproduction ofcorticosteroid hormones by the adrenal gland), and ex-cessive use of corticosteroids to treat inflammation.

    Cortisol is a corticosteroid that works with insulinto maintain a stable concentration of glucose in theblood. Cortisol and insulin counteract one another, withinsulin promoting tissue glucose uptake and cortisol in-hibiting it. They do this by influencing the action of glu-cose transporters that are required for glucose to betaken up by various tissues in the body.

    Insulin resistance occurs when glucose transportersare overworked, fail, and become resistant to insulin,for example, through overfeeding. When this happens,glucose uptake remains low despite elevated levels ofcirculating insulin.This is well known as type II diabetesin humans. The syndrome is known as equine metabolicsyndrome in horses. This problem has not been fully in-vestigated; however, it is thought to be a consequenceof obesity and overfeeding of carbohydrates.

    Corticosteroids inhibit glucose uptake in many tis-sues by a direct mechanism that limits the movementof glucose transporters to the cell surface. Corti-

    costeroids also can trigger a complex series of short-term and long-term hormonal events that limit glucoseuse in tissues.

    Healthy hoof tissue requires glucose and uses it atan exceptionally fast rate. Without glucose, or in thepresence of glucose uptake inhibitors, the connectionsbetween the hoof wall (lamellae) and underlying struc-tures break down rapidly.

    This project aims to understand the mechanismsthat control glucose uptake in the hoof and to identifythe factors associated with endocrine (hormone) andmetabolic abnormalities that lead to impaired glucoseuptake and laminitis.

    The project is a collaboration of researchers whohave extensive skills in different areas. Professor MartinSillence is an experienced endocrinologist (endocrinol-ogy is the study of the glands that secrete hormones).Dr Catherine McGowan is a specialist equine veteri-narian with a research interest in metabolic disorders,and Professor Chris Pollitt leads a well-establishedlaminitis research group. This collaboration will en-hance the productivity, scope, and effectiveness of thestudy.

    Current Progress

    The researchers have developed a technique tomeasure glucose uptake in hoof tissue. This techniqueis being used to study the uptake of glucose in the hoofand the response of glucose uptake in the presence ofinsulin.

    Progress has also been made in the study of thestress hormone adrenalin and its role in glucosemetabolism in the hoof. Adrenalin inhibits the uptakeof glucose in tissues, and the impact of adrenalin de-pends on the number and type of adrenalin receptors intissues.The researchers have found that hoof tissue con-tains at least two types of adrenalin receptors. Studiesare underway to determine how many adrenalin recep-tors are present in healthy hoof tissue. This will help theresearchers determine whether any change occurs inthe number of adrenalin receptors in horses that de-velop laminitis.

    Equine Projects Update, September 2005Australian Government

    Rural Industries Research and Development Corporation (RIRDC)

    0737-0806/$ - see front matter 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jevs.2006.05.008

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  • Volume 26, Number 7 327

    EPAXIAL MUSCULATURE AND ITSRELATIONSHIP WITH BACK PAIN IN THEHORSE

    Dr. Catherine McGowan, School of Animal Studies,University of Queensland

    Back pain is common in horses and can reoccur fre-quently in some animals. Back pain from any causeoften can result in changes to muscles along the spine.These muscles may weaken and atrophy in associationwith back pain stemming from spine or joint disease, orfrom a poorly fitting saddle, or as a result of disease, forexample, a generalized muscular disorder.

    Monitoring muscle size is very important in physio-therapy practice, because measurements taken over timecan show changes that represent the effectiveness of thetreatment. The muscles along the back of the horsewhere atrophy is most visible are the epaxial (back) mus-cles, consisting of the longissimus dorsi and the multi-fidus, which lie along the top of the back. Signs that thesemuscles have decreased in size can be seen as an in-creased prominence of the bones along the spine and adipping away of the muscles on the sides of the spine.Themultifidus muscle is deep, lying beneath the longissimusdorsi, and therefore determining which of these musclesis wasting by visual means alone is not possible.

    Ultrasound imaging has been successfully used inhumans to measure the size of the multifidus muscle;however, this technique has not been validated in horses.In humans restoration of the multifidus size and functionis an important factor in prevention of the recurrence ofback pain in people with acute back injuries. The recov-ery of this muscle is usually only possible in humanswhen they receive physiotherapy after the injury. Thesefindings lead to a speculation that the diagnosis, treat-ment, and monitoring of multifidus atrophy would havemerit in cases of acute back pain in the horse.

    This project aims to measure objectively the equineback muscles and the response of equine epaxial mus-cles to back pain syndromes. The project also aims todevelop ultrasonography as a simple tool for diagnosisof muscular abnormalities associated with back painand for monitoring response to therapy.

