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Critical review of acquired urinary incontinence in neutered female dogs

Author Name: Gemma Gormley, BSc (Hons) Veterinary Nursing Science

 

Abstract

It is important for veterinary nurses to advise owners on the risks associated with neutering bitches, therefore they need to possess an understanding of acquired urinary incontinence (AUI) and the risk factors. There have been reports of 3.14-7.3% of female canines suffering with AUI. However, the data is limited for England. The exact cause of AUI is still unknown, although there have been links made between neutering and reduced bladder contractility. This has further been linked to a reduction in mRNA expression for luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) receptors, post neutering. Furthermore, there has been findings of increased LH and follicle stimulating hormone (FSH) plasma concentrations following neutering. This supports the theory that the cause of AUI is associated with the hormonal changes recorded post neutering.

Studies have found that neutering bitches significantly increases the risk of AUI development. There is also a common theme that later spaying, heavier bodyweight and increasing age all increase the risk of AUI. Furthermore, reports also suggest that large and giant breeds have an increased risk, although this cannot be dissociated from the effects of a heavier weight. Common breeds found to have higher prevalence and risk for AUI include the Irish Setter, Doberman, Boxer and English Springer Spaniel. Based on the findings of the review, it is important that veterinary nurses ensure owners are aware that neutering female dogs may increase the risk of AUI development, particularly if the individual is a high risk breed, heavier in weight or older.

 

1.0  Introduction

Urinary incontinence (UI) is defined as the involuntary leakage of urine continuously or intermittently, which can occur when an animal is standing or recumbent (Gear and Mathie, 2011). Urinary incontinence can result from a number of causes including congenital anatomical deformities and hormonal changes, which can occur following neutering in female dogs (Moore, 2012). The routine procedure of neutering bitches is common in veterinary practices, therefore it is important for veterinary nurses to provide up to date information and advice on the risks (Smith, 2015). Acquired urinary incontinence (AUI) is common in neutered female dogs and can also be termed as urethral sphincter mechanism incompetence (USMI) (Byron, 2017).

UI can negatively affect relationships between owners and pets, with negative emotions expressed towards the condition including frustration and disappointment (Bleser et al., 2011). For these reasons, it is important to be able to understand the causes and risk factors associated with AUI, in order to effectively inform and advise owners about neutering their pet.

 

2.0  Critical Review

2.1 Incidence and causes of acquired urinary incontinence in the neutered female dog

Currently, data on the prevalence of UI in dogs in England is limited (O’Neill et al., 2017). Hart et al. (2016) state a 4.7%-7.3% prevalence of AUI within neutered German Shepherd dog (GSD) bitches. This study collected data using electronic patient records from the University California-Davis Veterinary Medical Teaching Hospital. As this data is only representative of one breed, the results cannot be generalised to include other breeds. However, a more recent study completed by O’Neill et al. (2017) found 3.14% prevalence of UI in female dogs, both neutered and intact. This study also utilised electronic patient records to estimate the prevalence of UI, however it did not distinguish between animals with congenital and acquired UI.

Coit et al. (2008) state that neutering dogs reduces bladder function in both males and females. The study of fifty-two dogs found that bladder biopsies of neutered animals have a reduced response to muscarinic stimulation compared to entire dogs. Carbochol, a non-specific muscarinic agonist, was used in addition to electrical field stimulation to measure bladder tissue biopsy contractility. Both tests for contractility resulted in a significant decrease of maximum contractile response within neutered dogs regardless of gender, weight or age of the animals. The suggestion that neutering is the cause of bladder contractility reduction is more reliable due to these variables being ruled out as the cause. The lowest maximum contractile response to Carbochol within the neutered female group was recorded from the biopsies belonging to two neutered females who had been suffering with acquired urinary incontinence (AUI). The study suggests that there must be other causes for AUI in neutered female dogs because male dogs’ bladder contractility is also reduced post neuter, however it is uncommon for them to develop AUI (Coit et al., 2008). The methodology of the study has been described in detail and could be repeated to test for validity. Coit, Dowell and Evans (2009) utilised the same method, which resulted in the same decrease in maximum bladder contractility in neutered dogs. Therefore, the reliability of the study has been increased because the latter study reports the same findings.

