Acquired Hemostatic Disorders

Johanna L. Watson

Acquired defects of hemostasis may be divided into those involving blood vessels, platelets, and coagulation factors, although some diseases affect more than one component.

Vasculitis

Vasculitis is a clinicopathologic process that involves inflam­mation and necrosis of blood vessel walls, regardless of size, location, or cause.¹ Vasculitis in large animals is generally a secondary manifestation of a primary infectious, toxic, or neoplastic disorder and has characteristics of hypersensitivity vasculitis in humans. The predominant involvement of small vessels in the skin (e.g., venules, arterioles) is the hallmark of hypersensitivity vasculitis.

Clinical manifestations of vasculitis include demarcated areas of dermal or subcutaneous edema that may progress to skin infarction, necrosis, and exudation.² Hyperemia, petechial and ecchymotic hemorrhages, and ulceration of mucous membranes are common. Although the skin and mucous membranes are predominantly involved, hemorrhage and necrosis may occur in any organ system, resulting in lameness, colic, dyspnea, and/or ataxia. Subclinical renal disease is not uncommon. Vasculitis is often associated with adverse sequelae such as cellulitis, thrombophlebitis, laminitis, and pneumonia. Char­acterized vasculitis syndromes with predominant cutaneous involvement include equine purpura hemorrhagica (EPH), equine viral arteritis (EVA), equine infectious anemia (EIA), and equine granulocytic anaplasmosis (EGA). There are also a number of vasculitis syndromes in horses for which cause, pathogenesis, and clinical course are poorly defined.^3,4 Vasculitis is apparently uncommon in ruminants but may accompany certain septicemic diseases (e.g., malignant catarrhal fever of cattle, bluetongue of sheep).⁵ Hematologic and serum biochemi­cal findings in vasculitis are determined by the underlying disease, length of illness, organ involvement, and secondary complications.

Definitive diagnosis of vasculitis is made by demonstration of the characteristic histopathology of involved vessels. Full­thickness punch biopsies (at least 6 mm in diameter) of skin in an affected area should be obtained and preserved in 10% formalin and Michel's transport medium. Multiple biopsies from different sites may be necessary to reach the diagnosis. The most common inflammatory pattern is neutrophilic infiltration of venules in the dermis and subcutaneous tissue, with nuclear debris in and around involved vessels (leukocy- toclasis) and fibrinoid necrosis. Considerable evidence suggests that most vasculitis syndromes are mediated by immunologic mechanisms—that is, a hypersensitivity reaction to a microbe, drug, toxin, or protein.¹ In some instances, an exogenous stimulus cannot be identified and an autoimmune pathogenesis is suspected. Immune complex deposition in vessel walls, with subsequent complement activation and chemoattractant produc­tion, seems to be the major pathogenic mechanism. Immuno­fluorescence on biopsies preserved in Michel's medium may reveal immune complexes. Infiltrating neutrophils and mac­rophages release proteolytic enzymes that cause vessel wall necrosis with subsequent edema, hemorrhage, and infarction of supplied tissues. Size and physiochemical properties of immune complexes, blood flow turbulence in sites of vessel bifurcation, and hydrostatic forces in dependent areas account for preferential formation of lesions in certain disease states and anatomic locations.

Equine Purpura Hemorrhagica

Purpura hemorrhagica (PH) is a noncontagious disease of horses characterized by vasculitis leading to extensive edema and hemorrhage of the mucosa and subcutaneous tissue. The disease has been recognized as a sequela to infection with or exposure to Streptococcus equi, Streptococcus zooepidemicus, Rbodococcus equi, Corynebacterium pseudotuberculosis, and others, as well as vac­cination against S. equi.^2,7

Young to middle-aged horses are commonly affected (mean, 8.4 years of age; range, 6 months to 19 years).⁷ Clinical signs usually develop acutely within 2 to 4 weeks of a respiratory infection.⁸ Predominant signs are well-demarcated subcutaneous edema of all four limbs, lethargy, anorexia, hemorrhages on mucous membranes, fever, and tachycardia. Other signs are tachypnea, reluctance to move, serum exudation from the skin, colic, and epistaxis.

The predominant laboratory abnormalities include anemia, neutrophilia, hyperproteinemia, hyperfibrinogenemia, hyper- globulinemia, and elevated muscle enzymes. Thrombocytopenia is rarely detected in horses with PH.

Skin biopsy shows evidence of acute leukocytoclastic or nonleukocytoclastic vasculitis with necrosis of blood vessels. The lesions are marked dermal and subcutaneous hemorrhage, protein-rich edema, and multifocal areas of dermal infarction. Small arteries and capillaries are swollen and infiltrated by degenerate or nondegenerate neutrophils. Hyaline thrombi may be observed.⁷ Immune complexes have been demonstrated to be primarily composed of immunoglobulin (Ig)M or IgA and streptococcal M protein and may be present in capillaries and small blood vessels of horses with PH, leading to a type III hypersensitivity reaction.⁹ Deposition of immune complexes in vessel walls and subsequent complement activation may result in cell death.

