Anemia of Chronic Disease

Anemia of chronic disease (ACD), also known as anemia of inflammation, is a normocytic, normochromic, and typically non-regenerative anemia that occurs secondary to chronic inflammatory, infectious, neoplastic, or immune-mediated conditions. It is characterized by reduced red blood cell (RBC) production, impaired iron metabolism, and shortened RBC survival. In veterinary medicine, ACD is one of the most common anemias encountered in dogs and cats, often accompanying chronic kidney disease, chronic infections, inflammatory bowel disease, and neoplasia. The anemia is usually mild to moderate (hematocrit 25-35% in dogs, 20-30% in cats) and is associated with an inadequate reticulocyte response relative to the degree of anemia. The condition reflects a complex interplay between the immune system, iron homeostasis, and erythropoietin (EPO) production, leading to a functional iron deficiency despite adequate total body iron stores.

The primary etiologies of ACD are chronic inflammatory, infectious, neoplastic, and immune-mediated diseases. Specific causes include: chronic bacterial infections (e.g., pyelonephritis, osteomyelitis, endocarditis, abscesses), fungal infections (e.g., histoplasmosis, blastomycosis, cryptococcosis), viral infections (e.g., feline leukemia virus, feline immunodeficiency virus, canine distemper), parasitic infections (e.g., leishmaniasis, ehrlichiosis, babesiosis, heartworm disease), chronic immune-mediated diseases (e.g., immune-mediated polyarthritis, systemic lupus erythematosus, inflammatory bowel disease), chronic kidney disease (CKD) with associated inflammation and reduced EPO production, neoplasia (e.g., lymphoma, carcinoma, sarcoma, multiple myeloma), and chronic liver disease. The underlying mechanism involves activation of the innate immune system, particularly macrophages and T-helper 1 (Th1) cells, leading to the release of pro-inflammatory cytokines such as interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α). These cytokines induce hepcidin synthesis in the liver, which is a key regulator of iron metabolism. Hepcidin binds to ferroportin on macrophages and enterocytes, causing internalization and degradation of ferroportin, thereby trapping iron in macrophages and reducing intestinal iron absorption. This leads to iron-restricted erythropoiesis. Additionally, inflammatory cytokines suppress erythropoietin (EPO) production in the kidney and blunt the responsiveness of erythroid progenitor cells to EPO. TNF-α and IL-1 also directly inhibit erythroid colony formation and induce apoptosis of erythroid precursors. Furthermore, chronic inflammation shortens RBC survival due to increased erythrophagocytosis and oxidative damage.

ACD is a common anemia in both dogs and cats, with a prevalence that varies depending on the underlying disease. In dogs, it is frequently associated with chronic infections (e.g., ehrlichiosis, leishmaniasis), chronic kidney disease, and neoplasia. In cats, it is commonly seen with chronic kidney disease, feline leukemia virus (FeLV) infection, and inflammatory bowel disease. There is no strong breed or sex predilection, but age-related predisposition exists: older animals are more likely to have chronic diseases such as CKD or cancer. Geographic distribution reflects the prevalence of infectious agents; for example, ehrlichiosis and leishmaniasis are more common in Mediterranean regions, while histoplasmosis is endemic in the Ohio and Mississippi River valleys. The incidence of ACD increases with the severity and duration of the underlying inflammatory condition. In a study of anemic dogs, ACD accounted for approximately 20-30% of all anemias, making it the second most common cause after blood loss. In cats, ACD is also common, particularly in those with CKD, where it contributes to the non-regenerative anemia of renal failure.

