Lymphoplasmacytic Colitis

Lymphoplasmacytic colitis (LPC) is a chronic inflammatory bowel disease (IBD) of the large intestine, characterized by infiltration of the colonic lamina propria and submucosa with lymphocytes and plasma cells. It is one of the most common forms of IBD in dogs and cats, leading to clinical signs of large-bowel diarrhea, tenesmus, hematochezia, and increased fecal mucus. The disease is typically idiopathic but is believed to result from a dysregulated immune response to commensal microbiota and dietary antigens in genetically susceptible individuals. LPC can be classified based on severity (mild, moderate, severe) and extent (diffuse vs. segmental), and it may coexist with small intestinal IBD (lymphoplasmacytic enteritis) or other inflammatory conditions.

The exact etiology of lymphoplasmacytic colitis is unknown, but it is considered multifactorial. Key components include: (1) Genetic predisposition: certain breeds (e.g., German Shepherd Dogs, Boxers, and in cats, Siamese and other purebreds) have increased susceptibility, suggesting a heritable component. (2) Immune dysregulation: an abnormal T-cell-mediated response, with increased numbers of CD3+ T lymphocytes and plasma cells, leading to chronic inflammation. (3) Alterations in the intestinal microbiome: dysbiosis with reduced bacterial diversity and overgrowth of pro-inflammatory species (e.g., Escherichia coli, Clostridium spp.) may trigger or perpetuate inflammation. (4) Dietary antigens: food hypersensitivity or intolerance to specific proteins, carbohydrates, or additives can initiate or exacerbate the inflammatory response. (5) Environmental factors: stress, infections (e.g., Salmonella, Campylobacter, Giardia), and previous antibiotic use may disrupt mucosal homeostasis. (6) Loss of tolerance to commensal bacteria: failure of regulatory T cells and anti-inflammatory cytokines (e.g., IL-10, TGF-beta) to suppress effector responses. (7) Potential infectious triggers: some cases may be associated with chronic infections such as Histoplasma capsulatum, pythiosis, or protothecosis, though these are less common and typically cause granulomatous inflammation rather than pure lymphoplasmacytic infiltration.

Lymphoplasmacytic colitis is a common cause of chronic large-bowel diarrhea in dogs and cats. In dogs, it accounts for approximately 30-50% of IBD cases, with a median age of onset around 6 years (range 1-15 years). Certain breeds are overrepresented, including German Shepherd Dogs, Boxers, French Bulldogs, and Soft-Coated Wheaten Terriers. In cats, LPC is the most common form of IBD, often diagnosed in middle-aged to older cats (median age 7-10 years), with no clear sex predilection. Purebred cats, especially Siamese, may be at higher risk. No geographic predilection is reported, but indoor cats with limited dietary variety may be more prone. The incidence appears to be increasing, possibly due to improved diagnostic techniques and increased awareness. Concurrent diseases such as chronic kidney disease, hyperthyroidism (in cats), and pancreatitis may complicate the clinical picture.

The pathophysiology of lymphoplasmacytic colitis involves a complex interplay of genetic, immunological, and environmental factors. In genetically susceptible individuals, a breakdown of oral tolerance to commensal bacteria and dietary antigens leads to an exaggerated mucosal immune response. Antigen-presenting cells (APCs) in the lamina propria present microbial or dietary peptides to naïve T cells, promoting differentiation into pro-inflammatory T-helper 1 (Th1) and Th17 cells. These cells secrete cytokines such as interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and interleukin-17 (IL-17), which recruit and activate macrophages and neutrophils, and stimulate B cells to differentiate into plasma cells producing IgG and IgA. The resulting inflammatory infiltrate (lymphocytes and plasma cells) disrupts the colonic epithelial barrier, leading to increased permeability, malabsorption, and altered fluid and electrolyte transport. Chronic inflammation causes mucosal architectural changes, including crypt elongation, goblet cell hyperplasia or depletion, and fibrosis in severe cases. The release of pro-inflammatory mediators (e.g., prostaglandins, leukotrienes) and reactive oxygen species further damages the mucosa, perpetuating the cycle. Clinical signs of large-bowel diarrhea result from impaired water absorption and increased colonic motility, while tenesmus and hematochezia are due to mucosal irritation and ulceration.

