Thrombocytopenia

Johanna L. Watson • Debra Deem Morris

Thrombocytopenia (platelet count blood coagulation proceeds in an integrated sequence that can be simplistically viewed as three key reactions: formation of activated factor X, formation of thrombin, and formation of fibrin.

Disseminated Intravascular Coagulation

The most common form of hemostatic dysfunction in large animals is a syndrome known variously as disseminated intra­vascular coagulation (DIC).^1,2 The pathologic process is characterized by widespread fibrin deposition in the micro- circulationand development of a hemorrhagic diathesis caused by the consumption of procoagulants and hyperactivity of fibrinolysis. Never a primary disease entity, DIC represents an intermediary mechanism of underlying disease. In large animals, DIC has been described in association with localized or systemic septic processes,^2-5 neoplasia,² GI disorders,⁶ renal disease,⁶ and hemolytic anemia.^5,7 Diffuse activation of the hemostatic system is particularly prevalent in horses with acute GI disorders that cause colic^5,6 and is a likely initiating factor for laminitis.^8,9 Clinical manifestations of DIC may fall anywhere on the spectrum from diffuse thrombosis leading to ischemic organ failure to severe hemorrhagic diathesis. The most important determinants are the rate of thrombin generation, adequacy of fibrinolysis, and functional state of the MPS, which is largely determined by peripheral circulation.

Renal involvement is common in DIC, which produces ischemic cortical necrosis followed by acute tubular necrosis. Renal disease may be manifested by oliguria, depression, and ileus caused by azotemia and electrolyte imbalances. GI microthrombosis may induce colic as a result of submucosal necrosis and superficial ulceration. Spontaneous GI hemorrhage caused by DIC may cause melena in ruminants and occult fecal blood loss in horses. Pulmonary function may rarely be compromised by microvascular thrombosis in DIC, causing tachypnea and variable hypoxemia. Altered consciousness, delirium, convulsions, or coma may follow cerebral microvas- cular thrombosis but is not common with DIC in large animals. Although reported in both horses¹⁰ and cattle,⁷ microangiopathic hemolysis is rare in large animals with DIC because of their small erythrocyte size.

Digital ischemia frequently accompanies DIC in horses and may play a role in the development of acute laminitis. Labora­tory evidence of DIC has been documented during the developmental phase of equine laminitis,⁹ and digital micro- vascular thrombosis occurs in horses that develop laminitis with colic or septic conditions.⁸ The tendency for thrombosis of major peripheral veins is another prominent manifestation of coagulopathy in horses.

As the thrombotic stimulus continues or intensifies, the tendency for hemorrhage develops because of clotting factors and platelet depletion or generation of excessive fibrinolytic by-products (FDPs). Petechial or ecchymotic hemorrhages on mucosae and sclerae and a tendency to bleed from venipuncture or after minor trauma are the principal signs.

Numerous laboratory tests of hemostasis may be abnormal during DIC, but no one test consistently or specifically pro­vides a definitive diagnosis. The lack of test sensitivity is due to the dynamic nature of DIC, the laboratory reflection of which is determined by the balance between coagulation and fibrinolytic forces, as well as MPS integrity at the time of blood sampling.

The most widely used hemostatic function tests for DIC in large animals include platelet count, plasma fibrinogen, PT, APTT, antithrombin (AT), and serum FDPs/D-dimers.¹¹ Because clinical manifestations of DIC vary widely, clarification of the most frequent laboratory abnormalities in large animals with DIC is hindered by lack of a definitive diagnosis in most instances. Repeated hemostatic testing is advised when there is strong suspicion for DIC. Serum FDPs are most often elevated by DIC, but they are usually normal in the early or compensated form of the disease. Hypofibrinogenemia is an uncommon manifestation of DIC in large animals and, when present, should strongly suggest concomitant liver dysfunction. Hemostatic function tests are totally unreliable unless blood samples are collected and handled properly.

Criteria used for diagnosis of DIC are extremely arbitrary, and laboratory results must be interpreted in light of the patient's underlying disease. The combination of thrombocytopenia with mild to moderate PT or APTT prolongation strongly suggests DIC. The clinician should seek laboratory assistance when considering the diagnosis of DIC but appreciate that the findings are often not helpful. Clinical signs and specific situations suggest the possibility of DIC, and laboratory tests are only used to provide support.