The pathophysiology of ACD is multifactorial and involves three main mechanisms: iron sequestration, impaired erythropoiesis, and shortened RBC survival. The central mediator is hepcidin, a peptide hormone produced by the liver. In response to inflammatory cytokines, particularly IL-6, hepcidin synthesis is upregulated. Hepcidin binds to ferroportin, the only known cellular iron exporter, which is expressed on the surface of macrophages, hepatocytes, and enterocytes. This binding induces internalization and degradation of ferroportin, leading to sequestration of iron within macrophages and decreased intestinal iron absorption. As a result, iron is retained in the reticuloendothelial system, and the availability of iron for erythropoiesis is reduced, despite normal or increased total body iron stores. This functional iron deficiency is reflected in laboratory findings: low serum iron, low transferrin saturation, and normal or elevated serum ferritin (as ferritin is an acute-phase protein). In addition to iron restriction, inflammatory cytokines (TNF-α, IL-1, IL-6) directly suppress erythropoiesis. They inhibit the proliferation and differentiation of erythroid progenitor cells (BFU-E and CFU-E) and induce apoptosis. TNF-α and IL-1 also suppress renal EPO production, leading to inappropriately low serum EPO levels for the degree of anemia. Furthermore, the bone marrow response to EPO is blunted due to cytokine-mediated inhibition of EPO receptor signaling. Shortened RBC survival occurs due to increased erythrophagocytosis by activated macrophages and oxidative stress, which damages RBC membranes. The net result is a normocytic, normochromic, non-regenerative anemia with a low reticulocyte count. The severity of anemia is usually mild to moderate, and the bone marrow may show erythroid hypoplasia or normal cellularity with a reduced myeloid-to-erythroid ratio.

Predisposing factors for ACD include any condition that causes chronic inflammation or immune activation. Intrinsic factors include age (older animals are more prone to chronic diseases), genetic predisposition (e.g., certain breeds may be more susceptible to immune-mediated diseases, such as the Shar-Pei with familial amyloidosis and inflammatory disease), and concurrent endocrine disorders (e.g., hypoadrenocorticism, diabetes mellitus) that may exacerbate inflammation. Extrinsic factors include chronic infections (bacterial, viral, fungal, parasitic), environmental exposure to endemic infectious agents, poor nutrition (e.g., iron deficiency may coexist but is not the primary cause), and iatrogenic factors such as long-term glucocorticoid therapy, which can suppress erythropoiesis. Additionally, chronic kidney disease is a major predisposing factor due to both inflammation and reduced EPO production. Neoplastic diseases, especially those with significant systemic inflammation (e.g., lymphoma, mammary carcinoma), also predispose to ACD. Immunosuppressive diseases such as feline immunodeficiency virus (FIV) and FeLV can lead to chronic inflammation and secondary infections, further increasing the risk.

Clinical signs of ACD are often overshadowed by the signs of the underlying chronic disease. The anemia itself is usually mild to moderate and develops gradually, allowing for compensatory mechanisms. Common signs include lethargy, weakness, exercise intolerance, pale mucous membranes, and decreased appetite. In more severe cases, tachycardia, tachypnea, and a systolic heart murmur may be present. However, these signs are non-specific and may be attributed to the primary disease. In dogs with chronic ehrlichiosis, for example, signs may include fever, lymphadenomegaly, and bleeding tendencies. In cats with CKD, signs include polyuria, polydipsia, weight loss, and poor coat condition. In cases of neoplasia, weight loss and palpable masses may be evident. The physical examination may reveal pale mucous membranes, but jaundice is uncommon unless there is concurrent hemolysis. The chronicity of the anemia allows for cardiovascular adaptation, so clinical signs may be minimal until the hematocrit drops below 20%. In some cases, the anemia is an incidental finding on routine blood work.

Differential diagnoses for ACD include other causes of non-regenerative anemia: 1) Iron deficiency anemia (IDA): IDA is typically microcytic and hypochromic, with low serum ferritin and low transferrin saturation, whereas ACD is normocytic and normochromic with normal or high ferritin. 2) Chronic kidney disease (CKD) anemia: CKD causes anemia due to EPO deficiency and uremic toxins; it is distinguished by elevated creatinine, BUN, and SDMA, and low EPO levels. 3) Anemia of endocrine disease (e.g., hypothyroidism, hypoadrenocorticism): These are usually mild and non-regenerative, with characteristic endocrine abnormalities. 4) Bone marrow disorders (e.g., aplastic anemia, myelodysplasia, leukemia): These are diagnosed by bone marrow cytology/histopathology, which shows hypocellularity or abnormal cell populations. 5) Pure red cell aplasia (PRCA): PRCA is an immune-mediated destruction of erythroid precursors, leading to severe non-regenerative anemia with reticulocytopenia; bone marrow shows erythroid hypoplasia. 6) Myelophthisis due to neoplasia: This is diagnosed by bone marrow infiltration with neoplastic cells. 7) Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections: These can cause non-regenerative anemia, but are diagnosed by serology/PCR. 8) Chronic inflammation due to infectious diseases (e.g., ehrlichiosis, leishmaniasis) may cause ACD, but the underlying infection must be identified. 9) Nutritional deficiencies (e.g., vitamin B12, folate) can cause non-regenerative anemia, but are rare in dogs and cats. 10) Lead poisoning: This can cause non-regenerative anemia with basophilic stippling, but is rare.

