What do we know about reptiles and antibiotics? Not nearly enough.

That's the word from Mads F. Bertelsen, DVM, DVSc, DACZM, DECZM, the chief veterinarian at the Copenhagen Zoo. Dr. Bertelsen spoke on ""Long-Acting Antibiotics in Reptiles -- What Works and What Doesn't" as part of the Association of Reptile and Amphibian Veterinarians (ARAV) track at the North American Veterinary Conference (NAVC) today in Orlando, Fla.

He focused on long-acting antibiotics because they typically can be administered at greater intervals, which can help avoid stressing a sick animal. Unfortunately, he said, what we do know about antibiotics and reptiles is limited, and what we may know about a particular species usually can't be extrapolated even to closely related species, let alone from an animal like a sea turtle to a snake to a lizard.

"Even in tortoises and turtles, there can as much as 10 times the difference in the half-life of a drug," he said.

Long-acting antibiotics also increase the risk of developing a drug-resistant infection. That's because, as the drug wears off over time, levels in the body gradually decrease from those that will inhibit the bacteria to those that no longer will. But because the drug is still present in sub-therapeutic levels, it puts pressure on the bacteria to develop resistance to it.

This is not a concern only in reptiles, Dr. Bertelsen said, but in all species.

One such drug, cefovecin (Convenia) is widely used in veterinary practice, including in exotic medicine. "We use it at the zoo," Dr. Bertelsen said, "and it's a good drug for many birds and reptiles."

The problem, he said, is a lack of documentation of efficacy and dosage across different species. What data there is demonstrates how wildly the drug's half-life can vary: In the Patagonian Sea Lion, the half-life of cefovecin is 13 days, which is considered exceptional. In the Green Iguana, it's 3.9 hours, in a chicken, it's 52 minutes, and in a Ball Python, it's 64 hours.

Dr. Bertelsen reviewed four categories of drug: The fluoroquinolones, like enrofloxacin (Baytril), a drug frequently used on reptiles; the third-generation cephalosporins like cefovecin; the macrolides like azithromycin, a drug he thinks shows promise in treating a variety of species; and the tetracyclines, which he said have been studied fairly extensively on sea turtles but not other species of reptile.

Until there's a much larger body of research, he said most reptile veterinarians are probably operating under the 10-10-80 rule when it comes to choosing antibiotics for their patients: "In exotic medicine, 10 percent of the animals we make better, 10 percent we make worse, and in 80 percent, what we do makes no difference."

Photo: Dr. Mads Bertelsen at the North American Veterinary Conference