Etiology

Immune-mediated hemolytic anemia (IMHA) is a clinical syndrome in which anemia results from the accelerated destruction of red blood cells (RBCs) by immune-mediated mechanisms (see Chapter 83). IMHA is the most common cause of hemolytic anemia in dogs but is much less common in cats. In primary IMHA (true autoimmune hemolytic anemia) antibodies are directed against RBC membrane antigens. These antigens have not been well characterized in the dog or cat, but antibodies directed against spectrin, band 3, and the family of erythrocyte membrane glycoproteins, known as glycophorins, have been identified. True autoimmune hemolytic anemia may also be a manifestation of systemic lupus erythematosus (SLE). In secondary IMHA an underlying disease is identified as a precipitating factor for the immune-mediated hemolytic process. Examples of causes of secondary IMHA include infection and neoplastic diseases (Box 104-1). Secondary IMHA may also occur after exposure to certain drugs, venoms, and possibly vaccines. Most studies in dogs suggest that primary autoimmune hemolytic anemia is more common than the secondary form, although the frequency of identification of a secondary cause likely depends in part on how thoroughly the clinician searches for it. Secondary IMHA is more common than primary IMHA in cats.

The most common antibody classes identified on the RBC in both dogs and cats with IMHA are immunoglobulin (Ig) G and IgM, with IgA being least common. Complement is usually also present on the RBC. In secondary IMHA antibodies may be directed against antigens that adsorb to the RBC membrane or against a microbial antigen combined with a self-determinant, with the RBCs destroyed as an “innocent bystanders.” Alternatively, previously hidden membrane antigens may be exposed by membrane damage from microbes or toxins, or microbial and drug antigens may be cross-reactive with self-determinants. Lastly, nonspecific activation of lymphocytes can result in formation of self-reacting lymphocytes in any chronic inflammatory process.

Recent vaccination has been implicated in the pathogenesis of IMHA. The occurrence of IMHA within 2 to 4 weeks of vaccination has been a clinical observation of concern to many owners and veterinarians. In one study of 58 dogs with IMHA, 26% of dogs had been vaccinated within 4 weeks of developing IMHA compared with a control group presenting for other disorders in which no increase in frequency of vaccination in the previous 4 weeks was observed (Duval et al., 1996). Mortality rates between the dogs that had been recently vaccinated and those that had not were not significantly different. In a later study that compared 72 dogs with IMHA to a control group, a temporal association between vaccination and development of IMHA was not identified (Carr et al., 2002). The importance of vaccination in the etiology of IMHA remains unclear.

IMHA clearly has a genetic predisposition, with the disease recognized more frequently in certain breeds (Box 104-2). The Cocker Spaniel appears to be the breed at greatest risk, accounting for as many as one third of cases. The presence of dog erythrocyte antigen 7 is associated with a protective effect in Cocker Spaniels (Miller et al., 2004). Female dogs and neutered dogs are overrepresented, suggesting a possible hormonal influence.

In IMHA the presence of antibody and/or complement on the RBC ultimately results in intravascular or extravascular hemolysis (see Chapter 83). Extravascular hemolysis is more common than intravascular hemolysis, is typically a less-acute process, and is commonly accompanied by spherocytes and hyperbilirubinemia (Figs. 104-1 and 104-2). Although hyperbilirubinemia is a common feature of IMHA, it does not occur in all cases and lack of hyperbilirubinemia does not rule out IMHA. Little clinical significance can be attributed to the relative proportions of conjugated and unconjugated bilirubin on the biochemical panel. Factors that determine the presence and severity of hyperbilirubinemia include the rate of hemolysis as well as hepatic function. In dogs with IMHA, hepatic function may be compromised by hypoxia and hepatic necrosis. In one study of 34 dogs that died of IMHA, 53% had moderate to severe centrilobular hepatic necrosis at necropsy (McManus et al., 2001).

FIG 104-1 Mixed-breed dog with moderate icterus of the sclera (A) and the oral mucous membranes (B).

FIG 104-2 Photomicrograph of a blood smear demonstrating presence of spherocytes.

Clinical Features

Dogs with primary IMHA are typically young to middle-aged adults, with a reported age range of 1 to 13 years and amedian age of 6 years. Females and neutered dogs of both sexes appear predisposed compared with sexually intact male dogs, and several breeds are overrepresented (see Box 104-2). Cats with primary IMHA tend to be younger than dogs, with a median age of 2 years. Males are slightly overrepresented, with no influence of neuter status (Kohn et al., 2006). Common clinical signs of IMHA are listed in Box 104-3. The duration of clinical signs before presentation to the veterinary hospital is typically short in both dogs and cats, with a median of 4 days. Seasonal increases in diagnosis of IMHA have been reported, although the findings are not consistent among studies. The majority of reports suggest an increased frequency of IMHA during the warmer months of the year.

Diagnosis

Diagnosis of IMHA relies on identifying abnormalities consistent with hemolytic anemia on a complete blood count (CBC), serum biochemistry panel, and urinalysis (Box 104-4) followed by identification of antibodies directed against the RBC membrane. Further diagnostic testing is then directed at establishing whether a secondary underlying cause for IMHA can be identified.

