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
- 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.
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.