Urinary catheterization is indicated for various medical conditions in veterinary species. Urinary catheters may increase the risk of bacterial contamination and colonization of the lower urinary tract via contamination of the urine collection system, allowing bacteria to gain access to the urinary tract through the catheter lumen or via catheter contact with the patient's perineal tissues, resulting in bacteria migrating along the outer surface of the catheter or within the mucous layer of the urethra.1
In dogs specifically, urinary catheterization has been significantly associated with hospital-acquired UTIs or bacteriuria.2,a In a veterinary referral hospital intensive care unit, the overall frequency of CAUTI development in dogs was reportedly 19% (26/137) but increased to 79% (11/14) for those that remained catheterized for 72 hours.3 In another study,4 the incidence of CAUTI development was 10% (4/39), and the probability of remaining UTI free by 4 days of catheterization was 63%. However, no attempt was made to distinguish bacteriuria from UTI in those studies.
Various strategies have been used in human medicine to reduce the incidence of CAUTI, including improvements in catheter care, maintenance of closed urine collection systems, and addition of antibacterial coatings to catheter surfaces.5,6 Silver has antibacterial properties, and meta-analyses7 and systematic reviews8,9 comparing silver-coated urinary catheters with standard urinary catheters have revealed a beneficial effect of silver alloy in reducing the incidence of CAB in humans. An in vitro study10 showed that silver-coated silicone urinary catheters had significantly less adherent canine urinary Escherichia coli at 24, 48, and 72 hours of placement in a bacterial broth and subjectively less biofilm formation than non–silver-coated urinary catheters. We are unaware of any reported studies in which silver-coated urinary catheters have been evaluated in dogs. The purpose of the study reported here was to determine the effects of a silver-coated versus non–silver-coated silicone urinary catheter on the incidence of CAB and CAUTI in dogs requiring urinary bladder catheterization for ≥ 24 hours.
Materials and Methods
Dogs
Any female dog admitted to the Ontario Veterinary College teaching hospital between May 26, 2012, and January 22, 2014, that required urinary catheterization for an expected period of ≥ 24 hours was eligible for inclusion in the study. Male dogs that weighed ≤ 8 to 10 kg (17.6 to 22 lb) were also included, but larger male dogs were excluded because of limitations in catheter length. Any dog that had a documented lower UTI (positive bacterial culture result with concurrent clinical signs of UTI or cytologic evidence of pyuria) at the time of catheterization was excluded from the study, as was any dog that required catheterization solely for surgery purposes or was expected to be catheterized for < 24 hours. Owner consent was obtained for all dogs. The study protocol was approved by the Ontario Veterinary College Animal Care Committee.
Procedures
Data were collected for each dog regarding signalment, reason for catheterization, and antimicrobial treatments administered (drug, dose, and duration). Eligible dogs were randomly assigned via a randomly generated table to receive a silver-coated Foley urinary catheterb (silver-coated catheter) or the same type of silicone Foley urinary catheter without the silver coatingc (control catheter). The silver-coated catheters had been extraluminally coated by the manufacturer with ionic silver complexed with an amino acid and dye. Catheters were available in 6F, 8F, or 10F sizes and were all 31 cm long; each catheter diameter was randomly assigned separately. All catheters were placed by an experienced veterinary technician or the attending veterinarian following a standardized aseptic catheterization protocol.
During catheter placement, data were collected regarding any breaks in aseptic technique and the number of placement attempts. A urine sample for bacterial culture and cytologic examination was collected at the time of catheter placement (time 0) and then daily while the dog was catheterized. Dogs remained enrolled and catheterized for 7 days unless catheter removal occurred earlier (because it was no longer deemed necessary for patient management) or because the catheter needed to be replaced (eg, because of blockage or removal by patient).