The first requirement for making a diagnosis of IMHA is the presence of anemia. The anemia is typically moderate to marked (median hematocrit of 13%) and is usually regenerative, although in approximately 30% of dogs and more than 50% of cats the anemia is nonregenerative. Reasons for nonregenerative anemia in IMHA include an acute onset and presentation before the bone marrow has had time to respond (typically takes 3 to 5 days for maximal regenerative response) and the presence of antibodies directed against bone marrow precursors. In the latter situation, reticulocytes are destroyed before they enter the peripheral circulation. In the absence of a regenerative response, a rapid fall in the hematocrit with little change in the serum total protein or albumin concentration should be considered suspicious for hemolysis. In anemia caused by decreased RBC production from the bone marrow, the hematocrit should not decrease by more than approximately 1% per day, whereas in blood loss anemia the drop in the hematocrit is usually accompanied by a concurrent decrease in the total protein or albumin (Table 104-1).

TABLE 104-1 Expected Changes on the CBC in Different Causes of Anemia

Most dogs with IMHA also have an inflammatory leukogram, often with a shift toward immature cells; thrombocytopenia; and abnormalities of the coagulation system, including prolongation of both the activated partial thromboplastin time (aPTT) and prothrombin time, elevations in d-dimer and fibrinogen degradation products, decreased antithrombin, and increased fibrinogen. Reasons for thrombocytopenia include the presence of antibodies directed against platelets as well as RBCs (Evans syndrome), disseminated intravascular coagulation, or sequestration in the spleen.

Identification of autoagglutination or spherocytosis (2+ or more) on a blood smear is considered definitive evidence of antibody-mediated RBC hemolysis (Fig. 104-3). Autoagglutination is detected by macroscopic or microscopic examination of the blood smear and is generally considered diagnostic for IMHA. Agglutination must be distinguished from rouleaux formation (see Chapter 83).

FIG 104-3 Blood smear showing gross (A) and microscopic (B) agglutination. Note the three-dimensional clustering of red blood cells on the microscopic view.

Spherocytes are formed by partial removal of antibody-coated RBC membranes by macrophages (see Fig. 104-2). This results in a loss of the normal discoid shape, decreased size, and loss of central pallor. Spherocytes are more rigid and less deformable than normal RBCs and are removed when they pass through the spleen. Spherocytes are readily identified in the dog but difficult to recognize in cats because of the lack of significant central pallor in their normal RBCs. Spherocytes are considered a hallmark morphologic change in IMHA, and when present in sufficient numbers (2+ or greater) may be regarded as diagnostic for IMHA in dogs. Of note, low numbers of spherocytes (1+) may be observed on a blood smear when damage to the RBC is nonimmune (e.g., zinc toxicosis, hypophosphatemia, oxidative damage, rickettsial diseases, neoplasia, microangiopathic anemia). Techniques for quantitation of spherocyte numbers are typically semiquantitative (Table 104-2). In retrospective studies approximately 90% of dogs with IMHA have spherocytes present on the blood smear; however, low numbers may be present in dogs with per-acute hemolysis. Ghost cells are remnant membranes of RBCs that have undergone intravascular lysis. Lysis can be induced by immune- or non–immune-mediated mechanisms, so ghost cells are not diagnostic for IMHA.

TABLE 104-2 Semiquantitative Scoring System for Numbers of Spherocytes on a Slide

The direct Coombs test with polyvalent antisera is the most commonly used diagnostic test for IMHA when autoagglutination or spherocytosis is not present; however, this test is neither particularly sensitive nor specific for confirming a diagnosis of IMHA. A positive Coombs test indicates that antibody, complement, or both are on the surface of the RBC but does not mean that the antibody is directed specifically against the RBC membrane or that the antibody is causing hemolysis. Approximately 60% to 80% of canine patients with IMHA have a positive Coombs test. Conversely, a positive Coombs test can occur in a variety of other inflammatory diseases causing false-positive results (see Chapter 102).

A search for secondary causes of IMHA should always be undertaken in a dog or cat with IMHA because the underlying disease may influence management strategy and prognosis. Potential secondary causes of IMHA are listed in Table 104-3. The diagnostic approach to ruling out secondary IMHA includes a thorough history of drug, vaccine, and toxin exposure; detailed physical examination, including rectal, ophthalmologic, and neurologic examinations; tests for specific infectious diseases; investigation into causes of chronic antigenic stimulation; and a search for evidence of neoplasia. Diagnostic tests to consider in addition to a CBC, biochemical panel, and urinalysis include urine culture, abdominal and thoracic radiographs, abdominal ultrasound, bone marrow cytology or histopathology (or both if the anemia is nonregenerative), and appropriate titers for infectious diseases.

TABLE 104-3 Secondary Causes of IMHA in Dogs and Cats

IMHA, Immune-mediated hemolytic anemia; IFA, immunofluorescent antibody; PCR, polymerase chain reaction.

Results of bone marrow evaluation in dogs with nonregenerative primary IMHA typically reveal erythroid hyperplasia with a low mycloid/erythroid (M/E) ratio, although maturation arrest at the rubricyte or metarubricyte stage may also be observed. Some dogs initially suspected to have IMHA based on the presence of spherocytosis or a positive Coombs test have pure red cell aplasia. Myelofibrosis can be detected on bone marrow core biopsy in many dogs with nonregenerative IMHA. In dogs with myelofibrosis, collection of adequate bone marrow elements by aspiration cytology is difficult. Myelofibrosis is likely a secondary response to bone marrow injury and may resolve in dogs that respond to treatment.

