Equine anaplasmosis

Veterinary advice should be sought before applying any treatment or vaccine.

Equine Anaplasmosis

Equine Ehrlichiosis, Equine Granulocytic Ehrlichiosis

Equine anaplasmosis is a common, seasonal tick-transmitted disease of horses caused by Anaplasma phagocytophilum. The hallmark clinical sign associated with equine anaplasmosis is the development of a fever that may be mistaken for a viral infection.

Prior to 2012, A. phagocytophilum was classified as Ehrlichia phagocytophilum, E. equi and associated with the disease ehrlichiosis. A. phagocytophilum is endemic in North America, Brazil, Europe, and North Africa. The first case of equine anaplasmosis was described in California in 1969. Most disease incidents in horses coincide with peak adult tick activity, specifically high populations of Ixodes spp. tick species. In the United States, A. phagocytophilum is transmitted predomiantely by the black-legged tick (I. scapularis) in the northeast, midwest, and southeast regions, and the western black-legged tick (I. pacificus) on the west coast.

Clinical signs of equine anaplasmosis vary depending on the age and immune status of the horse and duration of the illness. Horses that are 4 years of age and younger have milder signs than horses of other age groups. During the early stages of the disease (within the first 1 to 2 days of infection), horses usually have a very high, fluctuating fever from 102.9 to 106°F (39.4°C to 41.3°C). Horses may also show signs of depression, reluctance to move and ataxia when asked to move. By days 3 to 5, horses may develop mild swelling of the lower limbs, weakness, and in some cases staggering often causing horses to fall. The course of the disease usually ranges from 3 to 16 days. Horses that are treated early will have a reduced disease course.

Common laboratory findings in horses with equine anaplasmosis are thrombopenia, leukocytosis or leucopenia, anemia and hyperbilirubinemia. A definite diagnosis is made by detection of typical inclusions (moruale) in neutrophil granulocytes in a Giemsa-stained blood smear and detection of A. phagocytophilum DNA from EDTA-blood by PCR. However, there are some inconsistencies associated with available serologic tests. The main issues with serologic tests for A. phagocytophilum are:
  • Tests may differ in their ability to differentiate “active infection” from the serologic response following the horse's exposure to a pathogen.
  • The timing of sample collection from the horse, with respect to pathogen exposure may affect test results.
  • Some serologic tests, specifically whole-cell assays, may be more prone to cross-reactivity.
Horses become infected through tick bites from usually Ixodes spp., which serve as vectors of A. phagocytophilum. Common reservoir hosts include small rodents such as white-footed mice, chipmunks, and voles, as well as ruminants, raccoons, foxes, white-tailed deer and wild birds. In California, dusky-footed wood rats, lizards, birds, cervids, and white-footed mice have been proposed as potential reservoirs.

Incubation Period
The incubation period after exposure from infected tick bites in experimental trials in horses ranges from 8 to 12 days. The incubation period for natural infections is thought to be less than two weeks.

Horses treated with oxytetracylin (the gold standard for treatment in horses with anaplasmosis) usually show a rapid recovery. Full recovery including neurological signs may take up to 3 weeks to subside.


High, fluctuating or intermittent fever (102.9-106.3°F)
Partial loss of appetite
Increased heart rate (50-60 beats/min)
Reluctance to move
Lower limb edema in all four legs
Poor body condition
Base-wide stance


  • Clinical signs
  • Geographical region
  • PCR - positive for Anaplasma phagocytophilum
  • Indirect fluorescent antibody (IFA) titer greater than 160

While waiting for your veterinarian

  • Keep horse hydrated: Horses with a high fever often will drink and eat less than normal. When horses become dehydrated they have an increased risk of colic. Therefore, it is important to encourage horses to keep drinking by using various strategies such as soaking hay, adding water to feed, dropping treats or pieces of apples in water buckets, etc.



Oxytetracycline7 mg/kg of body weight IV q24h for 5 to 7 daysN Pusterla et al; M Restifo et al; F Uehlinger et al.
Management of high feverFlunixin meglumine at 0.5 mg/kg IV once initiallyF Uehlinger et al.
Supportive therapyLeg wraps applied to all 4 limbs and stall confinement to help with limb edema, minimize risk of secondary injury, and to monitor body temperature
Fluid and electrolyte therapyMay be indicated for severe cases.


  • Application of tick repellents
  • Keep horse pastures mowed
  • Fence off pastures from white tailed deer
  • Minimize rodent populations
  • Check horses daily for the presence of ticks


Prognosis is usually excellent for uncomplicated cases.

Scientific Research

General Overviews

Risk Factors

  • Not keeping pastures mowed, as ticks are found more frequently on long grass
  • High populations of white tailed deer present in or near horse pastures.



Causative agent

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