Table 3.
Minimum inhibitory concentrations of antimicrobials for Borrelia burgdorferi and comments regarding use in horses with Lyme disease.
| Drug | Dosage (reference) | MICa (µg/mL) for B. burgdorferi (reference) | Comments |
|---|---|---|---|
| Amikacin | 32–>128 (Hunfeld 2006) | NOT recommended (not effective against Borrelia) | |
| Amoxicillin |
0.05–0.39 (Kim 2006) 0.03–2 (Hunfeld 2006) |
NOT recommended for adult horses (low oral bioavailability Ensink 1992) | |
| Azithromycin | 0.003–0.03 (Hunfeld 2006) | NOT recommended for adult horses (risk of colitis) | |
| Cefotaxime | 25 mg/kg IV q6h (Orsini 2004) |
≤ 0.125 (Ates 2010) 0.01–1 (Hunfeld 2006) |
Expensive drug; higher dosages (eg, 50 mg/kg IV q 6 h) might be more effective for neuroborreliosis |
| Ceftiofurb |
Ceftiofur sodium 2.2 mg/kg IV q12h Ceftiofur crystalline free acid (CFA) 6.6 mg/kg IM Day 1, 4, then q7d |
< 0.04–0.08 (Caol 2017) | Serum ceftiofur and desfuroylceftiofur (DCA) combined concentrations remain >0.22 μg/mL throughout CFA administration at 1, 4, 7 days and weekly. Tissue concentrations of DCA in the uterus were maintained between 0.1 and 0.2 μg/g. (Scofield 2014) Concentrations in other tissues not reported. |
| Ceftriaxone | 25–50 mg/kg IV q12h (Ringger 1996) |
0.03 (Ates 2010) <0.01–0.125 (Hunfeld 2006) |
CAUTION: reported to cause life‐threatening gastrointestinal disease and anaphylaxis in some adult horses |
| Chloramphenicol | 50 mg/kg PO q6h | 1.25–2 (Hunfeld 2006) | No data on clinical use for borreliosis |
| Doxycyclineb | 10 mg/kg PO q12h (Bryant 2000) |
0.125–0.25 (Ates 2010) 0.06–2 (Hunfeld 2006) |
Commonly used for Lyme borreliosis in humans and horses. Peak synovial fluid concentrations can be similar or greater than serum concentrations. (Maher 2014, Schnabel 2010) Results in low or undetectable levels in the CSF or ocular fluids of healthy adult horses. (Bryant 2000) (Gilmour 2005) |
| Enrofloxacin | 12.5–50.0 (Kim 2006) | NOT recommended (not effective against Borrelia) | |
| Erythromycin | <0.007–1 (Hunfeld 2006) | NOT recommended for adult horses (risk of colitis) | |
| Metronidazole | 15–25 mg/kg PO q6–8h |
0.06–32 (Sapi 2011) 0.25–0.50 (Cao1 2017) |
No data on clinical use for equine borreliosis; theoretically more effective than other drugs at reducing round body (cystic) forms but clinical relevance uncertain. Poor in vitro efficacy against motile B. burgdorferi. |
| Minocyclineb | 4 mg/kg PO q12h (Schnabel 2012) |
0.03–1 (Hunfeld 2006) 0.4–0.8 (Caol 2017) |
Superior aqueous humor and CSF penetration to doxycycline. Mean CSF concentration is 69% of corresponding peak plasma concentration. Aqueous concentration was 0.9 μg/mL in healthy horse eyes. (Schnabel 2012) Reported trough synovial fluid concentrations of minocycline in horses were 0.33 μg/mL or less. |
| Penicillin Gb | 22,000–44,000 IU/kg IV q6h | 0.03–8 (Hunfeld 2006) | Highest dosage recommended for neuroborreliosis |
| Tetracyclineb | Oxytetracycline 5.0–6.6 mg/kg IV q12 or 24h (Brown 1981, Dowling 2000) |
0.25 (Ates 2010) 0.01–20 (Hunfeld 2006) |
IV oxytetracycline often used for Borrelia infection in horses. Oral tetracycline/oxytetracycline NOT recommended. |
| Tilmicosin | ≤0.01 (Kim 2006) | NOT recommended in horses—fatalities reported after injection; no data on clinical use for equine borreliosis | |
| Trimethoprim/sulfa‐methoxazole | 25 mg/kg PO q12h | >256 (Baradaran‐Dilmaghani, 1996) | NOT recommended for treatment of motile B. burgdorferi. Sulfonamides may have some activity against B. burgdorferi persisters (Feng 2014) |
a
Measurements of MIC can serve as a treatment guide but in vitro results cannot be directly applied to in vivo situations because of differences in pharmacokinetics and dynamics of each drug must be considered along with the immune responses of the patient.
b
Antibiotics with MIC values <1 μg/mL against B. burgdorferi and having good safety data for use in adult horses.