Etiology

Determining the underlying cause of weight loss may prove to be challenging and elusive, testing the skills of even the most experienced clinician. Generating a list of differential diag­noses based upon the history and physical examination find­ings will enable the clinician to proceed in a logical manner.

Firstly, it must be determined whether the weight loss oc­curred acutely or gradually. Acute weight loss typically occurs secondary to an increased demand for energy or as a result of massive nutrient loss. In both situations, the quantity of energy intake is insufficient to meet the patient’s needs.

Secondly, the patient’s appetite during the course of the weight loss needs to be established. If the patient is inappetent, it must be established if the inappetence is episodic or ongoing. If episodic, the duration of each episode, the interval between episodes, and the progressiveness of the episodes need to be established. Specifically, it should be determined if the fre­quency and duration of each inappetence episode is increasing and the time interval between episodes is decreasing.

Causes of weight loss accompanied by inappetence include intestinal disorders (e.g., gastrointestinal ulceration, neoplasia, or IBD), infectious disorders (e. g., pyometra, septicemia, pneu­monia, or systemic fungal infections), inflammatory disorders (e. g., immune-mediated disorders or pancreatitis), disorders of the oral cavity (e. g., neoplasia, foreign bodies, uremia-induced oral ulceration, gingivitis, or glossitis), hypoadrenocorticism, cardiac disorders, and various systemic disorders (e.g., renal, hepatic, pancreatic, or neoplastic disorders). Dietary-related is­sues can also be a factor. Feeding a diet of low palatability or a diet that has spoiled may cause inappetence with accompany­ing weight loss.

Weight loss occurring in a polyphagic patient may indicate a hypermetabolic state with an increased energy demand.

Weight loss maybe accompanied by polyphagia and may occur secondary to excessive loss of energy. This situation occurs in disorders such as PLE, PLN, or diabetes mellitus. Protein­losing enteropathies occur when the small intestine is severely diseased resulting in protein leakage into the intestinal lumen. The cause is either one of severe mucosal disease causing in­creased permeability or defective lymphatic drainage, or a combination of both. When the rate of gastrointestinal protein loss exceeds protein synthesis, hypoproteinemia ensues. Lym­phangiectasia is a common cause of PLE. Primary lym­phangiectasia is an idiopathic disorder of the lymphatic system in which the normal absorption and flow of lymph is im- peded.^4,5 Secondary lymphangiectasia develops in adult ani­mals. The lymph stasis occurs secondary to obstructive lesions. This can be a local phenomenon as is the case in patients with infiltrative intestinal diseases, such as severe IBD or alimentary lymphoma (LSA) or as a result of the development of lipo­granulomas within and around the lymphatics.^6,7 Additionally, IBD and alimentary LSA can also cause PLE through small intestinal mucosal injury. Alternatively, systemic venous hyper­tension resulting from right-sided heart failure can cause lym­phatic stasis by interference of normal flow of lymph into the venous circulation.

Protein-losing nephropathy results from disruption of the glomerular filter.

Glucosuria also creates a state of increased energy loss. The most common cause of glucosuria is diabetes mellitus. The weight loss associated with diabetes mellitus is multifactorial in origin but is, in part, caused by loss of glucose in the urine. Glucosuria can also occur during disorders of the proximal renal tubules, such as Fanconi Syndrome or congenital pri­mary renal glucosuria.

Nutrient malassimilation may result from gastrointestinal dis­orders. Documented causes include IBD, small intestinal bac­terial overgrowth (SIBO), EPI, neoplasia, and endoparasitism. Additionally, the feeding of a low quality diet with diminished nutrient bioavailability may prevent proper nutrient assimila­tion.

It should also be considered that the client may not view the historical occurrence of infrequent or subtle gastrointestinal signs as relevant. These signs may include occasional bouts of vomiting, diarrhea, flatulence, borborygmus, abdominal pain (e. g., assuming the prayer position or walking with a hunched back), belching, or pica. If gastrointestinal signs are present, once again it should be established if the occurrence is epi­sodic or continuous in nature. Also, it should be determined if the patient is clinically normal with the exception of periods when gastrointestinal signs are present or if coexisting clinical signs such as lethargy or weakness are also present.

If diarrhea is present, it should be further classified and worked up (see 2.1.3 and 2.3).

Establishing if pyrexia is present further narrows the list of dif­ferential diagnoses. The causes of weight loss accompanied by fever belong to one of the following categories: inflammatory, infectious, immune-mediated, or neoplastic.

Inflammatory and infectious disorders that can cause weight loss include pancreatitis, viral infections such as canine distem­per virus (CDV), FIP, FeLV, and FIV accompanied by second­ary infectious or neoplastic disorders.

Patients with immune-mediated disorders such as systemic lupus erythematosus (SLE), rheumatoid arthritis, and im­mune-mediated polyarthropathy may also present with fever and weight loss.

Alimentary LSA, adenocarcinoma, and leiomyosarcoma are the most common primary malignant neoplastic diseases of the intestines.^8-11 Primary intestinal neoplasia and intestinal metastatic disease may both lead to weight loss. The weight loss may result from neoplasia-induced malassimilation of nu­trients or may be the result of the cancer cachexia syndrome.

Metabolic disorders resulting in weight loss typically involve one or more major organs, including the kidneys, liver, or pancreas. The biochemical and systemic consequences of ure­mia include renal secondary hyperparathyroidism, metabolic acidosis, anemia, oral and gastric ulceration, proteinuria, and systemic hypertension, all of which have the potential to con­tribute to weight loss. Hepatic disorders that can potentially cause weight loss include inflammatory (e. g., cholangiohepa- titis and chronic active hepatitis), infectious (e. g., canine ade­novirus, leptospirosis), neoplastic (primary or metastatic), and vascular (congenital or acquired) diseases. Pancreatic diseases that can cause weight loss include inflammatory (e. g., acute or chronic pancreatitis), neoplastic, and exocrine and endocrine insufficiency.

Various cardiac abnormalities can induce weight loss. The oc­currence of this is referred to as the cardiac cachexia syn- drome.¹² Infectious (e.g., endocarditis), inflammatory (e.g., myocarditis or idiopathic pericarditis), neoplastic (primary or metastatic), parasitic (e. g., Dirofilaria immitis,Borrelia burgdorferi, or Trypanosoma cruzi) and various forms of primary myocardial dysfunction can induce the cardiac cachexia syndrome.

2.4.4