Diseases initiate DIC by two major mechanisms: (1) genera­tion of excessive procoagulant activity within the blood or (2) contact of blood with abnormal surfaces. Many diseases that produce DIC have the propensity to cause endotoxemia. The intestinal tract in large animals normally contains large quantities of endotoxins, only a small part of which is absorbed through the portal vein and removed by the liver. Condi­tions that cause intestinal mucosal edema or disruption allow endotoxins to gain access to the peripheral circulation and initiate many morbid sequelae, one of which is DIC. Intes­tinal strangulating obstruction, thromboembolic infarction, and severe colitis induce mucosal abnormalities, allowing endotoxemia to occur. The proliferation of gram-negative bacteria within tissues and the blood is also accompanied by endotoxemia.

Gram-negative endotoxins are capable of direct factor XII activation. However, most studies indicate that the procoagu­lant effects of endotoxin are primarily mediated by cytokine production by mononuclear phagocytes.¹² After endotoxin stimulation, phagocytes produce a platelet-activating factor (PAF), tissue factor, prostaglandins, interleukins (ILs), tumor necrosis factor (TNF), and other mediators with procoagulant activity.¹³

The net result of any triggering mechanism for DIC is exaggerated generation of systemic thrombin, which causes widespread microcirculatory thrombosis. In addition to the cleavage of fibrinogen to produce fibrin monomers, thrombin activates factor XIII to render fibrin more resistant to fibri­nolysis, enhances the cofactor activity of factors V and VIII, and induces platelet aggregation and exposure of platelet phospholipid. Circulatory obstruction produces organ hypo­perfusion, leading to ischemic necrosis.

The counterbalance fibrinolytic system is also activated by DIC, and plasmin contributes to factor consumption by destroying factors V, VIII, XIIa, IX, and XI, in addition to fibrin and fibrinogen.

The MPS plays a vital role in the pathogenesis of DIC. The tissue-fixed macrophages of the spleen and liver normally remove FDPs and activated clotting factors from the peripheral circulation, and FDPs only increase when their rate of formation exceeds the ability of the MPS to clear them. Shock and hypoperfusion of the liver and spleen or diseases associated with excessive tissue debris that must be removed by the MPS (e.g., sepsis, metastatic neoplasia) reduce its ability to function effectively and predispose to or perpetuate DIC.

Therapy for DIC is highly controversial, and the only undisputed modalities are those directed toward identification and treatment of the primary disorder, along with general supportive measures to combat shock and maintain tissue perfu­sion., Intravenous fluid administration helps prevent organ dysfunction after microvascular thrombosis and can correct existing acid-base or electrolyte imbalances. Septic conditions should be treated with appropriate antimicrobial agents, and necrotic tissue or purulent exudate should be removed whenever possible (e.g., immediate surgical intervention to resect nonviable bowel). Flunixin meglumine mitigates the deleterious effects of endotoxin caused by eicosanoids and is used in horses at a dosage of 0.25 mg/kg IV q8h). Corticosteroids may worsen DIC because they reduce the phagocytic action of the MPS and potentiate the vasoconstrictor effects of catecholamines.

Significant life-threatening hemorrhage is rare in large animals with DIC, but if it occurs, fresh plasma should be administered (15 to 30 mL/kg) to replace used coagulant and anticoagulant proteins. The use of low-molecular-weight heparin, over unfractionated heparin, in DIC has been recom­mended in various regimens to stall the disseminated microvas- cular thrombosis that precipitates organ failure.^11,16,17 In dogs, minidose heparin therapy (50 to 75 units/kg subcutaneously [SC] three times daily, or 5 to 10 units/kg/h) is commonly used with blood products to treat DIC.¹⁴ Efficacy of heparin for DIC in horses is unproven.

The prognosis for DIC in large animals depends largely on the nature and severity of the underlying disease and how effectively the latter is treated. Once DIC has progressed to the stage where signs of coagulation dysfunction predominate, the prognosis is generally poor.

Warfarin Toxicosis

Horses may develop a hemorrhagic diathesis due to warfarin toxicosis.¹⁸ This coumarin-derivative anticoagulant has been used by some to treat horses with navicular disease.¹⁹ Rarely, horses and other animals may be exposed to coumarins used as rodenticides in grains or other feedstuffs. The clinical signs of warfarin toxicosis include hematomas, ecchymoses of mucous membranes, epistaxis, and hematuria. The earliest laboratory indication of warfarin toxicosis is prolongation of the PT, because the plasma half-life of factor VII is shorter than the other clotting factors.²⁰ As the disease progresses, the APTT becomes prolonged and the animal may develop blood-loss anemia and hypoproteinemia. The diagnosis of warfarin toxicosis is based on a history of exposure, clinical signs of large-vessel hemorrhagic diathesis, and prolonged PT with or without APTT and with no other abnormalities of the clotting profile.