In dogs without the classic morphologic changes of immune-mediated hemolysis (regenerative anemia, autoagglutination, spherocytes), confirming a diagnosis of IMHA may be challenging. A positive direct Coombs test should be interpreted cautiously in such cases because false-positive results may occur. The logical approach is to rule out other causes of anemia (see Chapter 83) and use the Coombs test and other indications of hemolysis as supporting evidence of IMHA if no other cause of anemia is identified.

Treatment

Choosing an appropriate treatment regimen for dogs with IMHA is a frustrating task for the clinician (Fig. 104-4). Lack of prospective studies of treatment efficacy, the poor prognosis associated with the disease, and the high cost of treatment and supportive care are some reasons for this frustration. In addition, serious complications such as pulmonary thromboembolism and disseminated intravascular coagulation are relatively common occurrences but hard to predict in individual patients. Because of the lack of prospective studies of treatment efficacy, recommendations for approach to treatment in dogs with IMHA are based primarily on clinical experience rather than objective data.

FIG 104-4 Flow diagram showing approach to treatment in dogs with immune-mediated hemolytic anemia.

When planning the management of a dog with IMHA, the goals of treatment should include prevention of RBC hemolysis, alleviation of tissue hypoxia by blood transfusion, prevention of thromboembolism, and provision of supportive care.

Blood transfusion

Most dogs and cats with acute, severe IMHA need oxygen-carrying support while waiting for the anemia to improve. Oxygen supplementation alone is of limited benefit. The need for blood transfusion depends on the severity of anemia, the rapidity of onset and chronicity of the anemia, and the presence and severity of concurrent disease such as pulmonary thromboembolism and gastrointestinal blood loss. No specific hematocrit level is necessary as a transfusion trigger; rather, each patient should be considered individually. In general, transfusion should be considered when the dog has problems with tachycardia, tachypnea, anorexia, lethargy, or weakness while at rest. Most dogs with acute IMHA and a hematocrit level less than 15% have some degree of tissue hypoxia and will benefit from a blood transfusion regardless of how the dog appears to be doing clinically. Severe tissue hypoxia likely exacerbates the complications of IMHA, such as hepatic necrosis, disseminated intravascular coagulation, and thromboembolism.

Options for providing oxygen-carrying support include transfusion of packed RBCs (pRBCs) or a hemoglobin-based oxygen carrier (HBOC) such as Oxyglobin (Biopure, Cambridge, Mass.). Transfusion of whole blood is acceptable but less ideal because the plasma component is not necessary and may increase the risk of transfusion reactions. Disadvantages of HBOC include the short duration of effect (74 to 82 hours at the 30-mL/kg dose) and the discoloration of serum caused by the transfused hemoglobin, which interferes with many analytes on the biochemical profile (see Table 104-4 for a list of valid analytes). In dogs with hematocrit levels less than 8%, HBOC alone will not provide adequate oxygen-carrying support, and additional support with pRBCs or whole blood is recommended. (See Chapter 83 for more information about blood transfusions and HBOCs.)

TABLE 104-4 Valid Analytes by Instrumentation after Oxyglobin Administration (Plasma Hemoglobin = 4.0 g/dL)

Prognosis

Reported mortality rates of dogs with primary IMHA range from 26% to 70%, with thromboembolism being the cause of death in at least 30% to 60% of cases. Other common causes of death include infection, disseminated intravascular coagulation, and failure to control anemia. Factors that clinically appear to confer a good prognosis in dogs with IMHA include a rapid response to treatment with glucocorticoids, ability to maintain the packed cell volume at greater than 25% to 30% with glucocorticoids alone, and identification of a treatable secondary cause. The prognosis is more guarded in dogs that require multiple drugs to control the disease and those with persistent autoagglutination, an elevated bilirubin concentration, marked thrombocytopenia, and severe leukocytosis. If a major TE does occur in a dog with IMHA, particularly if blood supply to a major organ is disrupted, the long-term prognosis is typically very poor. Contrary to popular opinion the prognosis in Cocker Spaniels with IMHA does not differ from that of other breeds. In approximately 60% of dogs with IMHA, medications can ultimately be discontinued after a slow tapering of the dose. The remaining dogs require long-term immunosuppressive therapy.

Classification/Etiology

Immune-mediated thrombocytopenia (idiopathic thrombocytopenic purpura [ITP]) is a clinical syndrome in which thrombocytopenia results from antibody-mediated accelerated destruction of platelets. Immune-mediated thrombocytopenia is diagnosed in approximately 3% to 18% of cases of thrombocytopenia and is the most common cause of severe thrombocytopenia in dogs (Table 104-6). Immune-mediated thrombocytopenia is classified as primary or secondary. In primary thrombocytopenia (true autoimmune thrombocytopenia) antibodies are directed against platelet antigens, presumably because of an underlying defect in immune regulation. Antibodies directed against platelet membrane glycoproteins IIb and IIIa have been identified as target antigens in dogs, although others may be important as well. Primary ITP is a common cause of thrombocytopenia in dogs but is rare in cats. Environmental factors suspected to precipitate ITP in some cases include stress, changes in environmental temperature, hormonal changes, vaccination, and surgery.

