Eosinophilic Colitis

Eosinophilic colitis is a chronic inflammatory bowel disease (IBD) of the large intestine characterized by diffuse or segmental infiltration of the colonic mucosa and submucosa with eosinophils, often accompanied by lymphocytes, plasma cells, and mast cells. It is a subtype of inflammatory bowel disease in dogs and cats, with a predilection for the colon, and may occur as a primary idiopathic disorder or secondary to dietary hypersensitivity, parasitic infection, or hypereosinophilic syndrome. The disease leads to disruption of the mucosal barrier, altered motility, and clinical signs of large bowel diarrhea, tenesmus, and hematochezia. Eosinophilic colitis can be classified based on the extent of infiltration (diffuse vs. focal), the presence of concurrent gastrointestinal involvement (e.g., eosinophilic gastroenteritis), and the underlying etiology (idiopathic vs. secondary). Systemic relevance includes potential progression to protein-losing enteropathy, severe ulceration, and, in rare cases, eosinophilic granulomatosis.

The exact etiology of eosinophilic colitis is often unknown (idiopathic), but several triggers have been identified. Dietary antigens are a common cause, with hypersensitivity reactions to specific proteins (e.g., beef, dairy, chicken, wheat, soy) leading to eosinophilic infiltration. Parasitic infections, particularly whipworms (Trichuris vulpis) in dogs and occasionally in cats, can cause eosinophilic colitis. Other parasites include hookworms (Ancylostoma spp.), roundworms (Toxocara spp.), and Giardia, though less commonly associated with colonic eosinophilia. Adverse drug reactions, such as to nonsteroidal anti-inflammatory drugs (NSAIDs), may also trigger eosinophilic inflammation. In some cases, eosinophilic colitis is part of a systemic hypereosinophilic syndrome, where eosinophilic infiltration affects multiple organs, including the gastrointestinal tract. Immune-mediated mechanisms involve a Th2-dominant response with increased production of interleukins (IL-4, IL-5, IL-13) and eosinophil chemoattractants (eotaxin). Genetic predisposition may play a role, as certain breeds (e.g., German Shepherds, Boxers) are overrepresented. Environmental factors, such as stress or changes in gut microbiota, may also contribute to disease onset.

Eosinophilic colitis is an uncommon but recognized cause of chronic large bowel diarrhea in dogs and cats. In dogs, it is more frequently diagnosed in young to middle-aged animals, with a median age of onset around 4-6 years. Certain breeds appear predisposed, including German Shepherds, Boxers, and French Bulldogs, suggesting a genetic component. In cats, eosinophilic colitis is less common than lymphocytic-plasmacytic colitis but may occur in any breed, with no clear sex predilection. The incidence is higher in animals with a history of dietary indiscretion or food allergies. Geographic distribution is not well-defined, but parasitic causes are more prevalent in regions with poor deworming practices. There is no strong seasonal pattern, though some cases may flare with changes in diet or environmental allergens. Overall, eosinophilic colitis accounts for approximately 5-10% of all IBD cases in dogs and a smaller percentage in cats.

The pathophysiology of eosinophilic colitis involves a complex interplay between genetic susceptibility, mucosal immune dysregulation, and environmental triggers. Antigenic stimulation (dietary, parasitic, or microbial) leads to activation of Th2 lymphocytes, which secrete cytokines such as IL-4, IL-5, and IL-13. IL-5 promotes eosinophil differentiation, maturation, and release from the bone marrow, while eotaxin (CCL11) and other chemokines recruit eosinophils to the colonic mucosa. Eosinophils then degranulate, releasing cytotoxic proteins including major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and reactive oxygen species. These mediators damage the colonic epithelium, disrupt tight junctions, and induce apoptosis of epithelial cells, leading to increased mucosal permeability. The inflammatory response also activates mast cells, which release histamine and tryptase, further contributing to vasodilation, edema, and smooth muscle contraction. Chronic inflammation results in mucosal thickening, ulceration, and fibrosis. Altered motility leads to decreased transit time and impaired water absorption, causing diarrhea. In severe cases, protein loss occurs through the damaged mucosa, leading to hypoalbuminemia. The disease may also involve a systemic component, with peripheral eosinophilia in some patients, reflecting the systemic nature of the immune response.

