The "RBCs" of feline anemia (Proceedings)

Objectives

• Discuss the diagnostic workup of cats who present for anemia

• Focus on infectious causes of anemia

• Briefly review the evidence for primary immune-mediated hemolytic anemia in cats

Key Points

• The complete blood count determines the course of the workup by dividing the possible rule-outs into those causing a regenerative vs. non-regenerative anemia

• Infectious diseases remain an important cause of anemia in cats

• Primary immune-mediated anemia may be more common than previously thought.

Introduction

Anemia is a common blood work abnormality in many species, including cats. Determining the regenerative nature of the anemia guides the workup of the case. Regenerative anemias suggest blood loss or red cell lysis. Red cell lysis can be due to toxins, infectious agents, neoplasia (as a secondary immune-mediated phenomenon) or primary immune-mediated hemolytic anemia. Non-regenerative causes of anemia include iron-deficiency, anemia of inflammatory disease, renal disease, bone marrow disorders, neoplasia, infectious disease, and immune-mediated disorders directed at erythrocyte precursors.

Clinical Presentation and Historical Findings

Classic signs of anemia include lethargy, weakness, inappetence, pale mucous membranes or icteric membranes if hemolysis is occurring. Given that infectious disease and neoplasia are two major rule-outs for anemia, fever may be present. Since many of the infectious agents are vector-borne, flea and tick exposure should be determined. Outdoor status increases the chance of contact with other cats and thereby exposure to the retroviral agents. Exposure to drugs or toxins such as onions and acetaminophen is important to ascertain. Other systemic signs, such as PU/PD with renal disease, can indicate the presence of other chronic diseases.

Diagnostic Evaluation

A complete blood count, including reticulocyte count, will determine whether the anemia is regenerative or non-regenerative. Acute blood loss and red blood cell lysis will not be regenerative for 2-4 days and so clinical and clinicopathologic parameters must be taken into account when making an assessment of the regenerative nature of the anemia. Clinical evidence of bleeding and a low plasma protein should differentiate acute blood loss from hemolytic anemia. Spherocytes are difficult to identify in feline blood smears and are therefore not a reliable means to diagnose hemolytic anemia. Presence of reticulocytes and/or nucleated red blood cells and macrocytosis with or without autoagglutination is diagnostic for regenerative anemia. An iron-deficiency anemia will be characterized as microcytic and FeLV-induced non-regenerative anemia will often be macrocytic. A careful evaluation of blood smears can reveal intra- or extracellular organisms. Presence of Heinz bodies may indicate an exposure to toxins that cause oxidative damage such as onions, propylene glycol, etc. A direct Coombs' test may be performed, but a negative test does not rule out an immune-mediated anemia since its sensitivity and specificity are low in cats. An osmotic fragility test is indicated in breeds such as Abyssinians and Somalis which can have a hereditary erythrocyte membrane defect. A biochemistry panel is necessary to rule in or out other causes of anemia, such as renal disease and other systemic illnesses. Hyperbilirubinemia may be present if hemolysis is severe. Other abnormalities in the biochemistry panel depend on the specific chronic disease, infectious agent or neoplastic process. FeLV antigen/FIV antibody testing should be done and thoracic and abdominal radiographs (or abdominal ultrasound) should be performed to help rule-out neoplasia.

Toxins

A variety of toxins and drugs are associated with hemolytic anemia in cats. Zinc, onions, propylene glycol, acetaminophen, methylene blue, methionine, methimazole and benzocaine preparations are few of the most common agents. The majority of these toxins induce Heinz body formation and some, such as acetaminophen toxicity, cause methemaglobinemia as well. Supportive care is the primary therapy for Heinz body anemia.

Infectious Agents

Hemoplasmosis

Mycoplasma haemofelis, 'Candidatus M. haemominutum' and 'Candidatus M. turicensis' are the organisms responsible for hemoplasmosis. Virulence is attributed primarily to M. haemofelis and M. turicensis. The disease has worldwide distribution with rates of prevalence varying from 4%-23% for M. haemofelis, depending on the region.1 Since the flea is the likely vector, prevalence rates vary according to climate. Other modes of transmission include blood transfusion (and by extension between cats if blood contamination occurs) and from the queen to her kittens (in utero or via lactation). Clinical signs are those associated with anemia and fever is often present. Identification of the gram-negative, epierythrocytic organisms on a blood smear is an unreliable means of diagnosis because of cyclic parasitemia, staining artifacts and sample handling. PCR for organismal DNA is the most sensitive and specific test for the hemoplasmas. The standard treatment is doxycycline (5 mg BID x 14 days), but there is substantial evidence that the fluoroquinolones are equally efficacious. In several studies, enrofloxacin (5-10 mg PO q 24 hrs x 14 days),2 marbofloxacin (2.75 mg PO q24 hrs)3 and pradofloxacin4 (a novel fluoroquinolone not yet available in the U.S.) have demonstrated clearance of the organism by PCR weeks to months after therapy. In cats in which organism clearance has not occurred, recrudescence during times of stress is possible.

