05/26/2026
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One Tick. One Bite. One Very Sick Horse.
Anaplasmosis in horses is one of those diseases that can look terrifying at first — high fevers, swollen legs, jaundice, lethargy — but thankfully with prompt treatment, most horses recover very well.
Anaplasmosis is caused by a bacteria called Anaplasma phagocytophilum and is spread primarily through tick bites. Deer ticks (the same ticks associated with Lyme disease) are the main culprit. Horses become infected when an infected tick feeds on them and transfers the bacteria into the bloodstream. Cases are most common in spring and fall when ticks are especially active, but they can occur anytime ticks are present.
One important thing to understand is that there is typically an incubation period between the tick bite and when the horse actually starts showing symptoms. The incubation period for Anaplasmosis is usually around 6-12 days after exposure to an infected tick. This means horses can appear completely normal for days or even weeks before suddenly becoming sick. Because of this delay, owners often never actually see the tick responsible for the infection.
Once inside the body, the bacteria infect white blood cells and trigger a widespread inflammatory response. Symptoms can range from mild to severe depending on the horse’s age, immune system, and how quickly treatment is started.
Common symptoms include:
• Sudden high fever
• Depression or extreme lethargy
• Loss of appetite
• Reluctance to move or stiffness
• Swollen legs (especially hind legs)
• Ataxia/incoordination in more severe cases
• Yellowing of the gums or eyes (jaundice/icterus)
• Petechiae (small red pinpoint hemorrhages on gums)
• Increased heart rate
• Colic-like discomfort in some horses
Many horses look profoundly sick very quickly. Owners often describe them as going from “normal” to “seriously ill” in 24–48 hours.
Bloodwork is extremely helpful in diagnosing Anaplasmosis. Typical findings often include:
• Low platelet count (thrombocytopenia)
• Low white blood cell count
• Mild anemia
• Elevated inflammatory markers such as SAA or fibrinogen
• Elevated bilirubin causing jaundice
Veterinarians can sometimes actually see the bacteria inside white blood cells on a blood smear, but the most accurate diagnostic test is usually a PCR test run on blood. This detects the DNA of the bacteria and confirms infection.
The good news is that Anaplasmosis generally responds very well to treatment. The antibiotic of choice is usually oxytetracycline given intravenously. Many horses improve dramatically within 24–48 hours of starting treatment. In milder cases or after IV treatment, horses may also be placed on oral doxycycline.
Supportive care is also important and may include:
• Anti-inflammatories to control fever and discomfort
• Fluids if dehydrated
• Careful monitoring of appetite, hydration, and temperature
Recovery expectations are generally excellent when treatment is started early. Most horses recover fully within a couple of weeks, although some may take longer to regain full energy and muscle condition after being very sick. Severe untreated cases can occasionally become life-threatening, particularly in older horses or horses with complications, which is why early veterinary intervention is so important.
One important thing to know is that horses do not directly spread Anaplasmosis to each other. The disease is transmitted through ticks, not horse-to-horse contact.
The best prevention is good tick control:
• Daily tick checks
• Keeping pastures trimmed
• Reducing brush and wooded overgrowth
• Using veterinarian-recommended tick repellents or prevention products
• Monitoring horses closely during heavy tick seasons
If your horse suddenly develops a high fever, swollen legs, depression, or jaundice — especially during tick season — Anaplasmosis should absolutely be on the list of possibilities to discuss with your veterinarian. The earlier it’s caught, the smoother recovery usually is.
I’d love to hear of other symptoms you’ve experienced with your Anaplasmosis positive horses.