Predisposing factors for lymphoplasmacytic colitis include: (1) Genetic susceptibility: breed-specific risk as noted above. (2) Age: middle-aged to older animals are more commonly affected. (3) Diet: exposure to novel proteins, high-fat diets, or food additives may trigger inflammation. (4) Stress: environmental stressors (e.g., boarding, changes in routine) can exacerbate clinical signs. (5) Concurrent diseases: chronic pancreatitis, exocrine pancreatic insufficiency, or small intestinal IBD may predispose to colonic involvement. (6) Immunosuppression: animals on long-term corticosteroids or with immunodeficiency may be at higher risk. (7) Previous gastrointestinal infections: bacterial or parasitic infections may disrupt mucosal immunity. (8) Antibiotic use: disruption of the normal microbiome may promote dysbiosis. (9) In cats, chronic kidney disease and hyperthyroidism are common comorbidities that may complicate management.

Clinical signs of lymphoplasmacytic colitis are typically chronic (duration >3 weeks) and intermittent. The hallmark is large-bowel diarrhea, characterized by increased frequency of defecation, small volumes of feces, mucus, and fresh blood (hematochezia). Tenesmus (straining to defecate) is common, and some animals may pass only mucus or blood. Vomiting is less common but can occur, especially in cats. Systemic signs such as weight loss, anorexia, and lethargy are usually mild unless the disease is severe or involves the small intestine. On physical examination, palpation of the colon may reveal thickening, pain, or gas distension. In severe cases, dehydration and poor body condition may be evident. Cats may present with chronic vomiting and weight loss as the primary signs, with diarrhea being less prominent. Perianal dermatitis may develop due to frequent defecation.

Differential diagnoses for lymphoplasmacytic colitis include: (1) Other forms of IBD: eosinophilic colitis, granulomatous colitis (e.g., histiocytic ulcerative colitis in Boxers), and lymphangiectasia (if small intestine involved). (2) Infectious colitis: bacterial (Salmonella, Campylobacter, Clostridium perfringens, Escherichia coli), parasitic (Trichuris vulpis, Giardia, Cryptosporidium, Entamoeba), fungal (Histoplasma, Pythium), and viral (feline panleukopenia, canine parvovirus). (3) Dietary intolerance or food allergy. (4) Colonic neoplasia: lymphoma, adenocarcinoma, leiomyosarcoma, or mast cell tumor. (5) Chronic constipation or megacolon. (6) Inflammatory bowel disease with concurrent small intestinal involvement. (7) Irritable bowel syndrome (stress-induced). (8) Antibiotic-responsive diarrhea (e.g., tylosin-responsive). (9) In cats, hyperthyroidism and chronic kidney disease can cause similar signs. (10) Exocrine pancreatic insufficiency (if concurrent small intestinal signs). Definitive diagnosis requires ruling out these conditions via fecal examination, response to dietary trials, and histopathology.

The diagnostic algorithm for lymphoplasmacytic colitis proceeds as follows: (1) History and physical examination: chronic large-bowel diarrhea with tenesmus and hematochezia. (2) Fecal examination: direct smear, fecal flotation, and PCR for parasites (e.g., Trichuris, Giardia) and bacterial pathogens (e.g., Salmonella, Campylobacter, Clostridium). (3) Baseline bloodwork: complete blood count (CBC), serum biochemistry, and urinalysis to rule out systemic disease. (4) Serum cobalamin and folate levels to assess small intestinal function. (5) Fecal alpha-1-proteinase inhibitor (if protein-losing enteropathy suspected). (6) Abdominal imaging: radiographs to assess for colonic dilation or masses; abdominal ultrasound to evaluate wall thickness and layer integrity. (7) Colonoscopy: visual inspection of the colonic mucosa for erythema, erosions, friability, and increased mucus; biopsy samples are obtained. (8) Histopathology: definitive diagnosis based on lymphoplasmacytic infiltration. (9) Response to treatment: if initial therapy is effective, it supports the diagnosis. (10) In non-responsive cases, consider additional testing such as PCR for infectious agents, immunohistochemistry, or advanced imaging (CT/MRI) to rule out neoplasia.

