Petsavers – Feline subclinical bacteriuria

Clinical signs of lower urinary tract (LUT) disease occur frequently in cats, yet urinary tract infections (UTI) confirmed by bacterial culture represent only a small percentage of cases presenting with such signs.¹ In contrast, up to 30% of urine cultures from cats without any current or historical signs of lower urinary tract disease may be positive for bacterial growth.² These apparent contradictory findings can make antimicrobial prescribing challenging for veterinary practitioners. Clear evidence-based guidelines exist in human medicine outlining when antimicrobial therapy is warranted in cases of positive urine cultures.³ Although guidelines do exist in veterinary medicine, the evidence base is somewhat lacking, and many recommendations have been extrapolated from human data.⁴

Antimicrobial therapy is not a benign treatment, and its unnecessary use has been associated with the selection of multidrug-resistant bacteria, alterations of the gastrointestinal microbiome and a range of other gastrointestinal side effects.⁵ The close physical relationship between companion animals and their owners may play a role in the dissemination of antimicrobial resistance, impacting on both human and animal health.⁶ In this era of One Health, antimicrobial stewardship is therefore of utmost importance in both human and veterinary medicine. This research project aimed to identify the prevalence of subclinical bacteriuria and UTI in a population of cats at one referral centre hospital, and to identify any results of urinalyses that could discriminate between subclinical and clinical infections. Urinalysis, including sediment examination and culture and susceptibility testing, was performed on voided urine samples and cystocentesis samples to determine whether results were valid for both methods of collection.

Research methods

A prospective observational cohort study of cats referred to the Langford Vets Small Animal Hospital was conducted. Aerobic urine culture and antimicrobial susceptibility testing, and urinalysis including sediment examination, UPC measurement and cytological examination were performed on urine samples obtained by cystocentesis and voided urine from the same cat. Cystocentesis samples were obtained at the request of the attending clinician as part of a diagnostic workup. The study was approved by the University of Bristol Research Ethics Committee.

Results and discussion

Fifty-six cats were included in the study, of which seven presented with LUT clinical signs. The median age at the time of enrolment was 11.5 years (range, 1–18 years). Of the 56 cats, 28 were male and 28 were female. Urine obtained by cystocentesis was culture-negative in all seven cats with LUT clinical signs, whilst voided samples from three of the seven cats were culture-positive. A positive urine culture was obtained from just one cystocentesis sample from a cat with no LUT clinical signs. Three positive urine cultures were obtained from voided urine samples from cats with no LUT clinical signs. Of the six total positive urine cultures obtained from voided urine samples, three were thought to represent contamination, two were considered to represent genuine UTI cases and one was considered to represent genuine subclinical bacteriuria. The number of colony forming units and number of individual organisms isolated with respect to the method of urine collection were used to differentiate between contamination and genuine growth.

Unfortunately, the low prevalence of UTI and subclinical bacteriuria in the study population precluded meaningful risk factor analysis being performed. However, patient signalment, presenting clinical signs and urinalyses results were compared to known risk factors and predictive urinalyses results reported in the literature for UTI and subclinical bacteriuria.

All cats considered to have either subclinical bacteriuria or UTI were female, which is consistently reported as a risk factor for their development.^2,7 Out of 36 hyperthyroid cats screened, one case (2.7%) of subclinical bacteriuria was suspected. This is much lower than the reported prevalence from retrospective studies of positive urine cultures in hyperthyroid cats of 12%–17%.^8,9 However, a recent prospective study of over 390 hyperthyroid cats reported a much lower prevalence rate of subclinical bacteriuria of 4.6%.¹⁰ In the study, the prevalence of subclinical bacteriuria did not differ significantly from that of the euthyroid control population and it was concluded that hyperthyroidism is therefore not a risk factor for subclinical bacteriuria.

A profuse growth of an Enterococcus sp. was documented in one confirmed case of subclinical bacteriuria in this study. Urine sediment microscopy for this case is shown in the figure. Enterococcus spp. are reported to account for up to 27% of positive feline urine cultures and are significantly more likely to be isolated from cats with subclinical bacteriuria compared to other pathogens.^8,11 In a review of Enterococcus spp. isolated from canine and feline urine samples, it was reported that no clinical signs of LUT disease were recorded in over 54% of cats with E. faecalis-positive urine cultures.¹² Interestingly, a more recent study suggested enterococcal infections in cats with SUB devices may be more associated with lower urinary tract clinical signs, although clinical signs of lower urinary tract disease were not reported in the case in this study.¹³

Urinary sediment examination is a crucial part of any urinalysis, particularly in identifying pyuria in cats given the poor specificity (high rate of falsepositive) of the leukocyte pad on standard urine dipsticks. However, sediment examination cannot discriminate between cats with UTI or subclinical bacteriuria. The identification of bacteria in combination with white blood cells and red blood cells correlates poorly with clinical signs, so their identification on a sediment examination does not always necessitate antimicrobial therapy.⁴ Clinicians should bear in mind that the presence of a positive urine culture in the absence of LUT clinical signs rarely warrants treatment and so the reason for urine culture should always be considered before sampling. Cases where urine culture may be considered in the absence of LUT signs are listed in Table 1.⁴

^{Table 1. Situations where urine culture can be considered in the absence of lower urinary tract clinical signs. Adapted from 4}

Due to the apparent low prevalence of subclinical bacteriuria, the recruitment of a sufficiently powerful sample size within a reasonable study period does not seem feasible in many situations. This difficulty was also encountered in a prospective study of 179 cats in which only 11 were identified with subclinical bacteriuria.¹⁴ To overcome this, collaborative studies between multiple centres have the potential to achieve larger recruitment targets. Finally, as a new graduate in practice, subclinical bacteriuria was not something I was hugely aware of, having been taught that urinalysis including urine culture was a standard part of many medical workups. In recent years, there has been a turnabout on this thought process, largely due to evidence from human medicine and the now growing evidence in the veterinary literature. One longitudinal study of 67 cats documented that untreated subclinical bacteriuria was not adversely associated with survival.¹⁵ However, this study only included non-azotaemic, non-pregnant cats, and further prospective studies are needed to evaluate the effect of non-treatment in these patient groups.

Funding and acknowledgments

This Master’s Degree by Research was funded by BSAVA PetSavers. The project was supervised by Christina Maunder (University of Bristol), Marta Costa (Idexx Laboratories), Professor Matthew Avison (University of Bristol) and Professor Severine Tasker (The Linneaus Group and University of Bristol), to all of whom I am incredibly grateful for their support and guidance throughout the project and beyond. I am also thankful to the clinical pathology laboratory team at Langford Vets for their technical expertise in sample processing.