John E. Madigan, Nicola Pusterla
Equine Granulocytic Anaplasmosis (Formerly Ehrlichiosis)
Equine granulocytic anaplasmosis (EGA) is a common, seasonal, rickettsial, tick-transmitted, noncontagious disease of horses seen where Ixodes spp ticks occur. The reclassification of the agent on the basis of DNA sequence analysis from Ehrlichia to Anaplasma has created some confusion; other previous names are Ehrlichia equi, Anaplasma phagocytophilum, and foothill tick fever of horses. The etiologic agent is a member of the A phagocytophilum genogroup, containing gram-negative cocci with a tropism for granulocytes. Clinical manifestations may include these clinical signs alone or together: fever, partial anorexia, depression, limb edema, petechiation, icterus, ataxia, and reluctance to move. Hematologic changes are thrombocytopenia, high plasma icterus index, low packed cell volume, and marked leukopenia, which involves first lymphopenia and then neutropenia. Transmission by a tick vector is responsible for the seasonal appearance of the disease, which is being diagnosed with increasing frequency in the United States, Canada, Brazil, and northern Europe.
Etiologic Agent
The causative agent of EGA is A phagocytophilum (formerly known as E equi). The organisms are found in membrane-lined vacuoles in the cytoplasm of infected eukaryotic host cells, primarily granulocytes. These inclusion bodies consist of one or more coccoid or coccobacillary organisms and can range in size from approximately 0.2 µm in diameter to large granular structures called morulae that are approximately 5 µm in diameter. Organisms are visible under high, dry, or oil immersion with light microscopy. They stain deep-blue to pale blue-gray with Giemsa or Wright-Leishman stains.
The A phagocytophilum genogroup includes the agent of tickborne fever of ruminants in Europe (formerly known as Ehrlichia phagocytophilum) and the recently reported agent of human granulocytic anaplasmosis (HGA), formerly called human granulocytic ehrlichiosis (HGE) in the United States and Europe. Members of this genogroup have similar morphology and neutrophil cell tropism and are very closely related serologically and genetically. The DNA sequences of the 16S rRNA gene from the peripheral blood of naturally infected horses in Connecticut and California are identical to those of the HGE agent. Moreover, injection of infective human blood from HGE patients into horses causes typical equine ehrlichiosis, which can be transmitted to other horses. It induces protection in horses to subsequent challenge with E equi. These data suggest that the agents of EGA and HGA are probably conspecific, and were at the origin of reclassification of the agents under the name A phagocytophilum.
Epidemiology
The horse represents an aberrant host, and it seems unlikely that infected horses could serve as effective reservoirs of A phagocytophilum because the presence of the organism in an affected animal is limited to the acute phase of the disease. Horses of any age are susceptible, but the clinical manifestations are less severe in horses younger than 4 years. Horses from endemic areas have a higher seroprevalence of antibody against A phagocytophilum than do horses from nonendemic areas, which suggests the occurrence of subclinical infection in some animals. Furthermore, horses introduced into an endemic area are more likely to develop EGA than are native horses. Persistence of A phagocytophilum has not been demonstrated in naturally or experimentally infected animals. The disease is not contagious, but infection can be transferred readily to susceptible horses with transfusion of as little as 20 mL of blood from horses with active infection. Most often, one infected horse is observed in groups of horses in the same pasture. The disease has been reported in horses in Colorado, Illinois, Minnesota, Connecticut, Florida, Wisconsin, and—outside the United States—in Canada, Brazil, and northern Europe.
In recent years, EGA has been experimentally transmitted by the Western blacklegged tick (Ixodes pacificus) and the deer tick (Ixodes scapularis). Furthermore, an epidemiologic study in California revealed that the spatial and temporal pattern of EGA cases closely paralleled the well-characterized life history and distribution of I pacificus, but not of other ticks commonly associated with horses. In the East and Midwest of the United States, I scapularis is the vector of granulocytic anaplasmosis; small rodents such as white-footed mice, chipmunks, and voles, as well as the white-tailed deer, are potentially important reservoirs. In California, white-footed mice, dusky-footed wood rats, cervids, lizards, and birds have been proposed as reservoirs.
