Equine Endocrinology

François-René Bertin, Natalie S Fraser

• Publisher: CAB International
• Year: 2020
• ISBN:
  9781789241112
• Topic: Medicine
• Subtopic: Equine Veterinary Science

This book provides a practical, clinical approach to diagnosing, treating, and managing endocrine diseases in the horse. Each chapter uses the same structure to form a user-friendly tool of information and advice on aetiology, pathophysiology, clinical presentation, diagnosis and treatment for each endocrine disorder. This book covers: - approaches to endocrine disorders;- diseases of the hypothalamo-pituitary-thyroid axis;- disorders in calcium regulation and diseases of the parathyroid gland;- diseases of the hypothalamo-pituitary-adrenocortical axes;- diseases of the endocrine pancreas and Equine Metabolic Syndrome;- hyperlipaemia and lipid metabolism disorders; and- endocrine disorders associated with the female and male reproductive systems.This book also includes material on additional endocrinopathies, such as diabetes insipidus and pheochromocytoma, and is dedicated to the fast-moving field of equine endocrinology. Written by world-leading international experts, it collates their insights and experience into approaches that prove invaluable for general equine practitioners.

1 Clinical Approach to Equine Endocrinology

Equine endocrinopathies present with a variety of clinical signs. While some clinical signs, like hypertrichosis, are quite obvious and almost pathognomonic, others, like decreased fertility or weight loss, are subtle and might require a more thorough evaluation (Frank et al., 2010b; McFarlane et al., 2011). In addition, several endocrinopathies can be present in the same animal at the same time, and only early recognition and proper management of both conditions will allow a successful outcome (McGowan et al., 2013b). Therefore, a comprehensive and systematic examination is recommended when evaluating patients.

History

A detailed history is a key step to approach cases in which an endocrinopathy is suspected.

Signalment

Close attention to signalment should be paid as there are age-related endocrinopathies, such as pituitary pars intermedia dysfunction (PPID), and some breeds known to be predisposed to some endocrine disorders, such as insulin dysregulation (ID) in ponies or Morgans (McGowan et al., 2013a; Bamford et al., 2014). Gender can also be an important element to consider as endocrine disorders such as equine metabolic syndrome (EMS) have been shown to decrease conception rates (Sessions-Bresnahan and Carnevale, 2014). Thus, a close examination of a mare’s reproductive performance can indicate subclinical endocrinopathies.

Complaint

The primary complaint is often a clear suggestion of an endocrine disorder; however, some owners might overlook minor changes, such as delayed coat shedding or increased water consumption, and only seek veterinary advice when the endocrinopathy impairs the function of the horse and causes more severe signs such as lameness (Horn et al., 2019). Although the awareness of endocrine disorders has increased in the last two decades, early signs of endocrinopathies can be mistaken for normal age-related changes and might not therefore trigger veterinary attention (Miller et al., 2016).

Season

As many hormones have a circannual rhythm, clinical signs can vary with seasons (Frank et al., 2010a; Funk et al., 2011; Borer-Weir et al., 2013). Therefore, collecting history on the previous 12 months and comparing with recent years can be critical when assessing some cases. For example, horses with early signs of PPID can only show delayed coat shedding; comparing the time of shedding over consecutive years might give an indication of early endocrinopathies. Similar considerations are valid for horses with EMS with seasonal episodes of laminitis (Coleman et al., 2018).

Diet

Diet is a paramount element in the assessment of a patient suspected of an endocrine disorder. The clinician should focus on the type of feed offered to the horse (pasture vs hay vs concentrate), the quality of the feed (calcium/phosphorous imbalance can lead to secondary hyperparathyroidism; excessive non-structural carbohydrates, NSC, can lead to ID) and the quantity actually ingested by the horse (Toribio, 2011; Frank and Tadros, 2014). In critical cases such as severely hyperinsulinaemic patients, regular diet analysis is essential to minimize post-prandial hyperinsulinaemic peaks and limit lamellar damage. It is also important to remember that horses and ponies have different eating behaviours and time on pasture might not adequately represent the amount of food ingested: large amounts of grass can be ingested in a short period of time, mainly when the time is restricted. Water intake should also be recorded as increased water intake might suggest a disorder of the hypothalamo−pituitary−adrenal axis (Schott, 2011). Although difficult to properly measure with automatic waterers, a water intake above 3 ml/kg/h in an adult horse warrants further investigation.

