Why Do Some Dogs Keep Getting Urinary Tract Infections?

If your dog has ever had a urinary tract infection (UTI), you know how uncomfortable it can be. For some dogs, though, UTIs keep coming back, even after treatment. Veterinarians have long debated why this happens. Is each new infection caused by a new strain of bacteria, or does the same bacteria stick around and flare up again later?

A new multicenter study helps answer this question by closely examining recurrent Escherichia coli (E. coli) infections in dogs across the United States.

What the Researchers Wanted to Find Out

E. coli is the most common cause of UTIs in dogs. When infections recur, there are two main possibilities:

• Reinfection, where a dog picks up a new strain of E. coli after treatment
• Persistence, where the original strain is never fully eliminated and continues to cause problems

In everyday veterinary practice, it is very difficult to tell which of these is happening. The researchers wanted to know which scenario is more common and whether certain bacterial traits make recurrence more likely.

How the Study Worked

The research team collected 98 E. coli samples from dogs with repeated positive urine cultures. These samples came from five veterinary diagnostic laboratories located in different regions of the United States.

Using advanced genetic tools, the scientists compared bacteria from the same dog over time to see whether they were genetically identical or clearly different. They also looked for genes linked to bacterial virulence, meaning traits that help bacteria attach to and survive in the urinary tract.

The Key Finding: Often, It Is the Same Bacteria

The results were striking:

• About 62 percent of dogs with recurrent E. coli bacteriuria were infected repeatedly by the same strain of E. coli
• In some dogs, the same strain was detected over several months, in one case up to eight months
• A few dogs experienced both symptomatic infections and symptom-free bacteriuria caused by the exact same bacterial strain

These findings suggest that, in many cases, E. coli is not being fully cleared from the urinary tract. Instead, it may persist quietly and cause problems later.

Why Some Strains Stick Around

The study found that E. coli strains involved in persistent infections were more likely to carry P-fimbrial adhesin genes, specifically papA and papC. These genes help bacteria attach firmly to the lining of the urinary tract.

This ability to cling to bladder tissue may allow E. coli to hide from antibiotics and the immune system, creating a long-term reservoir inside the bladder.

Antibiotic Resistance Was Not the Main Issue

One surprising result was that persistent strains were not necessarily more resistant to antibiotics. Many were still susceptible to commonly used treatments when tested in the laboratory.

This suggests that treatment failure may not always be due to resistance, but instead due to where and how the bacteria survive in the body. Some bacteria may persist inside bladder cells or in biofilm-like communities that antibiotics cannot easily reach.

What This Means for Dog Owners and Veterinarians

This research helps explain why recurrent UTIs can be so frustrating to manage. Key takeaways include:

• Repeated infections are often caused by the same bacteria rather than new ones
• Common clinical clues, such as time between infections or changes in antibiotic resistance, are not reliable for telling persistence apart from reinfection
• Subclinical bacteriuria, where bacteria are present without obvious symptoms, may still matter because it can later develop into a clinical infection

The Bigger Picture

The findings suggest that the urinary bladder itself may act as a long-term hiding place for E. coli in some dogs. Similar patterns have already been observed in human medicine and experimental animal models.

The authors emphasize the need for future studies that follow dogs over time and combine clinical data with deeper genetic analysis. This approach could help veterinarians better understand how these bacteria persist and how best to eliminate them.

The full study is available as an open-access article in Veterinary Microbiology for readers who want to explore the research in more detail.