Lyme Disease
Basic Information 
Epidemiology
• No known breed, age, or sex predilection
• Seroprevalence ranges from less than 1% to 68% in endemic regions.
• Highest seroprevalence in mid-Atlantic and northeastern states, Minnesota, and Wisconsin (30%–40% of ticks are infected with B. burgdorferi)
• Rare positive horses in the Rocky Mountain states, North and South Dakota, and Nebraska (only 1%–3% of tick vector population are infected with B. burgdorferi)
• Occurs primarily in the spring, summer, and fall, with peak incidence in June and July in most endemic climates
• Not directly zoonotic; unknown if horses serve as source of infection for ticks
Clinical Presentation
• History of tick exposure or residing in a Lyme disease endemic area
• Vague signs of lameness or stiffness (generally in more than one limb)
• Lethargy, depression, decreased performance
• May observe muscle pain, hyperesthesia, or behavioral changes
• Polysynovitis usually with minimal joint effusion
• Possible muscle wasting and pain over thoracolumbar area with severe cases
Etiology and Pathophysiology
• The spirochete B. burgdorferi is the etiologic agent of Lyme disease.
• Prolonged attachment and feeding (>24 hours) of infected adult Ixodes spp. ticks transmits the organism.
• Invades the connective tissues, muscle, skin, and nerves and blood vessels near synovial membranes
• A lymphocytic plasmocytic reaction in tissue occurs in association with the organism.
• Unknown whether polysynovitis is the result of an immune-mediated reaction (immune complex deposition) or infection of the joint
Diagnosis 
Differential Diagnosis
• Arthritis: Trauma, osteoarthritis, other immune-mediated arthritis
• Neurologic signs: Equine herpes virus-1, equine protozoal encephalomyelitis, rabies, cervical vertebral malformation, cauda equine syndrome, West Nile virus, Eastern or Western equine encephalitis
• Distal limb edema: Purpura hemorrhagica, equine viral arteritis, equine infectious anemia, cellulitis, hypoproteinemia, Anaplasma phagocytophilum infection, or normal “stocking up”
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