Warfarin acts through competitive inhibition of vitamin K, which is necessary for liver production of clotting factors II, VII, IX, and X.¹⁹ Factor activity is reduced in the blood at a rate dependent on its individual half-life. In most species, factors VII, IX, X, and XI have increasingly greater half-lives, accounting for the greater sensitivity of PT for the early diagnosis of warfarin toxicosis. After GI absorption, warfarin is highly bound to plasma proteins. Drugs that are normally protein bound (e.g., phenylbutazone) can enhance warfarin’s toxicity by allowing a greater proportion of the administered drug to be unbound and active.²¹ In the same manner, hypo- albuminemia may increase the likelihood of warfarin toxicosis. Corticosteroids and thyroxin can lower the necessary therapeutic dose of warfarin by increasing both receptor affinity and clot­ting factor catabolism. Drugs that induce hepatic microsomal enzyme activity (e.g., barbiturates, rifampin, chloramphenicol) can accelerate warfarin metabolism and reduce therapeutic response to a given dose. Any reduction in hepatic function or content of vitamin K in the diet can also precipitate warfarin toxicosis.

Treatment of warfarin toxicosis depends on clinical signs. Warfarin therapy should be stopped if the PT exceeds twice the pretherapeutic value. Vitamin K₁ (0.5 to 1 mg/kg SC or IM) must be given every 6 hours until the PT is again normal and stable. Significant hemorrhage can be controlled by giving fresh plasma to provide necessary clotting factors. If the anemia is life threatening, whole blood transfusion should be considered. Although warfarin is eliminated rapidly, some potentiated coumarins have a prolonged half-life, requiring a longer course of vitamin K therapy. With early diagnosis and prompt vitamin K treatment, the prognosis for warfarin toxicosis is good. However, it is imperative that vitamin K₃ not be used because it has poor therapeutic action and is highly nephrotoxic for horses.²²

Prevention of warfarin toxicosis is based on limiting access of livestock to rodenticides and carefully monitoring the therapeutic use of warfarin in horses. Warfarin’s benefits for horses are highly controversial, and many question whether its advantages outweigh the risks.

Sweet Clover Toxicosis

Sweet clover (Melilotus spp.) toxicosis is caused by ingestion of moldy sweet clover hay or silage containing dicoumarol. Natural coumarins in sweet clover can be converted to dicou- marol when hay or silage is improperly cured and mold forms. The toxin persists in moldy hay or silage and is palatable. This disease can occur in all species but is most commonly seen in cattle fed sweet clover hay in the Northern Plains states. Early signs include epistaxis and melena, with the development of subcutaneous hematomas and periarticular swellings as the disease progresses. Visible swellings occur at points of trauma such as the brisket, tuber coxae, or carpi and are not hot or painful, although they may cause stiffness and disinclination to move. Accidental and surgical wounds cause severe hemor­rhage and may precipitate fatal blood-loss anemia.

Clinical pathology is similar to that described for warfarin toxicosis, with prolonged PT being the earliest abnormality (detected before clinical evidence of hemorrhage). The platelet count remains normal, which differentiates this syndrome from DIC and bracken fern toxicosis. Other diagnostic ruleouts include mycotoxicosis and toxicosis from trichloroethylene- extracted soybean meal. In the absence of fever and anorexia, coagulopathy should make moldy sweet clover toxicosis a strong tentative diagnosis in animals with a history of access. Chemical analysis for dicoumarol in suspected feed or in the blood and liver of affected animals aids in the diagnosis,²³ but the disease cannot be excluded if dicoumarol is not detected.

The pathogenesis of moldy sweet clover toxicosis is identical to that of warfarin toxicosis. Dicoumarol interferes with hepatic synthesis of clotting factors II, VII, IX, and X by inhibiting vitamin K. Usually the syndrome appears in cattle 2 to 7 days after they ingest the moldy hay. Lower levels of dicoumarol (may prolong the onset of signs for up to 3 months.

Grazing sweet clover is not dangerous. Because of its high forage yield, sweet clover is usually harvested as silage, which should carry less danger of molding when properly cured. The toxic level of dicoumarol in sweet clover feed samples is 10 mg/ kg of feed.²³

Treatment of sweet clover toxicosis involves discontinuing the use of contaminated feed and administering vitamin K₁.²¹ Dosages between 0.5 and 1 mg/kg of body weight should be administered SQ or IM every 6 hours; response occurs within 24 hours. Animals with severe blood-loss anemia or ongoing hemorrhage should be treated with plasma or whole fresh blood. Sweet clover toxicosis can be prevented by careful forage preparation, followed by the inspection of hay and silage before feeding. When the disease is suspected, questionable feed should be removed from the diet.