Equine Borreliosis (Lyme Disease) Study Guide

Overview and Clinical Importance

Equine borreliosis (Lyme disease) is a tick-borne infection caused by the spirochete bacterium Borrelia burgdorferi. While exposure to this organism is common in horses residing in endemic regions, clinical disease is considered rare. The distinction between exposure (seropositive) and clinical disease is crucial for the NAVLE. This condition represents a significant diagnostic challenge because clinical signs are vague and nonspecific, and positive serology only indicates exposure, not active disease.

The disease is primarily transmitted by Ixodes scapularis (blacklegged or deer tick) in the northeastern and midwestern United States and Ixodes pacificus on the West Coast. Understanding the documented clinical syndromes, appropriate diagnostic approach, and treatment protocols is essential for board examination success.

Etiology and Pathophysiology

Causative Agent

Borrelia burgdorferi is a Gram-negative spirochete bacterium characterized by its distinctive spiral or corkscrew shape. Key microbiological features include flagella enclosed between outer and inner membranes (endoflagella), a flexible cell wall measuring approximately 0.3 micrometers wide and 5-20 micrometers in length, and the ability to alter outer surface proteins (Osp) depending on host environment.

Transmission and Vector Biology

Pathogenesis

After transmission, B. burgdorferi enters the skin and undergoes a sophisticated process of upregulating and downregulating specific outer surface protein antigens. The spirochetes then migrate through the skin and connective tissue to joint and synovial membranes, meninges (brain and spinal cord coverings), and eyes. The most common histopathologic lesion is lymphohistiocytic and plasmacytic infiltrate in affected tissues.

Key pathogenic concepts: The disease severity is believed to be related to the host inflammatory response rather than direct damage by the spirochete. Antibodies can be detected 3-6 weeks after infection. Most infected horses remain asymptomatic and never develop clinical disease.

Clinical Presentation

Documented Clinical Syndromes

The ACVIM consensus statement identifies three well-documented clinical syndromes attributed to B. burgdorferi infection in horses. These are the only syndromes with strong scientific evidence linking them to Borrelia infection.

Undocumented but Possible Clinical Signs

The following signs are often attributed to equine Lyme disease but lack strong scientific documentation. Many other diseases are MORE LIKELY causes of these signs:

• Shifting leg lameness and joint swelling/effusion
• Muscle stiffness and soreness
• Low-grade fever
• Chronic weight loss and poor performance
• Behavioral changes (irritability, depression)
• Laminitis (rare association)

Exam Focus: Neuroborreliosis signs can mimic EPM (Equine Protozoal Myeloencephalitis), cervical vertebral stenotic myelopathy (wobblers), and EHV-1 myeloencephalopathy. These conditions must be ruled out before diagnosing Lyme disease. The key distinguishing feature of neuroborreliosis is meningeal thickening (dural thickening) seen on postmortem examination.

Diagnosis

Diagnostic Criteria

Lyme disease is a DIAGNOSIS OF EXCLUSION. Diagnosis requires ALL of the following criteria:

• Horse lives in or has traveled to an endemic region for Ixodes ticks
• Clinical signs compatible with documented Lyme disease syndromes
• Positive serologic test for B. burgdorferi antibodies
• Other causes of clinical signs have been RULED OUT

Serologic Testing

Lyme Multiplex Assay Interpretation

Diagnosing Neuroborreliosis

For horses with neurologic signs suspicious for neuroborreliosis:

• CSF analysis: Typically shows neutrophilic or lymphocytic pleocytosis
• Paired serum and CSF: Submit simultaneously for Lyme Multiplex; local antibody production in CNS supports diagnosis
• Rule out differentials: EPM, EHV-1, wobblers (cervical vertebral malformation), WNV
• Postmortem: Meningeal thickening (dural inflammation), lymphohistiocytic and plasmacytic infiltrates

Treatment

Antibiotic Therapy

Treatment should ONLY be initiated in horses with clinical signs compatible with Lyme disease, positive serology, AND after ruling out other causes. The ideal treatment regimen is unknown due to lack of controlled clinical trials.

Treatment Duration and Protocols

Recommended protocol: IV oxytetracycline for 7 days followed by oral doxycycline or minocycline for 3-6 weeks. Treatment duration is often longer in horses than humans because infection is typically present longer before diagnosis.

For neuroborreliosis: Consider parenteral beta-lactams (penicillin G, ceftriaxone, cefotaxime) that cross the blood-brain barrier, similar to human protocols. However, successful treatment outcomes remain rare.

Adjunctive Therapy

• NSAIDs: Flunixin meglumine or firocoxib for pain, lameness, or uveitis
• Dexamethasone: Consider in acute, severe neuroborreliosis or uveitis (controversial; mixed outcomes in humans)
• Joint support: Chondroprotective agents if joint involvement present

Prevention

Tick Control and Environmental Management

• Environmental modification: Mow pastures regularly, remove brush/leaf litter, eliminate rodent habitats
• Topical repellents: Permethrin-based products applied regularly
• Daily tick checks: Thorough grooming and inspection, especially ears, mane, tail base, and ventral areas
• Prompt tick removal: Remove ticks within 24 hours using fine-tipped tweezers; grasp close to skin and pull straight out

Vaccination

There is NO approved Lyme vaccine for horses. Off-label use of canine Lyme vaccines is practiced by some veterinarians in endemic areas. Studies suggest proper vaccination could prevent infection, but data on equine-specific efficacy is limited. Many horses show low and short-lasting antibody responses to canine vaccines.

Prognosis

Memory Aids

LYME = "Lots of Your tests Mean Exposure only" L - Lives in endemic region Y - Yes to compatible clinical signs M - Must have positive serology E - Exclude other diseases FIRST

The "NUP" Syndromes (Documented Equine Lyme Disease): N - Neuroborreliosis (POOR prognosis) U - Uveitis (POOR prognosis for vision) P - Pseudolymphoma (GOOD prognosis)

Tick Transmission Timeline: "24 to Transmit" Ticks must be attached for at least 24 hours to transmit B. burgdorferi. Daily tick checks can prevent infection!