Bladder contractility and mRNA expression for luteinizing hormone (LH), and gonadotropin-releasing hormone (GnRH) receptors in the canine bladder have been correlated. Coit, Dowell and Evans (2009) state that neutering dogs increases the levels of mRNA expression for LH and GnRH receptors in the bladder, with neutered females having the highest levels found. Furthermore, neutered females that had suffered with AUI were found to have the highest expression levels overall. However, the increase was not statistically significant as demonstrated by ANOVA analysis. This study also found that as mRNA expression for LH and GnRH increases, bladder contractility significantly decreases. The data supports that the increases in mRNA expression for LH and GnRH receptors may be important in the development of AUI in neutered female dogs because there is a relationship with bladder contractility (Coit, Dowell and Evans, 2009). In both studies, Coit et al. (2008) and Coit, Dowell and Evans (2009), there were limited numbers of neutered females that had suffered with AUI, with only two animals within this category in each study. To increase the reliability and validity of the results, future studies could include a greater sample of females diagnosed with AUI. This would allow for generalisation to the wider population due to a reduced chance of bias as a result of a smaller sample size.

In contrast to the findings of Coit, Dowell and Evans (2009), Reichler et al. (2007) state that there is no significant difference between the expression of mRNA for LH and GnRH receptors between neutered and intact female dogs. Bladder biopsies were harvested from a total of thirty-seven female dogs by laparotomy and necropsy. The study provides a detailed account of where the biopsies were harvested from the bladder, allowing the experiment to be easily repeated. In comparison, Coit, Dowell and Evans (2009) state that biopsies were taken from the dome region of the bladder, which is less specific and therefore less repeatable. The studies used different methods of storing the biopsies before analysis and different methods of PCR to extract the mRNA, which may have an effect on the results recorded by each study. Biopsies harvested from the proximal second quarter of intact females’ urethras show a reduced concentration of LH receptor mRNA. However, the limited number of intact females were influenced by progesterone levels at the time of biopsy (Reichler et al., 2007). Due to a decrease in LH levels when progesterone levels are high (Adams, 2011), the results cannot be relied upon because of the hormonal interaction. Additionally, there are concerns with bias due to the small sample size of intact female dogs.

Reichler et al. (2004) and Reichler et al. (2005) studied LH and follicle-stimulating hormone (FSH) plasma levels by taking blood samples from female dogs. Ten Beagle bitches were studied over a fifty-five week period. After two weeks of the study they all underwent ovariectomy. Analysis of their blood plasma showed that concentration levels of LH and FSH increased rapidly post ovariectomy, then stabilised at week forty-two (Reichler et al., 2004). In comparison, Reichler et al. (2005) sampled the blood of 191 intact and 308 neutered female dogs of varying breeds. It is stated that efforts were made to sample neutered Boxers and GSD females, which could have affected the results as Boxers especially have been reported to have increased odds of developing AUI (Bleser et al., 2011). This study found that LH and FSH plasma concentration levels rapidly increased in bitches during the first year post neutering, then stabilised at a level ten times higher than the level of intact bitches. Both studies, Reichler et al. (2004) and Reichler et al. (2005) used the same method to analyse FSH and LH within the plasma. Before analysis, anoestrus was confirmed in the intact females within both studies, which increases the reliability of the findings. Similarly, Coit, Dowell and Evans (2009) confirmed anoestrus within the entire female group by assessing the ovaries during necropsy. In contrast, Reichler at al. (2007) studied bitches that were considered to be anoestrus in the ovulation phase and metestrus. This was determined by examination of the vulva, the time since the last heat and the level of progesterone from the blood sample.

Reichler et al. (2005) state that neutered incontinent females have lower LH and FSH concentrations than neutered continent females. In contrast to this, Coit, Dowell and Evans (2009) reported that neutered female canines, who suffered with AUI, had the highest levels of LH receptor mRNA. The difference within the findings could be due to the contrasting experimental designs. Reichler et al. (2005) studied LH and FSH levels by collecting and analysing blood samples, whereas Coit, Dowell and Evans (2009) studied bladder biopsies for LH receptor mRNA. Using bladder biopsies focusses specifically on LH receptor mRNA in the urinary system which could increase the relevance of the results.

The exact pathophysiological causes of AUI in neutered female dogs is still unknown, however it is a common theme that female dogs experience endocrine changes such as gonadotropin concentration levels (Coit, Dowell and Evans, 2009). There are a number of factors that have been reported to have an effect on the likelihood of AUI occurring in female dogs, such as neuter status, weight, breed, age and age of spaying (Bleser et al., 2011).

 

2.2  Risk factors associated with acquired urinary incontinence

O’Neill et al. (2017) suggest that neutering female dogs increases the development of AUI by 2.23 times. This study also found that out of the female dogs with AUI studied, 95.9% were neutered. In comparison, Hart et al. (2016) found that out of all intact female dogs, none presented AUI. Whereas, within the neutered female dog group, 7% presented with AUI, a significant difference. These studies suggest that neutering is a significant risk factor for the development of AUI.

