Evidence Update- Cranberry Extracts and Urinary Tract Infections in Dogs

I’ve written in the past about the subject of cranberry extracts used to prevent or treat urinary tract infections. (1, 2) There is some in vitro and lab animal research that suggests these products might reduce the ability of a limited set of bacteria (fimbriated E. coli) to stick to the lining of the urinary tract, and might thus have some value in preventing, though not treating, urinary tract infections. However, the latest Cochrane systematic review on the subject did not find convincing evidence these products actually are effective in UTI prevention in human patients. There is little research in veterinary patients, and unfortunately a new study in the American journal of Veterinary Research (AJVR) doesn’t add much information to help in deciding whether or not such products are useful.

Hsin-I Chou, Kuan-Sheng Chen, Hsien-Chi Wang, Wei-Ming Lee. Effects of cranberry extract on prevention of urinary tract infection in dogs and on adhesion of Escherichia coli to Madin-Darby canine kidney cells. American Journal of Veterinary Research Apr 2016, Vol. 77, No. 4, Pages 421-427.

This study consisted of two parts. The first involved treating dogs with a history of frequent urinary tract infections with either an antibiotic or a cranberry supplement and monitoring them for development of UTIs. The other involved mixing E. coli and canine kidney cells with urine from dogs given the cranberry supplement to see if there was any antibacterial effect or any impact on the adhesion of the bacteria to the kidney cells.

In the first part of the study, 12 dogs were split into two groups (it’s not clear if this was done randomly or not). These were dogs who had previously had recurrent UTIs, at least three times in the previous year. Six of the dogs were given a 2-week course of an antibiotic, and the other six were given a daily cranberry supplement for 6 months. All dogs had urine samples taken to look for infection once a month. None of the dogs developed a UTI during the 6-month monitoring period.

In the in vitro portion of the study, the urine from dogs given the cranberry supplement did not appear to have any inhibitory effects on bacterial growth. It did, however, seem to reduce the adhesion of these bacteria to canine kidney cells.

The design of this study is a little strange. There was no report of blinding or placebo control, and it is unclear if the subjects were randomly assigned to the two groups, so the risk of bias is difficult to assess. The strange thing, though, is that the comparison was between the cranberry supplement and a short course of an antibiotic rather than a placebo supplement. One would not expect a 2-week course of antibiotic to protect dogs from a urinary tract infection for 6 months. If resolving recurring UTIs were this easy, they would never happen since everyone would do this.

So in having no UTIs, the cranberry supplement group was no different from a group that received a treatment one wouldn’t expect to work, but which wasn’t a true placebo. It is very difficult to interpret this kind of comparison, but it certainly isn’t appropriate to say, it proves the supplement prevents infections, as the authors do. It is also not evidence that the supplement was safer than the short course of antibiotics, which the authors also claim. The lack of UTIs in all dogs may have been due to unexpected and dramatic effects from the antibiotics and the cranberry supplement which occurred despite these sorts of effects not having been seen in past studies. However, this may also reflect some other change in how these dogs were managed or simply the natural course of their disease. The purpose of controlled research is to help separate these possible causes, but this particular comparison doesn’t effectively accomplish that.

The second part of the experiment was consistent with past research showing that cranberry products do not inhibit bacterial growth but may reduce adhesion to urinary tract cells to some extent. Since kidney cells were used, it isn’t clear whether this effect would apply to cells in the bladder, where most urinary tract infections occur, or in actual living patients.

Bottom Line
The in vitro portion of this study is consistent with existing research that suggests cranberry extracts may reduce the ability of some bacteria to stick to the lining of the urinary tract. This could theoretically help prevent some urinary tract infections, though clinical research in human patients suggests this doesn’t really work to a significant extent in living people.

The portion of the study looking at prevention of UTIs in actual dogs, unfortunately, doesn’t help establish what benefit, if any, this product might have. No infections occurred in any of the dogs regardless of the treatment they got, and the comparison was between 6months of daily use of the cranberry product and a 2-week course of an antibiotic, which seems unlikely to effectively prevent UTIs for 6 months. This raises questions about whether either of the treatments were really responsible for the lack of UTIs in the dogs or if this was the result of the Hawthorne effect or some other source of error.

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4 Responses to Evidence Update- Cranberry Extracts and Urinary Tract Infections in Dogs

  1. L says:

    My 15 year old small breed had emergency surgery to remove several calcium oxalate stones at age 11, about a year prior to that he was treated for struvite stones/UTIs.

    The follow up visits became expensive and I was told he might be to old to tolerate another surgery. Since then I feed him 4 small meals per day soaked with water and water added. Take him out for frequent bathroom breaks and monitor his urinary habits. I have not had him x-rayed or checked his PH, he did go in for an annual recently and his bloodwork was good. The vet said as long as he is not having any symptoms, no need for x-rays and PH testing.
    I also give him a potassium citrate/cranberry supplement twice a day. So far so good (4 years stone/UTI free)
    I suspect the increase in H2O and frequent opportunities to urinate is doing the most good.
    I hesitate to stop the supplements in case they are helping, I have cut down on the amount though.

  2. Jan says:

    The 2-month course of antibiotics seems to be in error (Clinician’s Brief Capsule for July 2016?). The actual study abstract indicates both the antibiotic and the extract were given for 6 months.

  3. skeptvet says:

    This is the description of the experiment from the Methods section:

    Dogs were allocated into
    2 groups (6 dogs/group). One group comprised 4
    Schnauzers and 2 Toy Poodles (3 spayed and 3 sexually
    intact; age of the dogs ranged from 7 to 14 years
    [mean, 9.8 years], and body weight ranged from 2.8
    to 7.4 kg [mean, 5.6 kg]). These dogs received cephalexina
    (20 mg/kg, PO, q 12 h for 14 days). The second
    group comprised 4 Schnauzers and 2 Chihuahuas (4
    spayed and 2 sexually intact; age of the dogs ranged
    from 5 to 12 years [mean, 8.0 years], and body weight
    ranged from 2.5 to 7.3 kg [mean, 5.3 kg]). Dogs in the
    second group received powdered cranberry extractb
    daily for 6 months.

  4. Jan says:

    I guess I misread this statement in the abstract (I didn’t access the full study): “12 dogs with a history of recurrent UTI received an antimicrobial (n = 6) or cranberry extract (6) orally for 6 months.” I took it to mean each group received their “treatment” for 6 months. Thanks for the clarification.

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