Changes in Borrelia burgdorferi ELISA antibody over time in both antibiotic treated and untreated horses.

Changes in ELISA serology are frequently used to determine antibiotic treatment success for Lyme disease in horses. This concept was based upon a previous report showing a marked decline in ELISA values in experimentally infected and antibiotic-treated ponies. Changes in Lyme serology following antibiotic treatment in naturally infected horses have not been reported. The objective of this study was to compare Borrelia ELISA antibody concentrations in naturally exposed horses both before and following antibiotic treatment for Lyme disease. A retrospective study was performed comparing oxytetracycline- or doxycyclinetreated (n = 68) and untreated (n = 183) horses from a single equine practice and their change in Borrelia ELISA values over a similar time period. Antibiotictreated horses had a decline in ELISA values in comparison to control horses (P ≤ 0.05) and untreated horses were twice as likely to have their ELISA values increase (OR = 0.5; 95% C.I. = 0.3-0.9) compared to treated horses. The magnitude of the decline in ELISA units following treatments was small compared to that previously reported in experimentally infected and treated ponies. Field-exposed horses with high Borrelia burgdorferi ELISA values who are treated with either oxytetracycline or doxycycline can be expected to have only a small decline in ELISA values following treatment. Persistently high ELISA titres following appropriate treatments for Lyme disease may not, without appropriate clinical signs, be a reason for more prolonged treatment.