Protein-Losing Enteropathy

Protein-losing enteropathy (PLE) is a syndrome characterized by excessive loss of plasma proteins, primarily albumin and globulins, into the gastrointestinal lumen, leading to hypoalbuminemia and hypoglobulinemia. It is not a single disease but a manifestation of various underlying disorders that cause increased intestinal permeability or lymphatic obstruction. PLE can occur in both dogs and cats, with dogs being more commonly affected. The condition results in a net loss of protein exceeding the body's synthetic capacity, leading to a panhypoproteinemia. Clinically, PLE presents with chronic diarrhea, weight loss, peripheral edema, and effusions (pleural, pericardial, or abdominal). The syndrome is associated with significant morbidity and mortality, particularly when underlying causes are inflammatory bowel disease (IBD) or intestinal lymphangiectasia. Early recognition and aggressive management are crucial for improving outcomes.

The etiologies of PLE are diverse and can be categorized into inflammatory, neoplastic, infectious, vascular, and infiltrative disorders. Primary inflammatory causes include inflammatory bowel disease (IBD), which encompasses lymphocytic-plasmacytic enteritis, eosinophilic enteritis, and granulomatous enteritis. Neoplastic causes include intestinal lymphoma (especially alimentary lymphoma in cats and T-cell lymphoma in dogs), adenocarcinoma, and leiomyosarcoma. Infectious etiologies include histoplasmosis (Histoplasma capsulatum), pythiosis (Pythium insidiosum), protothecosis (Prototheca zopfii), and severe parasitism (e.g., hookworms, whipworms, Giardia). Vascular causes include intestinal lymphangiectasia, which may be primary (congenital) or secondary to right-sided congestive heart failure, constrictive pericarditis, or lymphatic obstruction due to neoplasia or granulomatous inflammation. Other causes include chronic intussusception, gastrointestinal ulceration, and amyloidosis. In some cases, PLE is idiopathic. The underlying mechanism involves either increased mucosal permeability (e.g., IBD, neoplasia) or lymphatic obstruction (e.g., lymphangiectasia), leading to protein loss into the gut lumen.

PLE is most commonly diagnosed in dogs, with a higher prevalence in certain breeds such as the Soft-Coated Wheaten Terrier (where it is often associated with protein-losing nephropathy), Norwegian Lundehund, Basenji, and Yorkshire Terrier. In cats, PLE is less common but can occur, particularly with alimentary lymphoma or IBD. There is no strong age predilection, but middle-aged to older animals are more frequently affected. Sex predilection is not well-established. Geographic distribution may influence the likelihood of infectious causes (e.g., histoplasmosis in the Mississippi River Valley). The incidence of PLE is not precisely known, but it is considered an important differential in any animal with chronic diarrhea and hypoalbuminemia. In Soft-Coated Wheaten Terriers, a familial predisposition has been identified, suggesting a genetic component. In Norwegian Lundehunds, PLE is part of a syndrome of intestinal lymphangiectasia and IBD. Overall, PLE represents a significant clinical challenge due to its multifactorial etiology and variable prognosis.

The pathophysiology of PLE involves two primary mechanisms: increased mucosal permeability and lymphatic obstruction. In inflammatory or neoplastic conditions, the intestinal mucosa becomes damaged, leading to disruption of tight junctions and increased paracellular transport of proteins into the lumen. Additionally, mucosal erosion or ulceration can cause direct loss of blood and protein. In lymphangiectasia, the intestinal lymphatic vessels are dilated or obstructed, causing leakage of protein-rich lymph into the intestinal lumen. This leads to loss of albumin, globulins, and lymphocytes, resulting in hypoalbuminemia, hypoglobulinemia, and lymphopenia. The loss of albumin leads to decreased oncotic pressure, causing edema and effusions. Loss of immunoglobulins and lymphocytes may contribute to immunosuppression, increasing susceptibility to infections. Additionally, loss of antithrombin III can predispose to thromboembolism, a serious complication. The intestinal loss of protein is often accompanied by malabsorption of nutrients, leading to weight loss and deficiencies of fat-soluble vitamins, iron, and calcium. The systemic inflammatory response may also contribute to a hypercoagulable state. The severity of clinical signs correlates with the degree of hypoalbuminemia and the underlying disease process.

