The question of whether cranberries, in some form, have value in treating or preventing urinary tract infections (UTIs) is a pretty old one. Mark Crislip at Science-Based Medicine has written a nice summary of the topic, and he has found over 100 references dating back to 1962. Like most herbal remedies, it has traditionally been thought helpful for a wide variety of unrelated disorders, but it is now pretty firmly established in most people’s minds as useful for UTIs.
Unfortunately, as usual the subject is more complicated than is generally realized. The theoretical justification for using cranberry to treat UTIs used to be that it acidified urine and made it less hospitable for bacteria, however that is no longer believed to be true. The current theory is that chemicals called proanthocyanidins interfere with the attachment of bacteria to the bladder wall, making it easier for the body to eliminate these bacteria and harder for infections to get started.
This is certainly a plausible mechanism established by in vitro studies. The trouble is that the proanthocyanidins inhibit attachment only for E.coli with little hairs called fimbriae on them. There are many other bacteria that can cause UTIs, and there isn’t yet any evidence that proanthocyanidins affect these. And in humans fewer than 20% of E.coli in bladder infections have fimbriae, so theoretically, these chemicals should be only be useful in preventing recurrent infection in a small minority of cases. (Interestingly, the vast majority of E.coli from kidney infections are fimbriated, so cranberry could possibly be more useful in these cases). There is also the problem that no one has actually proven that oral cranberry leads to proanthocyanidins getting into the urine, or being biologically active when they get there.
As Dr. Crislip points out, though, the theoretical mechanisms are not so important if the remedy doesn’t actually work in clinical trials. So does it? Well, there is still no consensus, since some trials show and effect and others don’t. The best that can be said as of now is that cranberry probably isn’t useful for treating UTIS and it may or may not be useful for preventing them.
Despite this uncertainty, the popular belief that cranberry products are useful for urinary tract infections in humans makes it inevitable that such products will be marketed to pet owners for UTIs in dogs and cats. As in humans, many UTIs are caused by bacteria other than E.coli, and I am not aware of any research on the proportion of fimbriated E.coli in canine and feline UTIs, so the theoretical rationale for this remedy is even weaker than in humans. But a quick Google search shows plenty of veterinary versions on the market anyway, often with pretty confident claims.
I recently came across some marketing materials for one of these, Crananidin from Nutramax. They are careful to avoid any treatment or prevention claims, since that would violate the Dietary Supplement Health and Education Act (DSHEA). However, they try pretty hard to suggest a benefit and even superiority over their competitors despite the absence of any clinical trial evidence.
The Nutramax literature has a nifty little graph of bioactivity in the dog purporting to show that “by 7 days the average bioactivity is over 78% at inhibiting E.coli.” They cite two articles to support this statement. One is an NMR study of the molecular structure of proanthocyanidin, and the only bioactivity data is from mixing fimbriated E.coli with human red blood cells and some cell-surface-receptor coated plastic beads in vitro (interestingly, this study was funded by Ocean Spray). There is no testing or discussion of clinical effects, and no data concerning dog urine. There really seems no way to interpret this citation other than being deliberately misleading.
The other reference is “Data on file, Nutramax Laboratories,” so I presume it is an unpublished in-house experiment. Unpublished data from a company selling a product that “proves” the product works, and is better than the competition, ought to be viewed somewhat skeptically, needless to say.
The marketing literature also has a little chart showing their product “passing” a test of bioactivity and all the competing products failing it. The references for this are the same, and since the published article has nothing to do with this issue, apparently the only evidence for this claim is unpublished in-house data from Nutramax. I’m not aware of any published clinical trials investigating this or any other cranberry product for use in UTIs in dogs or cats.
There also isn’t any evidence concerning the safety of cranberry products in dogs and cats. In humans, one concern is that cranberry juice has a lot of salicylic acid, so people with aspirin allergies or on anti-coagulant medications aren’t supposed to take a lot of it. But those are pretty rare issues in veterinary medicine. And some studies have shown it increases oxalate in the urine by up to 43%, so I wouldn’t recommend it for patients with a history of oxalate urinary tract stones (even though, again, this data is for people, and there are no studies I can find in dogs or cats).
There is weak theoretical justification for using cranberry products for UTIs, though none of the supporting preclinical evidence involves dogs or cats. There is conflicting clinical trial evidence in humans, and no clinical studies in dogs and cats. There are weak theoretical safety concerns. And, of course, there is abundant marketing making bold statements unsupported by the little evidence that exists. You pays your money and you takes your chances. Fortunately (for Nutramax, if not for our pets), anecdotes claiming a benefit are easy to find, so there should be little trouble selling the products even without convincing evidence of safety or efficacy.