Sample collection and handling
Urine samples were aseptically collected via sterile syringe from the catheter end at the time of placement and subsequently from the aseptically prepared sampling port of the collection system. Each urine sample was injected into 2 plain evacuated tubesd (1 tube for cytologic examination and 1 for bacterial culture) through the aseptically prepared top via a sterile needle affixed to the syringe. Samples for cytologic examination were prepared within 15 minutes after collection by centrifugation of 200 μL of urine in the collection tube at 400 × g for 5 minutes. The supernatant was discarded, the pellet was resuspended in the remaining 20 to 50 μL of fluid, and a feather-edge smear was prepared on a glass slide. The slide was air-dried, stained with Wright stain, and evaluated via light microscopy by the same observer (WRG), who was unaware of the type of catheter the associated dog had received. This observer evaluated each sample for cytologic evidence of pyuria, hematuria, and microorganisms.
When collected during working hours, urine samples for bacterial culture were submitted for testing immediately after collection. Samples collected after hours on weekdays or on weekends were placed in a preservative medium,e refrigerated at 4°C, and processed within 12 to 36 hours after collection. For bacterial culture, 0.01- and 0.001-mL loops were used to transfer a small amount of each urine sample onto blood agar plates for quantitative analysis. A sterile swab was used to apply urine to MacConkey agar plates. Plates were then incubated at 35°C, with blood agar plates incubated in 5% CO2 and MacConkey agar plates incubated in ambient atmospheric conditions. Plates were checked for growth after 24 and 48 hours of incubation. Bacterial colonies were identified by means of matrix-assisted laser desorption-ionization–time-of-flight mass spectrometry or biochemical tests in accordance with standard operating procedures.
Outcomes
Dogs were considered to have a UTI if they had positive results of urine bacterial culture and concurrent clinical signs (eg, dysuria, pollakiuria, stranguria, hematuria, or malodorous urine) or cytologic evidence (pyuria) of UTI at any time during the study. If the dog had a UTI at time 0, it was removed from the study. Dogs were considered to have developed a CAUTI when the aforementioned UTI criteria were met after catheterization for at least 24 hours. Once a UTI was confirmed, the affected dog was treated as determined by the attending clinician and no further study data were collected.
Dogs were considered to have bacteriuria only (and not UTI) when bacteria were cultured from (ie, culture-detected bacteriuria) or cytologically identified (ie, cytologically detected bacteriuria) in a urine sample, but no clinical or cytologic (pyuria) evidence of UTI was detected at that time point. Dogs were considered to have CAB when both urine bacterial culture and cytologic results were initially negative, but urine samples collected after ≥ 24 hours of catheterization yielded positive results for either test (and dogs still had no clinical or cytologic evidence of UTI).
Statistical analyses
Statistical softwaref was used for all statistical analyses. Assessed outcome variables included cytologically detected bacteriuria, culture-detected bacteriuria, overall CAB, and CAUTI. Logistic regression was performed to evaluate whether the exposure characteristics age, sex, catheter type, duration of catheterization, and antimicrobial administration were associated with a positive bacterial culture result, bacteriuria, and UTI. When data were normally distributed, the Student t test was used to detect differences in means between catheter groups. When data were not normally distributed, the Wilcoxon rank sum test was used to detect differences in medians. The χ2 test was used to compare proportions of dogs in each catheter group by sex and antimicrobial use. Values of P < 0.05 were considered significant.
Results
Dogs
Forty-four dogs were enrolled in the study. For 6 dogs, the urinary catheter was in place for < 24 hours, so these dogs were excluded from analysis. An additional dog had documented contamination of the urinary catheter (detachment from the collection system), and another had evidence of UTI (ie, pyuria and bacteriuria identified via cytologic examination and bacterial culture) at the time of catheter placement (time 0); both dogs were also excluded. Therefore, 36 dogs were included in the analysis.
Fifteen included dogs received a silver-coated catheter and 21 received a control catheter, for a median duration of 48 hours (range, 24 to 144 hours). Characteristics of dogs in each group were summarized (Table 1).