Predisposing factors for PLE include breed-specific genetic predispositions, such as in Soft-Coated Wheaten Terriers, Norwegian Lundehunds, and Basenjis. Chronic inflammatory conditions like IBD are major risk factors. Infectious agents, particularly Histoplasma and Pythium, are more common in certain geographic regions. Dietary factors, such as food allergies or intolerances, may trigger or exacerbate IBD. Concurrent diseases, such as chronic heart failure or liver disease, can lead to secondary lymphangiectasia. Immunosuppressive therapy (e.g., corticosteroids) may increase the risk of infectious causes. Age is a factor, as older animals are more prone to neoplasia. Stress and environmental changes may exacerbate underlying gastrointestinal conditions. Additionally, any condition causing increased venous pressure, such as right-sided heart failure or constrictive pericarditis, can predispose to lymphatic dilation and PLE. Early recognition of these risk factors can aid in prompt diagnosis and management.

Clinical signs of PLE are often chronic and progressive. The most common signs include chronic diarrhea (which may be watery or fatty, steatorrhea), weight loss, vomiting, and decreased appetite. Peripheral edema, particularly of the limbs and ventral abdomen, is a classic finding due to hypoalbuminemia. Effusions (pleural, pericardial, or abdominal) may occur in severe cases, leading to respiratory distress or abdominal distension. Some animals may present with signs of thromboembolism, such as acute dyspnea or neurological deficits. In early stages, signs may be subtle, with only mild weight loss or intermittent diarrhea. Physical examination may reveal poor body condition, muscle wasting, pallor, and ascites. In cases of lymphangiectasia, there may be evidence of chylous effusions. Cats with PLE may present with more pronounced vomiting and weight loss, and diarrhea may be less prominent. The clinical signs can be categorized by stage: peracute (rare, often due to acute severe inflammation or obstruction), acute (sudden onset of diarrhea and vomiting), subacute (progressive signs over weeks), chronic (persistent signs over months), and terminal (severe debilitation, marked effusions, and thromboembolic events).

Differential diagnoses for PLE include: 1) Protein-losing nephropathy (PLN) – characterized by hypoalbuminemia but with proteinuria (urine protein:creatinine ratio > 2.0) and normal intestinal signs; 2) Hepatic insufficiency – hypoalbuminemia due to decreased synthesis, with elevated liver enzymes, hyperbilirubinemia, and abnormal bile acids; 3) Malnutrition or starvation – history of inadequate dietary intake, with no intestinal signs; 4) Chronic blood loss (e.g., gastrointestinal bleeding) – may cause hypoproteinemia but often with anemia and melena; 5) Exudative skin disease (e.g., severe burns, wounds) – protein loss through skin; 6) Lymphoma – can cause PLE but also may present with peripheral lymphadenopathy and other organ involvement; 7) Inflammatory bowel disease – a common cause of PLE, but not all IBD cases have protein loss; 8) Intestinal lymphangiectasia – often diagnosed via intestinal biopsy; 9) Histoplasmosis – systemic fungal infection with respiratory signs and cytological evidence of organisms; 10) Pythiosis – granulomatous lesions in the gastrointestinal tract, often with palpable masses. Definitive diagnosis requires ruling out these conditions through appropriate testing, including urinalysis, liver function tests, and intestinal biopsies.

The diagnostic approach to PLE should be systematic. Step 1: Perform a thorough history and physical examination, with attention to body condition, edema, and abdominal palpation. Step 2: Baseline laboratory tests: complete blood count (CBC), serum biochemistry profile, urinalysis, and fecal examination. Key findings include hypoalbuminemia, hypoglobulinemia, lymphopenia, and possibly hypocalcemia and hypocholesterolemia. Urinalysis is essential to rule out protein-losing nephropathy (urine protein:creatinine ratio). Step 3: If PLE is suspected, measure serum concentrations of albumin, globulins, and possibly antithrombin III. Step 4: Abdominal ultrasound is recommended to assess intestinal wall thickness, lymphadenopathy, and the presence of effusions. Step 5: If no contraindications, perform endoscopic evaluation with intestinal biopsies (duodenum, ileum, and stomach) to obtain histopathological diagnosis. Alternatively, full-thickness surgical biopsies may be indicated if ultrasound suggests a focal lesion or if endoscopic biopsies are non-diagnostic. Step 6: Additional tests may include serum cobalamin and folate levels to assess small intestinal function, and specific tests for infectious agents (e.g., histoplasmosis antigen, PCR for Pythium). Step 7: In cases with effusions, thoracocentesis or abdominocentesis with fluid analysis (characterize as transudate or modified transudate) can support the diagnosis. Step 8: Consider cardiac evaluation (echocardiography) if right-sided heart failure is suspected. The diagnostic algorithm should be tailored to the individual patient, but the goal is to identify the underlying cause to guide treatment.

