Anaplasmosis, Lyme Disease and EPM

Lyme Disease, Anaplasmosis, and EPM: How to Tell the Difference in Horses

Neurologic signs, unexplained lameness, fever, or a sudden change in performance can be alarming for any horse owner. Several diseases can present with overlapping symptoms, but three that are commonly discussed—and often confused—are Lyme disease, anaplasmosis, and equine protozoal myeloencephalitis (EPM).

This article explains the key differences between these conditions, how they are transmitted, what signs to watch for, and why an accurate diagnosis is essential for effective treatment.

At a Glance: Key Differences

Lyme Disease in Horses

What Is It?

Lyme disease is caused by the bacterium Borrelia burgdorferi and transmitted through the bite of an infected tick, most commonly the deer tick.

Common Signs

• Intermittent or shifting lameness

• Stiffness or poor performance

• Muscle soreness

• Behavioral changes

• Sensitivity to touch

• Neurologic signs in some cases

Many horses may test positive for Lyme exposure without showing clinical disease, which can complicate diagnosis.

Diagnosis and Treatment

Diagnosis involves a combination of blood testing, clinical signs, and response to treatment. Antibiotics are the primary treatment, but duration and protocol depend on disease severity. Complimentary therapies such as acupuncture and some Chinese herbs can help treat lingering affects of Lyme Disease.

Anaplasmosis in Horses

What Is It?

Anaplasmosis is a tick-borne bacterial infection caused by Anaplasma phagocytophilum that targets white blood cells.

Common Signs

• Sudden onset fever

• Lethargy

• Limb swelling

• Inappetence

• Ataxia or stiffness

• Low platelet counts on bloodwork

Unlike Lyme disease, anaplasmosis typically causes acute illness, and horses often appear obviously sick.

Diagnosis and Treatment

Diagnosis is based on CBC changes, PCR testing, and clinical signs. Horses usually respond rapidly to appropriate antibiotics, often improving within 24–48 hours. They also rarely exhibit lasting effects of the disease.

EPM (Equine Protozoal Myeloencephalitis)

What Is It?

EPM is a neurologic disease caused by protozoal organisms, most commonly Sarcocystis neurona. Horses become infected through ingestion of feed or water contaminated with opossum feces.

Common Signs

• Incoordination or weakness

• Muscle atrophy

• Asymmetric gait abnormalities

• Cranial nerve deficits

• Difficulty backing or turning

• Behavioral changes

EPM signs often progress over time and may vary widely depending on the area of the nervous system affected.

Diagnosis and Treatment

Diagnosis relies on neurologic examination and specific testing, often including cerebrospinal fluid (CSF) analysis. Treatment involves long-term antiprotozoal medication and supportive care. Paired blood and CSF samples are the only way to definitively diagnose EPM. Many presumed cases are not actually EPM, so an accurate test can save the client time and money on costly treatments.

Why These Diseases Are Often Confused

These conditions share overlapping signs such as lameness, stiffness, or neurologic changes. Seasonal timing and exposure history help narrow the possibilities, but no single symptom definitively identifies the disease.

Veterinary evaluation and targeted diagnostics are critical before starting treatment. Using medications off-label or treating for EPM without proper workup can delay correct diagnosis and put your horse at risk. Thoughtful testing ensures the treatment prescribed truly addresses the underlying cause rather than just perceived performance issues.

When Should You Call Your Veterinarian?

Contact your veterinarian promptly if your horse shows:

• Fever during tick season

• Sudden limb swelling or lethargy

• Unexplained or shifting lameness

• Any neurologic abnormalities

• Rapid changes in performance or attitude

Early diagnosis improves outcomes and prevents unnecessary or ineffective treatments.

Prevention and Risk Reduction

• Perform regular tick checks during warm months

• Use veterinarian-recommended tick control products

• Manage feed storage to discourage wildlife access

• Maintain clean feeding and watering areas

• Seek veterinary advice promptly for concerning signs

Key Takeaway

Lyme disease, anaplasmosis, and EPM are very different diseases that can look similar early on. Accurate diagnosis is essential to ensure your horse receives the right treatment at the right time. Avoid the temptation to treat based solely on perceived performance issues or popular trends; careful evaluation protects your horse’s health and long-term performance.

If your horse is showing signs that concern you, our team is happy to help determine the cause and guide the next steps in care.

Clear diagnosis, early treatment, and informed prevention are the best tools we have to protect horses from these complex diseases.