TABLE 104-6 Causes of Thrombocytopenia in Dogs and Cats

In secondary ITP antibody-mediated platelet destruction occurs as a result of an underlying inflammatory or neoplastic disease. Causes of secondary immune-mediated thrombocytopenia in dogs and cats are listed in Table 104-6. Immune-mediated thrombocytopenia may also be a component of SLE and may occur in conjunction with IMHA (Evans syndrome).

Clinical Features

Dogs with primary ITP range in age from 8 months to 15 years, with a median age of 6 years. Females are affected twice as often as males, and although any breed can be affected the Cocker Spaniel, Poodle (all varieties), German Shepherd dog, and Old English Sheepdog are overrepresented. Common findings include sudden onset of petechial and ecchymotic hemorrhages in the skin and mucous membranes, epistaxis, hematochezia, hematemesis, easy bruising, lethargy, weakness, and anorexia. Additional findings on physical examination may include melena, hematuria, hyphema, retinal hemorrhage, and pale mucous membranes (Fig. 104-5). Neurologic signs and blindness may occur from bleeding into the CNS and eye, respectively. Because rapid-onset, life-threatening hemorrhage is rare in dogs with ITP, anemia is usually initially mild and slowly progressive unless IMHA is concurrent. As affected dogs become moderately to severely anemic, lethargy, exercise intolerance, tachypnea, tachycardia, and a heart murmur may be evident. In some dogs with ITP clinical signs of hemorrhage are not present and thrombocytopenia is an incidental finding on bloodwork performed for another reason. The platelets present in dogs with ITP are often larger and may be hemostatically more competent, which may explain why not all dogs with severe ITP bleed spontaneously. However, platelet dysfunction (impaired aggregation) has been documented in normal canine platelets after incubation with serum from dogs with ITP, suggesting that antibodies or other factors in the serum impair platelet function in some dogs with ITP. Certain breeds, such as the Cavalier King Charles Spaniel and the Greyhound, are known to have lower platelet counts than other dogs and do not appear to have increased risk of bleeding.

FIG 104-5 Photographs of three dogs with immune-modulated thrombocytopenia and ecchymotic hemorrhage. A, Note the ecchymotic hemorrhages in the skin of the abdomen. B, Note the hemorrhage into the anterior chamber of the eye. C, Note petechial hemorrhage in the oral mucous membranes.

Diagnosis

Because immune-mediated thrombocytopenia can occur in association with many other disorders (see Table 104-6 and Chapter 87), a diagnosis of primary ITP can only be made by ruling out other causes of thrombocytopenia. Dogs with ITP usually have severe thrombocytopenia (less than 50,000 platelets per μL), and platelet fragments (microthrombocytosis) may be present on the blood smear. Platelet fragments are reported to be a specific but insensitive indication of ITP. Platelet fragments may be present as a result of immune injury or because larger platelets are preferentially removed from circulation. The presence of enlarged platelets on the blood smear supports the presence of increased bone marrow production of platelets, but this is not specific for a regenerative response because bone marrow injury may also cause enlarged platelets.

Diagnosis of ITP is confirmed by ruling out other cause of severe thrombocytopenia (see Table 104-6 and Chapter 87). Spurious thrombocytopenia from platelet clumping, other technical problems, and breed-related thrombocytopenia should be considered in dogs that do not have clinical signs of bleeding. In dogs with thrombocytopenia, examination of a bone marrow aspirate should be performed early in the diagnostic workup to rule out disorders such as myelophthisis, neoplasia, megakaryocytic aplasia, and aplastic anemia (see Chapter 87). Bone marrow aspiration and biopsy can be safely performed even in severely thrombocytopenic dogs because hemorrhage can be controlled with local pressure. In most dogs with ITP normal to increased numbers of megakaryocytes are present on a bone marrow aspirate. Decreased numbers of megakaryocytes in the bone marrow have been associated with a poorer prognosis in dogs with ITP. Megakaryocytic aplasia is a rare disorder in which aplasia of the megakaryocytic cell line results in severe thrombocytopenia. This disease may be a primary immune-mediated disease or occur secondary to infections such as Ehrlichia canis and Borrelia burgdorferi. Immune-mediated megakaryocytic aplasia has a poor prognosis unless it is caused by underlying infection.

The presence of a positive assay for platelet-bound antibody (see Chapter 102) is highly sensitive but not specific for a diagnosis of ITP. A diagnosis of ITP is unlikely if the test result is negative. A positive test result is not specific for ITP because immune-mediated mechanisms are responsible for many causes of thrombocytopenia in dogs, including Babesia canis, Dirofilaria immitis, E. canis, myelodysplasia, SLE, drug reactions to trimethoprim sulfadiazine, and various forms of neoplasia.

Treatment

Prognosis

The prognosis for dogs with ITP is good to guarded, with a mortality rate of approximately 30%. Most dogs respond to medical treatment, although relapse is common, occurring in as many as 50% of dogs. Dogs with megakaryocytic hypoplasia have a more guarded prognosis. The prognosis for dogs with concurrent IMHA and ITP is poor, with a reported mortality rate of 80% or higher. See Chapter 87 for more information on this topic.

