Equine granulocytic anaplasmosis

Kristiina Ruotsalo

Animal Health Laboratory, University of Guelph, Guelph ON.

AHL Newsletter 2021;25(1):15.

A 16-year-old quarter horse gelding presented to the referring veterinarian with a 5 day history of lethargy, inappetence, and weakness.  Pyrexia was identified during physical examination, feces were normal, and no proprioceptive deficits were identified.  Samples for a comprehensive CBC, equine biochemistry profile, fecal flotation, and equine herpes virus 1 PCR were submitted to the Animal Health Laboratory.  Herpes virus PCR was negative and fecal flotation revealed the presence of both strongyles and Anoplocephala perfoliata.

The CBC demonstrated a marked thrombocytopenia of 38 x109/L.  A moderate lymphopenia was identified, however, remaining leukocyte and erythrocyte values were within reference intervals.  Review of the peripheral blood smear demonstrated the presence of one or more loose aggregates of blue-gray to dark blue coccoid to coccobacillary organisms within the cytoplasm of numerous neutrophils.  These aggregates were consistent with morulae of Anaplasma phagocytophilum bacteria, the causative agent of equine granulocytic anaplasmosis (Fig. 1).  A duplex quantitative PCR for both A. phagocytophilum and Borrelia burgdorferi was performed by the AHL molecular biology lab using EDTA blood; a strong positive result for A. phagocytophilum was obtained, and B. burgdorferi was negative.

Concurrent significant changes in the serum biochemistry profile included a marked increase in serum amyloid A (6431 mg/L), along with alterations in electrolyte concentrations.  This patient responded well to antibiotic treatment.  No follow-up laboratory data was available.

These laboratory findings were similar to those previously published (AHL Newsletter December 2015) which described A. phagocytophilum infection in an Appaloosa mare.  Interestingly, both horses were from eastern Ontario.  A 2017 OAHN (Ontario Animal Health Network) equine disease surveillance project evaluating the seroprevalence of B. burgdorferi and A. phagocytophilum in Ontario horses identified that 1% (5/551) of serum samples submitted from clinically healthy horses were positive for A. phagocytophilum antibodies utilizing a snap 4Dx ELISA test.  Two of these horses originated from eastern Ontario, whereas the remaining three were distributed throughout the province.

A. phagocytophilum, formerly known as Ehrlichia equi, is the causative agent of equine granulocytic anaplasmosis (EGA).  It is most likely transmitted in Ontario by the deer tick, Ixodes scapularis, which is also the tick that transmits B. burgdorferi, the causative agent of Lyme disease.  EGA occurs most commonly in the late fall, winter, and spring.  Horses are likely aberrant hosts, and do not appear to serve as reservoirs of A. phagocytophilum, as the presence of the organism is generally limited to the acute phase of disease.  Naturally infected horses appear to have persistent humoral immunity for at least two years.  The occurrence of subclinical infection in horses from endemic areas has been suggested.

EGA can have a variable presentation ranging from mild fever in young horses (<4 years of age), to more severe clinical signs including fever, limb edema, ataxia, jaundice and petechial hemorrhages secondary to vasculitis and thrombocytopenia.  Body effusions and myopathies have occasionally been reported.  Moderate to severe morbidity is occasionally seen with EGA, and occasional mortality has been reported.  The disease is often self-limiting, and clinical signs usually last 7-14 days.  Anemia, variable leukopenia and thrombocytopenia are usually identified with clinical cases.

The pathogenesis of EGA is poorly understood, however following inoculation by tick bites, the bacteria invade the hematopoietic and lymphoreticular systems.  Peripheral sequestration, consumption, and destruction of peripheral blood components are all proposed as mechanisms of cytopenias.

Diagnosis relies upon clinical awareness of geographic areas for infection, consistent clinical signs, and supportive laboratory changes.  Evaluation of a peripheral blood smear is imperative, although the number of granulocytes containing morulae may vary from 1% to 50% by day 3-5 of infection.  If the horse is severely neutropenic, evaluation of a buffy coat may be helpful.

PCR can confirm the clinical diagnosis and may be particularly helpful in both early and late stages of the disease when morulae are difficult to detect.  The AHL molecular biology lab offers a qPCR test for A. phagocytophilum (test code: lyPCR) which can be applied to whole blood, tissue, and ticks.   AHL

Figure 1. A neutrophil containing three A. phagocytophilum morulae.

Figure 1. A neutrophil containing three A. phagocytophilum morulae.

1. Arroyo M et al. Seroprevalence of Borrelia burgdorferi and Anaplasma phagocytophilum in Ontario horses. Ontario Animal Health Network, Equine Expert Network Quarterly Report 2017(10).
2. Ruotsalo K et al. Equine granulocytic anaplasmosis (Anaplasma phagocytophilum). Animal Health Laboratory Newsletter  2015(19):43.
3. Pusterla N, Madigan J. Anaplasma phagocytophilum infection. In: Equine Infectious Diseases, 2nd ed. Sellon D and Long M, eds. Elsevier 2014:344-346.