    Current Progress

    The researchers have confirmed that MagneticResonance Imaging (MRI) is a reliable tool for mea-suring back muscles in the horse. Therefore, MRI hasbeen used as a standard in the studies investigating ul-trasound as a noninvasive measurement tool.

    Basic ultrasound data have been collected and ana-lyzed, and the ultrasonography studies are continuing.The ultrasound comparisons between different breedsof horses have been completed for Thoroughbreds andstockhorses, and the researchers are currently collectingdata for other breeds. The effect of conditioning is con-tinuing with a group of Thoroughbred yearlings, anddata collection has begun on aged horses. The re-searchers have begun collecting ultrasound data fromhorses displaying back or hind limb problems.

    PENETRATION OF PHARMACOLOGICALAGENTS THROUGH EQUINE SKIN

    August 2003May 2006 Dr. Paul Mills, School of Veterinary Science,

    University of Queensland

    The skin is the protective covering of the body. Itconsists of two layers with the outermost layer beingthe main barrier to movement of substances throughthe skin. This layer comprises skin cells surrounded bylipid (fat) that serve to hold the cells together. For adrug to penetrate the outer layer of skin, it needs asmall molecular structure and an affinity with fat. Manycurrent medicines have been developed for topical use,and the number is increasing. The advantages of thismode of drug delivery are the ease of administration,the fact that the drug does not pass through the liver be-fore it reaches the area needing treatment, and the re-duction in the occurrence of irritation of the gastroin-testinal tract.

    Although many drugs are applied topically, very lit-tle is known about the mechanisms that control themovement of drugs through the equine skin. Concern isincreasing about inadvertent positive results from swabsamples collected during equine competition after top-ical administration of drugs. Much of the available in-formation regarding products such as topical corticoste-roid creams and nonsteroidal anti-inflammatory gels isbased on results from human or laboratory animal stud-ies. Extrapolation of transdermal (through the skin)drug movement between species is unreliable. Adverseor unexpected results may occur when these prepara-tions are applied to horses.

    This project aims to develop methods to measurethe movement of drugs through equine skin and tomeasure the differences in drug penetration when applied to different parts of the body. The drugs under investigation are commonly used on horses inAustralia and include nonsteroidal anti-inflammatories

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  • 328 Journal of Equine Veterinary Science July 2006

    (NSAIDs) such as phenylbutazone, corticosteroids, andanalgesics (pain relievers). The researchers also aim todetermine the effects of the chemicals that are mixedwith drugs to enable the drugs to move through theskin. The effects of skin damage (rashes) and alteredmicroenvironment (saddles, bandages) on the transder-mal movement of drugs will be studied.

    Current Progress

    The researchers have developed laboratory tech-niques that will enable the study of drug movementthrough skin. The model is based on the Franz-type dif-fusion cell. A skin sample is placed between two glassreceptacles. After placing the drug of interest in the topreceptacle, the bottom receptacle can be tested overtime to see how much of the drug has diffused throughthe skin sample.

    Several drugs have been studied using the devel-oped techniques, and the experiments are ongoing. Theresearchers have found that the drugs they have studiedpenetrate the skin on different areas of the body at dif-ferent rates. Some drugs that are the same molecularsize have been shown to have different rates of skinpenetration.

    Future experiments in horses will involve topicalapplication of drugs and testing of blood and urine sam-ples.The researchers have developed a technique of mi-crodialysis in which probes are inserted into the skinand drug concentrations can be measured in the skinand underlying tissue.

    DEVELOPMENT OF IMPROVED TREATMENTAND PREVENTION STRATEGIES FORINFLAMMATORY AIRWAY DISEASE OF HORSES

    January 2003December 2005Associate Professor Jennifer Hodgson

    University Veterinary Centre Camden, University of Sydney

    Inflammatory airway disease (IAD) can causecoughing and it is a common cause of poor performancein young horses worldwide. The researchers of thisstudy have shown in previous studies that up to onethird of all horses will develop airway inflammationwithin 2 weeks of entering a stable environment.

    A dramatic transformation has occurred in our un-derstanding of equine lower respiratory disease inAustralia in recent years. Work funded by RuralIndustries Research and Development Corporation(RIRDC) has been pivotal to this understanding. Thiswork has shown that environmental factors have an im-

    portant influence on the lower respiratory health ofhorses. In the past, viruses were thought to be the majorcause of respiratory disease in young racingThoroughbreds, but this is not the case, and researchhas shown that viruses cause only a relatively smallnumber of cases of IAD.

    This project involves the study of environmentalfactors to assess the best treatment and control mea-sures for inflammatory airway disease in Thoroughbredhorses.