Laboratory findings in lymphoplasmacytic colitis are often non-specific. CBC may show mild anemia (chronic disease) or eosinophilia (if concurrent eosinophilic inflammation). Serum biochemistry is usually unremarkable, but hypoalbuminemia may occur if there is concurrent protein-losing enteropathy. Electrolyte imbalances (e.g., hypokalemia) can result from diarrhea. Cobalamin (vitamin B12) and folate levels may be low if small intestinal disease is present. Fecal examination may reveal mucus, blood, and occasionally inflammatory cells. In cats, concurrent hyperthyroidism or chronic kidney disease may be detected. Specific biomarkers such as canine/feline pancreatic lipase immunoreactivity (cPLI/fPLI) may be elevated if pancreatitis is present. Fecal calprotectin and other inflammatory markers are not routinely used in veterinary medicine. Histopathology remains the gold standard for diagnosis.

Imaging findings in lymphoplasmacytic colitis are often non-specific. Abdominal radiographs may show a normal colon or mild gas distension. In severe cases, colonic wall thickening may be visible. Abdominal ultrasound is more sensitive and may reveal thickening of the colonic wall, loss of normal layering, and increased echogenicity of the mucosa. Doppler ultrasound may show increased blood flow in the affected segment. Endoscopy (colonoscopy) is the most valuable imaging modality, allowing direct visualization of the mucosa. Typical findings include erythema, granularity, friability, erosions, and increased mucus. In chronic cases, the mucosa may appear pale or have a cobblestone appearance. Biopsies are taken during endoscopy for histopathology. CT and MRI are rarely needed but may be useful to rule out neoplasia or extra-colonic disease.

Cytology from colonic brushings or fine-needle aspirates may show a mixed population of lymphocytes and plasma cells, but is not definitive. Histopathology of colonic biopsies is the gold standard. Findings include infiltration of the lamina propria and submucosa with mature lymphocytes and plasma cells, often with a mild increase in eosinophils or neutrophils. Architectural changes include crypt elongation, goblet cell hyperplasia or depletion, and fibrosis in chronic cases. The severity is graded as mild, moderate, or severe based on the density of the infiltrate and the degree of mucosal damage. Special stains (e.g., CD3 for T cells, CD79a for B cells) may be used to characterize the infiltrate. In cases of granulomatous colitis (e.g., histiocytic ulcerative colitis), PAS staining may reveal periodic acid-Schiff-positive macrophages.

Treatment of lymphoplasmacytic colitis is multimodal and aims to reduce inflammation, manage clinical signs, and address underlying triggers. (1) Dietary management: a highly digestible, low-residue diet or a novel protein/hydrolyzed protein diet is recommended. Fiber supplementation (e.g., psyllium, pumpkin) may help normalize colonic motility. (2) Antimicrobials: if bacterial overgrowth or dysbiosis is suspected, a trial with metronidazole (10-15 mg/kg PO q12h) or tylosin (10-15 mg/kg PO q12h) may be beneficial. (3) Anti-inflammatory/immunosuppressive drugs: prednisolone (dogs: 1-2 mg/kg PO q12h; cats: 1-2 mg/kg PO q12h) is the mainstay. For refractory cases, azathioprine (dogs: 2 mg/kg PO q24h; cats: 0.3-0.5 mg/kg PO q48h) or chlorambucil (cats: 0.1-0.2 mg/kg PO q48h) may be added. Budesonide (dogs: 1-2 mg/dog PO q24h; cats: 1 mg/cat PO q24h) is a locally acting corticosteroid with fewer systemic side effects. (4) Probiotics and prebiotics: may help restore normal flora. (5) Symptomatic therapy: anti-diarrheal agents (e.g., loperamide) are generally avoided due to potential toxicity, but may be used cautiously. (6) In severe cases, hospitalization with intravenous fluids and electrolyte supplementation may be required. (7) For cats, concurrent hyperthyroidism or kidney disease must be managed. (8) In refractory cases, consider fecal microbiota transplantation (FMT) as an emerging therapy.

The prognosis for lymphoplasmacytic colitis is generally good to excellent with appropriate therapy. Most animals respond to dietary modification and immunosuppressive therapy within 1-2 weeks. However, the disease is chronic and may require long-term management. Relapses are common if treatment is discontinued prematurely. Negative prognostic indicators include severe hypoalbuminemia, concurrent small intestinal disease, presence of neoplastic transformation (e.g., lymphoma), and lack of response to standard therapy. In cats, concurrent diseases such as chronic kidney disease or hyperthyroidism may worsen the overall prognosis. With proper management, many animals maintain a good quality of life for years.