Etiology

Autoimmune causes of neutropenia are rare in dogs and cats, accounting for approximately 0.4% of cases of neutropenia (see Chapter 85). In immune-mediated neutropenia (also called idiopathic neutropenia or steroid-responsive neutropenia), serum antineutrophil IgG antibodies can be detected by flow cytometry in the serum (Weiss, 2007). Antibody and complement directed against myeloid cells within the bone marrow have also been identified. In most cases of suspected immune-mediated neutropenia, the diagnosis is one of exclusion because commercial testing for antineutrophil antibodies is not readily available. As with other immune-mediated disorders, immune-mediated neutropenia may be a primary disorder or occur secondary to drug therapy, neoplasia, or other immune-mediated disorder (Table 104-7). The majority of canine cases reported in the literature have been primary. Only one case of suspected immune-mediated neutropenia in a cat has been reported.

TABLE 104-7 Causes of Severe Neutropenia in Dogs and Cats

Clinical Features

In a retrospective report of 11 dogs with suspected immune-mediated neutropenia, a variety of breeds were represented and eight of 11 cases were female (Brown et al., 2006). Affected dogs were typically young, with a median age of 4 years. Clinical signs included fever, lameness, anorexia, and lethargy and the duration of clinical signs ranged from 3 to 180 days. Common abnormalities detected on CBC, serum biochemistry panel, and urinalysis included severe neutropenia (median 110 cells/μL), mild anemia, hyperglobulinemia, and increased alkaline phosphatase level. Further evaluation of affected dogs with bacterial culture, infectious disease serology, and imaging did not reveal a cause for the neutropenia. Bone marrow cytology and histopathology revealed myeloid hyperplasia in the majority of affected dogs and myeloid hypoplasia in two dogs. All dogs had resolution of neutropenia 1 to 18 days after initiation of treatment with glucocorticoids.

Diagnosis and Treatment

A clinical diagnosis of immune-mediated neutropenia is made by exclusion of other causes of neutropenia and by rapid response to treatment with glucocorticoids at an initial dose of 2 to 4 mg/kg/day of prednisone. Gradual withdrawal of corticosteroid therapy is possible without relapse in most dogs; however, some dogs require long-term immunosuppression. Routine monitoring is important to detect recurrence of neutropenia and monitor for infection. See Chapter 85 for more information on this topic.

Etiology

Immune-mediated polyarthritis is defined as chronic synovial inflammation in two or more joints, failure to isolate an organism from the joint fluid, and a positive response to immunosuppressive therapy. Immune-mediated polyarthritis is primarily a type III immune complex hypersensitivity disorder (see Chapter 101) in which immune complexes are deposited in the synovial membrane, initiating local inflammation and release of proteolytic enzymes and cytokines, with resultant cartilage degeneration. In rheumatoid arthritis type IV hypersensitivity may also be present with perivascular infiltration of mononuclear cells into the synovial membrane (see Chapter 101). Immune-mediated polyarthritis may be classified as primary or secondary. In secondary polyarthritis immune complex deposition in the joints is secondary to an underlying inflammatory or neoplastic disease. Infectious agents are an important cause of secondary polyarthritis. Chronic bacterial infections may cause secondary or reactive polyarthritis, and Anaplasma spp., Ehrlichia spp., and Borrelia burgdorferi also cause polyarthritis, although they cannot usually be visualized in or cultured from affected joints. Administration of live calicivirus vaccine also causes transient polyarthritis in cats.

In primary immune-mediated polyarthritis no underlying cause of polyarthritis can be identified. This form of polyarthritis is believed to be attributable to an underlying immune system dysfunction or imbalance (true autoimmunity; see Chapter 101). The most commonly recognized forms of polyarthritis in the dog and cat are idiopathic nonerosive polyarthritis, reactive nonerosive polyarthritis secondary to underlying inflammatory disease (gastrointestinal disease, chronic inflammation, neoplasia, or infection), rheumatoid arthritis, and proliferative polyarthritis (Table 104-8). A number of breed-specific syndromes are recognized in dogs. A nonerosive polyarthritis is also a prominent feature of SLE. See Chapter 74 for a more detailed discussion of the various forms of polyarthritis.

TABLE 104-8 Forms of Polyarthritis Recognized in Dogs and Cats

SLE, Systemic lupus erythematosus; FeFSV, feline synctium-forming virus.

Clinical Features

The clinical hallmark of immune-mediated polyarthritis is the presence of nonseptic inflammation within the synovial membrane of two or more joints. Consequently the diagnosis is made by analysis of synovial fluid collected from joints suspected to be affected. Common clinical signs are listed in Box 104-5. In some cases neurologic disease is initially suspected because the animal is unable to ambulate; however, the neurologic examination in dogs with polyarthritis is normal. Many dogs and cats with polyarthritis have clinical signs of systemic illness, including fever, anorexia, and lethargy. In some cases joint pain and swelling may be mild or not clinically detected and fever is the only clinical sign. Polyarthritis is one of the most common causes of unexplained fever in dogs. Joint pain from polyarthritis may also cause cervical pain, and concurrent meningeal inflammation has been reported in dogs with polyarthritis (Webb et al., 2002). Polyarthritis should therefore be considered in any dog or cat presenting with cervical pain without neurologic deficits. Cats with polyarthritis may appear to have generalized hyperesthesia and resist handling. Cats may also present for decreased activity, and the owners often note that the animal has become withdrawn, often hiding in inaccessible locations. In the less-common erosive forms of polyarthritis, affected joints may become distorted or collapsed as the disease progresses, resulting in a severe gait abnormality. These changes are typically irreversible.

