Regulatory bodies sometimes issue “Dear Veterinarian” (or similar) notices about important issues pertaining to drugs and rug use, like the recent FDA’s letter to veterinarians about extralabel use of fenbendazole in dogs.  These notices are used to get important information out to veterinarians, to try to head of future problems.

My topic du jour doesn’t deal with a drug per se, but it’s an important issue that’s outside of any regulatory control. So I’m issuing my own “Dear Veterinarian” letter about the antibiotic nitrofurantoin. It’s not about risks from the drug itself, but rather the risks from how veterinarians use it – largely because of poor information received from diagnostic laboratories.

FOR IMMEDIATE RELEASE (from the desk of JS Weese)

June 17, 2025

Dear Veterinarian:

Nitrofurantoin is an antibiotic that can be very effective for treatment of lower urinary tract infections (e.g. bacterial cystitis) in dogs and cats. However, it only achieves useful drug levels in urine. Therefore, it is not useful for treatment of infections outside of the bladder.

Standard practice is for diagnostic laboratories to not report nitrofurantoin susceptibility for non-urine samples. Unfortunately, they often do anyway. This can lead to inappropriate use of this drug, with potentially serious consequences for the animal.

Sometimes, this occurs because veterinarians fail to indicate what specimen they’ve submitted for culture (e.g. urine vs wound swab). This highlights the importance of veterinarians providing  laboratories with basic specimen information and patient history so the lab can 1) determine whether to report nitrofurantoin susceptibility and 2) determine which breakpoints to use for reporting susceptibility to other antimicrobials (since breakpoints for a few drugs vary between urine and non-urine).

However, all too often laboratories still report nitrofurantoin susceptibility in specimens that are clearly non-urine. This can lead to use of nitrofurantoin by veterinarians for conditions where there is no potential for the drug to be effective, such as wound infections. This can lead to a delay in providing appropriate treatment, progression of disease, and in some situations, death.

While is it the responsibility of veterinarians to understand the drugs that they use, inappropriate reporting of nitrofurantoin susceptibility for specimens not associated with lower urinary tract disease is a major contributing factor to the inappropriate use of this drug that needs to be avoided. Veterinarians need to be aware of this issue to avoid unnecessary animal disease and death.

Sincerely,

Signed: Someone with no regulatory power but who’s tired of dealing with patients that have been harmed by misuse of this drug.

PS: Diagnostic laboratories, if you can’t figure out how to selectively report nitrofurantoin susceptibility only for lower urinary tract specimens, can you please at least put a ‘*’ with the “S” to note that this only applies to treating cystitis?

I’d love to see labs come out with a policy that “if we report something wrong, the test is free.” That seems reasonable to me, and it would provide an impetus for labs to do better at reporting and motivate veterinarians to better understand testing and results, and encourage reporting of issues to labs.

Laboratories aren’t perfect, and neither are their tests. While we’d like them to be perfect, that’s a very high bar. That said, there is some low hanging fruit that would certainly get us closer: things like better nitrofurantoin susceptibility reporting are straightforward and would save some lives.