Summary data for dogs requiring urinary bladder catheterization for ≥ 24 hours that received a silver-coated (n = 15) or non–silver-coated (21; control) silicone Foley urinary catheter.
| Variable | Control catheter | Silver-coated catheter | P value |
|---|---|---|---|
| Age (y) | 4.6 (1–14) | 3.6 (0.5–13) | 0.41 |
| Body weight (kg) | 9.9 (3.5–26) | 9.2 (3–38) | 0.76 |
| Sex | 0.08 | ||
| Female* | 12 (57) | 7 (87) | — |
| Male | 9 (43) | 2 (13) | — |
| Antimicrobial treatment during catheterization | 0.50 | ||
| Yes | 11 (52) | 10 (67) | — |
| No | 10 (48) | 5 (33) | — |
| Duration of catheterization (h) | 48 (24–120) | 48 (24–144) | 0.41 |
Data for age, body weight, and duration of catheterization represent median (range); data for sex and antimicrobial treatment represent number (%) of dogs in each group. Dogs were catheterized for a maximum of 7 days.
Ten females that received control catheters were spayed, and 2 were sexually intact. For dogs that received silver-coated catheters, these numbers were 7 and 6, respectively.
— = Not applicable.
Outcomes
Only female dogs developed cytologically detected bacteriuria (6/25 [24%]), culture-detected bacteriuria (4/25 [16%]), CAB (6/25 [24%]), and CAUTI (4/25 [16%]) during the study period. Three dogs (2 with a silver-coated catheter and 1 with a control catheter) had a positive bacterial culture result at time 0 with no other evidence of UTI; only one of these dogs (with a control catheter) went on to develop a CAUTI as defined.
Treatment group comparisons
No significant differences in evaluated exposure characteristics were identified between the 2 catheter groups (Table 1). Four of the 15 (27%) dogs with a silver-coated catheter and 2 of the 21 (10%) dogs with a control catheter developed cytologically detected bacteriuria during the study period (P = 0.37). No significant difference (P = 0.93) was identified between these groups in median time to development of cytologically detected bacteriuria (48 hours for silver-coated catheters and 60 hours for control catheters; Supplementary Table S1, available at avmajournals.avma.org/doi/suppl/10.2460/javma.253.10.1289).
Positive results of urine bacterial culture (excluding those positive at time 0) were obtained for 2 of 13 (15%) dogs with a silver-coated catheter and 2 of 20 (10%) dogs with a control catheter (P = 1.00). Median time to development of culture-detected bacteriuria (36 hours and 96 hours, respectively) did not differ significantly (P = 0.33) between these groups.
Excluding culture-positive dogs at time 0, 4 of 13 (31%) dogs with a silver-coated catheter and 2 of 20 (10%) dogs with a control catheter met the definition of having developed CAB during the study period (P = 0.18). Median time to development of CAB (36 and 96 hours, respectively) differed significantly (P = 0.03) between these 2 groups.
Overall, 2 of 15 (13%) dogs with a silver-coated catheter and 2 of 21 (10%) dogs with a control catheter met the definition of having developed a CAUTI during the study period (P = 1.00). Median time to development of CAUTI (60 and 72 hours, respectively) did not differ significantly (P = 0.83) between these 2 groups.
Associations with outcome for both groups combined
Overall, CAUTI developed in 3 of 21 (14%) dogs that received antimicrobials while catheterized, compared with 1 of 15 (7%) dogs that received no antimicrobials (P = 0.63). Excluding dogs that had a positive bacterial culture result at time 0, CAB (positive cytologic or culture result but without evidence of CAUTI) developed in 4 of 19 dogs (21%) that received antimicrobials and 2 of 14 (14%) dogs that received no antimicrobials (P = 0.68).
