Urinary tract infections (UTIs) in dogs most commonly result from ascending bacterial infection of the lower urinary tract. Female dogs are affected more frequently than males, and typical clinical signs include pollakiuria, dysuria, stranguria, hematuria, inappropriate urination, and excessive licking of the perineal area (1). In many cases, UTIs are uncomplicated and respond well to appropriate antimicrobial therapy. However, a subset of dogs experience recurrent or relapsing infections that can be challenging to manage.
What Are Recurrent UTIs?
Recurrent UTIs are generally defined as two or more infections within six months or three or more infections within one year. These cases are commonly categorized as (2):
- Reinfections: new infections caused by different bacterial species or strains
- Relapsing infections: recurrence of infection with the same bacterial strain, typically within six months of treatment
Relapsing infections often prompt investigation into underlying causes such as anatomic abnormalities, urolithiasis, endocrinopathies, neoplasia, chronic prostatitis, or inappropriate antimicrobial selection or duration. In addition to these well-recognized factors, emerging evidence suggests that bacterial persistence mechanisms—such as biofilm formation or intracellular survival—may also contribute in some cases (3).
Biofilms and Their Potential Role in Persistent UTIs
Biofilms are structured communities of bacteria encased within a self-produced extracellular matrix. This matrix can protect bacteria from host immune defenses and reduce susceptibility to antimicrobial agents. In the context of UTIs, biofilms have been demonstrated in vitro and in association with indwelling devices, and they are well recognized in human urology.
In dogs, uropathogenic Escherichia coli (UPEC) is the most frequently implicated organism in recurrent UTIs and has demonstrated the ability to form biofilms under experimental conditions (3,4). Within biofilms, bacteria may adopt a metabolically less active state and intermittently shed planktonic organisms. This behavior may contribute to treatment failure or recurrence and may be associated with intermittently negative or low-count urine cultures, particularly in dogs recently treated with antibiotics.
It is important to note, however, that the prevalence and clinical significance of biofilms on the canine urothelium in naturally occurring UTIs remain incompletely defined, and biofilm formation should be considered one of several possible contributors rather than a universal explanation for recurrence.
Intracellular Bacterial Communities: Emerging Evidence
In addition to biofilms, UPEC has been shown in murine and human models to invade urothelial cells and form intracellular bacterial communities (IBCs) that can persist in a dormant state. These intracellular reservoirs are protected from host immune responses and from antimicrobials with poor intracellular penetration.
Recent experimental work using a canine urothelial cell model demonstrated that UPEC can form IBC-like structures in canine cells, supporting biological plausibility for this mechanism in dogs (5). As infected urothelial cells exfoliate and regenerate, dormant bacteria may reactivate and potentially contribute to relapse.
At present, however, the role of IBCs in naturally occurring canine UTIs remains an area of active investigation. Their true prevalence, diagnostic significance, and impact on clinical outcomes in dogs have not yet been fully established.
Diagnostic Considerations in Recurrent and Relapsing UTIs
Urinalysis and quantitative urine culture remain the cornerstones of UTI diagnosis and antimicrobial stewardship in veterinary medicine (6). In most cases of recurrent disease, appropriately collected and timed cultures will still yield clinically actionable results.
That said, bacteria associated with biofilms or intracellular reservoirs may not be continuously shed into urine, particularly during or shortly after antimicrobial therapy. In select cases, this may contribute to discordance between clinical signs and culture results, complicating diagnosis and treatment decisions. These limitations highlight the importance of interpreting diagnostic findings in the context of clinical history, treatment response, and underlying risk factors.
The Potential Role of Next-Generation Sequencing (NGS)
Next-generation sequencing (NGS)–based diagnostics, such as MiDOG’s test, offer a culture-independent approach to microbial detection by identifying bacterial DNA present in a urine sample. Because this method does not rely on bacterial growth, it may detect organisms present at low abundance or in samples collected during antimicrobial therapy.
NGS testing may provide adjunctive information in select cases of persistent or relapsing UTIs where standard diagnostics have repeatedly failed to identify a causative organism. However, detection of bacterial DNA does not necessarily confirm organism viability or clinical relevance, and results must be interpreted cautiously. At present, NGS should be viewed as a complementary tool rather than a replacement for urine culture, susceptibility testing, and thorough evaluation for underlying disease.
Implications for Treatment and Case Management
Repeated empiric or broad-spectrum antimicrobial therapy without clear identification of the underlying cause can contribute to antimicrobial resistance and ongoing recurrence. Effective management of relapsing UTIs requires:
- Careful distinction between reinfection and relapse
- Investigation and correction of predisposing conditions
- Judicious antimicrobial selection based on culture and susceptibility when possible
- Appropriate treatment duration and follow-up monitoring
In cases where bacterial persistence mechanisms are suspected, treatment strategies should be individualized and grounded in antimicrobial stewardship principles, recognizing the current limitations of available evidence.
Conclusion
Biofilms and intracellular bacterial survival represent biologically plausible and increasingly studied mechanisms that may contribute to recurrent or relapsing UTIs in some dogs. While emerging research has expanded our understanding of these processes, their clinical significance in canine patients is not yet fully defined. A balanced, evidence-based approach that integrates standard diagnostics, careful case evaluation, and emerging tools when appropriate remains essential for optimizing outcomes in dogs with challenging UTIs.
References:
[1] Teh, H. (2022). A review of the current concepts in canine urinary tract infections. Australian veterinary journal, 100(1-2), 56-62.
[2] Foster, JD (2020) Urinary Tract Infections in Dogs. https://todaysveterinarypractice.com/urology-renal-medicine/urinary-tract-infections-in-dogs/#:~:text=Patients%20with%20recurrent%20UTIs%20often,the%20most%20likely%20risk%20factors.
[3] Kern, Z. T., Jacob, M. E., Gilbertie, J. M., Vaden, S. L., & Lyle, S. K. (2018). Characteristics of dogs with biofilm‐forming Escherichia coli urinary tract infections. Journal of Veterinary Internal Medicine, 32(5), 1645-1651.
[4] Rodrigues, I. C., Ribeiro-Almeida, M., Ribeiro, J., Martins, L. L., Prata, J. C., Pista, Â., & Costa, P. M. Biofilm formation by urinary E. coli: the critical role of glucose in persistent urinary tract infections. In RESUMOS EM LIVRO DE ATAS DE CONGRESSO (p. 42).
[5] Gilbertie, J. M., Sheahan, B. J., Vaden, S. L., & Jacob, M. E. (2025). Canine urothelial cell model to study intracellular bacterial community development by uropathogenic Escherichia coli. PloS one, 20(1), e0316834.
[6] Bartges, J. W. (2004). Diagnosis of urinary tract infections. Veterinary Clinics: Small Animal Practice, 34(4), 923-933.