    Current Progress

    The researchers have conducted a questionnaire ofveterinarians and trainers to provide an overview ofhow they approach the problem of IAD in terms of di-agnosis, treatment, and management. The researchersthen designed an experiment to determine how wellcurrent medical treatments work. In this experiment,IAD was induced in horses, and the treatment regimensunder study were tested. The next phase of the researchwas to assess different management strategies (eg, al-tering bedding, wetting feed) on air quality in the sta-bled horses environment.

    The outcome of the studies conducted by the re-searchers have led to some important recommenda-tions that can be applied to the management of stabledhorses. These recommendations include:

    Stable bedding that consists of sawdust or woodshavings is less likely to contribute to IAD than strawor rice hulls.

    Ventilation to minimize dust accumulation is im-portant in the management of IAD in stabled horses.

    Horses should be removed from stables whenperforming activities such as mucking out or addingbedding material.

    A randomized clinical trial to investigate currentdrug treatments for IAD is underway, and the re-searchers have planned a further trial to investigate in-haled treatments for IAD. These trials will not only as-sess treatment options but will yield importantinformation on drug elimination times. This informa-tion will enable the development of recommendationson withdrawal times when IAD treatments are used inracing horses.

    When this study is complete, the researchers antic-ipate that they will be able to deliver up-to-date infor-mation on strategies for the management of inflamma-tory airway disease. This information will benefit allhorse owners and managers who stable their horses andhave to manage the difficult problem of coughinghorses.

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  • Volume 26, Number 7 329

    RIRDC shall not be responsible in any mannerwhatsoever to any person who relies, in whole or inpart, on the contents of these reports unless authorizedin writing by the Managing Director of RIRDC.

    Reprinted from RIRDC website:http://www.usyd.edu.au/rirdc/projects/study17.html

    Please address any questions or comments to: MellisaOfford, mofford@vetsci.usyd.edu.au (Assistant Editor of the

    RIRDC Equine Research News).

    AQHA EQUINE RESEARCH FUNDING

    Approved by the AQHA Executive Committee, March 2006

    The following projects for equine research werefunded for a total of $501,302:1. University of California DavisIdentification of

    the Causative Mutation for HERDA in theAmerican Quarter Horse for $57,623.

    2. University of GeorgiaIn Vitro Models ofCellular Level Processes in the Early Pathogenesisof Black Walnut Induced Laminitis for $50, 890.

    3. University of GeorgiaAcute InflammatoryChanges in Skin and Laminar Tissue During theOnset of Acute Laminitis for $47,979.

    4. University of IllinoisGrowth Factor EnhancedMesenchymal Stem Cell-Based Therapies toPromote Tendon Healing for $32,495.

    5. Michigan State UniversityIntersection Re-modeling and Decreased Tissue Perfusion inNavicular Syndrome for $27,410.

    6. University of MinnesotaGenetic Analysis ofGlycogen Storage Disorder in Quarter Horses for$55,997.

    7. Mississippi State UniversityFurther Char-acterization of Hyperelastosis Cutis in QuarterHorses and Confirmation of an AutosomalRecessive Mode of Inheritance for $18,934 pend-ing resolution of patent issues.

    8. University of FloridaEffects of Acetylcysteineon Recovery of Equine Large Colon FromIschemic Injury for $29,566.

    9. University of KentuckyDevelopment of Real-Time PCR-Based Diagnostic Assays for Detectionof Virus Infections in Horses for $27,751.

    10. Purdue UniversityThe DomperidoneResponse Test: A Novel Method to InvestigateEquine Pituitary Pars Intermedia Dysfunction(Cushings Disease) for $37,638.

    11. Washington State UniversityNavicularSuspensory Ligament Desmotomy for $68,436.

    12. Virginia/Maryland Regional CollegeApoptosisin the Equine Intestine Following Ischemia-Reperfusion Injury for $46,583.

    Contact: Jody ReynoldsSenior Manager of Direct Response Marketing

    American Quarter Horse Association1600 Quarter Horse Drive

    Amarillo, TX 79104(806) 376-4888 ext. 4326

    (806) 349-6403 (fax)jreynolds@aqha.org

    POSSIBLE ROLE OF ADIPOSE TISSUE IN THEDEVELOPMENT OF LAMINITIS

    by Mandi Vick and Barry Fitzgerald, Maxwell H. GluckEquine Research Center, University of Kentucky

    Laminitis is associated with obesity as well as sepsisand postoperative conditions in horses in which levels ofinflammation are elevated. The precise mechanisms thatlead to laminitis are unknown; however, several studiesdocument the involvement of inflammation in the patho-genesis of laminitis. In addition, recent studies using an invitro approach to studying laminitis using hoof wall ex-plants have shown that removal of glucose from the cul-ture medium leads to rapid separation of hoof lamellatissue when subject to stress forces. A similar mechanismmight occur in natural cases of laminitis where there is in-sulin resistancea suppressed ability of insulin to induceglucose uptake into the cell.

