The primary differential diagnosis for a neonatal foal observed to have evidence of anemia and intense icterus during the first week of life is neonatal isoerythrolysis (NI). Incompatible blood group antigens between the mare and stallion combined with maternal exposure to fetal erythrocytes during gestation results in maternal production of antigen specific antibodies targeted against the foal’s red blood cells, if the foal inherited the sire’s red blood cell phenotype. The most common blood types associated with NI in foals are Aa and Qa. Foals with NI most commonly suffer from extravascular hemolysis, although rare intravascular hemolysis occurs concurrently. A positive direct Coombs’ test provides a presumptive diagnosis of neonatal isoerythrolysis. However, the most sensitive diagnostic technique for identification of surface-associated immunoglobulin (Ig) molecules on suspect foal red blood cells involves a new direct immunofmorescence (DIF) assay using flow cytometry. In this assay, isotype specific antibodies bind to red blood cells that contain surface-associated antibodies, thus providing a quantitative measure of erythrocytes in the circulation that are bound with antibody. Site provides more information on the pathophysiology and treatment of neonatal isoerythrolysis.
Equine ehrlichiosis is caused by the rickettsial organism A. phagocytophila (formerly known as Ehrlichia equi), which belongs to the phagocytophilia complex of ehrlichial agents. A. phagocytophila, is the causative agent of tick-borne fever of ruminants that reside in Europe and human granulocytic ehrlichiosis (HGE) in Europe and America. This organism has DNA sequence homology with A. phagocytophila recovered from horses in Connecticut and California and HGE. Blood from people with HGE given to horses induces disease and imparts protective immunity to future challenge with A. phagocytophila. Equine ehrlichiosis is most common in the northern California foothills; however, ehrlichiosis has been reported in other regions of the United States, including the upper Midwest and the northeastern United States.
Clinical Signs and Diagnosis
Clinical signs include fever, depression, petechiae, ventral edema, and ataxia, with reluctance to move. Differential diagnoses for these signs include purpura hemorrhagica, equine viral arteritis, and encephalitis. Granulocytopenia, anemia, and thrombocytopenia are common abnormalities detected on hematology. Horses that are younger than 3 years of age exhibit a less severe form of the disease. Diagnosis of A. phagocytophila may be made from identification of morula in circulating granulocytes. Seroconversion (fourfold rise in serum antibodies in a convalescent sample) can be detected with an indirect fluorescent antibody test. A polymerase chain reaction assay has been developed for A. phagocytophila and has improved sensitivity and specificity in comparison to conventional diagnostic tests.
Supportive care for affected patients should include NSAID therapy, intravenous fluids, and lower limb sweat wraps. Intravenous oxytetracycline (7 mg/kg diluted in 1 liter of saline IV q24h x 5-7 days) is effective for clearance of equine granulocytic ehrlichiosis. Response to therapy is extremely rapid, thus supporting the diagnosis in suspect cases. Carrier phases have not been recognized; in fact, once-infected horses acquire strong immunity for up to 2 years, thus providing resistance to reinfection.