Laboratory findings in PLE typically include: Hematology: lymphopenia (due to loss of lymphocytes), mild anemia (non-regenerative, due to chronic disease or blood loss), and occasionally neutrophilia or eosinophilia depending on the underlying cause. Serum biochemistry: marked hypoalbuminemia (often < 2.0 g/dL), hypoglobulinemia (total protein low), hypocalcemia (due to albumin-bound calcium loss and vitamin D malabsorption), hypocholesterolemia (due to malabsorption of lipids), and possibly elevated liver enzymes if concurrent hepatopathy. Urinalysis: typically normal, but a urine protein:creatinine ratio should be measured to rule out protein-losing nephropathy; if UPC > 2.0, renal protein loss is likely. Blood gas analysis may reveal metabolic acidosis or alkalosis depending on diarrhea severity. Specific biomarkers: serum cobalamin (vitamin B12) and folate levels may be low due to small intestinal disease; cobalamin deficiency is common in distal small intestinal disease. Inflammatory markers such as C-reactive protein (CRP) may be elevated. Serology and PCR for infectious agents (e.g., Histoplasma antigen, Pythium antibodies) may be positive in endemic areas. Antithrombin III levels may be decreased, indicating a hypercoagulable state. Fecal alpha-1-proteinase inhibitor (α1-PI) concentration can be measured to confirm intestinal protein loss, but this test is not widely available. Overall, the hallmark is panhypoproteinemia with no evidence of renal or hepatic loss.

Imaging plays a crucial role in the diagnostic workup of PLE. Abdominal radiography may show loss of serosal detail due to effusion, hepatomegaly, or splenomegaly, but is often unremarkable. Thoracic radiography is indicated if pleural effusion is suspected, showing blunting of costophrenic angles or lung lobe retraction. Abdominal ultrasonography is the most valuable imaging modality. Findings may include: diffuse or segmental thickening of the small intestinal wall (often > 3 mm in dogs), loss of normal layering, hyperechoic mucosal striations (suggestive of lymphangiectasia), mesenteric lymphadenopathy, and the presence of free abdominal fluid. In cases of lymphangiectasia, dilated lymphatic vessels may be visualized as anechoic tubular structures. Ultrasonography can also guide aspiration of effusions or fine-needle aspiration of enlarged lymph nodes. Computed tomography (CT) may be useful for detecting masses or evaluating the extent of disease, but is not routinely performed. Magnetic resonance imaging (MRI) is rarely used. Endoscopy allows direct visualization of the intestinal mucosa, revealing erythema, erosions, friability, and sometimes white spots (dilated lacteals). Fluoroscopy may be used to assess gastrointestinal motility, but is not specific. Echocardiography is indicated if cardiac disease is suspected as a cause of secondary lymphangiectasia. Overall, imaging helps narrow the differential list and guide biopsy site selection.

Cytological evaluation of effusions (pleural, pericardial, or abdominal) typically reveals a modified transudate with low cellularity and high protein content, consistent with protein loss. In chylous effusions, the fluid may appear milky and contain triglycerides. Fine-needle aspiration of enlarged mesenteric lymph nodes may reveal reactive hyperplasia or neoplastic cells (e.g., lymphoma). Histopathological examination of intestinal biopsies is the gold standard for diagnosing the underlying cause of PLE. Endoscopic biopsies are often obtained from the duodenum and ileum. Histopathological findings may include: lymphocytic-plasmacytic enteritis (characterized by infiltration of lymphocytes and plasma cells in the lamina propria), eosinophilic enteritis (eosinophilic infiltration), granulomatous enteritis (with macrophages and giant cells), intestinal lymphangiectasia (dilated lacteals in the villi), and lymphoma (neoplastic lymphocytes). Special stains may be needed to identify infectious agents (e.g., GMS stain for Histoplasma, PAS for Prototheca). In cases of lymphangiectasia, the villi may be blunted and contain dilated lymphatic vessels. Full-thickness surgical biopsies may be necessary to obtain deeper tissue samples, especially for diagnosing lymphoma or for evaluating the muscularis. The histopathological diagnosis is essential for guiding treatment and determining prognosis.