Diagnosis

Diagnosis of immune-mediated polyarthritis is made by documentation of inflammation within the synovial fluid, synovial membrane, or both (Fig. 104-6). Synovial fluid for cytologic evaluation and culture should be collected from at least three and preferably four joints. Synovial fluid should be collected from the more distal joints (carpus, tarsus, stifle) because these are the most commonly affected. The approach to joint fluid collection is discussed in Chapter 73. Joint fluid may be grossly turbid, with decreased viscosity and increased volume. Cytologic evaluation reveals neutrophilic inflammation with no evidence of sepsis. Fluid should always be collected for culture and sensitivity to rule out an occult infection (especially likely if the animal has been previously treated with antibiotics). Once inflammation within multiple joints has been documented, the next step is to identify the type of polyarthritis (see Table 104-8) and whether it is from a primary autoimmune disease or secondary to underlying inflammation, infection, or neoplasia. Diagnostic tests should include a CBC, biochemistry profile, urinalysis, urine culture, thoracic radiographs, abdominal ultrasound, and infectious disease titers or SNAP test (E. canis, A. phagocytophilum, B. burgdorferi) (SNAP test, IDEXX, Westbrook, Maine). In some cases blood cultures may also be indicated. In dogs with suspected rheumatoid arthritis, a rheumatoid factor test should be performed (see Chapter 102). In dogs and cats with evidence of multiple organ involvement, an antinuclear antibody (ANA) titer is indicated to investigate for SLE (see Chapter 102).

FIG 104-6 Direct smear of synovial fluid from a dog with idiopathic immune-mediated polyarthritis. Note the presence of increased numbers of nondegenerate neutrophils.

Treatment

Treatment of secondary immune-mediated polyarthritis depends on identification of an underlying cause. Polyarthritis usually resolves with appropriate treatment and use of antiinflammatory doses of glucocorticoids or nonsteroidal antiinflammatory drugs. In dogs with primary (autoimmune) polyarthritis, immunosuppressive dosages of glucocorticoids are the initial treatment of choice (2 to 4 mg/kg/day). Additional immunosuppressive treatment is necessary in dogs that do not respond to corticosteroids alone or that relapse as glucocorticoids are withdrawn. Drugs that are useful include azathioprine, cyclophosphamide, and cyclosporine. Azathioprine is typically the first drug added to the treatment regimen. More aggressive immunosuppression is often necessary in polyarthritis from SLE, in polyarthritis seen in Akitas, and in rheumatoid arthritis.

Response to treatment should be monitored by assessment of clinical signs and cytologic changes within the joint fluid. Joint fluid should be cytologically normal before tapering immunosuppressive therapy. Failure to establish cytologic remission in addition to clinical remission may result in disease relapse or progressive injury to the joints that ultimately results in degenerative joint disease. Approximately 80% of dogs with idiopathic nonerosive polyarthritis treated with prednisone alone respond well to initial treatment, and half of these dogs can be weaned off therapy after 3 to 4 months. The prognosis for idiopathic nonerosive polyarthritis is good, with a mortality/euthanasia rate of less than 20%. Relapses are common, however, and some dogs require life-long therapy. The prognosis for other forms of immune-mediated polyarthritis varies with the different forms of the disease. See Chapters 73 and 74 for more information on this topic.

Etiology

SLE is a multisystemic immune disorder in which antibodies to specific tissue proteins (type II hypersensitivity) and immune complex deposition (type III hypersensitivity) result in immune-mediated damage to multiple organs. Type IV mechanisms (delayed hypersensitivity) may also contribute to tissue damage. The underlying cause of SLE is still poorly understood, but an increased CD4/CD8 ratio, increased expression of a T-cell activation marker, and marked lymphopenia have been reported in dogs with active disease. These findings suggest that T-suppressor cells may be defective in dogs with SLE. The disease is heritable although not by simple autosomal mechanisms. Breeds that are predisposed include the German Shepherd dog, Shetland Sheepdog, Collie, Beagle, and Poodle. Several colonies of dogs with a high predisposition toward SLE have been established, and an association with certain MHC (DLA) types exists. Other risk factors likely include environmental factors and exposure to certain infectious agents and drugs.

Clinical Features

The disease is uncommon in dogs and rare in cats. In dogs SLE most commonly occurs in middle-aged dogs (age range, 1 to 11 years), and there is no sex predisposition. Because any organ system may be affected in SLE, a wide range of clinical signs is possible. The most common signs are fever (100%), lameness or joint swelling from nonerosive polyarthritis (91%), dermatologic manifestations (60%), and signs of renal failure such as weight loss, vomiting, polyuria, and polydipsia. Proteinuria from glomerulonephritis is detected in 65% of patients. The dermatologic lesions often involve areas of skin exposed to sunlight, with photosensitization being common. The dermatologic manifestations are highly variable, with alopecia, erythema, ulceration, crusting, or hyperkeratosis common. Mucocutaneous lesions may also occur. Other clinical manifestations may include hemolytic anemia, PRCA, thrombocytopenia, leukopenia, myositis, pleuropericarditis, and central nervous system dysfunction. A similar spectrum of disease manifestations has been reported in cats with SLE. SLE typically has a relapsing and remitting course, and different organ systems may be involved with subsequent relapses. For example, a dog initially presenting with clinical signs predominantly relating to the neuromuscular system (polyarthritis or myositis) may later relapse with signs of IMHA or ITP.

