Causes of lower urinary tract disease in Norwegian cats

Abstract

A study was made on causes of lower urinary tract disease in cats, and to investigate whether demographic data and factors related to husbandry might influence the occurrence of a particular diagnosis. The study was a prospective, descriptive, and analytical study of primary cases of feline lower urinary tract disease (FLUTD) in Norway. Only cats sampled by cystocentesis were included in the present study. Of the 119 cats included, 28.6% were diagnosed with obstructive FLUTD. The majority of cats were diagnosed with feline idiopathic cystitis (FIC) (55.5%). Urethral plugs were the second most common diagnosis (21.0%), whereas bacterial cystitis and urolithiasis each were diagnosed in 11.8%. Nearly one-third (28.6%) of the cats diagnosed with urolithiasis had significant bacteriuria. Thus, significant bacteriuria was diagnosed in a total of 15.1% of the cats. There were no significant differences in the urine specific gravity, pH and amount of epithelial cells in the urine sediment in the different aetiological categories of FLUTD. There was a higher amount of red blood cells in the urine sediment in cats diagnosed with urethral plugs and urolithiasis, whereas cats with bacterial cystitis and urolithiasis had a higher amount of white blood cells in their sediment. Regarding demographic data and factors related to husbandry, cats diagnosed with FLUTD were more often males and kept strictly indoors, when compared with a ‘reference population’.

Feline lower urinary tract disease (FLUTD) includes disorders affecting the urethra and/or urinary bladder. Infectious agents, urethral plugs, uroliths, tumours, malformations and trauma can all be causes of FLUTD. If no cause is found after thorough evaluations, a diagnosis of feline idiopathic cystitis (FIC) is made. ¹ Stranguria, dysuria, haematuria, pollakiuria and inappropriate urination are typical signs in cats with FLUTD, but these signs are rarely indicative of a particular aetiology. ^2–5

Several authors have concluded that FIC is the most common cause of FLUTD, but the majority of these studies have been performed in the USA. ^2,4,6–8 A study from Switzerland has also reported FIC as the most frequent cause of FLUTD. ⁹ On the other hand, in the USA bacterial cystitis has usually been diagnosed in only 1–3% of FLUTD cases, ^2,4,8 whereas studies from Europe have reported a considerably higher occurrence of bacterial cystitis, ranging from 8% to 25%. ^9–11 These differences might be explained by differences in study population characteristics, related to geographic location, signalment data, whether first opinion versus referral cases are studied, and the presence or not of concurrent disorders in the cats studied. Also, factors related to study design, like whether the study is prospective or retrospective, the inclusion and diagnostic criteria applied on the study sample, and urinary sampling techniques used, all have the power to influence the findings reported.

The small animal clinic at the Norwegian School of Veterinary Science (NSVS) receives a substantial number of cats with FLUTD each year and the vast majority of them are first opinion cases. ¹¹ A prospective study on FLUTD was performed at the NSVS in the period from January 2003 to December 2007 and a database regarding FLUTD was established from these cases. The aims of this paper are to report the causes of lower urinary tract disease in cats diagnosed with FLUTD in this period, and to investigate whether demographic data and factors related to husbandry might potentially influence the occurrence of a particular diagnosis. To avoid potential bias from the urine sampling method, only cases with urine obtained by cystocentesis were included.

Materials and methods

Study population

This study was undertaken from January 2003 to December 2007 at the Department of Companion Animal Clinical Sciences, NSVS. Cats presented with signs of lower urinary tract disease (stranguria, dysuria, haematuria, pollakiuria and inappropriate urination) and that had a diagnosis consistent with FLUTD (urethral plugs, uroliths, lower urinary tract infections, idiopathic cystitis, bladder neoplasia, urinary incontinence, or trauma, malformations or behavioural disorders affecting the bladder) were eligible for inclusion in the study. No breed, age or gender restrictions were made. Cats receiving treatment that could interfere with the diagnostics, including antimicrobial treatment, glucocorticoid treatment, urinary tract catheterisation in the month before presentation and previous urethrostomy surgery were excluded. Also, cats with concurrent diseases, like chronic kidney disease, diabetes mellitus and hyperthyroidism, were excluded. The study population consisted of client-owned cats from Oslo and the surrounding areas, and cats were only included if the owner's informed consent was obtained. A sub-sample of the study population has been included in a previous study. ¹¹