Concurrent diseases

By nature, endocrine disorders can affect multiple body systems as hormones regulate metabolism, immunity and homeostasis. For example, by modulating immunity, hypothalamo−pituitary−adrenal axis dysregulation can result in recurrent infections or non-healing wounds (Hofberger et al., 2015; McFarlane et al., 2015; Horn et al., 2019). In addition, some endocrinopathies can be the consequence of systemic diseases. For instance, in foals with sepsis, the hypothalamo−pituitary−adrenal axis can be depressed and in adult horses with systemic inflammatory response syndrome, the endocrine pancreas function can be impaired (Hart et al., 2009b; Bertin et al., 2018). Accordingly, a comprehensive evaluation of the horse is required.

Concurrent treatments

Hormones have systemic effects and their action can be modified by drug administration. For example, phenylbutazone has been identified as the cause of endocrine dysregulation in horses: by either altering the protein binding of thyroxine (T4) or by directly decreasing thyroid-stimulating hormone (TSH) secretion, phenylbutazone administration can result in decreased total and free T4 (Ramirez et al., 1997). In addition, phenylbutazone has been shown to extend the preovulatory period of mares by the inhibition of prostaglandin (Lima et al., 2015). As some drugs, such as non-steroidal anti-inflammatory drugs or sedatives, are routinely administered by owners or veterinarians, the clinician should be careful when examining a horse or interpreting a laboratory result (Kritchevsky et al., 2020).

Physical Examination

A thorough physical examination is an essential step of the diagnostic approach for all diseases. For complaints that are likely to have an endocrine cause, particular attention should be paid to level of alertness and activity, body weight and body condition, haircoat, musculoskeletal system and reproductive system.

Alertness and activity

The hypothalamus is the link between the nervous and the endocrine systems. As part of the limbic system, the hypothalamus integrates environmental and intrinsic stimuli and controls alertness, behaviour and emotions through the secretion of releasing hormones (Clarke, 2015). Therefore, changes in activity level can suggest an endocrine disorder at the level of the hypothalamus. For example, in horses with PPID, decreased alertness is reported and can be reversed with proper treatment (Donaldson et al., 2002).

Body weight and body condition

Morphometric measurements and their changes over time are key elements in the evaluation of patients with endocrine disorders. Whether the horse is overweight or underweight, a careful evaluation of fat distribution and muscle mass is warranted. For example, EMS is associated with increased adiposity, whereas PPID is associated with muscle loss (Aleman et al., 2006; Pleasant et al., 2013). Although body weight is a relevant piece of information, it is a poor indicator of muscle/fat ratio and obtaining an accurate body weight can be challenging in equine practice. A 9-point body condition score, initially developed on morphometric measurements from 12 Quarter Horse mares, is now commonly used in practice for all breeds and has been shown to be a useful tool to predict laminitis in horses with EMS (Henneke et al., 1983; Coleman et al., 2018). The score is based on the adiposity in specific areas such as neck, withers, shoulders, ribs, back and tailhead as well as on the density of the fat deposits (Table 1.1 and Figs 1.1–1.9).

In addition to a full body condition score, a cresty neck score has been developed and is mainly relevant in cases of EMS, as the neck is a common site of fat deposition in horses (Table 1.2 and Figs 1.10–1.15) (Carter et al., 2009; Martin-Gimenez et al., 2016).

Haircoat

Aspect, quality, length and shedding pattern of hair-coat should be assessed when evaluating patients with endocrine conditions, in which the aspect and the quality of the hair can be altered. For example, in foals with congenital hypothyroidism, the hair can appear silky; in geriatric patients with PPID, the abnormal ratio of hair in anagen and telogen phases results in hypertrichosis and delayed or absent shedding (Fig. 1.16) (Breuhaus, 2011; Innera et al., 2013). Close examination of the haircoat will also allow the identification of an abnormal sweating pattern with both hyperhidrosis and anhidrosis reported in horses with PPID (Rohrbach et al., 2012; Spelta and Axon, 2012). Finally, close examination of the integument can allow the identification of non-healing wounds or chronic ulcers.

Table 1.1. Description of individual body condition scoring system. From Henneke et al., 1983.

Score	Definition	Description
1	Poor	Animal extremely emaciated. Spinous processes, ribs, tailhead, tuber coxae and ischiae projecting prominently. Bone structure of withers, shoulders and neck easily noticeable. No fatty tissue can be felt.
2	Very thin	Animal emaciated. Slight fat covering over base of spinous processes, transverse processes of lumbar vertebrae feel rounded. Spinous processes, ribs, tailhead, tuber coxae and ischiae prominent. Withers, shoulders and neck...