Neutering

is the process of surgical removal of the gonads or reproductive organs in male and female animals. It may also be termed castration in male animals and ovariohysterectomy or spaying in females. This surgical procedure will result in an abrupt and lifelong absence of endogenous gonadal hormones and will prevent unwanted pregnancy. It has become routine practice in many countries including the US and many European countries, with neutered animals comprising up to 54% of the pet population in the UK. For many rescue and adoption organisations, neutering is mandatory either before or after the animal has found a new home.

Benefits of Neutering

Population control:
The main indication for neutering is population control and promotion of responsible pet ownership. Abandonment of animals, unexpected vet bills and increasing numbers of puppies and kittens can all be addressed by promotion of neutering when the pet is young. Most vets recommend a minimum age of 6 months to perform the surgery currently but many tailor those recommendations especially when discussing the procedure in medium to large breed dogs. Given this is an elective procedure with both positive and negative lifelong implications, it is important for vets to feel confident in their advice to owners, based on recent and reliable evidence.

Behaviour:
Undesirable behaviours commonly observed in entire animals include roaming, territory marking, vocalising, fighting and humping. Socialisation with other animals can be a challenge and must be carefully managed when female cats and dogs are in season to avoid unwanted pregnancy.
With respect to male cats, neutering can help to prevent injuries from fighting- one retrospective study showed a decrease in the incidence of abscesses and injury in male cats neutered before 5.5 months of age .  
Neutered pets often adapt more favourably than an entire cat or dog to an indoor or more restricted environment.

Health:
Neutering also has health benefits- the procedure removes the risk of uterine infections in females and decreases the incidence of prostatic disease in male animals.
Neutering has been long considered an effective way to decrease the incidence of certain neoplasms, given that exposure to sex hormones have been recognised as a risk factor [1, 2]. Hormone receptors are present in neoplastic tissue in both dogs and cats including mammary and perianal gland tumours [2]. Both dogs and humans with mammary tumours have been noted to have higher levels of serum oestrogen compared to healthy controls of the same age and oestrogen has been demonstrated to have a pro-proliferative effect [1].

Evidence on long-term effects post gonadectomy
Research investigating the optimal age to neuter an animal has revealed several trends seen in neutered animals versus intact animals. Retrospective epidemiological studies have been used to compare a population of entire to neutered pets to investigate the long-term health effects of gonadectomy.

Lutenising hormone:

• Once an animal has been neutered, the normal feedback pathway from oestrogen and testosterone on the hypothalamus and pituitary is no longer present, resulting in elevated levels of luteinising hormone (LH) [3]. This release is continuous and levels are elevated beyond the levels observed in entire animals- by up to 30-fold [3]. LH receptors exist on organs not associated with reproductive function including the gastrointestinal tract, urinary tract, thyroid and adrenal glands. Certain neoplastic tissues also express LH receptors. The long-term effect of this sustained release of excess LH is currently an area of research and investigation and may affect multiple body systems.

Effects on body weight:

• It has been documented that neutering can lead to obesity in pets. Neutered cats have been quoted as being 3.4x more likely to become obese than intact cats [4]. Gonadectomy induces a change in metabolic rate and appetite resulting in energy expenditure decreasing and food intake increasing within the first 3 months following the procedure [5]. A consequence of the neutering procedure is an alteration in hormonal profile- with decreases in oestradiol and testosterone, and increases in insulin-like growth factor 1 (IGF-1), prolactin and leptin. Leptin, in particular, has effects on hunger and the feeling of satiety. If exogenous oestradiol is given to cats following gonadectomy, food intake remains stable post-surgery indicating this hormonal disruption has a direct association with food intake [6].

Behaviour and Ageing

• Neutering can cause an increase in the rate of onset of cognitive dysfunction. Cognitive dysfunction syndrome is a disorder of older animals characterised by memory loss, confusion and disruption of normal sleep-wake patterns [3]. The absence of oestrogen is associated with accelerated brain ageing in females and elevated LH levels are implicated in the development of Alzheimer’s disease in humans [3, 7].