Logistic regression revealed that as age increased, so did the incidence of subclinical culture-detected bacteriuria (OR, 1.28; 95% CI, 1.00 to 1.75; P = 0.047), such that for every 1-year increase in dog age, the odds of developing culture-detected bacteriuria increased by 28%. No association with age was identified for the incidences of cytologically detected bacteriuria (P = 0.27), CAB (P = 0.32), or CAUTI (P = 0.13). Duration of catheterization was not significantly associated with the incidence of cytologically detected bacteriuria (P = 0.72) or CAUTI (P = 0.53) but was significantly associated with the incidences of culture-detected bacteruria (OR, 1.04; 95% CI, 1.01 to infinity; P = 0.03) and CAB (OR, 1.04; 95% CI, 1.003 to infinity; P = 0.04). No association with sex was identified for any outcome (P ≥ 0.15).
Discussion
Results of the present study suggested that there may not be a significant benefit to the use of silver-coated silicone urinary catheters for short-term (median of 48 hours) urinary bladder catheterization in dogs. The incidence of CAB development during the study period was 17% (6/36), which is higher than previously reported for catheterized dogs (9.8%).11 This discrepancy could have been related to different patient characteristics or methods, such as the use of urine cytologic examination and bacterial culture to diagnose bacteriuria in our study versus different culture-based criteria in the other study.11 Antimicrobial treatment; different standard procedures for urine collection (cystocentesis at time 0 vs catheter collection), catheter placement, and management; and urine collection systems may also have influenced the degree of contamination and subsequent incidence of bacteriuria in the present study.
Subclinical bacteriuria has been recognized in people and dogs and may occur intermittently without clinical importance.12–14 In the present study, bacteriuria was identified in 3 dogs at time 0 on the basis of urine bacterial culture. These dogs were not removed from the study because they had no history of UTI, signs of urinary tract disease, or evidence of active sediment (pyuria) on urine cytologic examination; therefore, no treatment for UTI was indicated.14 However, these dogs were not considered to have developed CAB. Only 1 of these 3 dogs went on to develop a CAUTI during the study period, which supported the supposition that bacteriuria can be identified but have no clinical importance.
Although no significant difference between catheter groups was identified regarding median time to cytologically or culture-detected bacteriuria, median time to CAB was significantly longer with control catheters (96 hours) than with silver-coated catheters (36 hours). This result contradicted findings in human medicine8,15 that a significant delay in the onset of bacteriuria is associated with silver alloy urinary catheters, and whether such an association exists in veterinary medicine may require further investigation.
The overall incidence of CAUTI in the present study (4/36 [11%]) was comparable to previously reported findings for dogs (10% to 19%),3,4 and this incidence did not differ significantly between catheter groups. However, the incidence of CAUTI in dogs catheterized for ≥ 72 hours (13 dogs) was lower in the present study (2/13 [15%]) than in a previous study3 (79%) performed at the same institution. In that previous investigation, a latex urinary catheter (vs a silicone catheter) but the same catheterization protocol were used. Standardized catheter placement and catheter care protocols were followed for all but 1 dog in the present study, and this dog was excluded from analysis. It is unknown whether similar stringent criteria were used in the previous study.
A high proportion of dogs in the present study (21/36 [58%]) received antimicrobial treatment during catheterization, and this treatment may have affected the assessed outcomes. Similarly, in other studies4,11,16 of CAUTI in dogs, antimicrobial administration rates varied from 48.6% to 87.1%. In a previous study16 of catheterized dogs, antimicrobial treatment increased the odds of acquiring a UTI (as diagnosed via bacterial culture) substantially, whereas no such association was identified in the present study. Indeed, antimicrobial use in the present study was common and unassociated with any evaluated outcome.
