Endocrine Diseases

Contents

Preface: Endocrine Diseases xi

Ramiro E. Toribio

Hypothalamic-Pituitary Gland Axis Function and Dysfunction in Horses 1

Samuel D.A. Hurcombe

Hypothalamic-pituitary (HP) dysfunction has been documented in a limitedcapacity in horses and foals associated with critical illness, stress, andpain. This article reviews species-specific details of anatomy, function,hormones, receptors, and testing of the HP axis in the horse. A discussionof critical care medicine relevant to HP dysfunction in the horse with somereference to current understanding in human medicine is made, focusingprimarily on current and relevant literature. A brief mention of other condi-tions described in human and veterinary medicine is also provided for ref-erence only, such as syndrome of inappropriate antidiuretic hormonesecretion and other conditions.

Adrenocortical Insufficiency in Horses and Foals 19

Kelsey A. Hart and Michelle H. Barton

The adrenal cortices produce various steroid hormones that play vital rolesin several physiologic processes. Although permanent adrenocortical in-sufficiency is rare in all species, emerging evidence in both human andequine medicine suggests that transient reversible adrenocortical dys-function resulting in cortisol insufficiency frequently develops during criti-cal illness. This syndrome is termed relative adrenal insufficiency (RAI) orcritical illness–related corticosteroid insufficiency (CIRCI) and can contri-bute substantially to morbidity and mortality associated with the primarydisease. This review discusses the mechanisms, diagnosis, and clinicalconsequences of adrenocortical insufficiency, with particular focus onthe current understanding of RAI/CIRCI in horses and foals.

Endocrine Dysregulation in Critically Ill Foals and Horses 35

Ramiro E. Toribio

Critical illness challenges many endocrine homeostatic systems to over-come diseases, stress, and hostile conditions that threaten survival. Coor-dinated and consecutive responses by the autonomic nervous system,endocrine metabolic adaptations to mobilize and conserve energy andelectrolytes, cardiovascular adjustments to maintain organ perfusion,and immunomodulation to overcome infections and inflammation arerequired. Because most admissions to equine intensive care units arerelated to horses with gastrointestinal disease and septic foals, most endo-crine information during critical disease are generated from these popula-tions. This article presents an overview on endocrine responses to criticalillness in horses and foals and also some comparative information.

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Endocrinology of the Equine Neonate Energy Metabolism in Healthand Critical Illness 49

Rosa J. Barsnick and Ramiro E. Toribio

Hormonal control of energy metabolism plays an important role in the peri-partum development and health of the equine neonate. The endocrinesystem is generally functional at birth, but the maturation of the endocrinesystem and the associated energy metabolism is delayed and continuesduring the postnatal period. The energy metabolism is susceptible todisturbances, especially when illness occurs. Hormones involved in en-ergy metabolism have recently been studied in healthy and critically illneonatal foals. Understanding these hormones in the equine neonate willsupport appropriate therapeutic interventions as well as prognosticassessment of the sick foal.

Equine Hyperlipidemias 59

Harold C. McKenzie III

Hyperlipidemia is the presence of elevated lipid concentrations in theblood and is associated with periods of negative energy balance and phys-iologic stress. In increased concentrations, circulating lipids typicallyoccur in the triglyceride form, which may interfere with numerous normalphysiologic functions, particularly by reducing insulin sensitivity. Althoughthe hyperlipidemia risk is greatest in ponies, miniature horses, and don-keys, all equids are at risk if they are in a situation involving negative energybalance. The sedentary lifestyle of many modern horses and the frequentfeeding of high-carbohydrate diets contribute substantially to the risk ofexcessive fat mobilization and the development of hyperlipidemias.

Equine Metabolic Syndrome 73

Nicholas Frank

The concept of an equine metabolic syndrome (EMS) was first proposed in2002. This concept has developed over time, and EMS was recentlydescribed in a consensus statement released by the American Collegeof Veterinary Internal Medicine. In human medicine, metabolic syndrome(MetS) refers to a set of risk factors that predict the risk of cardiovasculardisease, including obesity, glucose intolerance and insulin resistance (IR),dyslipidemia, microalbuminuria, and hypertension. EMS shares some ofthe features of MetS, including increased adiposity, hyperinsulinemia, IR,but differs in that laminitis is the primary disease of interest.

