Anaplasma Phagocytophila Infection in Horses
John E. Madigan
Definition and Etiology
Equine granulocytic ehrlichiosis (EGE) was first reported in the late 1960s in the foothills of northern California.1 The disease is caused by Anaplasma phagocytophila.
This organism was formerly known as Ehrlichia equi, but it was reclassified in the Anaplasma genus on the basis of genetic analysis.2 Recently, the agents of human granulocytic ehrlichiosis (HGE), Ehrlichia phagocytophila and E. equi, were grouped into a single2
species and named A. phagocytophila.2 These organisms are identical based on 16S rRNA gene sequences, and they have similar morphology, host cell tropism, and indirect fluorescent antibody (IFA) response.3
The organism is found within vacuoles (1.5 to 5 μm in diameter) in the cytoplasm of infected granulocytes, primarily neutrophils and eosinophils. These vacuoles or inclusion bodies are pleomorphic and contain one or more coccobacillus or large granular aggregates called morulae. The organisms are visible under light microscopy as deep blue to pale blue gray with Giemsa or Wright-Leishman stains.
Epidemiology
Since the disease was first reported in California,4 cases have been diagnosed in Colorado, Connecticut,5 Florida, Illinois, Minnesota, New Jersey, New York, Oregon, Washington, Wisconsin, Canada, Brazil, northern Europe, Poland, Italy, Pakistan, South Korea, Ukraine, and Israel.6 Equine cases occur most commonly during late fall, winter, and spring. There is no apparent gender or age predilection, but the disease appears to be less severe in younger horses. Persistent, chronic, or latent infections or carrier status have not been demonstrated and are unlikely to occur, since the presence of A. phagocytophila is limited to the acute phase. Therefore it is also unlikely that infected horses could serve as reservoirs.
Anaplasma phago- cytophilum is a zoonotic infection with human infection occurring via tick bite. The disease is not contagious but could be readily transmitted through the administration of infected blood. The vectors of granulocytic ehrlichiosis are Ixodes pacificus in California, Ixodes scapularis in the eastern and midwestern United States, and Ixodes ricinus in Europe.7-10 Potential or proposed reservoirs are white-footed mice, chipmunks, whitetailed deer, dusky-footed wood rats, cervids, lizards, and birds.10Pathogenesis
The pathogenesis of the disease is unknown. Entry of the organism occurs after inoculation from a biting tick and is presumed to be spread via blood and/or lymph. The organism has cell tropism toward neutrophils and eosinophils, where it replicates within vacuoles, forming characteristic morulae. Presumed cytolysis, induction of inflammation, cell sequestration, consumption, or destruction result in the observed clinical signs and pancytopenia.11 Modulation of clinical signs in experimental infection in horses occurs with dexamethasone treatment.12 This indicates that the proinflammatory response plays a role in worsening disease severity and that disease severity can be decreased by modulating proinflammatory response.12
Cell- and humoral-mediated immune responses develop in affected animals. Antibody titers peak at 19 to 81 days after the onset of clinical signs, and immunity likely persists for a long period of time (>2 years).13
Clinical Signs
Clinical signs vary depending on the stage of infection. Early signs are fever of unknown origin and partial anorexia. Further signs develop over 24 to 48 hours and include reluctance to move, fever ranging from 39.4° to 41.3° C (102.9° to 106.3° F), mild to moderate tachycardia (50 to 60 beats per minute [bpm]), lethargy, decreased appetite, limb edema, petechiation, icterus, weakness, ataxia, and recumbency (reported in one case).
Secondary trauma may result from falling in severely ataxic horses. Additional presentations besides the classic signs described may occur. Several reports of cardiac arrhythmias in clinical cases have been noted with premature ventricular contractions observed. Severe rhabdomyoliss may be the presenting clinical presentation in some cases.14,15 The clinical signs appear to be less profound in younger horses. The incubation period after natural infection is believed to be fewer than 14 days. The prepatent period after experimental exposure to infected ticks or inoculation with infected blood is 8 to 12 days and 3 to 10 days, respectively. The disease is self-limiting and nonfatal provided no complications develop. However, affected horses may be predisposed to secondary bacterial, fungal, and viral infections.1 Abortions and laminitis have not been reported in affected horses.Clinical Pathology
Laboratory alterations include anemia, leukopenia characterized by granulocytopenia and lymphopenia, and thrombocytopenia. Morulae will be observed within the cytoplasm of neutrophils and eosinophils during the infection.
Diagnosis
Definitive diagnosis is based on the presence of characteristic morulae within the cytoplasm of neutrophils and eosinophils, or positive PCR assay for A. phagocytophila in peripheral blood (buffy coat).16 Morulae may be observed in less than 1% of cells in the initial stages of the infection, rising to 20% to 50% of cells a few days later. False-positive morulae identification can occur when Dohle bodies in toxic neutrophils are mistaken for morulae. Therefore careful screening of a blood film and identification of several morulae is suggested. The history of Ixodes sp tick exposure should be a critical part of the history in suspected cases. A fourfold or greater increase in IFA titer of paired samples confirms recent exposure.17
Pathologic Findings
Petechiae and ecchymosis of subcutaneous tissues, as well as edema of the ventral abdomen, limbs, and prepuce, are characteristic in infected animals.
Proliferative and necrotizing vasculitis, thromboses, and perivascular cuffing in subcutaneous tissue, fascia, kidneys, heart, brain, lungs, ovaries, and testes have been reported.1,11Treatment, Prognosis, and Prevention
The treatment of choice is IV administration of oxytetracycline at 7 mg/kg, or oral administration of doxycycline at 20 mg/ kg of body weight once a day for 5 to 7 days. Prompt response to treatment is seen within the first 24 hours. Supportive therapy may be necessary in some cases. In cases with severe ataxia, use of dexamethasone may hasten resolution of clinical signs. The disease can be self-limiting in 2 to 3 weeks if untreated. Prognosis is excellent provided no secondary complications develop. In experimental cases and reports of natural infection, there are no reports of laminitis associated with the infection. At present, prevention is limited to tick control.