Horses with infarctive PH can present with colic, lameness, muscle swelling, and stiffness.¹⁰ Extensive GI infarction results in severe colic signs. Significant findings in these horses include leukocytoclastic vasculitis and necrosis of various tissues, neutrophilia with a left shift, hypoalbuminemia, and high serum creatine kinase.¹⁰

Treatment should aim to address the primary cause if identified, reduce the immune and inflammatory response, initiate antimicrobial therapy in cases of active infection or prevent infection if indicated, provide supportive care, and prevent complications. Horses with known streptococcal infection should receive penicillin (22,000 IU/kg of intramus­cular [IM] procaine penicillin G twice daily, or IV potassium penicillin G q6h) or other antibiotic with excellent efficacy against streptococcal organisms for at least 2 weeks. Hydro­therapy, limb bandages, and light exercise (hand walk) have been helpful in reducing limb edema. Fluids administered IV or via nasogastric tube may be necessary for animals that become severely lethargic and fail to drink or those that develop dysphagia from laryngeal edema. Stridor and dyspnea may indicate the need for tracheostomy. Prolonged treatment with corticosteroids (2 to 4 weeks) has resulted in favorable outcome and low relapse rate. However, some horses may require a longer course of therapy than 4 weeks. Depending on severity of clinical signs, proposed dosages are 0.04 to 0.2 mg/kg of dexamethasone (IV, IM, or PO) once (morning) or twice a day, or 0.5 to 1 mg/kg of prednisolone PO once (morning) or twice a day, with gradual reduction of the dosage.⁷ It has been suggested that antimicrobials be used throughout the course of corticosteroid therapy to reduce the occurrence of secondary sepsis.²

Outcomes will depend on early detection, early aggressive treatment, and extent of organ involvement.

Equine Viral Arteritis

Equine viral arteritis (EVA) is an infectious disease characterized by panvasculitis, edema, hemorrhage, and abortion in pregnant mares. EVA is caused by an enveloped, spherical, positive- stranded RNA virus with a diameter of 50 to 70 nm. Equine arteritis virus (EAV) is a nonarthropod-borne virus classified as a member of the new order Nidovirales within the family Arteriviridae.¹¹ EAV was first isolated from fetal lung collected during an epizootic of abortion in Bucyrus, Ohio.¹² Clinical signs may be absent or may develop 1 to 10 days after infection and include pyrexia, lethargy, anorexia, limb edema, stiffness, rhinorrhea, epiphora, conjunctivitis, rhinitis, and abortion. Edema of several regions can be observed, including periorbital, supraorbital, ventrum, mammary gland, scrotum, and limbs.¹³ Other signs include urticarial rash, abortion, respiratory signs, ataxia, mucosal eruptions, submaxillary lymphadenopathy, and intermandibular and shoulder edema. EAV can present as epidemic abortion, with occasional fatalities in foals and adults. With natural exposure, abortion rate varies widely (60%) and can occur from 3 to 10 months of gestation.¹⁴ Infected mares do not become EAV carriers or chronic shedders and do not appear to have fertility problems.

Laboratory abnormalities are variable and not diagnostic for EVA. Experimentally infected mature horses had a consistent leukopenia due to neutropenia and lymphopenia.¹⁵ After infec­tion, EAV can be localized in macrophages and lymph nodes within 24 and 48 hours, respectively. Various tissues are affected, but blood vessels are the principal target of EAV.

The virus is mainly transmitted through aerosols from respiratory, urinary, or reproductive tract secretions of acutely infected animals. The other route of transmission is via semen from persistently infected stallions; the virus remains viable in fresh, chilled, and frozen semen.¹⁴ Horizontal transmission via fomites is possible.¹⁶ Natural EAV exposure results in long-term immunity to the disease. Mares and geldings eliminate virus within 60 days, but 30% to 60% of acutely infected stallions will become persistently infected, temporarily or permanently shedding virus in semen.¹³ The virus is maintained in the accessory organs of the male reproductive tract (ampullae, vasa deferentia).¹⁴ Seroprevalence for EAV is common in the Standardbred and warmblood breeds and, more recently, EAV infection has been established within some Quarter Horse populations.¹⁷

Laboratory diagnosis of EVA can be based on virus isolation, viral nucleic acid detection (polymerase chain reaction [PCR] assay), or serology. In the case of serology, a fourfold or more increase in serum neutralizing antibodies between acute and convalescent samples (3 weeks apart) is required for diagnosis. Stallions with positive titers of 1:4 should be tested for per­sistent infection by virus isolation from sperm-rich ejaculate.¹⁴ Viral isolation and PCR can be attempted from respiratory secretions or fetal and placental tissues. Semen can be tested for viral shedding by culture, isolation, or PCR. Identification of carrier stallions is crucial in preventing dissemination of EAV. A modified live (ML) vaccine is commercially available (ARVAC [Zoetis Animal Health Inc, Kalamazoo, Mich., USA]). Vaccination will result in development of a serum titer that will be detected on EVA testing and cannot be distinguished from active infection. Horses vaccinated for the first time should have serum submitted to a U.S. Department of Agriculture (USDA)-approved laboratory to confirm seronegative status. After vaccination, these animals should be isolated for 28 days because they may temporarily shed the modified virus; however, there is no evidence that stallions vaccinated with the ML vaccine develop the carrier state.¹⁸

Equine Infectious Anemia

See section on hemolytic anemia.