Efficacy of silver-coated urinary catheters for reducing urinary tract infection in dogs

Indwelling urinary catheters are frequently used in practice, however urinary catheters have been associated with bacteriuria and catheter-associated urinary tract infections (UTIs) in dogs (Tissot et al, 2001; Ogeer-Gyles et al, 2006). Antimicrobial coating of urinary catheters can reduce catheter-associated UTIs through the initial prevention of bacterial attachment (Beattie and Taylor, 2011). Silver has antibacterial properties, with historical studies identifying its benefit in reducing bacteriuria in humans (Liedberg and Lunderberg, 1990; Riley et al, 1995).

This knowledge summary uses a PICO (Population, Intervention, Comparator, and Outcomes) to investigate the question: in hospitalised dogs, does the use of silver-coated urinary catheters reduce the incidence of UTI when compared to silicone Foley urinary catheters?

A clinical scenario from the perspective of a veterinary nurse was considered and a search of the literature was performed (Table 1) based on this scenario. Exclusion and inclusion criteria are discussed in Tables 2 and 3. Three papers were critically reviewed. Two were prospective, double-blind, randomised controlled clinical trials and one was a controlled, in vitro study.

Clinical scenario

You are a small animal registered veterinary nurse working in a referral practice. Your practice has seen a high incidence of UTIs in canine patients with indwelling urinary catheters. Recognising that UTIs can hinder patient recovery and increase mortality and morbidity, you are concerned that urinary catheter care in your practice may not be effective. Therefore, you decide to investigate the effectiveness of silver-coated urinary catheters as an implementation that could reduce the risk of UTIs in the catheterised patient.

Strength of evidence

Critical appraisal of the selected papers meeting the inclusion criteria found that they provide moderate evidence in terms of their experimental design and implementation.

The evidence

Two prospective, double-blind, randomised controlled clinical trials compared the efficacy of silver-coated Foley urinary catheters in reducing the incidence of UTIs as compared to a silicone Foley urinary catheter (Ogilvie et al, 2018; Akcam et al, 2019). The study conducted by Akcam et al (2019) (Table 4) evaluated the effectiveness of silver-coated Foley urinary catheters against bacteriuria in human patients admitted to the intensive care unit (ICU), requiring catheterisation for >24 hours. Similarly, the study conducted by Ogilvie et al (2018) (Table 5) evaluated the effectiveness of silver-coated Foley urinary catheters on the incidence of bacteriuria and UTIs in canine patients requiring a urinary catheter for >24 hours. The third study (Table 6) evaluated the impact of silver coating of UCs on the adherence of E. coli in vitro (Ogilvie et al, 2015); however, the clinical application of these findings is limited by the in vitro study design, suggesting that the numerous factors that contribute to the complex environment of urine and the urinary bladder, such as periodic voiding and immune response, were not taken into account (Desai et al, 2010).

Summary of evidence

UTI detection

Urine culture is gold standard for UTI detection in humans and canines (Chu and Lowder, 2018). Akcam et al (2019) and Ogilvie et al (2018) confirm bacteriuria in their human and canine population, respectively, with a culture exam. The culture exams differ between studies, with Akcam et al (2019) not outlining the urine collection procedure. The handling of urine samples can affect culture results, suggesting that Akcam et al's (2019) results must be considered with caution due to ambiguity surrounding possible sample contamination (Padilla et al, 1981). Chu and Lowder (2018) argue that the process for sample collection is irrelevant if it is sterile and standardised, reducing the likelihood of contamination.

Urine samples were analysed immediately after collection, confirmed by Patterson et al (2016) as providing optimum results; however, the term ‘immediate’ is non-specific (Ogilvie et al, 2018; Akcam et al, 2019). The distance between the laboratory and practice could cause the time surpassed between sample collection and culture exam to differ between studies. The storing of urine samples collected out of hours by Ogilvie et al (2018) increased the processing time by 12–36 hours, with Patterson et al (2016) identifying the unreliability of urine samples stored incorrectly for 24 hours. It would have been beneficial for the authors to specify the processing time of the sample.