Methods

The study design was a descriptive and analytical study of FLUTD among cats diagnosed at the NSVS. Cases were included in a prospective manner in the study-period and a primary database containing data regarding various aspects of FLUTD was established. In the present study only cats that had urine for culture obtained by cystocentesis were included.

The signalment and history were recorded from a standardised questionnaire. Among other things, information regarding the cat's living-conditions and feeding-regimen were collected.

All the cats included in the study went through a physical examination, and blood samples were collected for routine haematological analysis and clinical chemistry, with analysis being performed at the Central Laboratory, NSVS. Based on the general impression (level of consciousness, behaviour, posture, locomotion) and results of the physical examination the cat's general condition were classified as normal, mildly depressed, depressed and severely depressed.

Standard urinalysis was performed and included commercial urine dipstick analysis (Krulab; Kruuse, Marslev, Denmark), specific gravity measured with a refractometer (URC-Ne; Atago, Tokyo, Japan) and microscopic examination of the sediment (native samples and samples stained with Sternheimer-Malbins). Approximately 10 ml of urine was centrifuged for 10 min at 450×g (1500 rpm), and findings (epithelial cells, red blood cells, white blood cells) in the sediment were scored semi-quantitatively.

Quantitative urine bacteriology was performed by streaking 1 μl of urine onto blood agar plates and qualitatively by cultivation from the sediment after centrifugation. The bacterial isolates from urine samples were also analysed for susceptibility to antibacterial agents on Mueller-Hinton (MH) agar (Difco; Detroit, MI) with antimicrobial discs (NeoSensitabs; Rosco, Taastrup, Denmark). Whenever possible, the urine sample was cultured on the same day that it was collected. Urine samples that had to be stored were refrigerated for a maximum of 24 h.

All the cats included received an ultrasound examination (preferably) and/or a radiological examination of the urinary tract. A combination of medetomidine (Domitor vet; Orion, Turku, Finland) 30 μg/kg SC and methadone hydrochloride (Metadon inj; Rikshospitalets Apotek, Oslo, Norway) 0.2 mg/kg SC was used for sedation.

Based on results of examinations and in line with defined diagnostic criteria, the FLUTD cases were divided into diagnostic groups. A case was considered obstructed if the cat was unable to urinate freely and the urinary bladder was distended. Uroliths were diagnosed on ultrasonographic and/or radiological examination. If more than 10³ colony-forming units per millilitre (CFU/ml) urine (cut-off value) were detected in the bacteriological analyses, the cat was diagnosed with significant bacteriuria. A diagnosis of bacterial cystitis was made when a significant bacteriuria was diagnosed in the absence of uroliths. A diagnosis of urethral plugs was made in an obstructed cat, when a plug was detected at catheterisation and in the absence of a significant bacteriuria. A diagnosis of FIC was made if there was no significant bacteriuria, an absence of uroliths on ultrasonographic and/or radiological examination and an absence of urethral plugs. Whether crystalluria was present was also noted in each individual case.

The results are expressed as prevalence in percent with 95% confidence intervals (CI) in brackets constructed by using the theory of simple binomial sequences. ¹² The assumed continuously distributed variables are presented as mean values with 95% CI in brackets. Prevalence data were considered not significantly different from each other at a significance level of 5% if there was an overlap in their respective 95% CI. Comparison of groups with regard to variables assumed to be continuously distributed were performed using analysis of variance (ANOVA). Categorical data were compared by contingency analysis. In all cases, differences were considered significant if P-values were less than or equal to 0.05.

