Vasculitis can be due to infectious or immune-mediated causes. Vasculitis can result in thrombocytopenia due to local platelet aggregate formation or can initiate DIC.
- Infectious causes: Rickettsia rickettsii, the etiologic agent of RMSF, is an obligate intracellular parasite transmitted principally by the vector ticks Dermacentor variabilis and Dermacentor andersoni. The organism invades vascular endothelial cells leading to cell necrosis, increased vascular permeability, and perivascular hemorrhage and edema. The developing vasculitis is accompanied by thrombocytopenia and variable activation of the coagulation mechanism. Early signs may include petechial and ecchymotic hemorrhages of the skin and mucous membranes, retinal hemorrhage, epistaxis, melena, and hematuria. Severely affected individuals may develop DIC. The most useful test for diagnosing RMSF is serologic testing. Although cross reactions do develop between other rickettsial organisms, the titer is usually highest for the specific rickettsia causing the infection. Paired acute and convalescent serum samples (10 to 14 days apart) are recommended for testing. Tetracycline is the recommended therapy. Intravenous fluid therapy should also be used with caution due to the presence of increased vascular permeability.
Although the incidence is sporadic, herpesvirus infection can result in the rapid death of puppies usually between the ages of 7 and 21 days. The disease is characterized by multiple hemorrhages throughout numerous tissues including the liver, kidney, brain, gastrointestinal tract and lung as a consequence of a viral-induced necrotizing vasculitis. Puppies usually die within 24 hours; treatment is often unsuccessful.
- Immune-mediated vasculitis: Deposition of immune complexes in small vessels can cause damage to the wall with perivascular leakage of plasma and blood cells that mimics thrombocytopenia. The antigens involved in the complexes can be autoantigens or antigens of infectious agents. Petechial and ecchymotic hemorrhages and marked edema of affected areas are typical signs. The most well known syndrome in domestic animals is purpura hemorrhagica of horses. This syndrome is one of acute onset of edema of the head and limbs with petechiae in the mucous membranes. The syndrome is usually a sequela of infectious diseases; many affected horses have recently had strangles or Streptococcus equi infection. Horses with purpura hemorrhagica have normal platelet counts.
A necrotizing vasculitis has been reported in young Beagle and Bernese Mountain dogs. Thrombocytopenia is variable in these disorders.