Antimicrobial treatment

Antimicrobial use can increase the UTI risk in dogs (Bubenik et al, 2007). When studied in vitro, silver-coating reduced the risk of UTIs, a finding that was not mirrored by Akcam et al (2019) and Ogilvie et al (2018). This discrepancy could be owed to Akcam et al (2019) and Ogilvie et al (2018) including patients receiving antimicrobials in their population. Ogilvie et al (2018) concluded that antimicrobial use did not increase UTI risk, contradicting Bubenik et al (2007). This could be due to the majority of patients receiving antimicrobials, leaving too few dogs untreated to determine a significant association. Contrarily, as the majority of dogs received antimicrobials, their influence on the development of a UTI could be uniform for both catheter groups. Although identified as a confounding factor, it is common for patients undergoing surgery to be administered prophylactic antimicrobials, highlighting the applicability to the hospitalised canine (Bubenik et al, 2007).

The antimicrobial used was not disclosed by Ogilvie et al (2018), and it is unclear whether urine samples were subject to antimicrobial sensitivity testing. Differing antimicrobials could justify the discrepancy between studies, as the effect on UTI development differs based on the antimicrobial used (Olin and Bartges, 2015). To reduce the impact of confounding factors and improve reliability of future studies, all dogs receiving antimicrobials should be excluded. However, the applicability to practice due to the varied requirements for antimicrobials in the hospitalised patient would be reduced. This highlights the relevance of Akcam et al (2019) and Ogilvie et al's (2018) findings to clinical practice, despite the variation in antimicrobials.

Duration of catheterisation

The duration of catheterisation can impact the UTI risk in both humans and dogs (Bubenik and Hosgood, 2008). Akcam et al (2019) and Ogilvie et al (2018) identified a significant association between the duration of catheterisation and increased bacteriuria, regardless of the urinary catheter used. Canine studies have demonstrated an increased UTI risk associated with catheterisation for over 3 days, with a 27% increase in risk for each additional day (Smarick et al, 2004; Bubenik and Hosgood, 2008). Ogeer-Gyles et al (2006) identified a similar trend specific to catheter-associated UTIs, where infection rose from 19% following catheterisation for 12 hours to 79% after 72 hours. Ogilvie et al (2018) found the association between catheterisation length and catheter-associated UTI to not be significant; however, their median catheterisation length was 48 hours, compared to 72 hours in Ogeer-Gyles et al's (2006) study. Smarick et al (2004) define catheter-associated UTI as a UTI in canine patients catheterised for greater than 24 hours, highlighting the relevance of Ogilvie et al's (2018) findings to practice.

Ogilvie et al (2015) identified reduced bacterial adherence on silver-coated urinary catheters, with fewer bacteria identified after 72 hours. The in vitro study design differs to the complex environment of the urinary bladder, with bacterial adherence inevitably differing in the laboratory setting, highlighting the requirement for in vivo evaluation of silver-coated urinary catheters on canine UTIs.

Uropathogen

E. coli is the prevailing bacteria present in canine and human UTIs; however, it is not the sole causative urophathogen (Stiffler et al, 2006; Chuang and Tambyah, 2021). Akcam et al (2019) identified higher rates of uropathogens other than E. coli in patients with silver-coated urinary catheters. Kędziora et al (2021) identify E. coli as having greater sensitivity to silver ions, explaining why Ogilvie et al (2015), having only tested the efficacy of silver-coated urinary catheters on E. coli isolates, found there to be a significant impact on bacterial adherence. The conduction of a further in vitro study comparing the sensitivity of different uropathogens to silver could assess their effectiveness in the canine patient, where E. coli is often not the sole causative uropathogen (Chuang and Tambyah, 2021).

Ogilvie et al (2018) omit to identify the uropathogens isolated, however the previous identification of E. coli as the primary isolate in urinary catheters suggests that it was likely the causative uropathogen in their study (Chuang and Tambayah, 2021). The identification of uropathogens by Ogilvie et al (2018) could have compared the effectiveness of silvercoating against E. coli and other uropathogens.

Sample size

Sample size is imperative in determining whether a significant difference between treatments can be detected (Handler and Boninger, 2014). During the planning stage, a sample size power calculation should be performed, as a too small sample size can result in reduced validity, while a too large sample size is unnecessary and unethical (Handler and Boninger, 2014). Akcam et al (2019) and Ogilvie et al (2015) performed a sample size power calculation, however Ogilvie et al (2015) does not report the results and thus cannot be externally verified. The robustness should be considered alongside sample size when determining the validity of a study; however, when being applied to a large population such as canine patients with urinary catheters, studies with a small sample size should be handled with caution.