Treatment of PLE is multifaceted and depends on the underlying cause. Emergency stabilization may be required for animals with severe hypoalbuminemia, effusions, or thromboembolism. Fluid therapy with colloids (e.g., hetastarch) or human serum albumin may be considered, but human albumin carries risks of adverse reactions. Diuretics (e.g., furosemide) may be used to manage effusions, but caution is needed to avoid dehydration. The primary medical therapy for inflammatory causes (e.g., IBD) includes immunosuppressive doses of corticosteroids, such as prednisone or prednisolone at 1-2 mg/kg/day PO, tapered over weeks to months. Additional immunosuppressive agents (e.g., cyclosporine, azathioprine, chlorambucil) may be added if response is inadequate. For lymphangiectasia, a low-fat diet is crucial to reduce lymphatic flow; dietary therapy includes highly digestible, low-fat, and moderate-protein diets, often with medium-chain triglycerides (MCTs) as a fat source. Antibiotics (e.g., metronidazole at 10-15 mg/kg PO q12h) may be used to address bacterial overgrowth. For infectious causes, specific antifungal (e.g., itraconazole for histoplasmosis) or antiparasitic (e.g., fenbendazole) therapy is indicated. Surgical intervention may be necessary for focal lesions (e.g., intussusception, neoplasia). Supportive care includes nutritional support (e.g., enteral feeding tubes if anorexic), vitamin B12 supplementation (e.g., cobalamin 250-500 µg SC weekly), and management of complications such as thromboembolism (e.g., low-molecular-weight heparin). The treatment plan should be individualized and monitored closely.

The prognosis for PLE varies widely depending on the underlying cause and response to therapy. In cases of inflammatory bowel disease, the prognosis is generally fair to good with appropriate immunosuppressive therapy, but some animals may require long-term management. Intestinal lymphangiectasia carries a guarded prognosis, as it is often progressive and may not respond fully to dietary modification. Neoplastic causes, such as lymphoma, have a poor to guarded prognosis, with median survival times of months despite chemotherapy. Infectious causes, if diagnosed early and treated aggressively, may have a better prognosis. Negative prognostic indicators include severe hypoalbuminemia (< 1.5 g/dL), presence of effusions, thromboembolic events, and lack of response to initial therapy. The mortality rate is significant, with some studies reporting up to 50% mortality within the first year. However, with early diagnosis and aggressive management, some animals can achieve remission and maintain a good quality of life. Regular monitoring of serum albumin and clinical signs is essential to assess response and adjust therapy.

Follow-up care for PLE is critical. Initially, re-evaluation should occur every 1-2 weeks until clinical improvement is noted. Serum albumin and total protein should be monitored serially to assess response to therapy. Once stabilized, re-checks may be extended to every 1-3 months. Repeat abdominal ultrasound may be performed to evaluate intestinal wall thickness and lymphadenopathy. If the patient is on immunosuppressive therapy, blood pressure and urine protein:creatinine ratio should be monitored to detect adverse effects. Cobalamin levels should be rechecked and supplemented as needed. Dietary management should be continued long-term, and any changes should be made gradually. Owners should be educated on signs of relapse, such as weight loss, diarrhea, or edema. In cases of thromboembolism, anticoagulant therapy may require monitoring of coagulation parameters. The underlying disease may require specific follow-up, such as repeat biopsies in cases of lymphoma. Overall, a structured follow-up plan is essential for optimizing outcomes and minimizing complications.