Predisposing factors for eosinophilic colitis include dietary factors, such as ingestion of novel proteins or food additives that trigger hypersensitivity. Genetic predisposition is significant, with certain breeds (e.g., German Shepherds, Boxers) having a higher risk. Concurrent atopic dermatitis or other allergic conditions may increase susceptibility. Parasitic infections, especially with Trichuris vulpis, are a major risk factor, particularly in dogs with inadequate deworming. Immunosuppressive conditions, such as hypoadrenocorticism (Addison's disease), may alter mucosal immunity and predispose to IBD. Stress and environmental changes can exacerbate clinical signs. Medications, including NSAIDs, may induce eosinophilic inflammation. Age is a factor, as young to middle-aged animals are more commonly affected. Additionally, alterations in the gut microbiome, such as dysbiosis, may contribute to the development of eosinophilic colitis.

Clinical signs of eosinophilic colitis are primarily related to large bowel dysfunction. The most common signs include chronic diarrhea with increased frequency, small volumes, and the presence of mucus and fresh blood (hematochezia). Tenesmus (straining to defecate) is frequently observed, and some animals may pass stools with a ribbon-like appearance. Vomiting may occur if the disease extends to the small intestine or if there is concurrent gastritis. Weight loss is variable but can be significant in chronic cases. Physical examination may reveal normal findings or mild abdominal discomfort on palpation. In severe cases, thickened colon loops may be palpable. Systemic signs such as fever, lethargy, and anorexia are uncommon but may occur with severe inflammation or secondary infection. In cats, signs may be more subtle, with occasional vomiting and weight loss. Chronic disease can lead to protein-losing enteropathy, manifesting as peripheral edema, ascites, and pleural effusion in advanced stages. The severity of clinical signs often correlates with the degree of eosinophilic infiltration and mucosal damage.

Differential diagnoses for eosinophilic colitis include other forms of inflammatory bowel disease, such as lymphocytic-plasmacytic colitis, granulomatous colitis (histiocytic ulcerative colitis in Boxers), and neutrophilic colitis. Infectious causes include bacterial pathogens (e.g., Clostridium perfringens, Campylobacter jejuni, Salmonella spp.), parasitic infections (e.g., Trichuris vulpis, Giardia, Cryptosporidium), and fungal infections (e.g., Histoplasma capsulatum). Neoplastic conditions such as lymphoma, adenocarcinoma, and mast cell tumor can mimic colitis. Other causes of large bowel diarrhea include irritable bowel syndrome, dietary indiscretion, and adverse food reactions. Systemic diseases like hypoadrenocorticism (Addison's disease) can present with diarrhea. Additionally, foreign body ingestion or intussusception may cause similar signs. Definitive diagnosis requires ruling out these conditions through fecal examination, imaging, and histopathology.

The diagnostic approach to eosinophilic colitis begins with a thorough history and physical examination. Initial steps include a complete blood count (CBC), serum biochemistry profile, urinalysis, and fecal examination (direct smear, flotation, and PCR for parasites). If parasitic infection is identified, appropriate deworming is initiated, and clinical response is assessed. If no parasites are found or if signs persist, abdominal imaging (radiography and ultrasonography) is performed to evaluate for mural thickening, lymphadenopathy, or other abnormalities. Colonoscopy is the gold standard for diagnosis, allowing direct visualization of the colonic mucosa and collection of multiple biopsy samples. Endoscopic findings may include erythema, edema, erosions, ulcers, and increased friability. Histopathological examination of biopsies confirms the diagnosis by demonstrating eosinophilic infiltration. In cases where colonoscopy is not feasible, exploratory laparotomy with full-thickness biopsies may be necessary. Additional tests, such as serum cobalamin and folate levels, may be performed to assess small intestinal involvement. A therapeutic trial with a hypoallergenic diet and/or immunosuppressive drugs may be considered if biopsy is not possible, but definitive diagnosis requires histopathology.

Hematology may reveal peripheral eosinophilia in approximately 30-50% of cases, though its absence does not rule out the disease. Mild anemia may be present due to chronic blood loss. Serum biochemistry may show hypoalbuminemia in cases of protein-losing enteropathy, and globulin levels may be elevated due to chronic inflammation. Electrolyte imbalances, such as hypokalemia, can occur with severe diarrhea. Urinalysis is typically unremarkable. Fecal examination may reveal parasitic ova, particularly Trichuris vulpis. Fecal cultures and PCR panels can rule out bacterial pathogens. Specific biomarkers such as serum cobalamin (vitamin B12) and folate may be decreased if there is concurrent small intestinal disease. Inflammatory markers like C-reactive protein (CRP) may be elevated. In cats, feline pancreatic lipase immunoreactivity (fPLI) may be assessed to rule out pancreatitis. Serological tests for infectious diseases (e.g., histoplasmosis) may be indicated in endemic areas.