Effects on the urinary tract

• Sphincter Mechanism Incompetence has a known association with early neutering in female dogs- resulting in urinary incontinence particularly in large breed dogs [3]. LH receptors are found in all regions of the lower urinary tract in the neutered female dog and in far greater numbers when compared to intact females [3]. Treatment for this condition is to provide synthetic oestrogen supplementation or GnRH agonists and these medications will result in a decrease in circulating LH [3].
• Urolith development- the effect of neutering on urolith development depends on the type of urolith. Cystine uroliths are more likely to be associated with entire dogs, neutering is routinely offered as a preventative action in these cases [8]. However when considering the incidence of calcium oxalate and struvite stones, a study by Banfield Pet Hospital found that calculi were 3x more likely to develop in neutered dogs (both male and female) [3].

Effects on the Musculo-skeletal system

• Recent research has shown than there can be an increase in risk of certain degenerative joint diseases (DJD) associated with neutering in dogs [9]. These include hip dysplasia (HD), cranial cruciate ligament injury (CCL), elbow dysplasia (ED) and osteochondrosis dissecans (OCD).
• The increase in DJD in neutered animals is most significant when considering cases of CCL [9] and the hypothesised theory is that neutering before growth plate closure and skeletal maturity delays the process. Extra growth can result in alterations to the tibial plateau angle and this is known to be a risk factor in CCL [9]. German Shepherd dogs, Labrador Retrievers and Golden Retrievers have been identified as being most at risk of this increased incidence of CCL [10].
• Fractures in cats- capital physeal fractures (fractures of the growth plates in the proximal femur) occur spontaneously and one study reported 96% of cats noted with this injury were neutered males [11]. Again delayed physeal closure has been hypothesised to contribute to this injury [11].

Effects on development of neoplasms

• The association of neutering female dogs and the decreased incidence of malignant mammary tumours has historically been a reason to recommend the procedure- however the evidence to support this relationship is limited. Further research in this area is required to determine whether neutering should be recommended before the first or second season particularly in breeds where other health problems have been described [12].  
• Epidemiological studies have reported an increased incidence of some cancers including lymphosarcoma (LSA), osteosarcoma (OSA) and mast cell tumours (MCT) in dogs that undergo early neutering [1, 2]. These neoplastic tissues express LH receptors in differing frequencies and the effect of increased levels of LH is being investigated. Research is ongoing as to variations between breeds, sexes and whether the time of neutering affects the incidence of these neoplasms in later life [3].
• An increased risk of haemangiosarcoma has been reported in several studies tracking the long-term health consequences of early spaying. This is thought to be due to the protective effect of oestrogen when considering the incidence of non-oestrogen receptor neoplasms, as is also seen in human studies [1].

Summary

Considerations for whether to neuter a dog and at what age should be made on a case-by-case basis based upon lifestyle, demands of working and breed of the patient [2]. This approach is recommended by the American Veterinary Medical Association based upon the recent literature demonstrating an association between early neutering and orthopaedic and neoplastic disease [13]. A recent retrospective study at the University of California Davis analyzed data from 35 different breeds of dog to determine whether neutering, and age at which procedure was performed, affected incidence of DJD or cancer [10]. Records from a 15-year period were reviewed and included over 15,000 dogs.

This study is presented to provide clinicians an easy- to-access reference detailing the evidence and risk assessment in each of the 35 breeds. Timing of neutering can be advised to the pet owner based on this reference and recent evidence [7, 10]. Neutering in every case should involve a discussion between the owner and the veterinarian to determine what best suits their family and their pet.