Results

Study population

A total of 119 cats with FLUTD were included and all were first opinion cases. The study population consisted of 16 pure-breed cats (13.4%; 95% CI: 8.4–20.7) and 103 domestic short-, semi- or long-haired cats (86.6%; 95% CI: 79.3–91.6). The pure-breed cats represented seven different breeds: five Persian cats, four Norwegian Forest cats, two Birmans, two British Shorthairs, one Maine coon, one Abyssinian and one Burmese. Eighty-eight cats were males (73.9%; 95% CI: 65.4–81.0) and 31 (26.1%; 19.0–34.6) were females. Seventy-eight of the males were castrated (88.6%; 95% CI: 80.1–94.4), whereas 24 (77.4%; 95% CI: 58.9–90.4) of the female cats had been spayed. The mean age was 5.6 years (95% CI: 5.0–6.3). There were significantly more males than females (P<0.001) and significantly more neutered than intact cats in the study population (P<0.001).

Findings and diagnosis

Seventy-three (62.4%; 95% CI: 53.4–70.6) cats were suffering from their first episode of FLUTD and in 44 (37.6%; 95% CI: 29.4–46.6) cases the cats had experienced previous episode(s).

Thirty-four (28.6%; 95% CI: 21.2–37.3) were diagnosed with obstructive FLUTD and 85 (71.4%; 95% CI: 62.7–78.7) with non-obstructive FLUTD. Of the cats with obstructive FLUTD, 10 (29.4%; 95% CI: 15.1–47.5) and 31 cats (91.2%; 95% CI: 76.3–98.1) had elevated serum creatinine (>180 μmol/l) and urea (>10.0 mmol/l) concentrations, respectively; the mean values of creatinine and urea in these cats were 693.8 μmol/l (95% CI: 490.1–897.4) and 43.4 mmol/l (95% CI: 31.2–55.5). Of the cats with non-obstructive FLUTD, 17 (20.0%; 95% CI: 12.1–30.1) and 21 cats (24.7%; 16.0–35.3) had elevated serum creatinine and urea concentrations, respectively; the mean values of creatinine and urea in these cats were 210.3 μmol/l (95% CI: 178.4–242.2) and 13.8 mmol/l (95% CI: 11.9–15.7). Of the obstructed cats, 13.8% (95% CI: 3.4–31.7) had normal general condition, 34.5% (95% CI: 17.9–54.3) were mildly depressed, 41.4% (95% CI: 23.5–61.1) were depressed and 10.3% (95% CI: 2.2–27.4) were severely depressed, whereas the corresponding frequencies in the cats without obstruction were 63.3% (95% CI: 49.9–75.4), 30.0% (95% CI: 18.9–43.2), 6.7% (95% CI: 1.9–16.2) and 0.0% (95%: 0.0–6.0).

The vast majority of the study population were examined by ultrasound or ultrasound and radiology (97.5%; 95% CI: 92.8–99.1). More than half of the cats were diagnosed with FIC (55.5%; 95% CI: 46.5–64.1). A urethral plug was the second most common diagnosis (21.0%; 95% CI: 14.7–29.2) whereas bacterial cystitis and urolithiasis each were diagnosed in 11.8% (95% CI: 7.1–18.9). An overview of the diagnoses and additional findings, like crystalluria, are presented in Table 1. Struvites were the most commonly detected crystals (84.6%; 95% CI: 73.5–92.3). Oxalate crystals were identified in three cases (4.6%; 95% CI: 1.0–12.9), whereas seven cats had crystals that could not be identified. Of the cats with urethral plugs 80.0% (95% CI: 59.3–93.2) had crystalluria, compared to 64.3% (95% CI: 35.1–87.2) of the cats with urolithiasis, 57.1% (95% CI: 28.9–82.3) of the cats with bacterial cystitis and 42.4% (95%CI: 30.3–55.2) of the cats with FIC. Nearly one-third (28.6%; 95% CI: 8.4–58.1) of the cats diagnosed with urolithiasis had significant bacteriuria. Thus, significant bacteriuria was diagnosed in a total of 15.1% (95% CI: 9.8–22.6) of the cats (Table 1).