    Whereas numerous mechanisms contribute to thedevelopment of insulin resistance, growing evidencepoints to both a correlative and causative relationshipbetween inflammation and insulin resistance. In addi-tion to sepsis and surgery, obesity is also considered aninflammatory state that some studies relate to a local-ized form of Cushings syndrome. Circulating concen-trations of inflammatory moleculesincluding acutephase proteins and inflammatory cytokinesare ele-vated in obese humans, with even higher elevations inobese patients with insulin resistance and type II dia-betes. Further evidence indicating an inflammatory rolein insulin resistance is derived from studies demonstrat-ing a reversal of insulin resistance by administration ofanti-inflammatory salicylates, such as aspirin.

    Several key molecules involved in the develop-ment of insulin resistance have come to the forefrontin recent years and have led researchers to investi-gate adipose tissue as a major player in the regula-tion of insulin sensitivity. Adipose tissue, once

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  • 330 Journal of Equine Veterinary Science July 2006

    considered only a reservoir for energy storage, hasemerged as an endocrine organ and an active partici-pant in whole body energy homeostasis. A number offactors directly contributing to insulin resistance,such as the inflammatory cytokine tumor necrosisfactor alpha (TNF-), are synthesized and secretedby adipose tissues. Additionally, studies have shownthat adipose tissues contribute a considerable per-centage of the circulating inflammatory moleculesinterleukin-6 (IL-6) and TNF-, with degree of con-tribution correlating to degree of obesity in humans.Thus, the significance of excess adipose tissue as amajor contributor to the development of insulin re-sistance cannot be understated.

    Preliminary studies in our laboratory have em-ployed a model for inducing inflammation. Thismodel investigates (a) the relationship between in-flammation and insulin resistance, and (b) the possi-ble role of adipose tissue as a contributor of

    inflammatory molecules in insulin resistance in thehorse. Administration of lipopolysaccharide (LPS),part of the cell wall of the bacteria Escherichia coli, isa commonly used model for inducing inflammation.LPS induces an acute but transient inflammatory re-sponse characterized by elevation in heart rate, tem-perature, respiration, and insulin levels over a periodof several hours. That administration of LPS in thehorse induces increased circulating levels of the in-flammatory cytokine TNF- in blood is well docu-mented. The objectives of the study were todetermine whether insulin resistance results from adirect inflammatory stimulus in the horse and mea-sure inflammatory cytokine mRNA expression in adi-pose tissue in response to LPS.

    Mares treated with an inflammatory stimulus notonly developed a dramatic decrease in insulin sensitiv-ity but also had marked increases in the levels of cy-tokine gene expression in adipose tissue. These resultssuggest that inflammation is associated with reducedinsulin sensitivity in the horse and that adipose tissuemight be a significant contributor of inflammatory cy-tokines in obese horses. Clearly laminitis can be trig-gered in a number of different situations involving anumber of different factors. However, growing evi-dence points to both insulin resistance and inflamma-tion as major players in the onset of this severelydebilitating disease. Impaired glucose uptake associ-ated with inflammation in obesity and postoperativeconditions might weaken the layers of the hoof lamel-lae and predispose the horse to laminitis. In the future,therapeutic drugs that increase insulin sensitivity andthose that act as anti-inflammatory agents might workin concert to effectively treat laminitis.

    Author: Mandi Vick, Graduate Research Assistant,859-257-4757, mmvick2@uky.edu, Maxwell H. GluckEquine Research Center, University of Kentucky,Lexington, KY.

    Reprinted from Equine Disease Quarterly,

    Department of Veterinary Science, Maxwell H. Gluck Equine Research Center,

    University of Kentucky, Lexington, KY 40546; 859-257-4757;www.uky.edu/Agriculture/VetScence/

    gluck1.htm.

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    Equine Projects Update, September 2005INVESTIGATING THE ROLE OF IMPAIRED GLUCOSE UPTAKE IN LAMINITISCurrent Progress

    EPAXIAL MUSCULATURE AND ITS RELATIONSHIP WITH BACK PAIN IN THE HORSECurrent Progress

    PENETRATION OF PHARMACOLOGICAL AGENTS THROUGH EQUINE SKINCurrent Progress

    DEVELOPMENT OF IMPROVED TREATMENT AND PREVENTION STRATEGIES FOR INFLAMMATORY AIRWAY DISEASE OF HORSESCurrent Progress

    AQHA EQUINE RESEARCH FUNDINGPOSSIBLE ROLE OF ADIPOSE TISSUE IN THE DEVELOPMENT OF LAMINITIS