Anorexia
The lack or loss of appetite for food; appetite is psychological and its existence in animals is assumed. Hunger is physiologically aroused by the body’s need for food. Anorexia may be partial (hyporexia) or complete. Anorexia results in decreased food intake, which then leads to weight loss. Pseudoanorexia is associated with the inability to prehend or swallow food rather than the actual loss of appetite.
The control of appetite is a complex interaction between the central nervous system and the periphery.
The hypothalamus and brainstem contain peptidergic feeding-regulatory neurons that act as input stations for sensory and metabolic signals. These cell populations project to several brain regions and interconnect extensively.
Sensory signals that affect appetite include the odor, taste, texture, and temperature of food as well as gastric and duodenal distension.
Metabolic signals for hunger and satiety include a variety of peptides and hormones released during the fasting and fed states as well as plasma concentrations of glucose and fatty acids interacting with nutrient-specific receptors in the liver and gastrointestinal tract.
Leptin is primarily produced by adipocytes and acts on specific hypothalamic receptors to decrease metabolism and decrease appetite.
Neuropeptide Y release from the gastrointestinal tract induces hunger and hyperphagia and decreases energy expenditure after food restriction.
Ghrelin produced by the stomach is a prokinetic and decreases leptin and increases neuropeptide Y production. • Cholecystokinin and bombesin released from the gastrointestinal tract decrease appetite.
Serotonin is an important and perhaps final mediator centrally via a serotonergic tract that passes near the ventromedial hypothalamus.
Dopaminergic tracts in the hypothalamus help regulate food intake and are closely associated with the lateral hypothalamus (classical feeding center).
Environmental factors including the location and timing of meals as well as learned behaviors and circadian rhythms modulate appetite and may override other signals for satiety and hunger.
Appetite is stimulated by aldosterone and corticosterone and suppressed by glucagon and somatostatin.
Inflammatory and neoplastic disease can cause hyporexia by releasing proinflammatory cytokines such as interleukin-1, tumor necrosis factor, and interferon.
The expected upregulation of dietary intake in response to elevated energy expenditure is frequently absent in cancer patients.
Exogenous and endogenous toxins (e.g., renal and liver failure) cause hyporexia.
Any disorder that decreases cerebral arousal will potentially decrease food intake.
Gastroparesis associated with neoplasia, metabolic disorders, and primary gastrointestinal disease is associated with decreased appetite.
Fear, pain, and stress may decrease appetite.
SIGNS
HISTORICAL FINDINGS
Refusal to eat is a common presenting complaint because pet owners strongly associate poor appetite with illness.
Patients with disorders causing dysfunction or pain of the face, neck, oropharynx, and esophagus may display an interest in food but cannot eat. These patients are referred to as being pseudoanorectic.
Animals lacking a sense of smell (anosmia) often show no sniffing behavior.
Weight loss may be noted.
PHYSICAL EXAMINATION FINDINGS
Clinical signs in animals with anorexia/hyporexia vary depending on the underlying cause but may include fever, pallor, icterus, pain, changes in organ size, ocular changes, abdominal distention, dyspnea, muffled heart and lung sounds, adventitious lung sounds, cardiac murmurs, and masses. Weight loss and muscle wasting may be evident depending upon the extent and duration of decreased food intake.
Pseudoanorectic patients commonly display weight loss, halitosis, excessive drooling, difficulty in prehending and masticating food, and odynophagia (painful swallowing).
CAUSES
ANOREXIA/HYPOREXIA
Almost any systemic disease process can cause anorexia/hyporexia.
Psychological—unpalatable diet, food aversion, stress, alterations in routine and environment.
Pain.
Toxicities and drug side-effects.
Gastrointestinal disease.
Acid-base disorders.
Cardiac failure.
Endocrine and metabolic disease.
Neoplasia.
Infectious disease.
Immune-mediated disease.
Respiratory disease.
Musculoskeletal disease.
Neurologic disease.
Miscellaneous (e.g., motion sickness, high environmental temperature).
PSEUDOANOREXIA
Any disease-causing painful or dysfunctional prehension, mastication, and swallowing.
Stomatitis, glossitis, gingivitis, pharyngitis, and esophagitis (e.g., physical agents, caustics, bacterial or viral infections, foreign bodies, immune-mediated diseases, uremia).
Retropharyngeal disorders (e.g., lymphadenopathy, abscess, hematoma, sialocele).
Dental disease or periodontal disease.
Retrobulbar abscess.
Oral, glossal, pharyngeal, or esophageal neoplasia.
Neurologic disorders (e.g., rabies; neuropathies of cranial nerves V, VII, IX, X, XII; and central nervous system lesions).
Musculoskeletal lesions (e.g., masticatory myositis, temporomandibular joint disease, fractures, craniomandibular osteopathy, myasthenia gravis, botulism, and cricopharyngeal achalasia).
Salivary gland neoplasia or inflammation.
DIFFERENTIAL DIAGNOSIS
Perform a nutritional assessment. Gather information about the patient’s diet (including all foods fed to the patient), food intake (current and normal) and obtain body and muscle condition scores.
Elicit a thorough history regarding the patient’s environment, changes in routine, people, or other pets to help identify potential psychological etiologies.
Question owners about the patient’s interest in food and ability to prehend, masticate, and swallow food.
A complete physical examination is required to determine the presence of systemic disease.
Perform a thorough ophthalmic, dental, oropharyngeal, facial, and cervical examination (sedation or anesthesia may be required) in addition to observing the patient eating to rule out pseudoanorexia.
A database including a complete blood count, serum biochemistry panel, urinalysis, heartworm serology, retrovirus serology, abdominal, thoracic, and cervical imaging studies, endoscopy, and histologic/cytologic examination of tissue/cell samples are often required to make a definitive diagnosis.
Only if the history, physical examination, and database strongly suggest psychologic anorexia should further diagnostic work-up be forgone; in such cases, daily contact with the pet owner is essential until the anorexia has resolved.
DIAGNOSTIC PROCEDURES
Vary with underlying condition suspected.
Endoscopy may be useful for visualization of the pharyngeal and esophageal structures.
TREATMENT
The mainstay of treatment is aimed at identifying and correcting the underlying disease.
Symptomatic therapy includes attention to fluid and electrolyte derangements, control of pain and/or nausea, reduction in environmental stressors, and modification of the diet to improve palatability.
Palatability can be improved by adding flavored toppings such as chicken and beef broth, seasoning with condiments such as garlic powder, increasing the moisture, fat or, protein content of the food, and warming the food to body temperature.
When learned food aversion is suspected, food should be offered cautiously and removed immediately at the first signs of aversion. A patient showing signs of aversion to its normal diet may accept novel foods.
Medications the patient is receiving should be reviewed for possible side effects leading to reduced food intake.
Significantly malnourished dogs and cats are immediate candidates for assisted feeding (enteral or parenteral feeding). Well-nourished patients with debilitating diseases should not go without food for longer than 3–5 days before assisted feeding is started.
The decision to institute enteral or parenteral feeding can be influenced by several factors. In animals with inadequate food intake that have ≥ 10% body weight loss, hypoalbuminemia, poor body condition score, evidence of muscle wasting, and/or chronic disease processes, supplemental nutrition should be considered.
Techniques for providing enteral nutrition include coax feeding and placement of a nasoesophageal, esophagostomy, gastrostomy, or jejunostomy tube. Force-feeding should be avoided, particularly in cats in light of the association with conditioned food aversions.
PREVENTION/AVOIDANCE
Maximize patient comfort and wellbeing.
Enhance the palatability of the diet.