Table 1.

The occurrence of diagnostic categories of FLUTD in the study population and the frequency of obstruction within each category. Also, additional findings, like crystalluria, are reported. Findings are reported as percentages with 95% CI in brackets.

Diagnosis	Obstructive	Diagnosis with additional findings in brackets
FIC (n=66) 55.5% (46.5–64.1)	0.0% (0.0–5.4)	FIC 31.9% (24.2–40.8)
FIC (crystalluria) 23.5% (16.8–31.9)
Urethral plug (n=25) 21.0% (14.7–29.2)	100.0% (86.3–100.0)	Urethral plug 4.2% (1.8–9.4)
Urethral plug (crystalluria) 16.8% (11.2–24.5)
Bacterial cystitis (n=14) 11.8% (7.1–18.9)	21.4% (4.7–50.8)	Bacterial cystitis 4.2% (1.8–9.5)
		Bacterial cystitis (crystalluria) 5.0% (2.3–10.6)
		Bacterial cystitis (plug) 0.8% (0.01–4.6)
Bacterial cystitis (plug, crystalluria) 1.7 (0.5–5.9)
Urolithiasis (n=14) 11.8% (7.1–18.9)	42.9% (17.7–71.7)	Urolithiasis 2.5% (0.9–7.2)
		Urolithiasis (bacteriuria) 1.6% (0.4–5.9)
		Urolithiasis (crystalluria) 5.9% (2.9–11.6)
Urolithiasis (bacteriuria, crystalluria) 1.6% (0.4–5.9)
100.0%		100.0%

Urinary findings

There were no significant differences in the urine specific gravity, pH, and amount of protein in the different diagnostic groups of FLUTD. Regarding the amount of red blood cells in the urine sediment there was a significant difference with cats diagnosed with urethral plugs and urolithiasis displaying the highest amounts (P=0.02). Furthermore, there was a higher amount of white blood cells (P=0.002) in the urine sediment of cats diagnosed with bacterial cystitis and urolithiasis. The amount of epithelial cells in the urine sediment did not differ significantly (data not shown).

Eighteen urinary samples yielded a bacterial growth exceeding 10³ cfu/ml; in six samples the bacterial growth exceeded 10⁴ cfu/ml, in six samples the bacterial growth exceeded 10⁵ cfu/ml and in five samples the bacterial growth exceeded 10⁶ cfu/ml urine. In all cases a single bacterial species was isolated, most commonly Escherichia coli (44.4%; 95% CI: 21.5–69.2). The rest of the bacteriological results was: Staphylococcus species (two), Enterococcus species (two), Proteus mirabilis (one), Pasteurella multocida (one), Pseudomonas aeruginosa (one), Hemophilus species (one), Streptococcus species (one) and Citrobacter species (one).

Signalment data and husbandry

In all the diagnostic groups, domestic cats out-numbered the pure-breed cats. The number of pure-breed cats in the bacterial cystitis group was significantly higher than in the urethral plug group. In all groups, except the bacterial cystitis group, the number of male cats was significantly higher than the number of female cats. No female cats were diagnosed with plugs, and significantly more female cats were diagnosed with bacterial cystitis than FIC and urolithiasis (Table 2).

Table 2.

Breed, gender and age within the four different diagnostic groups of FLUTD and the total study population (n=119). Results are given as percentages with 95% CI or as the mean with 95% CI.