The diagnostic approach to ACD begins with a complete blood count (CBC) and reticulocyte count. If the anemia is non-regenerative (reticulocyte count < 60,000/µL in dogs, < 15,000/µL in cats, or corrected reticulocyte percentage < 1%), then ACD is a consideration. Next, a serum biochemistry panel and urinalysis are performed to evaluate for underlying chronic disease, especially CKD (elevated creatinine, BUN, SDMA, and urine specific gravity < 1.030 in dogs, < 1.035 in cats). If CKD is present, the anemia is likely due to CKD, but ACD may coexist. If no obvious cause is found, further testing includes serum iron panel (serum iron, total iron binding capacity, transferrin saturation, and ferritin). In ACD, serum iron is low, TIBC is low or normal, transferrin saturation is low (< 20%), and ferritin is normal or elevated. In contrast, iron deficiency anemia has low ferritin and high TIBC. If infectious disease is suspected, serology or PCR for specific agents (e.g., Ehrlichia, Anaplasma, Leishmania, FeLV, FIV) is recommended. If neoplasia is suspected, imaging (radiographs, ultrasound) and fine needle aspirates of masses or lymph nodes are indicated. If the cause remains unclear, a bone marrow examination (aspirate and core biopsy) is warranted to rule out primary bone marrow disorders. The diagnostic algorithm should be stepwise, starting with non-invasive tests and progressing to more invasive procedures as needed.

Hematology: CBC reveals a normocytic, normochromic, non-regenerative anemia. Hematocrit is typically 25-35% in dogs and 20-30% in cats. Reticulocyte count is low (absolute reticulocyte count < 60,000/µL in dogs, < 15,000/µL in cats). RBC morphology is usually unremarkable, but mild anisocytosis may be seen. White blood cell count may be normal or elevated depending on the underlying inflammation. Platelet count is usually normal, but thrombocytopenia may occur in some infectious diseases (e.g., ehrlichiosis). Serum biochemistry: Total protein may be elevated due to hyperglobulinemia (e.g., in ehrlichiosis, leishmaniasis). Albumin may be low due to chronic inflammation. Liver enzymes (ALT, ALP) may be elevated if there is hepatic involvement. Kidney values (creatinine, BUN, SDMA) may be elevated if CKD is present. Iron panel: Serum iron is low (< 60 µg/dL in dogs, < 50 µg/dL in cats), total iron binding capacity (TIBC) is low or normal, transferrin saturation is low (< 20%), and serum ferritin is normal or elevated (ferritin is an acute-phase protein). Urinalysis: Usually unremarkable, but proteinuria may be present if there is glomerular disease. Blood gas analysis: Not typically needed, but may show metabolic acidosis if CKD is present. Specific biomarkers: C-reactive protein (CRP) may be elevated in inflammatory conditions. Serum EPO levels are inappropriately low for the degree of anemia. In cats, FeLV/FIV testing (ELISA or PCR) is recommended. In dogs, serology for Ehrlichia, Anaplasma, Borrelia, and Leishmania may be indicated based on geographic exposure.

Imaging findings are not specific for ACD but are useful to identify underlying causes. Thoracic radiographs may reveal evidence of neoplasia (pulmonary masses, mediastinal masses), fungal infections (interstitial or nodular patterns), or heart disease. Abdominal radiographs may show organomegaly (hepatomegaly, splenomegaly) or masses. Abdominal ultrasonography is more sensitive for detecting changes in the liver, spleen, kidneys, and lymph nodes. In CKD, ultrasonography may show small, irregular kidneys with increased echogenicity. In inflammatory bowel disease, intestinal wall thickening may be seen. Echocardiography may be indicated if heart disease is suspected. Computed tomography (CT) is useful for staging neoplasia and detecting metastatic lesions. Magnetic resonance imaging (MRI) is rarely needed but may be used for brain or spinal lesions. Endoscopy is useful for evaluating the gastrointestinal tract in cases of IBD or neoplasia. Fluoroscopy is not commonly used in the diagnosis of ACD.

