DOMESTIC EQUINES · OWNER EDUCATION
Lyme Disease in Equines
Lyme disease is associated with infection by Borrelia bacteria transmitted by ticks. Horses can be exposed without becoming ill, so a positive antibody result is not proof that Lyme disease explains current signs.
THE SHORT VERSION
What matters for your animal?
Start with the warning signs and when-to-contact sections if your animal is unwell. The same outward change can have several causes, so this guide helps you prepare questions rather than identify a diagnosis on your own.
What is this condition?
Lyme disease is associated with infection by Borrelia bacteria transmitted by ticks. Horses can be exposed without becoming ill, so a positive antibody result is not proof that Lyme disease explains current signs.
A positive Lyme exposure result does not automatically explain every change in an equine's behavior or performance. Clinical signs and alternative causes must be evaluated alongside the test. This is particularly important for neurologic or eye problems, where prompt assessment is needed regardless of exposure history. Ask what supports active disease and how response to any proposed treatment will be assessed objectively.
Which species does this guide cover?
The species listed above are the focus of this guide, rather than a complete list of every animal that can develop this condition. Causes, testing and treatment may differ between species and individual patients.
Why can it happen?
- Tick habitat and regional exposure matter. Other causes of pain, neurologic changes and eye disease must be evaluated.
An infectious illness depends on both the organism and the animal's ability to resist it. Exposure does not always produce the same severity of illness in every animal. Age, previous immunity, nutrition, stress, and concurrent disease can influence the outcome. The infection may also cause complications that need treatment separately from the organism itself. For example, correcting fluid loss or supporting breathing can be essential even when a targeted medicine has already been started.
What might an owner notice?
- Possible signs include neurologic or eye abnormalities and other nonspecific changes. The clinical picture is more difficult to establish than exposure alone.
An eye can be painful even when there is little discharge. Holding it partly closed, avoiding light, rubbing the face, or becoming reluctant to move through familiar surroundings can be important clues. Surface injury, inflammation inside the eye, and abnormal pressure can look similar from outside. Looking at a photograph cannot replace examination of the structures and measurements that distinguish them. A sudden change in sight or a painful eye deserves prompt attention because some eye problems progress quickly.
How is the problem investigated?
History, examination and testing for competing diagnoses are combined with appropriately interpreted Lyme tests.
Molecular testing, such as PCR, looks for genetic material from a target organism in the submitted sample. It can be useful, but the meaning depends on the sample site, timing, and disease. Detecting material is not always the same as proving that the organism explains every sign, while a negative result may not exclude infection if sampling or timing was unsuitable. The veterinarian interprets the result alongside the examination and other findings rather than treating it as an isolated verdict.
A neurologic examination helps locate where a problem may be occurring, such as the brain, spinal cord, peripheral nerves, or balance system. The location then guides the choice of tests. Weakness caused by low blood sugar or another body-wide problem can resemble a primary neurologic disorder, so blood testing may be important even when movement is the main concern. Imaging can provide structural information, but it is selected in the context of the examination rather than ordered identically for every animal.
Blood-cell testing and a chemistry profile provide complementary information. Cell counts can help assess anemia, inflammation, and platelets; chemistry measurements help assess organs and internal balance. Results are interpreted against the species, examination, hydration, medication history, and previous values. A value just outside the reference interval is not automatically the cause of illness. Trends can be more useful than one result, particularly when the aim is to monitor a chronic problem or check that treatment remains safe.
What might treatment involve?
Treatment is veterinarian-directed when clinical evidence supports disease. An exposure result alone does not justify indefinite antibiotic treatment.
This is an overview, not a prescription or a recommendation for a specific individual animal.
What can owners do at home?
Keep a timeline of signs, travel and tick exposure and follow reassessment plans.
Avoid actions that could make care less safe.
- Do not attribute every behavioral or performance change to Lyme disease or use unprescribed long courses of antibiotics.
What can the longer-term outlook involve?
Outcome depends on whether Lyme disease is truly the cause and which organs are involved.
A recheck is not simply a repeat of the first appointment. It assesses whether the original problem is improving, whether complications are appearing, and whether the treatment remains appropriate. The useful measures may include comfort, function, intake, body weight, examination findings, or selected tests. Ask which changes are expected before the visit and which should trigger an earlier call. Improvement in one sign does not always mean that every part of the disease has resolved.
Can risk be reduced?
Tick control and pasture management reduce exposure. Discuss region-specific prevention with the veterinarian.
When should you arrange care?
- Seek urgent assessment for neurologic changes or a painful eye, regardless of whether Lyme exposure is known.
Seek urgent veterinary advice if you notice:
DO NOT DELAY URGENT CARE- Seek urgent assessment for new neurologic changes or a painful eye.
What information helps the care team?
Record which animals are affected, feeding or housing changes, recent arrivals, treatments and any losses. For a flock or herd problem, the number affected and the pattern over time may be as important as one animal's examination.
How should you understand the tests?
Ask what each proposed test is intended to answer. Some tests identify a cause; others assess severity, organ function or treatment safety. Results are interpreted with the examination and history. A normal result does not exclude every problem, and an abnormal result does not always explain every sign.
Questions to ask before leaving
- What is confirmed, and which other explanations are still being considered?
- What changes should we expect with the proposed plan, and when should we reassess if they do not occur?
- Which observations should I record, and which changes mean I should call sooner?
- What medicine or home-care problems should I report, and what are the instructions for a missed dose or suspected side effect?
- When is the recheck, what will it assess, and whom should I contact after hours?
A care plan may be adjusted as results or the response become clearer. Keep the written instructions and tell the team if cost, handling or administration difficulty makes the plan hard to follow.
Sources behind this guide
Lyme Disease (Lyme Borreliosis) in Horses - Horse Owners - Merck Veterinary ManualVeterinary, university or animal-health guidance ↗Content last reviewed: October 3, 2026. Review status: veterinary-informed; independent veterinarian review not yet completed.