There are several reasons why the Lyme disease may not be responding to
the doxycycline (which is a tetracycline). First and foremost
tetracyclines bind avidly to divalent metals such as calcium and
magnesium. For this reason, they MUST be given on an empty stomach. In
horses, which are continuous grazers this is virtually impossible since
they always have food in the intestines (which is where the drug is
absorbed from, not really from the stomach). If the food source is high
in calcium and magnesium, such as sweet feed or alfalfa, none of the drug
will be absorbed into the blood stream.
Secondly, bacterial resistance is common to tetracycline especially during
prolonged therapy.
Thirdly, tetracycline is the recommended first drug of choice only during
Stage 1 Lyme Disease, which is when the rash appears surrounding the tick
bite. For Stage 2 Lyme Disease, which is the development of arthritis,
neurological and cardiac problems, the first drug of choice in humans is
Ceftriaxone (brand name Rocephin) which is a third generation
cephalosporin (related to penicillins) and is very expensive. The second
drug of choice for Stage 2 is good old penicillin. CHEAP and readily
availble to all veterinarians. Tetracyclines are considered 3rd
alternative for Stage 2 and are usually used when the patient has a
penicillin allergy.
Discuss all this with your veterinarian. My expertise is in human
medicine, not equine, however the offending organism and its drug
sensitivity and resistance are the same. I also echo the recommendation
to contact Cornell University Veterinary School. This is simply meant to
give you additional information to discuss.
Lynn
L. M. Crespo, Ph.D.
Associate Professor and Chair
Department of Pharmacology
Nova-Southeastern University
lynn@hpd.acast.nova.edu