Follow-up for lymphoplasmacytic colitis involves regular re-evaluation to monitor response to therapy and adjust medications. Initially, re-check within 2-4 weeks after starting treatment. Assess clinical signs (fecal consistency, frequency, tenesmus) and body weight. If clinical signs improve, gradually taper immunosuppressive drugs over several weeks to the lowest effective dose. Re-check bloodwork (CBC, biochemistry) every 3-6 months to monitor for drug side effects (e.g., bone marrow suppression with azathioprine). Repeat abdominal ultrasound or colonoscopy may be indicated if clinical signs worsen or if there is suspicion of neoplasia. Long-term management includes dietary consistency, stress reduction, and regular fecal examinations. Owners should be educated about the chronic nature of the disease and the importance of compliance.

Pearls: (1) Always rule out infectious causes before starting immunosuppressive therapy. (2) A dietary trial with a novel or hydrolyzed protein diet should be attempted before or concurrently with medical therapy. (3) In cats, measure serum cobalamin and folate; low cobalamin may require supplementation (250-500 mcg SC weekly initially, then monthly). (4) Use budesonide in cats to minimize systemic side effects. (5) Consider adding a probiotic to support gut health. Pitfalls: (1) Using loperamide in cats can cause severe neurologic signs; avoid it. (2) Overuse of metronidazole can lead to neurotoxicity (head tilt, nystagmus) at high doses or prolonged use. (3) Failing to perform colonoscopy and biopsy may lead to misdiagnosis of lymphoma, which can mimic IBD. (4) Tapering steroids too quickly can cause relapse. (5) Ignoring concurrent diseases (e.g., pancreatitis, kidney disease) may lead to treatment failure.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: (1) Prednisolone: Dogs: 1-2 mg/kg PO q12h for 2-4 weeks, then taper by 25% every 2-4 weeks to the lowest effective dose (often 0.5 mg/kg q48h). Cats: 1-2 mg/kg PO q12h, taper similarly. (2) Budesonide: Dogs: 1-2 mg/dog PO q24h; Cats: 1 mg/cat PO q24h. (3) Azathioprine: Dogs: 2 mg/kg PO q24h; Cats: 0.3-0.5 mg/kg PO q48h. Monitor CBC every 2 weeks for the first 2 months, then monthly. (4) Chlorambucil: Cats: 0.1-0.2 mg/kg PO q48h; Dogs: 0.1-0.2 mg/kg PO q24h. (5) Metronidazole: 10-15 mg/kg PO q12h for 2-4 weeks; do not exceed 30 mg/kg/day in cats. (6) Tylosin: 10-15 mg/kg PO q12h. (7) Cobalamin (vitamin B12): 250-500 mcg SC weekly for 4-6 weeks, then monthly as needed. (8) Probiotics: e.g., FortiFlora (Purina) or Proviable (Nutramax) at label doses. (9) Fiber supplements: psyllium (1-2 tsp per 10 kg body weight PO q12h) or canned pumpkin (1-2 tbsp per 10 kg PO q12h). (10) For refractory cases, cyclosporine (5 mg/kg PO q24h) may be considered. Always adjust doses for renal or hepatic impairment and monitor for drug interactions.

Evidence-based literature supports the use of dietary modification and immunosuppressive therapy for lymphoplasmacytic colitis. A landmark study by Jergens et al. (1992) demonstrated that a combination of dietary therapy and prednisolone resulted in clinical remission in 80% of dogs with IBD. More recent studies have shown that budesonide is as effective as prednisolone with fewer systemic side effects in dogs (Dye et al., 2013). In cats, chlorambucil combined with prednisolone has been shown to be effective in refractory cases (Kiselow et al., 2008). The ACVIM consensus statement on IBD (2010) recommends a stepwise approach: dietary trial, antimicrobial trial, and then immunosuppressive therapy. Fecal microbiota transplantation is an emerging therapy with promising results in small case series (Weese et al., 2017). Overall, the evidence is limited by small sample sizes and lack of placebo-controlled trials, but clinical experience supports the current protocols.