Introduction

• A tick-borne, gram-negative obligatory intracellular Rickettsia bacterial infection of horses.
• Cause: Anaplasma phagocytophilum (formerly Ehrlichia equi) similar or identical to the causal agent of human granulocytic anaplasmosis (formerly human granulocytic ehrlichiosis) and closely related to the organism causing tick-borne fever in ruminants.
• Signs: fever, which later in infection may be fluctuating, mild depression, inappetence, icterus and limb edema.
• Diagnosis: identification of the organisms in granulocytes.
• Treatment: intravenous oxytetracycline (6.6-7 mg/kg SID) for 7 days.
• Prognosis: excellent if treatment started early; guarded if secondary infections develop.

Pathogenesis

Etiology

• A. phagocytophilum Anaplasma phagocytophilum (formerly E. equi).

Predisposing factors

General

• Immunologic status of the host.
• Stress including malnutrition and pregnancy.
• Virulence of the strain of the organism.

Specific

• Infestation with infected Ixodes spp ticks.

Pathophysiology

• A tick-transmitted infection in which granulocytes are parasitized by the rickettsial bacteria A. phagocytophilum.
• A. phagocytophilum is an obligate intracellular bacteria which multiplies inside cytoplasmic vacuoles in granulocytes, neutrophils, eosinophils and monocytes.
• The bacteria is able to evade the bacteriocidal effects of the granulocytes. Up to 30% of granulocytes may be infected.
• Bacteremia occurs 4-7 days after the tick bite. The primary site of replication is not certain, but may include the lungs and spleen. Bacteremia is accompanied by an initial high fever (>41°C/105.8°F). 
• Vasculitis and interstitial inflammation develops affecting various organs which contributes to the clinical manifestations:
  • Behavior: lethargy.
  • Hemopoietic system: lymphadenopathy, edema, mucosal hemorrhages, hemolytic anemia.
  • Musculoskeletal system: limb edema, lameness, reluctance to move.
  • Nervous system: ataxia.
  • Cardiac system: myocarditis, arrhythmias.
• Thrombocytopenia appears to result from bone marrow suppression of platelet maturation or release and the presence of anti-platelet antibodies.
• Concurrent infections may be exacerbated during the period of leukopenia.
• Chronically infected animals may show more vague signs.

Timecourse

• Incubation period ranges from 8-25 days following experimental inoculation and is believed to be less than 14 days following natural exposure.
• Clinical signs evolve over several days and last for 7-14 days - initially there is 1-3 days high fever and depression followed by 7 days of lower grade fever and progression to jaundice, edema of the limbs, formation of petechiae, ataxia and reluctance to move.
• A secondary fever may occur 2-4 weeks after the first.

Epidemiology

• A. phagocytophilum is transmitted by species of Ixodes ticks including Ixodes pacificus (Pacific coast), I. scapularis (primary vector in eastern and mid-western USA) andI. ricinusin Europe. The bacteria can survive in host ticks for over a year.
• Free-living rodents, white-tailed deer, grey squirrels, white-footed moose and some species of bird have been shown to act as reservoir hosts.
• Occurrence tends to be seasonal, related to tick activity.
• In endemic areas many horses appear to develop subclinical infections.
• Cell-mediated and humoral immune responses develop following natural infection and recovery.
• A carrier state does not appear to occur in horses, although has been reported in sheep.

Diagnosis

Treatment

Prevention

Outcomes