Pearls: 1) Always rule out protein-losing nephropathy and hepatic insufficiency before diagnosing PLE; measure urine protein:creatinine ratio and bile acids. 2) In any dog with chronic diarrhea and hypoalbuminemia, PLE should be high on the differential list. 3) Abdominal ultrasound is invaluable for identifying intestinal wall thickening and guiding biopsy. 4) Endoscopic biopsies are often sufficient, but full-thickness biopsies may be needed for definitive diagnosis of lymphoma. 5) Low-fat diet is crucial for managing lymphangiectasia; consider MCTs as a fat source. 6) Cobalamin supplementation is often necessary due to malabsorption. 7) Monitor for thromboembolism, as it is a life-threatening complication. Pitfalls: 1) Failing to perform a urinalysis, leading to misdiagnosis of protein-losing nephropathy as PLE. 2) Using human albumin for treatment, which can cause severe allergic reactions. 3) Overusing diuretics, which can worsen dehydration and electrolyte imbalances. 4) Not tapering immunosuppressive therapy slowly, leading to relapse. 5) Ignoring the possibility of infectious causes, especially in endemic areas. 6) Delaying biopsy, which can delay definitive diagnosis and treatment. 7) Assuming that all PLE cases are due to IBD, when other causes such as neoplasia or lymphangiectasia may be present.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are commonly used for PLE: 1) Corticosteroids: Prednisone or prednisolone at 1-2 mg/kg/day PO, divided q12h or given once daily, for initial therapy; taper gradually over 3-6 months. For severe cases, pulse therapy with methylprednisolone sodium succinate at 10-20 mg/kg IV once daily for 1-2 days may be used. 2) Immunosuppressive agents: Cyclosporine (modified) at 5-10 mg/kg/day PO, divided q12h; azathioprine at 2 mg/kg/day PO (dogs) or 0.3 mg/kg q48h (cats); chlorambucil at 0.1-0.2 mg/kg/day PO. These are often used in combination with corticosteroids for refractory cases. 3) Antibiotics: Metronidazole at 10-15 mg/kg PO q12h for 2-4 weeks to manage bacterial overgrowth; tylosin at 10-20 mg/kg PO q12h may also be used. 4) Antifungals: Itraconazole at 5-10 mg/kg/day PO for histoplasmosis; fluconazole at 5-10 mg/kg/day PO for systemic mycoses. 5) Antiparasitics: Fenbendazole at 50 mg/kg/day PO for 3-5 days for Giardia or other parasites. 6) Diuretics: Furosemide at 1-2 mg/kg IV or PO q8-12h for effusions, but use cautiously. 7) Anticoagulants: Low-molecular-weight heparin (e.g., enoxaparin) at 1-2 mg/kg SC q12h for thromboembolism prophylaxis; clopidogrel at 1-2 mg/kg/day PO may also be considered. 8) Nutritional supplements: Cobalamin (vitamin B12) at 250-500 µg SC weekly for 4-6 weeks, then monthly; vitamin E at 10-20 IU/kg/day PO; medium-chain triglycerides (MCTs) as a dietary supplement. 9) Colloids: Hetastarch at 10-20 mL/kg IV over 24 hours for severe hypoalbuminemia; human serum albumin is not recommended due to risk of adverse reactions. All dosages should be adjusted based on renal or hepatic function, and drug interactions should be considered, especially with immunosuppressive agents.

Evidence-based literature on PLE includes several key studies and consensus statements. A landmark study by Kimmel et al. (2000) evaluated the clinical features and outcomes of PLE in dogs, finding that IBD and lymphangiectasia were the most common causes, and that hypoalbuminemia < 1.5 g/dL was associated with a poorer prognosis. Another study by Craven et al. (2004) reported that dogs with PLE had a median survival time of 60 days, with those having IBD surviving longer than those with lymphangiectasia. A more recent study by Dossin et al. (2011) highlighted the importance of antithrombin III levels as a predictor of thromboembolism. The ACVIM consensus statement on the diagnosis and treatment of chronic inflammatory bowel disease in dogs and cats (2010) provides guidelines for immunosuppressive therapy, recommending a stepwise approach with corticosteroids as first-line therapy. Regarding lymphangiectasia, a study by Okanishi et al. (2014) demonstrated that a low-fat diet with MCTs improved clinical signs and serum albumin levels. For infectious causes, a retrospective study by Aulakh et al. (2012) on histoplasmosis in dogs showed that itraconazole therapy resulted in a good prognosis if initiated early. There is limited evidence for the use of human albumin, and a study by Cohn et al. (2007) reported adverse reactions in dogs, supporting the avoidance of this treatment. Overall, the literature emphasizes the need for a definitive diagnosis via biopsy and a multimodal treatment approach tailored to the underlying cause.