UTIs are caused by changes in a host’s immune defenses that allow microbes to adhere and persist within the urinary tract. In in vitro and in vivo studies, cranberries have been shown to affect bacterial adhesion to cells in the urinary tract wall. 

In the in vivo portion of this study, 12 client-owned dogs that had at least 3 UTIs in the previous year were divided into 2 groups. One group received cephalexin for 2 weeks; the other received powdered cranberry extract daily for 6 months. No dogs developed UTIs during the 6-month evaluation period, and no side effects were noted. 

In the in vitro portion, the bacteriostatic effects of cranberry extract and the impact on adherence of Escherichia coli to Madin-Darby canine kidney (MDCK) cells was tested on urine from 6 healthy dogs receiving cranberry extract for 60 days. After incubation with urine samples obtained at 30 and 60 days, mean E coli adherence to MDCK cells was significantly lower than adherence after incubation with the urine sample obtained before cranberry extract administration. The bacteriostasis assay showed no zone of inhibition around any of the canine urine samples. The results suggest cranberry extract does not inhibit bacterial growth but does decrease E coli adherence to MDCK cells, and that it therefore has potential clinical benefit in UTI prevention.

An exciting alternative to chronic antibiotic therapy, cranberry extract can help fight multidrug-resistant bacteria development in dogs with recurrent E coli UTIs. It is important to evaluate the literature behind a suggested cranberry product, as some are purported to have superior efficacy. Also, it may be prudent to prescribe cranberry extract only to dogs with recurrent E coli infections; there is a lack of evidence suggesting it will prevent all types of UTIs. As with all supplements, due diligence in finding the appropriate product is critical for success, as regulatory control is lacking.—Heather Troyer, DVM, DABVP, CVA, CVPP