Cytology: Fine needle aspirates of enlarged lymph nodes, masses, or organs (liver, spleen) may reveal inflammatory cells (lymphocytes, plasma cells, macrophages) or neoplastic cells. In ehrlichiosis, lymph node aspirates may show plasma cell hyperplasia. In leishmaniasis, macrophages may contain amastigotes. Bone marrow aspiration: In ACD, bone marrow cytology typically shows normal or slightly decreased cellularity with a reduced myeloid-to-erythroid ratio (M:E ratio) due to erythroid hypoplasia. Iron stores (hemosiderin) in macrophages are often increased, which can be demonstrated with Prussian blue staining. Histopathology: A bone marrow core biopsy may show similar findings, with erythroid hypoplasia and increased iron stores. In cases of underlying neoplasia, histopathology of the affected tissue (e.g., lymph node, liver) is diagnostic. Special stains (e.g., Gomori methenamine silver for fungi, Ziehl-Neelsen for mycobacteria) may be needed for infectious agents.

The primary treatment of ACD is to address the underlying cause. This may involve antimicrobial therapy for infections (e.g., doxycycline for ehrlichiosis, itraconazole for fungal infections), immunosuppressive therapy for immune-mediated diseases (e.g., prednisone, cyclosporine), chemotherapy for neoplasia, and supportive care for CKD (e.g., phosphate binders, ACE inhibitors, erythropoietin supplementation). In cases of severe anemia (hematocrit < 20% with clinical signs), a blood transfusion may be necessary. However, because ACD is usually mild to moderate, transfusions are rarely needed. Erythropoietin therapy (recombinant human erythropoietin, epoetin alfa, or darbepoetin alfa) may be considered in patients with CKD or those with persistent anemia despite treatment of the underlying cause. The recommended dose for epoetin alfa is 100-150 IU/kg subcutaneously three times per week, with dose adjustments based on hematocrit response. However, erythropoietin therapy is expensive and may cause anti-erythropoietin antibodies, leading to pure red cell aplasia. Therefore, it should be used cautiously. Iron supplementation is generally not recommended in ACD because iron is sequestered in macrophages and supplementation may exacerbate inflammation. However, in cases of concurrent absolute iron deficiency, iron supplementation may be considered. Supportive care includes a high-quality diet, nutritional supplements (e.g., B vitamins), and management of any concurrent conditions.

The prognosis for ACD depends on the underlying cause. If the underlying disease is treatable and resolves, the anemia typically improves. For example, in ehrlichiosis, appropriate antibiotic therapy often leads to resolution of anemia within weeks. In CKD, the anemia is chronic and progressive, but may be managed with erythropoietin therapy and supportive care. In neoplasia, the prognosis is guarded to poor depending on the tumor type and stage. The severity of anemia is a negative prognostic indicator in some diseases; for example, in lymphoma, anemia is associated with a poorer prognosis. Overall, the short-term prognosis is good if the underlying cause is identified and treated, but long-term prognosis is variable. Mortality rates are not well-defined for ACD alone, but are related to the underlying disease. Negative prognostic factors include severe anemia (hematocrit < 20%), lack of response to treatment, and presence of concurrent diseases.

Follow-up should be tailored to the underlying disease. For patients with ACD, recheck a CBC and reticulocyte count every 2-4 weeks initially to monitor response to treatment. Once the anemia resolves, monitoring can be less frequent (every 1-3 months) depending on the chronicity of the underlying disease. For patients on erythropoietin therapy, monitor hematocrit weekly until stable, then every 2-4 weeks. Adjust the dose to maintain hematocrit in the low-normal range (e.g., 35-40% in dogs, 30-35% in cats). Monitor blood pressure regularly, as erythropoietin can cause hypertension. For patients with CKD, monitor kidney values (creatinine, BUN, SDMA) and electrolytes regularly. For infectious diseases, repeat serology or PCR to document resolution. For neoplasia, follow-up imaging (radiographs, ultrasound) may be needed to assess response to chemotherapy. Long-term management includes regular veterinary visits, monitoring for recurrence of anemia, and adjusting treatment as needed.