Specific patient factors may lead to the development of CAUTI.17 All dogs with CAUTI in the present study had neurologic dysfunction that prevented them from urinating normally. However, this risk factor was common among study patients. In dogs with intervertebral disk disease necessitating urinary bladder management, UTI development is common (21%)18; however, the reason for catheterization was not predictive of CAUTI in a previous prospective study.16
In the study reported here, CAUTIs were noted at a median of 60 hours after catheterization with silver-coated catheters or a median of 72 hours after catheterization with control catheters. These findings were comparable to those of another prospective study6 in which 4 of 39 (10%) dogs developed a CAUTI (2 at 24 hours and 2 at 4 days after catheterization). Although a clinical trial19 involving catheterized humans yielded a CAUTI rate of 2.66 infections/1,000 patient days for those that received silver-coated catheters and 3.35 infections/1,000 patient days for those in the control group, use of silver-coated urinary catheters did not appear to delay the development of CAUTI in the dogs of the present study. Furthermore, the rate of CAUTI in dogs increased with increasing duration of catheterization in 2 previous studies3,4 that involved standard urinary catheters of various types, in which the incidence of CAUTI at 72 hours after catheterization was 79%3 and the probability of remaining CAUTI free at 4 days after catheterization was 63%.4 The overall median duration of catheterization in the present study was only 48 hours, which was briefer than that in the other studies and may have contributed to the low incidence of CAUTI observed. These briefer catheterization periods could have been attributable to a general practice of limiting the catheterization period to < 72 hours to reduce the likelihood of catheter-related complications.
For both types of catheters in the present study, older age was a significant predictor of dogs developing culture-detected bacteriuria but not CAB or CAUTI. This was consistent with previous veterinary18 and human19 studies in which age had no association with CAUTI. Duration of catheterization was also a significant predictor of developing culture-detected bacteriuria and CAB in the present study. Similarly, veterinary3,4,18 and human19 studies have shown that duration of catheterization is associated with CAUTI. The lack of a significant association with CAUTI in the present study could have been related to the low number of included dogs and the relatively low incidence of CAUTI.
Although females are at greater risk of CAUTI than males in human medicine,7,19 this has not held true in clinical studies3,4,18 of catheterized dogs, despite the apparent increased risk that females have of recurrent or persistent UTI.17 All dogs that developed CAUTI in the present study were female; however, no significant difference was identified between the sexes in the incidence of this outcome. Nevertheless, numbers of females (n = 25) and males (11) were uneven because of the limited catheter length (31 cm), and although not significantly different, there were substantially more males among dogs that received control catheters than there were among dogs that received silver-coated catheters.
The main limitations of the present study were the small number of included dogs and low incidences of CAB and CAUTI. The exclusion of male dogs weighing > 8 to 10 kg was a substantial hindrance to enrollment. Dogs expected to be catheterized for < 24 hours were also excluded, and this may have led to exclusion of dogs that were ultimately catheterized for a longer period. A post hoc power analysis revealed that with the incidence of CAUTI identified in our study and a power of 50%, 335 dogs would have been needed in each catheter group to detect a 5% difference in incidence between groups. Even so, the lower-than-expected catheterization rates and briefer duration of catheterization observed during the study period could have reflected progress in patient management given the known complications associated with urinary catheterization. A larger prospective study is required to definitively determine whether use of silver-coated urinary catheters should or should not be considered to reduce the risk of CAB or CAUTI in dogs.
Acknowledgments
Supported by the Ontario Veterinary College Pet Trust Fund.
The authors thank Covalon Technologies Ltd for providing the urinary catheters used in this study.
ABBREVIATIONS
| CAB | Catheter-associated bacteriuria |
| CAUTI | Catheter-associated urinary tract infection |
| CI | Confidence interval |
| UTI | Urinary tract infection |
Footnotes
McDonell J, Lefebvre S, Morley P, et al. Syndromic surveillance for hospital-associated infections in a small animal referral hospital (abstr), in Proceedings. 26th Annu Am Coll Vet Interm Med Forum 2008;788.
SilverCoat silicone Foley catheter, Covalon Technologies Ltd, Mississauga, ON, Canada.
Covalon Technologies Ltd, Mississauga, ON, Canada.
Vacutainer, Becton Dickinson and Co, Franklin Lakes, NJ.
CultureSwab, Becton Dickinson and Co, Franklin Lakes, NJ.
SAS/STAT, version 9.3, SAS Institute Inc, Cary, NC.
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