Diagnosis

A diagnosis of SLE should be suspected when evidence of involvement of more than one organ system is present in a dog or cat with immune-mediated disease. Because of the large number of organ systems that may be involved, the diagnostic testing required varies widely from patient to patient. Diagnostic tests that should be performed in all dogs and cats with suspected SLE include a CBC, serum biochemical profile, urinalysis, quantitation of urine protein, collection of synovial fluid for cytology and culture, and fundic examination. Additional tests that may be indicated include thoracic and abdominal radiographs (investigating fever), abdominal ultrasonography (investigating renal dysfunction), infectious disease titers (investigating fever, thrombocytopenia, hemolytic or nonregenerative anemia, proteinuria, or polyarthritis), Coombs test (in presence of hemolytic anemia), bone marrow aspirate and core (in cases of cytopenia), and skin or kidney biopsy if dermatologic or renal lesions are present. The extent of diagnostic testing for infectious disease will depend on the species and geographic location. For example, testing for feline leukemia virus, feline immunodeficiency virus, and feline infectious peritonitis should be considered in any cat with suspected SLE. In dogs in Europe, testing for leishmaniasis should be strongly considered because this disease can mimic SLE.

Numerous criteria for the diagnosis of SLE in dogs have been extrapolated from the literature in humans. The most commonly accepted and clinically applicable criteria are shown in Table 104-9. Measurement of serum ANA titers is a relatively sensitive test to confirm the diagnosis of SLE, although the sensitivity reported in the literature ranges from 50% to 100% (see Chapter 102). The variability in diagnostic sensitivity probably arises from variation in the diagnostic criteria for confirming the diagnosis as well as variations in the populations of dogs tested. When used in dogs that have appropriate clinical criteria for SLE, the ANA test is an excellent test; however, false-positive results can occur in dogs and cats with other inflammatory or infectious disorders or neoplasia. ANAs are detected in 10% to 20% of dogs with seroreactivity to Bartonella vinsonii, Ehrlichia canis, and Leishmania infantum. Dogs with seroreactivity to multiple pathogens are more likely to be ANA positive. A recent study of 120 dogs in which an ANA titer was measured emphasized the importance of appropriate patient selection for testing (Smee et al., 2007). In this study measurement of an ANA titer was not a useful diagnostic test in dogs without any major clinical or clinicopathologic abnormalities suggestive of SLE. Only one of 47 dogs tested that did not have any major signs of SLE had immune-mediated disease, and this dog was seronegative for ANA. Ten (21%) of 47 dogs were seropositive for ANA. Conversely, 13 of 16 dogs with two major signs compatible with SLE had immune-mediated disease, and ANA was positive in 10 of these dogs. These results emphasize that the positive predictive value of a diagnostic test is lower in a population of animals in which the disease prevalence is low.

TABLE 104-9 Criteria for Diagnosis of SLE

The LE test is rarely used clinically for diagnosis of SLE because of very low sensitivity. A number of other antibody tests have been investigated in groups of dogs with SLE, including antinative DNA antibodies, antiextractable nuclear antigen antibodies, and antihistone antibodies. None of these tests has been extensively evaluated in dogs, and none is currently commercially available.

Treatment

Immunosuppressive therapy for SLE begins with high doses of corticosteroids (1 to 2 mg/kg q12h). The dose is then tapered if disease remission is achieved. Addition of other cytotoxic drugs (e.g., azathioprine, cyclophosphamide, cyclosporine) is usually necessary to induce or maintain remission. Little information is available on the efficacy of drug protocols for treating SLE. One study reported a protocol of prednisone (0.5 to 1.0 mg/kg q12h) combined with levamisole (2 to 5 mg/kg [maximum 150 mg per patient] every other day; Chabanne et al., 1999b). The prednisone is tapered over a 1- to 2-month period and the levamisole continued for 4 months. In cases that relapse, levamisole is administered for a further 4 months. This protocol was effective in inducing remission in 25 of 33 dogs with SLE. The prognosis for dogs with SLE is guarded to poor. Relapse is common regardless of the drug protocol used, and long-term and often life-long immunosuppressive therapy is necessary to control the disease. Relapses may involve different organ systems and clinical signs than at initial presentation (e.g., hemolytic anemia initially and polyarthritis at relapse).