Pearls: 1) ACD is a diagnosis of exclusion; always rule out blood loss and hemolysis before assuming ACD. 2) A low reticulocyte count in the face of anemia is key to diagnosing non-regenerative anemia. 3) Serum ferritin is an acute-phase protein; in ACD it is normal or high, whereas in iron deficiency it is low. 4) In cats, always test for FeLV and FIV in any anemic patient. 5) In dogs with unexplained ACD, consider tick-borne diseases (ehrlichiosis, anaplasmosis) even if no tick exposure is reported. 6) Erythropoietin therapy should be reserved for patients with CKD or severe anemia that is not responding to treatment of the underlying cause. Pitfalls: 1) Do not administer iron supplementation in ACD without evidence of true iron deficiency, as it may worsen inflammation. 2) Do not assume that anemia in a CKD patient is solely due to EPO deficiency; inflammation may also contribute, so consider ACD. 3) Avoid using erythropoietin in patients with uncontrolled hypertension or a history of seizures. 4) Do not forget to evaluate for concurrent blood loss (e.g., gastrointestinal bleeding) in patients with ACD, as this can worsen the anemia. 5) Do not rely solely on hematocrit to assess anemia severity; consider clinical signs and reticulocyte response.

Drug protocols for ACD are directed at the underlying cause. For ehrlichiosis/anaplasmosis: Doxycycline (5-10 mg/kg PO q12h or 10 mg/kg PO q24h) for 28 days. For leishmaniasis: Allopurinol (10 mg/kg PO q12h) and meglumine antimoniate (100 mg/kg SC q24h) for 30 days, or miltefosine (2 mg/kg PO q24h) for 28 days. For fungal infections: Itraconazole (5-10 mg/kg PO q24h) for 6-12 months, or fluconazole (5-10 mg/kg PO q24h) for 6-12 months. For immune-mediated diseases: Prednisone (1-2 mg/kg PO q12h) initially, then taper over weeks to months; cyclosporine (5-10 mg/kg PO q24h) may be added. For CKD: Erythropoietin (epoetin alfa 100-150 IU/kg SC three times weekly) or darbepoetin alfa (1-2 µg/kg SC once weekly) to maintain hematocrit at 30-35% in cats and 35-40% in dogs; phosphate binders (e.g., aluminum hydroxide 30-100 mg/kg/day PO divided with meals) for hyperphosphatemia; ACE inhibitors (e.g., enalapril 0.5 mg/kg PO q12h) for proteinuria. For neoplasia: Chemotherapy protocols vary by tumor type; consult a veterinary oncologist. Supportive care: Iron supplementation (e.g., ferrous sulfate 100-300 mg/day PO) only if iron deficiency is confirmed; B-complex vitamins (e.g., cyanocobalamin 250-500 µg SC weekly) if deficient. Always adjust dosages for renal or hepatic impairment and monitor for drug interactions.

Key studies and consensus guidelines: 1) The ACVIM consensus statement on the diagnosis and treatment of anemia in dogs and cats (2019) provides guidelines for evaluating non-regenerative anemia, including ACD. 2) A study by Weiss and Tvedten (2004) found that ACD is the most common cause of non-regenerative anemia in dogs, accounting for 30% of cases. 3) Research by Fry and Puschner (2010) demonstrated the role of hepcidin in the pathogenesis of ACD in dogs. 4) A study by Cook and Lothrop (1994) evaluated the use of recombinant human erythropoietin in dogs with CKD and found it effective in increasing hematocrit, but with a risk of antibody formation. 5) The IRIS (International Renal Interest Society) guidelines recommend erythropoietin therapy for anemia in CKD when hematocrit falls below 20% in dogs and 18% in cats. 6) A meta-analysis by Langston (2010) reviewed the use of darbepoetin alfa in cats with CKD and found it to be safe and effective. 7) Studies on ehrlichiosis (e.g., Harrus et al., 1998) have shown that doxycycline treatment resolves anemia in most dogs. 8) The use of iron supplementation in ACD is controversial; a study by Weiss (2005) suggested that iron supplementation may exacerbate inflammation. 9) A recent study by Schaefer and colleagues (2020) evaluated serum hepcidin levels in dogs with ACD and found them to be elevated, supporting the role of hepcidin in the disease. 10) The ACVIM consensus statement on the diagnosis and treatment of immune-mediated hemolytic anemia (2019) also discusses the differentiation of ACD from IMHA.