All three studies identify sample size as a limitation (Ogilvie et al, 2015; Ogilvie et al, 2018; Akcam et al, 2019). The sample size was smaller than that calculated to be required to detect a significant difference, thus contributing to differences being statistically insignificant. This highlights the requirement for a larger study to determine whether silver-coated urinary catheters reduce the UTI risk in dogs.

Application

The frequency of UTIs in dogs with urinary catheters high-lights the importance of reducing this risk to improve patient care. Although not avoidable, a significant proportion of UTIs are preventable (Pratt et al, 2001). Silver-coated urinary catheters have proved to reduce UTIs in humans, however veterinary studies are limited. Where results are extrapolated from human studies, their applicability to the veterinary setting must be evaluated. Akcam et al's (2019) study design in humans is comparable to that of Ogilvie et al (2018) on a canine population. The complex environment of the urinary tract and the development of UTIs occurs analogously in both species. Having identified E. coli as the predominant causative bacteria for UTIs in humans and dogs, with a historic study by Low et al (1988) confirming the E. coli strains isolated from human and canine UTIs as identical, the effect of silver-coated urinary catheters is expected to be comparable in both species. This highlights the applicability of Akcam et al's (2019) study to the veterinary setting.

Urinary catheter placement and maintenance may differ between human and veterinary practice, with an increased risk of patient interference and contamination in dogs (Bubenik et al, 2007). Weese et al (2019) identify urinary catheter maintenance as a risk factor for UTIs. Akcam et al (2019) and Ogilvie et al (2018) omit to outline urinary catheter maintenance, however as conducted in a teaching hospital, it can be presumed that evidence-based veterinary medicine was practiced by Ogilvie et al (2018), where protocols are commonly extrapolated from human medicine. This suggests that urinary catheter maintenance was comparable between the studies, highlighting the applicability of Akcam et al's (2019) study to the canine population.

Ogilvie et al (2015) studied E. coli isolates from canine urine in vitro, investigating the adherence of E. coli to silver-coated urinary catheters. Bacterial adherence in the canine urinary tract is influenced by numerous factors, such as periodic voiding, immune response and attachment to the bladder wall (Desai et al, 2010). This creates a complex evironment that is impossible to replicate in vitro, limiting the applicability of the findings to the canine patient. It provides evidence supportive of the use of silver-coated urinary catheters, however the conduction of an in vivo study is necessary to justify their use in practice.

Conclusions

Overall, Ogilvie et al (2015) demonstrate moderate evidence that silver-coating reduces the adherence of E. coli to urinary catheters, however the in vitro nature limits its applicability to the canine patient. The complexity of the urinary system is made apparent by Akcam et al (2019) and Ogilvie et al (2018) who identify the ineffectiveness of silver-coating in reducing UTIs in humans and dogs. All studies concluded with caution because of study design or lack of statistical significance. No study was able to demonstrate that the use of silver-coated urinary catheters is superior to silicone urinary catheters in reducing the incidence of UTIs in dogs.

The available evidence does not support the hypothesis that silver-coated urinary catheters reduce the incidence of UTIs compared to silicone Foley urinary catheters (Ogilvie et al, 2015; Ogilvie et al, 2018; Akcam et al, 2019). Due to the limited evidence supporting the effectiveness of the antibacterial properties of silver-coated urinary catheters and the increased cost associated with them, their use in practice is currently not justifiable.

Given that all three studies were weakened by limitations, a further study conducted in hospitalised dogs not undergoing antimicrobial treatment, using a larger population size and standardised maintenance of silver-coated catheters, would be valuable.

KEY POINTS

• Indwelling urinary catheters are frequently used in practice, however urinary catheters have been associated with bacteriuria and catheter-associated UTIs in dogs.
• The frequency of UTIs in dogs with urinary catheters highlights the importance of reducing this risk to improve patient care.
• The available evidence does not support the hypothesis that silver-coated urinary catheters reduce the incidence of UTIs compared to silicone Foley urinary catheters.