Etiology

Acquired glomerulonephritis (GN) is more common in dogs than cats and results from the presence of immune complexes within the glomerular capillary walls. Immune complexes may be circulating antigen-antibody complexes that are deposited or trapped in the glomerulus or may form in situ when circulating antibodies react with either endogenous glomerular antigens or nonglomerular antigens within the glomerular capillary wall. Soluble circulating immune complexes formed in the presence of mild antigen excess, or when both antigen and antibody are present in approximately equal quantities, may be deposited along capillary walls resulting in a granular pattern observed on immunofluorescent or immunoperoxidase staining. Infectious and inflammatory diseases are common identifiable causes for deposition of immune complexes within the glomerulus (Box 104-6). Unfortunately in the majority of cases of GN, an underlying cause is not identified. When immune complexes form in situ, a smooth linear pattern is observed with immunofluorescent or immunoperoxidase staining. Causes of in situ deposition of immune complexes may be either true autoimmune disease when antibodies are directed against the basement membrane of the glomerular capillaries (not yet documented as a spontaneous disease in dogs and cats) or when antigen becomes localized in the glomerular capillary wall. For example, in dogs with heartworm disease, soluble Dirofilaria immitis antigens have been shown to adhere to the glomerular capillary wall by a carbohydrate-glycoprotein interaction.

Whatever the cause of immune complex deposition, the consequences are similar (see Chapter 43) and ultimately lead to severe proteinuria, systemic hypertension, renal failure, and predisposition to thromboembolism.

Clinical Features

The hallmark of GN is proteinuria, which is readily detected on routine urinalysis. In many cases proteinuria is initially identified as an incidental finding and the animal may have no obvious clinical signs or only subtle abnormalities (e.g., weight loss, lethargy, decreased appetite). In other cases animals present with clinical signs of renal failure (e.g., anorexia, weight loss, vomiting, polyuria, polydipsia), and proteinuria is identified in the course of the evaluation. In nephrotic syndrome, which is defined as the presence of proteinuria, hypoalbuminemia, hypercholesterolemia, and either edema or ascites, the clinical signs are more severe and often rapidly progressive. Other clinical signs in dogs with glomerulonephritis may relate to the presence of hypertension or hypercoagulability. Hypertension may result in retinal changes and blindness, whereas TEs may occur as a result of the hypercoagulable state.

Diagnosis

A diagnosis of protein-losing nephropathy is made by documentation of persistent proteinuria that cannot be explained by inflammation of the lower urinary tract or blood contamination of the urine. Initial dipstick estimates of urine protein should be evaluated in the light of the urine sediment and specific gravity of the urine. The severity of protein loss should then be quantitated by measurement of a protein/creatinine ratio, preferably on a urine sample with no inflammation or hematuria. A protein/creatinine ratio greater than 0.5 is abnormal; most dogs and cats with protein-losing nephropathy have a ratio greater that 2.0. Once persistent proteinuria has been documented, further testing is necessary to determine whether evidence of tubular dysfunction also exists and to investigate for the presence of underlying infectious or inflammatory diseases implicated as causes of GN. Diagnostic tests that should be performed include a CBC, serum biochemical profile, urinalysis and urine culture, blood pressure, and radiographs of the thorax and abdomen. Ultrasonography of the kidneys is useful to investigate for evidence of pyelonephritis, nephroliths, or other underlying renal disease, but it rarely detects changes associated with glomerulonephritis. An occult heartworm test should be performed and serum titers submitted for the infectious diseases discussed in Box 104-6. Testing for hyperadrenocorticism should be considered in dogs if the appropriate signalment and clinical signs are present. Renal biopsy should be considered if an underlying cause for the proteinuria cannot be identified. Tissue samples should be submitted for routine histopathology, electron microscopy, and immunopathology. Goals of renal biopsy should be to confirm the underlying disease process (specific type of GN, hereditary nephritis, glomerulosclerosis, amyloidosis), determine severity of the disease and, if possible, determine a prognosis as well as guide specific therapy.

Treatment

Therapy for immune-mediated glomerulonephritis should be directed at treating the underlying disease (if identified), decreasing protein loss in the urine, decreasing the likelihood of thromboembolism, and initiating appropriate dietary therapy and supportive care. Angiotensin converting enzyme inhibitors (ACEI) (e.g., enalapril 0.25-0.5 mg/kg q12-24h) are currently the most effective treatment for proteinuria. Anticoagulation is recommended to decrease the likelihood of thromboembolism in dogs with GN, especially in those with documented antithrombin deficiency (less than 70%). Low-dose aspirin (0.5 mg/kg q24h) may be beneficial for its anticoagulant effects and for decreasing the glomerular response to immune complexes. Other supportive measures include control of hypertension (if not controlled by ACEI alone); dietary sodium restriction; a low-protein, high-quality protein diet with n-3 fatty acid supplementation; and control of ascites and edema if present. Therapy for overt renal failure may also be necessary. See Chapter 44 for further details on general management of renal failure.

In theory, immunosuppression should be useful in idiopathic immune-mediated GN; however, no studies have documented beneficial responses to immunosuppressive therapy in dogs with GN, and the use of corticosteroids may exacerbate rather than ameliorate proteinuria. Immunosuppressive therapy is indicated when glomerulonephritis occurs as part of an immune-mediated disease known to respond to corticosteroids, such as SLE. Other indications for immunosuppressive treatment are currently poorly defined.

Careful monitoring of response to therapy with monthly measurement of protein/creatine ratios, blood urea nitrogen, creatine, and blood pressure is important to assess adequacy of therapy. Prognosis for GN varies depending on the severity of disease, underlying histopathology, and response to treatment. In general, the prognosis is guarded in animals that initially present with concurrent azotemia. The outcome is best in dogs with reversible causes of immune complex deposition and those that respond to diet and ACEI to control proteinuria. See Chapter 43 for more information on this topic.