Diagnosis	Breed percentages (95% CI)		Gender percentages (95% CI)	Age (years) mean (95% CI)
DSH+DLH	Pure-breed
FIC n=66	86.4 (75.7–93.6)	13.6 (6.4–24.3)	IF 6.1 (1.7–14.8) NF 22.7 (13.3–34.7) IM 4.5 (1.0–12.7) NM 66.7 (54.0–77.8)	5.5 (4.7–6.3)
Urethral plug n=25	92.0 (74.0–99.0)	8.0 (1.0–26.0)	IF 0.0 (0.0–13.7) NF 0.0 (0.0–13.7) IM 8.0 (1.0–26.0) NM 92.0 (74.0–99.0)	5.2 (3.9–6.6)
Bacterial cystitis n=14	71.4 (41.9–91.6)	28.6 (8.4–58.1)	IF 21.4 (4.7–50.8) NF 35.7 (12.8–64.9) IM 21.4 (4.7–50.8) NM 21.4 (4.7–50.8)	7.3 (4.5–10.0)
Urolithiasis n=14	92.9 (66.1–99.8)	7.1 (0.2–33.9)	IF 0.0 (0.0–23.2) NF 28.6 (8.4–58.1) IM 14.3 (1.8–42.8) NM 57.1 (28.9–82.3)	5.4 (3.4–7.3)
Total study population n=119	86.6 (79.3–91.6)	13.4 (8.4–20.7)	IF 5.9 (2.9–11.6) NF 20.2 (13.9–28.3) NM 65.5 (56.6–73.5) IM 8.4 (4.6–14.8)	5.6 (5.0–6.3)

DSH=domestic shorthair, DLH=domestic longhair, IF=intact female, NF=neutered female, IM=intact male, NM=neutered male.

In all diagnostic groups more than 50% of the cats were indoor cats. The proportion of cats living indoors did not differ significantly between the different groups, although there was a tendency for more cats with bacterial cystitis to be strictly indoor cats (Table 3).

Table 3.

Living-conditions within the four different diagnostic groups of FLUTD and the total study population (n=117). Results are given as percentages with 95% CI.

	FIC (n=65)	Urethral plug (n=25)	Bacterial cystitis (n=13)	Urolithiasis (n=14)	Total study population (n=117)
Indoor	56.9% (44.0–69.2)	64.0% (42.5–82.0)	69.2% (38.6–90.1)	64.3% (35.1–87.2)	60.7% (51.6–69.1)
Outdoor	43.1% (30.6–56.0)	36.0% (18.0–57.5)	30.8% (9.1–61.4)	35.7% (12.8–64.9)	39.3% (30.9–48.4)

The majority of cats in all diagnostic groups were fed a combination of dry and wet food or dry food only (Table 4).

Table 4.

Feeding regimens within the four different diagnostic groups of FLUTD and the total study population (n=115). Results are given as percentages with 95% CI.

	FIC (n=63)	Urethral plug (n=24)	Bacterial cystitis (n=14)	Urolithiasis (n=14)	Total study population (n=115)
Dry+wet food	57.1% (44.1–69.5)	62.5% (40.6–81.2)	50.0% (23.0–77.0)	42.9% (17.7–71.4)	55.7% (46.5–64.4)
Dry food only	38.1% (26.2–51.2)	37.5% (18.8–59.4)	42.9% (17.7–71.4)	50.0% (23.0–77.0)	40.0% (31.5–49.1)
Wet food only	4.8% (1.0–13.3)	0.0% (0.0–14.3)	7.1% (0.2–33.9)	7.1% (0.2–33.9)	4.3% (1.9–9.8)

Discussion

The present study provides new information on cats with FLUTD. Relatively few European FLUTD studies have been published, and furthermore most studies worldwide have been performed in referral cases, ie, secondary or tertiary populations. Thus, external validity of these studies could be low, and the reported findings not necessarily representative of the ‘typical’ cat that presents with FLUTD in primary care practice.