Cytauxzoonosis

Cytauxzoon felis is a tick-borne protozoal organism that is predominantly found in the south central, southeastern and mid-Atlantic states.5 The bobcat is the reservoir host and Dermacenter variabilis is the presumed tick vector. These signet ring-shaped organisms are located within the red blood cell during the "erythrocytic phase" of the infection. C. felis invades monocytes/macrophages which are distributed to the tissue for the "tissue phase" or "leukocytic phase" of the disease. Schizont-engorged monocytes clog vessels and account for the multi-organ failure and rapid death, usually within 1 week, associated with cytauxzoonosis. Considered to be 100% fatal in cats, recent reports of healthy carriers and cats which survive the infection have suggested that aggressive therapy may improve survival rates.6 A presumptive diagnosis is often made on acute onset of clinical signs (severe lethargy, anorexia, icterus, dyspnea) in an endemic region with possible exposure to ticks. A definitive diagnosis can be made if the parasite is visualized within the RBCs; PCR is also now available.7 A promising combination of the anti-malarial drug atovaquone (15mg/kg PO TID) and azithromycin (10mg/kg PO Q24), in addition to aggressive supportive care and heparin, is currently being recommended. With this combination, a survival rate of 64% was reported.8

Bartonellosis?

Bartonella spp. have been implicated in multiple diseases in humans, dogs and cats. In cats, the primary infective species are B. henselae and B. clarridgeiae. In humans, B. bacilliformis causes a hemolytic anemia in which nearly 100% of RBCs are infected.9 Although this organism does not infect cats, a case of human hemolytic anemia caused by Bartonella henselae has been documented.10 Studies have also demonstrated the intraerythrocytic location of B. henselae in naturally-infected cats using electron microscopy, making this organism a candidate for unexplained cases of feline anemia. Recently, however, a retrospective study attempting to correlate Bartonella spp. with cases of hemolytic anemia found no significant prevalence differences in the healthy cats versus anemic cats with a positive Bartonella PCR assay.11 As with many high-prevalence diseases, it will be difficult to make the association between Bartonella spp. and anemia.

Ehrlichiosis?

Ehrlichia and Anaplasma-like DNA have been amplified from naturally-infected cats and have been demonstrated to cause clinical signs in cats, including fever, lethargy, inappetence and, in some cases, anemia.12,13 Though these organisms are listed under hemolytic causes, the anemias in one case report were non-regenerative.14 In the retrospective study referenced above, however, no sample from any of the healthy or anemic cats was PCR-positive for Ehrlichia or Anaplasma spp.11 Given the fact that ehrlichiosis in cats is rare and that only some of these cats had anemia, it is unnecessary to routinely test for these organisms in cats with anemia.

FeLV/FIV

FeLV can cause both non-regenerative and regenerative anemias. The latter may be due to concurrent infectious with the hemoplasmas, but an immune-mediated process is also possible. FeLV antigen testing is therefore recommended for any cat presenting with hemolytic anemia. FIV infections are nearly always associated with a non-regenerative anemia. There are only a few controlled studies evaluating use of antiviral agents in these diseases and most show either no effect or are toxic. Feline interferon-? and human interferon-a showed some effect for FeLV and FIV, respectively.15

Primary Immune-Mediated Hemolytic Anemia

There is a subset of cats with hemolytic anemia for which neoplasia, toxins and the majority of infectious diseases have been ruled out, either by infectious disease assays or trial therapies with anti-bacterial agents. In one study of 19 cats, no identifiable infectious, neoplastic or toxic could be found as a cause of their anemia.12 A majority of these cats were Coombs' positive suggesting that anti-erythrocyte antibodies were present. In another retrospective study of anemic cats whose blood was submitted for extensive infectious disease testing, approximately 75% of cats with either non-regenerative or regenerative anemia and 60% of cats with non-regenerative anemia were negative for all organisms tested.11 This would suggest that primary IMHA may be more common than previously thought. Unfortunately, a definitive diagnosis may be difficult to obtain. The direct Coombs' test appears to be neither specific nor sensitive in cats. New techniques, such as flow cytometry and anti-erythrocyte antibody assays, may be helpful in identifying anti-erythrocyte antibodies in cats with primary hemolytic anemia but are currently not commercially available. Therefore, the diagnosis of primary IMHA cats is usually made by ruling out all other causes of anemia with an exhaustive workup and trial therapy. Treatment with immunosuppressive doses of steroids is the mainstay of therapy.

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