Immune-mediated neutropenia (Proceedings)

Immune-mediated neutropenia is an uncommon disease in small animal practice, although some have argued that it may be more prevalent than believed. In one retrospective study of neutropenia in dogs, immune-mediated neutropenia accounted for fewer than 0.5% of cases. The dilemma created by patients with immune-mediated neutropenia is the overlap in clinical presentation between this disease and infectious disease, and the fact that immune-mediated neutropenia is treated with immunosuppressive drugs. The notion of treating a febrile neutropenic dog with immunosuppressive drugs can incite cringes, but can be safely done with an appropriate diagnostic evaluation and exclusion of alternative causes of neutropenia.

Mechanisms of neutropenia

Compared to anemia and thrombocytopenia, the pathophysiological mechanisms that underlie the development of neutropenia are few. The three usual mechanisms of neutropenia are decreased production, increased peripheral consumption, and destruction. Sequestration of neutrophils is rare, and neutrophils are typically not lost (e.g. through hemorrhage) in sufficient numbers to account for a clinical neutropenia. The approach to the neutropenic patient then centers on identification of the pathophysiological mechanism driving neutropenia.

In the neutropenic patient, careful examination of other cell numbers is critical. Patients with pancytopenia are more likely to have bone marrow disease (decreased production), while primary bone marrow disease would be an uncommon cause of isolated neutropenia. That said, bone marrow cytology is an easily performed diagnostic test to exclude decreases in production as a cause of neutropenia, and is recommended for the majority of patients with isolated neutropenia not attributable to another obvious cause. One cannot rely on the absence of abnormal cells in circulation to exclude infiltrative bone marrow disease, adding emphasis to the importance of bone marrow examination in persistently neutropenic patients.

Acute inflammation is the most common cause of neutropenia by increased peripheral consumption. Recall that neutrophils enumerated in a blood count reflect cells in the circulating pool of neutrophils. In acute inflammation, there is a shift of neutrophils from the circulating pool to the marginating pool. Neutrophils in the marginating pool are poised to leave the circulation to migrate to a site of inflammation. With persistence of the inflammatory stimulus, the bone marrow releases pools of immature neutrophils (bands), and increases production of neutrophils leading to neutrophilia.

Like erythrocytes and platelets, neutrophils can be the targets of destruction by the patient's immune system. Immune-mediated neutropenia can be a primary, idiopathic process, or secondary to another disease or provocative stimulus. The specific antigen targets on neutrophils are not known, and the reasons for aberrant targeting of neutrophils by the immune system are also unknown. While there are assays that have been able to demonstrate antibody-coated neutrophils, for many clinicians, immune-mediated destruction of neutrophils is a mechanism deduced by exclusion of other mechanisms of neutropenia.

Clinical presentation

The literature suggests that immune-mediated neutropenia is more prevalent in dogs less than 4 years of age, although the diagnosis has been established in older dogs. The clinical signs associated with immune-mediated neutropenia are variable and non-specific, but commonly include lethargy and anorexia; fever is a very common physical examination finding. In the largest study of dogs with immune-mediated neutropenia (11 dogs), fever was seen in 8/11 and lethargy in 4/11 affected dogs; these were the most common clinical signs in affected dogs. Evidence of bleeding, lameness, and vomiting have also been described in dogs ultimately diagnosed with immune-mediated neutropenia. Other findings, equally non-specific as the preceding, have been described and could be encountered in affected patients. In the author's experience, many patients with immune-mediated neutropenia have clinical signs that persist over weeks to months before definitive diagnosis. 

Diagnostic approach

As with most problems, the diagnostic approach to the patient with immune-mediated neutropenia starts with a history. Particularly important would be travel history as some infectious diseases (e.g. chronic ehrlichiosis) can cause neutropenia, although often in conjunction with other peripheral cytopenias. Drug history is also a critical part of the history as a number of drugs/medications have been associated with neutropenia (Table 1) in dogs and cats. The number of drugs associated with neutropenia in people is considerably greater than that for veterinary species, and it would be reasonable to give at least some passing thought to a drug-induced response in a patient with neutropenia being treated with virtually any medication.

Table 1. Drugs associated with neutropenia in dogs and cats

Estrogens Chemotherapeutic agents Anticonvulsants (phenobarbital) Antibiotics (sulfonamides) Cardiovascular drugs (amiodarone) Methimazole NSAIDs (carprofen) Griseofulvin in FIV-positive cats

During the physical examination, careful attention should be paid to look for causes of inflammatory disease. The presence of fever is common in patients with immune-mediated neutropenia, so it does not discriminate causes of neutropenia. Petechial hemorrhage and peripheral edema could indicate vasculitis of infectious or non-infectious causes. New heart murmurs could raise a red flag for bacteremia. Diarrhea and/or vomiting could suggest the potential for gut-derived bacteremia or endotoxemia as a cause of acute inflammation. Joint pain or effusion, or muscle pain may be a part of a multisystemic inflammatory disease such as systemic lupus erythematosus. Organomegaly may be a clue to the existence of neoplastic disease, such as lymphoma, that could also cause bone marrow infiltration, or reflect hyperplasia of the mononuclear-phagocytic cell system common to immune-mediated cytopenias.