Equine Pituitary Pars Intermedia Dysfunction 93

Dianne McFarlane

Equine pituitary pars intermedia dysfunction (PPID), also known as equineCushing’s syndrome, is a widely recognized disease of aged horses. Overthe past two decades, the aged horse population has expanded signifi-cantly and in addition, client awareness of PPID has increased. As a result,there has been an increase in both diagnostic testing and treatment of thedisease. This review focuses on the pathophysiology and clinical syn-drome, as well as advances in diagnostic testing and treatment of PPID,

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with an emphasis on those findings that are new since the excellent com-prehensive review by Schott in 2002.

Disorders of the Equine Thyroid Gland 115

Babetta A. Breuhaus

Regulatory control of the thyroid gland in horses is similar to other spe-cies. Clinical signs of hypothyroidism in adult horses are minimal. Severaldrugs and physiologic and pathophysiological states can cause circulat-ing thyroid hormone concentrations to be low without actual pathology ofthe thyroid gland. Thus, nonthyroidal factors must be ruled out before adiagnosis of hypothyroidism can be made. Thyroid hormone supplemen-tation seems to be well tolerated, even in euthyroid horses. Neonatal foalshave very high circulating thyroid hormone concentrations, and defi-ciencies result in significant clinical signs. Unlike in adults, twosyndromes of hypothyroidism are well described in foals.

Disorders of Calcium and Phosphate Metabolism in Horses 129

Ramiro E. Toribio

Calcium and phosphate have structural and nonstructural functions, andtheir concentrations in the extracellular compartment are affected by thephysiologic status of the animal as well as diseases. Important progressin understanding calcium and phosphorus metabolism in healthy anddiseased horses and foals has been made in recent years. For example,several studies have confirmed that hypocalcemia is frequent in horseswith gastrointestinal disease and that calcium endocrine dysregulation isassociated with survival in foals. One critical point in the homeostasis ofthese minerals is their interaction and interdependence with other ions,including potassium and magnesium. In this review, the author providesa clinical overview on disorders of calcium and phosphate in the horse.

Magnesium Disorders in Horses 149

Allison J. Stewart

Magnesium (Mg) is an essential macroelement that is required for cellularenergy-dependent reactions involving adenosine triphosphate and for theregulation of calcium channel function. Subclinical hypomagnesemia iscommon in critically ill humans and animals and increases the severity ofthe systemic inflammatory response syndrome; worsens the systemic re-sponse to endotoxins; and can lead to ileus, cardiac arrhythmias, refractoryhypokalemia, and hypocalcemia. This article discusses the clinical signs,consequences, and treatment of hypomagnesemia in horses and de-scribes the association of Mg and endotoxemia, insulin resistance, andbrain injury.

The Endocrine Disruptive Effects of ErgopeptineAlkaloids on Pregnant Mares 165

Tim J. Evans

During equine gestation, ergopeptine alkaloid exposure is not uncommon,and pregnant mares are particularly sensitive to the endocrine disruptiveeffects of these compounds on lactogenesis and steroidogenesis.

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Agalactia, prolonged gestation, abortion, dystocia, and placental and fetalabnormalities are all clinical manifestations of changes in the endocrinemilieu induced by the ingestion of ergopeptine alkaloid-contaminatedfeedstuffs by mares during late gestation. An understanding of the endo-crine disruptive effects of gestational exposure to ergopeptine alkaloidsis necessary for the diagnosis of potential exposures to these compoundsand for effective prophylaxis and therapy.

Water Homeostasis and Diabetes Insipidus in Horses 175

Harold C. Schott II

Diabetes insipidus (DI) is a rare disorder of horses characterized by pro-found polyuria and polydipsia (PU/PD), which can be caused by loss ofproduction of arginine vasopressin (AVP). This condition is termed neuro-genic or central DI. DI may also develop with absence or loss of AVPreceptors or activity on the basolateral membrane of collecting-duct epi-thelial cells. This condition is termed nephrogenic DI. Equine cliniciansmay differentiate true DI from more common causes of PU/PD by a sys-tematic diagnostic approach. DI may not be a correctable disorder, andsupportive care of affected horses requires an adequate water source.

EndocrineAlterations in the EquineAthlete: An Update 197

Kenneth Harrington McKeever

Horses spend most of their day eating, standing, and occasionally exercis-ing. Exercise can range from running in a pasture to athletic training. Underresting conditions, horses easily maintain the internal environment. Theperformance of work or exercise is a major physiologic challenge, a distur-bance to homeostasis that invokes an integrative response from multipleorgan systems. The response to exercise involves endocrine and neuroen-docrine signaling associated with the short-term and adaptive control ofmany systems. The coordinated control of multiple physiologic variablesis essential for achieving regulation to maintain the integrity of the internalenvironment of the body.

Index 219