Nutrition following neutering

The choice of whether or not to neuter and when, is potentially more complex than previously considered. If the pet owner does wish to neuter, the veterinarian can advise how best to proceed following surgery. Nutrition is one parameter we can control with good compliance from owners. Epidemiological data has shown that neutering is a risk factor for obesity especially in male cats and dogs [5]. This is of particular importance as in this region a large proportion of cats are maintained completely or mostly indoors, due to the weather conditions and the urban environment in the UAE. This is another risk factor for obesity, due to lack of activity. Obese and overweight animals are much more at risk of orthopaedic disease, urinary tract disease and diabetes mellitus [5]. Given certain breeds show a significant increase in DJD post-surgery, vets and pet owners must be cautious to monitor for weight gain.

There are several points that can be highlighted to owners at the time of the procedure- prevention is better than cure! If owners are aware of the risk, they can pay closer attention to their pet and be proactive in management.

• Ad-lib feeding post the neutering procedure is associated with increase in body weight and body condition due to an increased food intake [5]. Portion management by weighing food daily should be recommended as standard practice.
• An adjustment of the diet to one with lower calorie density can avoid nutrient-deficiency and begging behaviour seen when maintenance diets are restricted.
• The addition of a wet food component into the diet may also be advantageous as wet food is less calorie-dense due to the moisture content and can provide a satiating effect.

References

1. Karin, U.S., et al., The estrogen effect; clinical and histopathological evidence of dichotomous influences in dogs with spontaneous mammary carcinomas. PLoS ONE, 2019. 14(10): p. e0224504-e0224504.
2. Smith, A.N., The Role of Neutering in Cancer Development. Veterinary Clinics of North America: Small Animal Practice, 2014. 44(5): p. 965-975.
3. Kutzler, M.A., Possible Relationship between Long-Term Adverse Health Effects of Gonad-Removing Surgical Sterilization and Luteinizing Hormone in Dogs. Animals (Basel), 2020. 10(4).
4. Effects of dietary fat and energy on body weight and composition after gonadectomy in cats. American journal of veterinary research, 2004. 65(12): p. 1708-1713.
5. Larsen, J.A., Risk of obesity in the neutered cat. Journal of Feline Medicine & Surgery, 2017. 19(8): p. 779-783.
6. Cave, N.J., et al., Oestradiol, but not genistein, inhibits the rise in food intake following gonadectomy in cats, but genistein is associated with an increase in lean body mass. Journal of animal physiology and animal nutrition, 2007. 91(9-10): p. 400-410.
7. Benjamin, L.H., et al., Assisting Decision-Making on Age of Neutering for Mixed Breed Dogs of Five Weight Categories: Associated Joint Disorders and Cancers. Frontiers in Veterinary Science, 2020. 7.
8. Florey, J., V. Ewen, and H. Syme, Association between cystine urolithiasis and neuter status of dogs within the UK. The Journal of small animal practice, 2017. 58(9): p. 531-535.
9. Hart, B.L., et al., Neutering of German Shepherd Dogs: associated joint disorders, cancers and urinary incontinence. Veterinary Medicine & Science, 2016. 2(3): p. 191-199.
10. Benjamin, L.H., et al., Assisting Decision-Making on Age of Neutering for 35 Breeds of Dogs: Associated Joint Disorders, Cancers, and Urinary Incontinence. Frontiers in Veterinary Science, 2020. 7.
11. Schwartz, G., Spontaneous capital femoral physeal fracture in a cat. The Canadian veterinary journal = La revue veterinaire canadienne, 2013. 54(7): p. 698-700.
12. Beauvais, W., J.M. Cardwell, and D.C. Brodbelt, The effect of neutering on the risk of mammary tumours in dogs–a systematic review. The Journal of small animal practice, 2012. 53(6): p. 314-322.
13. Alannah, J., et al., Attitudes of Veterinary Teaching Staff and Exposure of Veterinary Students to Early-Age Desexing, with Review of Current Early-Age Desexing Literature. Animals, 2017. 8(1): p. 3-3.

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Written on November 11, 2021