A laboratory data base will be obtained at some point in the evaluation of most patients with immune-mediated neutropenia. Neutropenia can be profound in affected patients, with total neutrophil counts often less than 500/ul; in the retrospective study by Brown et al. the median neutrophil count for the 11 dogs was 110/ul (range 0-2380). Band neutrophils may also be seen. It is critical to examine a blood smear in affected patients as moderate or marked neutrophil toxicity, or unusual morphologic forms of any cell lineage, should be viewed as possible indicators of diseases other than immune-mediated neutropenia.  The presence of either anemia and/or thrombocytopenia should prompt consideration of causes other than immune-mediated disease as the cause of the neutropenia, although it is possible that immune-mediated neutropenia could co-exist with immune-mediated anemia or thrombocytopenia.  Biochemical profile results are not often helpful in the diagnosis, but may suggest locations of inflammatory disease (e.g. liver, kidney) that could drive neutropenia. Hyperglobulinemia and increased serum alkaline phosphatase activity (including dogs that had been treated with glucocorticoids) were the most common biochemical profile abnormalities in Brown's retrospective study. A urinalysis is important to exclude acute pyelonephritis.

Bone marrow aspiration cytology is appropriate for immune-mediated neutropenia candidates. The granulocytic series can be hyperplastic, hypoplastic, or show an arrest in maturation; importantly, bone marrow cytology helps exclude infiltrative disease as a cause of neutropenia.

Ideally, patients with no identified cause of neutropenia would receive three-view thoracic radiographs, and abdominal ultrasonography, and if murmurs are present, cardiac ultrasonography to exclude occult neoplasia or inflammatory disease as the cause of neutropenia. Immune-mediated neutropenia, likely secondary to a thymoma, has been described in a cat.  Blood and urine cultures can be considered to exclude bacteremia. Testing for infectious disease (e.g. feline retroviruses, E. canis serology) may be necessary in patients with risk factors for infection with these and other organisms.

Patients with immune-mediated neutropenia are typically negative for infectious disease testing, and have unremarkable imaging results. Tests that allow definitive diagnosis of primary immune-mediated neutropenia are lacking at present time. Assays for the detection of neutrophil-bound antibodies, expected in cases of immune-mediated neutropenia, have been developed, but await validation in large numbers of dogs with immune-mediated neutropenia and dogs with neutropenia from other causes.

Treatment

Immune-mediated neutropenia is treated with immunosuppressive drugs. The notion of treating a neutropenic patient, often febrile, with immunosuppressive drugs can be unsettling, which is why in general care is taken to exclude infectious disease to the extent possible/reasonable before treating with immunosuppressive drugs. Prednisone (1-2 mg/kg PO q12h) is the mainstay of treatment; the author has also combined azathioprine (1-2 mg/kg PO q24h for 10-14 day, then every other day) with glucocorticoids, and relied upon azathioprine for long-term therapy when patients showed a tendency to relapse when treated with prednisone alone. Other drugs that can be used include cyclosporine (5-10 mg/kg q12h), and leflunomide should also be of benefit, although the author has no experience using this drug to treat immune-mediated neutropenia. Treatment should continue until there is confidence that neutrophil counts will remain above the neutropenic range before tapering drugs. The author typically will make a dose reduction after 2 weeks of acceptable neutrophil counts, then every 2-3 weeks thereafter as long as neutrophil counts remain acceptable. For patients receiving combination therapy, drugs are tapered or stopped based on owner wishes and concerns, typically coming down to side effects of glucocorticoids or expense of other drugs.  There is no evidence to suggest a “best” way to treat patients with immune-mediated neutropenia, so using principles applied to treatment of other immune-mediated diseases is reasonable.

Summary

While uncommon, immune-mediated neutropenia can be an important cause of clinical disease, particularly in dogs. A careful approach that excludes, to the best of one's abilities, infectious, neoplastic or drug-induced causes of neutropenia will often lead to a diagnosis of immune-mediated neutropenia. The prognosis for patients that response to immunosuppressive therapy is often good, although some patients will require long-term maintenance therapy with immunosuppressive drugs to control clinical signs and maintain normal neutrophil counts.

References and suggested reading

Brown CD, et al. Evaluation of clinicopathologic features, response to treatment, and risk factors associated with idiopathic neutropenia in dogs: 11 cases (1990–2002). J Am Vet Med Assoc 2006;229:87–91.

Brown RM, Rogers KS. Neutropenia in dogs and cats: a retrospective study of 261 cases. J Am Anim Hosp Assoc 2001;37:131–139.

Fidel JF, et al. Granulocytopenia associated with thymoma in a domestic shorthair cat. J Am Anim Hosp Assoc 2008; 44:210-217.

McManus PM, et al. Immune-mediated neutropenia in 2 dogs. J Vet Intern Med 1999;13:372–374.

Vargo CL, et al. Immune mediated neutropenia and thrombocytopenia in 3 giant schnauzers. Can Vet J 2007;48:1159–1163.