Findings and diagnosis

In the present study, the cats were non-referrals and the majority presented with their first episode of FLUTD. Most of the cats were not obstructed, which is in agreement with other reports, ^2,8,13 although referral clinics tend to receive a larger proportion of cats with urethral obstructions. ^9,14 Not surprisingly, significantly more cats with obstructive FLUTD were depressed. Also, creatinine and urea concentrations were much higher in obstructed than non-obstructed cats. Gerber et al ⁹ reported that 29% of cats with obstructive FLUTD had increased urea and creatinine concentrations. In the present study, some cats with non-obstructive FLUTD also were depressed and/or had increased creatinine concentrations. The latter most likely reflects a pre-renal azotaemia due to reduced eating and drinking possibly due to pain.

The majority of cats in the present study were diagnosed with FIC (55.5%) or urethral plugs (21.0%), whereas bacterial cystitis and urolithiasis each were diagnosed in 11.8%. No other diagnosis was made. Clearly, the number of cats within each diagnostic group is highly dependent on which criteria the diagnosis is based. These criteria could vary between different studies and thus hamper direct comparisons between studies. However, FIC is reported to be the most common cause of FLUTD in several studies. ^2,4,6–9 As recently reviewed by Kruger et al, ¹ many theories about the causes of FIC have been proposed, although the specific aetiological factor(s) are still unknown.

In contrast to uroliths, urethral plugs consist of large quantities of organic matter and varying amounts of minerals, mainly struvite. ¹⁵ The reported frequency of crystalluria in FLUTD varies considerably. ^2,4,9 In the present study the number of cats with crystalluria was considerable higher in the sub-sample with urethral plugs. Kruger et al ¹ suggest that males with FIC and concomitant crystalluria are at risk for the formation of urethral plugs and subsequent urethral obstruction.

The differentiation between obstructive FIC and urethral plugs can be difficult, as also highlighted by Gerber et al. ⁹ In their study, 55% of cats with FIC reportedly had urethral obstruction. Urethral obstruction in cats with FIC may result from inflammatory swelling of the urethra, urethral muscle spasm and reflex dyssynergia, in addition to the formation of matrix-crystalline urethral plugs. ¹ Also, urethral obstruction in FIC could be due to intraluminal accumulations of sloughed tissue, inflammatory cells, or red blood cells. ¹ Without performing plug analysis a distinction between the latter and urethral plugs is difficult, and hence, some cats diagnosed with urethral plugs in the present study could have been misclassified, and the proportion of cats with FIC may, therefore, have been underestimated.

Mayer-Roenne et al ¹⁶ reported a high occurrence of bacterial infections in the urinary tract of cats with hyperthyroidism (12%), diabetes mellitus (12%), and chronic kidney disease (22%). To avoid potential confounding from such diseases, and also previous urinary tract catheterisation and urethrostomy, in the present study cats were not included if any of these diseases/conditions were present. It is commonly stated in major textbooks that bacterial cystitis is uncommon in cats, unless defences are compromised because of concurrent diseases or treatments. ^17–19 Indeed, a recent text states that in cats less than 10 years of age at presentation with FLUTD signs, ‘less than 2% of cases will be associated with urinary infection’. ¹⁸ Our finding of a prevalence of significant bacteriuria of 15.1% in adult/mature cats indicates that there is a need for a re-evaluation of this dogma. Some other previous studies have also reported a relatively high occurrence of bacterial cystitis in cats presenting with signs of FLUTD: Kraijer et al ¹⁰ reported a prevalence of 22.2% (95% CI: 10.2–39.1) in primary cases (mid stream catheter urine, cut off 10⁴), Gerber et al ⁹ reported a prevalence of 8% (95% CI: 3–16%) in referred cases (cystocentesis and catheter urine samples, no cut off reported), whereas Eggertsdóttir et al ¹¹ reported a prevalence of 23% (95% CI: 12–41%) in primary cases (cystocentesis urine, cut off 10⁴). These studies, together with the current one, suggest bacterial cystitis may be more frequent than has previously been assumed in primary care practice.