 

2.3  Age of female canines at time of neuter and development of acquired urinary incontinence

Studies have reported that the age of bitches at the time of neutering can have an effect on the development and risk of AUI presentation. Veronesi et al. (2009) state that an increased age of bitches at the time of neuter results in an earlier onset of AUI. Although, the method of data collection lacked detail making the study less repeatable, further studies supported these findings (Bleser et al., 2009; Hart et al., 2016). Earlier neutering at the age of four to six months leads to a reduced occurrence of AUI (Bleser et al., 2011). However, it is important to note that the findings reported in this study were a trend and not statistically significant. Furthermore, Hart et al. (2016) suggest that as the age at the time of neuter increases, the occurrence of AUI significantly increases. Bitches neutered at an age below six months show a 4.7% occurrence of AUI, whereas bitches neutered at an age over six months show a 7.3% of AUI.

In contrast to these previous studies, Byron et al. (2017) state that there is no significant difference in the age at neutering and the development of AUI. This study comprised female canine case data, collected from sixteen veterinary practices in continental USA. In comparison, Hart et al. (2016) collected data from University California-Davis. The risk of AUI development are suggested, by O’Neill et al. (2017), to be increased by 3.86 times when the age of the female dog is between nine and twelve years. The study suggests that older female dogs are more likely to develop AUI than females under the age of three. However, this could be due to deterioration caused by aging, rather than pathophysiological changes caused by neutering.

 

2.4  Acquired urinary incontinence and associated breed predispositions

There have been recorded associations between particular breeds and AUI. Within a recent study, 78% of female dogs presenting with AUI were purebred (O’Neill et al., 2017). In contrast, an earlier study from North West Italy states that the highest occurrence (29%) of neutered bitches diagnosed with AUI post neuter were crossbreeds (Veronesi et al., 2009). However, this study does not specify the method of data collection or the sample size. In contrast O’Neill et al. (2017) collected electronic patient records from 119 veterinary practices in England.

O’Neill et al. (2017) suggest that Irish Setters, Dobermans, Bull Mastiffs, rough Collies, Dalmatians and Boxers have the highest odds of UI. Bleser et al. (2011) agree, stating that Dobermans, Irish Setters and Boxers have increased odds of AUI. Additionally it is noted that English Springer Spaniels and Gordon Setters have increased odds. However, the study does not state the odd ratios unlike O’Neill et al. (2017) which gives the latter study increased reliability and comparability. Furthermore, results published by Veronesi et al. (2009) show that crossbreeds, GSDs and Boxers make up the largest proportion of female neutered dogs diagnosed with AUI. Reichler et al. (2005) also state that Boxers have a risk of 64% to develop AUI. Large and giant breeds make up all but one of the purebred female dogs presenting with AUI (Veronesi et al., 2009). There are some concerns though as the proportion of large and giant breeds within the study has not been stated, therefore bias from the sample could affect the results. This finding could also be associated with the effects of weight, rather than the risk of large or giant breeds.

 

2.5  The effect of weight on the risk of acquired urinary incontinence development

A number of studies have provided evidence that heavier female dogs are more likely to develop AUI. For example, Reichler et al. (2005) state that females with a body weight of over twenty kilograms have an increased risk of developing AUI. However, it was noted that it is difficult to distinguish between the influence of breed and body weight. More recently, a study suggested that female dogs with a bodyweight over thirty kilograms have 2.94 times the odds of developing AUI (O’Neill et al., 2017). Furthermore, this study states that bitches weighing at or over the mean bodyweight for their breed have 1.13 times the odds of developing AUI, compared to bitches below the mean. In agreeance with these studies, Byron et al. (2017) state that the heavier the adult weight of the female dog, the higher the risk of AUI development. Most importantly, the study suggests that the weight at the time of diagnosis and the age at the time of neuter has a significant risk on AUI development. For example, the risk of AUI would decrease in a female dog weighing more than twenty-five kilograms, for every month that neutering is delayed with the first year.

 

3.0  Conclusion

AUI is common within female dogs, specifically the neutered population. The exact causes are still unknown. However, changes post neutering such as increased levels of LH and FSH, as well as increased mRNA expression of LH and GnRH have been reported. Links have been made between these hormonal changes and the development of AUI. Furthermore, bladder contractility has been reported to deplete with the increase of mRNA expression for LH and GnRH, with bladder tissue biopsies.

A number of risk factors including age, weight, breed, and age at neuter have been identified. A common theme is that the lower the age at the time of neuter, the lower the risk of AUI development and the later the onset. Furthermore, an increased risk of AUI development has been recorded in female dogs that weigh more. However, this cannot always be separated from breed influence. Common breeds such as Irish Setters, Dobermans, Boxers and English Springer Spaniels have an increased prevalence and risk of AUI. Based upon this review, veterinary nurses should be advising owners that neutering females does carry the risk of AUI development, caused by hormonal changes affecting bladder function. Earlier neutering and lower bodyweight has been found to carry less of a risk for AUI.

 

References

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