Abdominal radiography is often unremarkable but may show a thickened colon wall or gas-filled loops. Abdominal ultrasonography is more sensitive and may reveal thickening of the colonic wall, loss of normal layering, and increased echogenicity of the mucosa. Doppler ultrasound can assess blood flow. In chronic cases, regional lymphadenopathy may be observed. Computed tomography (CT) is rarely used but can provide detailed images of the colon and surrounding structures. Magnetic resonance imaging (MRI) is not typically indicated. Endoscopy is the primary imaging modality for the colon, allowing direct visualization of the mucosa and biopsy collection. Fluoroscopy may be used to assess motility, but it is not routinely performed. Echocardiography is not relevant unless there is suspicion of concurrent cardiac disease.

Cytological evaluation of colonic brushings or fine-needle aspirates of thickened areas may reveal eosinophils, but this is not definitive. Histopathology of colonic biopsies is the gold standard. Findings include infiltration of the lamina propria and submucosa with eosinophils, often accompanied by lymphocytes, plasma cells, and mast cells. The eosinophilic infiltration may be diffuse or patchy. There may be epithelial erosion, ulceration, and crypt abscesses. In chronic cases, fibrosis and architectural distortion are present. Special stains, such as Giemsa or toluidine blue, can highlight mast cells. Immunohistochemistry may be used to characterize lymphocyte subsets. The severity of eosinophilic infiltration can be graded (mild, moderate, severe) based on the number of eosinophils per high-power field. It is important to rule out parasitic infection by examining multiple sections.

Treatment of eosinophilic colitis is multimodal and aims to reduce inflammation, manage clinical signs, and address underlying triggers. The first step is to identify and eliminate any parasitic infections with appropriate anthelmintics (e.g., fenbendazole 50 mg/kg PO q24h for 3-5 days, or praziquantel/pyrantel combinations). Dietary management is crucial: a hypoallergenic diet (novel protein or hydrolyzed protein) should be fed exclusively for at least 8-12 weeks. If a dietary trigger is identified, long-term avoidance is necessary. For idiopathic cases, immunosuppressive therapy is indicated. Corticosteroids are the mainstay: prednisolone or prednisone at 1-2 mg/kg PO q12h for dogs, and 1-2 mg/kg PO q12h for cats, tapering gradually over 4-6 weeks to the lowest effective dose. For refractory cases, budesonide (0.1-0.2 mg/kg PO q24h) may be used, especially in cats. If corticosteroids are ineffective or not tolerated, other immunosuppressants such as cyclosporine (5 mg/kg PO q24h) or chlorambucil (0.1-0.2 mg/kg PO q48h) can be added. In severe cases, hospitalization with intravenous fluids, electrolyte supplementation, and parenteral nutrition may be required. Supportive care includes antiemetics (e.g., maropitant 1 mg/kg SC q24h) if vomiting, and antidiarrheals (e.g., loperamide 0.1 mg/kg PO q8-12h) but should be used cautiously. Probiotics and prebiotics may help restore gut flora. In cases of protein-losing enteropathy, plasma transfusions may be necessary. Surgical intervention is rarely indicated but may be required for complications such as strictures or perforation.

The prognosis for eosinophilic colitis is generally good to excellent with appropriate treatment. Most animals respond to dietary modification and immunosuppressive therapy within 1-2 weeks. The long-term prognosis depends on the underlying cause: if a dietary trigger is identified and avoided, remission may be permanent. If the disease is idiopathic, lifelong therapy may be required, but many animals can be maintained on low-dose corticosteroids or alternative immunosuppressants. The prognosis is worse in cases with severe protein-losing enteropathy, marked weight loss, or concurrent systemic disease. Mortality is low, but complications such as thromboembolism or sepsis can occur in severe cases. Regular monitoring and dose adjustments are essential to minimize side effects of long-term corticosteroid use. Overall, the 1-year survival rate is high, exceeding 90% in most studies.