Cystocentesis should be performed for urinary sampling for microbiological culture in cats, because contamination from the lower genitourinary tract is avoided. ^17,20,21 To minimise the number of false positive, a higher cut-off has been suggested for voided samples. ^17,20,22 In the present study only infections with a single organism were detected. Mixed urinary infections are reportedly associated more commonly with contamination. ²⁰ Also, our results indicate that in most cases of bacterial cystitis in the cat high numbers of bacteria are present. In fact, in only one cat diagnosed with bacterial cystitis was less than 10,000 CFU/ml detected, and in the majority of cases more than 100,000 CFU/ml were reported. Clearly, any bacteria in the bladder is regarded as abnormal (ie, suggests infection), but a cut-off value of 1000 CFU/ml in the cat has been suggested to avoid false positive test results due to contamination. ^17,19 Pressler and Bartges, ¹⁹ however, suggest that values between 100 and 1000 should also be interpreted as suspicious.

The bacterial isolates reported in the present study, are in line with findings reported in other studies. ^19,23 In an Australian study, E coli was most commonly isolated (37.3% of the isolates) and in another recent study from Europe, E coli represented 50% of the isolates. ^24,25

Unfortunately, no chemical analysis of the uroliths diagnosed in the present study was performed. In the USA, quantitative analysis of uroliths has shown that since the early 1980s and up until recently there has been a reduction in the number of struvite stones and an increase in the number of oxalate stones. ^15,26 A similar development has been observed in cats from Benelux (Belgium, the Netherlands, Luxembourg). ²⁷ Several risk factors have been suggested for the formation of feline uroliths, eg, breed, gender, age, urinary tract infection, urine pH, and diet. ²⁶ In the present study almost two-thirds of the cats with urolithiasis had concomitant crystalluria and nearly one-third had bacteriuria. In the majority of the cases, the crystals identified were struvite.

The degree of macrohaematuria was not recorded in the present study, but there was a higher amount of red blood cells in the urine sediment of cats diagnosed with urethral plugs and urolithiasis. Many of these cats were obstructed and according to Gerber et al ⁹ one may expect more bleeding in obstructed cats. A significantly higher amount of white blood cells was detected in the urine sediment of cats diagnosed with bacterial cystitis. An association between bacterial urinary infections and increased number of white blood cells in the sediment has also been reported in several previous studies. ^16,28,29

Signalment data and husbandry

Recently, a study was conducted aimed at describing important characteristics regarding demography and husbandry of Norwegian pet cats, and the population examined is suggested to represent an appropriate ‘reference population’ for ongoing scientific studies. ³⁰

Compared to the estimated number of pure-breed cats (approximately 10%) in the Norwegian pet cat population, ³⁰ a greater proportion of pure-breed cats were present in our study population and particularly in the group diagnosed with bacterial cystitis. This could be explained by either pure-breed cats being more prone to develop FLUTD, or perhaps their owners may be more willing to consult a veterinarian, when the cat displays clinical signs. ³¹

In the ‘reference population’, there were almost an equal number of female and male cats, ³⁰ whereas in our study population almost 80% were males. One explanation for this could be that the vast majority of cats with obstructive FLUTD are males. ^9,11 Cases of obstructive FLUTD constitute an emergency which necessitates veterinary consultation, whereas non-obstructive FLUTD cases such as FIC may display subtle clinical signs with a high rate of spontaneous recovery ^32,33 perhaps leading to fewer cases being presented to veterinary clinics. However, in the present study male cats were presented more often than female cats in all diagnostic groups, except in the bacterial cystitis group. Also, in another European study of FLUTD males were represented more often than females in the study population. ⁹ Buffington et al ³⁴ reported risk factors associated with clinical signs of lower urinary tract disease in indoor cats. In their study, more owners of cats with lower urinary tract disease perceived that their cats had fear, nervousness, and aggression. ³⁴ Also, in a study reporting signalment factors in behaviour diseases in cats, like aggression and house soiling, male cats and neutered cats were overrepresented. ³⁵ Thus, one could speculate that male cats have more difficulties in adapting to husbandry practices, like indoor housing, and that gender may increase the risk for the development of some cases of FLUTD. A shorter and wider urethra has been suggested to be a reason for higher occurrence of urinary tract infections in females. ²⁹ In line with findings reported for the ‘reference population’, the majority of both males and females in the present study were neutered and a higher proportion of male cats than female cats were neutered. ³⁰

