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University of Minnesota Extension

Tick diseases in horses

Quick facts

Ticks can transfer many disease-causing organisms to horses. Minnesota horse owners should be aware of two of the most common tick diseases:

  • Anaplasmosis

  • Lyme disease

Preventing tick disease

There are no vaccines available for anaplasmosis or Lyme disease, but there are prevention steps you can take.

  • Remove and destroy ticks as soon as possible.

  • Check horses for early signs of illness.

  • Reduce tick habitat near horses.

    • Clear brush out of pastures and along both sides of the fence line.

    • Keep pastures mowed.

  • Use a topical insecticide that includes a label claim for ticks.

    • Apply it to your horse before riding through long grass or brush.


Anaplasmosis is the most common tick-transferred disease to cause illness in horses. Humans and other animal species, including dogs and livestock can get a similar illness from ticks.

Deer ticks commonly transfer the bacteria from small mammals (deer mice and woodrats).

In horses, signs of illness usually appear 10 to 45 days after infection. Limb swelling and small bleeds normally accompany fever. Small bleeds appear in the following areas:

  • Nose

  • Mouth

  • Eyes

  • Vulva

Less common signs include incoordination, muscle swelling or gut pain.

Clinical signs set the base for diagnoses in horses with possible tick exposure during warm weather. Your veterinarian may confirm the diagnosis by drawing blood.

If you leave the disease untreated it can cause death.

Antibiotic treatment is usually effective if you treat the horse shortly after the signs of illness begin.

Oxytetracycline is the antibiotic of choice to treat anaplasmosis. This can be given intravenously at least once daily for 5 to 10 days. Your veterinarian may recommend oral doxycycline either following initial oxytetracycline treatment or in cases where daily veterinary visits aren’t feasible.

Both antibiotics carry a slight risk of diarrhea. Relapses may occur if antibiotic treatment is too short. Other supportive care often used include nonsteroidal anti-inflammatory drugs and leg support wraps.

Lyme disease

Borrelia burgdorferi is the organism that causes Lyme disease and is a much less frequent source of illness in horses. Tick bites expose horses to this organism. Few horses develop clinical illness, usually months after the bite.

Possible diagnosis of Lyme disease often occurs after ruling out the more common causes of:

  • Lameness

  • Joint swelling

  • Kidney disease

  • Moon blindness

  • Incoordination

Your veterinarian will likely take two blood samples 2 to 3 weeks apart to check for any changes that show a sign of active infection. Your veterinarian can also diagnose the disease after taking a tissue sample from an affected joint or lymph node.

If your veterinarian finds evidence of Lyme disease, they may try the same antibiotics as for anaplasmosis to see if this improves your horse's signs of illness.

Author: Julie Wilson, DVM

Reviewed in 2023

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