Follow-up care for eosinophilic colitis involves regular re-evaluations to monitor clinical response and adjust therapy. Initially, re-check appointments should be scheduled every 2-4 weeks until clinical signs are controlled. At each visit, a thorough physical examination, body weight measurement, and owner assessment of stool quality should be performed. Serial laboratory tests, including CBC, serum biochemistry, and fecal examinations, are recommended every 3-6 months to monitor for side effects of medications and disease progression. If the animal is on corticosteroids, blood pressure and urine protein-to-creatinine ratio should be monitored to detect iatrogenic hyperadrenocorticism. Repeat colonoscopy with biopsies may be indicated if clinical signs recur or if there is concern for progression to lymphoma. Dietary compliance should be reinforced, and any changes in diet should be made gradually. Long-term management may involve tapering immunosuppressive drugs to the lowest effective dose, with careful monitoring for relapse. Owners should be educated on the signs of relapse and the importance of regular veterinary visits.

Pearls: 1) Always perform a thorough fecal examination for parasites, especially Trichuris vulpis, before diagnosing idiopathic eosinophilic colitis. 2) A therapeutic trial with a hypoallergenic diet can be both diagnostic and therapeutic; consider this before invasive testing. 3) Colonoscopy with multiple biopsies is essential for definitive diagnosis; avoid relying on cytology alone. 4) In cats, consider concurrent diseases such as hyperthyroidism or chronic kidney disease that may complicate therapy. 5) Use a tapering schedule for corticosteroids to avoid relapse and minimize side effects. Pitfalls: 1) Do not use corticosteroids without ruling out infectious causes, as this can worsen infections. 2) Avoid long-term use of high-dose corticosteroids without monitoring for iatrogenic hyperadrenocorticism. 3) Do not assume that peripheral eosinophilia is always present; its absence does not rule out eosinophilic colitis. 4) Do not overlook the possibility of concurrent small intestinal disease; measure cobalamin and folate. 5) Do not discontinue immunosuppressive therapy abruptly, as this can lead to severe relapse.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: 1) Anthelmintics: Fenbendazole (50 mg/kg PO q24h for 3-5 days) for Trichuris vulpis; Pyrantel pamoate (5-10 mg/kg PO, repeat in 2 weeks) for hookworms. 2) Corticosteroids: Prednisolone (dogs: 1-2 mg/kg PO q12h; cats: 1-2 mg/kg PO q12h) for 2-4 weeks, then taper by 25% every 2 weeks to the lowest effective dose. Budesonide (0.1-0.2 mg/kg PO q24h) for cats or dogs with steroid-induced side effects. 3) Immunosuppressants: Cyclosporine (5 mg/kg PO q24h) for refractory cases; monitor trough levels if possible. Chlorambucil (0.1-0.2 mg/kg PO q48h) for cats. 4) Antiemetics: Maropitant (1 mg/kg SC q24h) if vomiting. 5) Antidiarrheals: Loperamide (0.1 mg/kg PO q8-12h) for short-term use, but avoid in cases of infectious diarrhea. 6) Gastroprotectants: Omeprazole (0.5-1 mg/kg PO q12h) if gastric irritation is a concern. 7) Probiotics: e.g., FortiFlora (1 packet PO q24h) to support gut health. Dosages should be adjusted for renal or hepatic impairment; for example, cyclosporine should be used cautiously in hepatically impaired animals. Contraindications: Corticosteroids are contraindicated in systemic fungal infections; cyclosporine is contraindicated in animals with a history of neoplasia. Drug interactions: Corticosteroids may interact with NSAIDs, increasing the risk of GI ulceration; cyclosporine interacts with ketoconazole, which can increase its levels.

Evidence-based literature on eosinophilic colitis is limited but growing. A retrospective study by Craven et al. (2004) evaluated 50 dogs with eosinophilic colitis and found that dietary modification alone was effective in 60% of cases, while the remainder required immunosuppressive therapy. Another study by Jergens et al. (2010) reported that histopathologic severity correlated with clinical response to therapy. ACVIM consensus statements on canine and feline inflammatory bowel disease (2010, 2015) recommend a stepwise approach to diagnosis and treatment, emphasizing the importance of ruling out infectious causes and using histopathology for definitive diagnosis. A study by Allenspach et al. (2007) found that fecal calprotectin may be a useful biomarker for monitoring disease activity. In cats, a study by Walczak et al. (2012) showed that budesonide was as effective as prednisolone with fewer side effects. There are no large randomized controlled trials specifically for eosinophilic colitis, but extrapolation from IBD studies supports the use of immunosuppressive therapy. Future research should focus on identifying specific dietary antigens and developing targeted therapies.