Approximately 60% of the cats in the present study were indoor cats. Surveys have estimated that only 7–9% of the Norwegian cat population are strictly indoor cats. ^30,36 There was, however, a large variation depending on the level of urbanisation with almost 30% of cats living in cities being held as indoor cats. ³⁰ In the present study a much higher proportion, up to 69%, of the cats lived strictly indoors. There are several possible explanations for this finding. Indoor housing or restricted access to outdoors is suggested to be a risk factor of FLUTD. ³⁷ Lue et al ³⁸ reported that owners with the strongest bond to their pets kept their pets indoors. Furthermore, owners with the strongest bond to their pets were more likely to seek higher levels of veterinary care for their pets. ³⁸ It is also likely that owners of strictly indoor cats spend more time with their cat, and that this enables them to more closely observe signs indicative of disease and seek veterinary care when deemed appropriate. The tendency for more cats diagnosed with bacterial cystitis living strictly indoors could be explained by the relatively higher number of intact female cats in this group.

In the ‘reference population’, the vast majority of cats were fed commercial dry food: 87% of the cats got more than 60% of their food as dry food. ³⁰ Although the results presented herein are not directly comparable, due to differences in definitions and type of registrations made, the majority of cats in the present study were fed dry food or dry and wet food. Thus, this finding most likely reflects how Norwegian cat owners feed their cats, rather than specifically indicating dry food as a risk factor for development of FLUTD. A diet high in dry food, however, has previously been reported to be a risk factor of FLUTD. ^37,39

The possibility that our results may be biased by the sample of cats studied needs to be discussed. In some situations cystocentesis is impractical or difficult to perform. In non-obstructed cases, the cat may continuously void small amounts of urine and, therefore, keep the bladder empty. ^11,20 Therefore, the selection of cats examined by only cystocentesis, could potentially influence both the number, and relative occurrences, of diagnoses. For example, cats with urinary tract infections tend to pass urine in small volumes frequently and tend to have a small bladder that is difficult to identify (leading to potential under-reporting of bacterial cystitis). ²³ Also, although no cats were referred, the owners could contact the out-patient clinic at the NSVS due to the fact that it is a part of a university clinic and perhaps due to knowledge of an ongoing study on FLUTD, leading to a selection towards more ‘complicated’ cases (leading to over-reporting of FIC). Also, with the diagnostic procedures applied on the study population, not all causes of FLUTD would necessarily have been identified. When comparisons were performed between the different diagnostic groups in the present study, regarding signalment data, husbandry, urinary findings, etc, in many cases only minor differences were detected. Firstly, the study could lack power to detect such differences (type II bias). Secondly, as discussed earlier, some cats could be misclassified. Thirdly, different diagnostic groups could also be a reflection of the same ongoing disease and each cat may display several different signs and diseases over a longer observation period. If so, differences between different diagnostic groups could be minor or even absent, although the cat is correctly diagnosed at the time of presentation.

Nevertheless, under these study conditions, we identified several important findings. Results from the present study indicate that also in primary cases of FLUTD in Norway, FIC constitutes the largest diagnostic group. A finding of a significant bacteriuria in approximately 15% of FLUTD cases warrants the routine culture of urine during diagnostic evaluation of Norwegian cats with clinical signs of lower urinary tract disease. In all cases of significant bacteriuria a single bacterial species was detected, and in the majority of cases more than 100,000 CFU/ml were detected. In these aspects, the results of our study could also be valid for populations of cats in general practice in other countries. Also, results of our study indicate that being male and/or indoor confinement could increase the risk of developing FLUTD.