Introduction

Overpopulation of domestic pets is a significant problem worldwide. Neutering programs are widely promoted by animal shelters to reduce indiscriminate pet breeding, but compliance with programs which operate after adoption is estimated to be < 60% (Spain et al. 2004). In order for sterilisation programs to be maximally effective at controlling overpopulation, animals should be neutered prior to adoption. Early age neutering (EAN) from 8–16 weeks of age was officially approved by the AVMA in 1993 but it has not met with uniform acceptance from veterinarians. In this session, we will look at the rationale behind EAN, the concerns raised about EAN, and examine the evidence relating to safety and long-term effects.

Hormonal and Skeletal Effects of Gonadectomy

In dogs, puberty varies from 6 months in small breeds to up to 24 months in some large breeds. In the early postnatal period, the hypothalamus is sensitive to negative feedback from testosterone and/or estrogens (Salmeri et al. 1991). As the animal matures, hypothalamic sensitivity declines, allowing gonadotrophin-releasing hormone to be secreted. The timing of neutering may affect the hypothalamic control of growth in poorly understood ways.

Although not essential for growth, gonadal hormones influence growth, development, maintenance, and aging of the skeleton. Delayed epiphyseal closure and subsequent increase in long bone length (eunuchoidal stature) have been noted in many domestic species, including cats (Stubbs 1996). Large and obese male cats are thought to be at greater risk of capital physeal slip fractures (SH1 femoral head) at the age of approximately 1 year. Duerr et al. (2007) reported that early neutering was a significant risk factor for development of excessive tibial plateau angle in large-breed dogs, a risk factor for cranial cruciate ligament rupture (CCLR). Barnes and Baltzer (2012) have since reported the relative risk of CCLD was 1.6x for dogs spayed < 6 months (95% [CI] = 1.28–1.99), cf > 6 months and there was a significant inverse linear trend toward decreased incidence of CCLD with increasing age at neutering.

Therefore, neutering is not benign, but the health benefits (elimination of the risk of pyometra/testicular neoplasia, reduction in the risk of mammary neoplasia [recently queried], decreased risk of trauma due to roaming) generally outweigh the negative effects.

Many veterinarians already recommend neutering prior to puberty, but after the primary vaccination course. With the advent of modern vaccines with less requirement for boosters, dogs and cats can now be fully vaccinated from 12 or 14 weeks. So should we be neutering prior to the traditional 5–6 months?

One common concern regarding EAN is the perceived anaesthetic risk in pediatric patients (6, 10, 12 weeks).

Specific concerns raised about EAN in cats include: Is there an increased risk of urethral obstruction? Does it increase the incidence of obesity and behavioural problems?

Specific concerns raised about EAN in dogs include: behavioural problems, alterations in growth rate, peri-vulval dermatitis, increased risk of urinary incontinence and obesity, as well as perceived increased anaesthetic risk.

1. Anaesthetic considerations

The stated reasons for increased risk include:

 Less reserve of physiological function

 Greater chance of drug overdose (usually due to drug calculation errors)

 Increased utilisation of oxygen and glucose

 Increased risk of hypothermia

A detailed discussion is beyond the scope of these proceedings; see Walsh and Worth 2008.

Summary of Anaesthetic Management and Support of the Paediatric Patient

2. Evidence of Short-Term Safety of EAN

Short-term results and complications of prepubertal gonadectomy in cats and dogs (Howe 1997)

 Prospective randomised study of 775 cats and 1213 dogs: divided into 3 groups < 12 wks, 12–23 wks, 24+ wks. Student surgeries Texas A&M

 More minor complications were noted in the > 24-wk group.

 No significant differences were noted in major complications (low = 3% all groups)

 Five cardiac arrests spread over the groups. Overall anaesthesia related complications 10/1988 = 0.5%

 Twelve died or were euthanized due to infectious disease (parvo & respiratory disease). Underscores the need to hold animals in shelters prior to surgery and quarantine

 Surgical time was lowest in youngest group

3. Evidence of Long-Term Effects - Cats

Howe et al 2000 studied 263 cats neutered as a part of the 4th year surgical program at Texas A&M. Cats were divided into two groups: neutered < 24 wks and ≥ 24 wks with a median followup of 3 years. Differences between age groups were not detected in the incidence of overall behavioural problems, destructive behaviour or inappropriate elimination. There were more reported urinary tract conditions in the older cats, but differences were not shown in the incidence of obstruction.

In cats, the largest study to date is Long term risks and benefits of early age gonadectomy in cats, Spain, et al. 2004. Similar design to the canine study by the same authors. 1660 cats with a median followup of 3.9 years (range 0.25 to 11.7 years). For most of the 47 outcomes studied, no association was found between frequency of the outcome and age at gonadectomy (< 5.5 or ≥ 5.5 months). Importantly, urethral obstruction, obesity, diabetes and long-bone fracture were not more likely in cats neutered younger. Incidence of asthma, gingivitis and abscesses (in the first few years of life) were significantly decreased by EAN. Shyness was increased and aggression towards veterinarians and urine spraying was decreased in EAN animals (these positive effects may decrease relinquishment of EAN animals).

4. Evidence of Long-Term Effects - Dogs

In dogs, the largest study to date concerning EAN was conducted on SPCA shelter animals in New York between 1989 and 1998 (Long term risks and benefits of early age gonadectomy in dogs, Spain, et al. 2004). It was a retrospective cohort study by owner survey and veterinary records where available, with a median followup of 4.5 years (range 0.3 to 11.3 yrs). Dogs were divided by age into < 5.5 months (EAN) versus > 5.5 months (traditional age neutering). Fifty-six outcomes were evaluated and analysed by logistical regression (behavioural) or survival analysis (medical conditions). Of 3501 adoptions, data was available on 1842.

Overall, 4 medical conditions and 8 behavioural conditions were significantly associated with age at gonadectomy:

Hazards ratio, confidence interval, P value

Of the dogs that developed cystitis, only 2 had a repeat bout, and none had chronic cystitis; therefore, the clinical relevance of the increased HR is doubtful.

The incidence of clinically relevant hip dysplasia was actually lower in the early neutered group so that HR may represent a type I error. Among female dogs, decreasing age at gonadectomy on a continuous scale was associated with increasing incidence of urinary incontinence that required medical treatment. Bitches gonadectomized before 3 months were at the greatest risk compared to those neutered at > 3 months (HR 3.46, P < 0.001). This corresponded to a 12.9% incidence in the first 6 years after gonadectomy for the < 3-month group versus 5.0% for the > 3-month-old group. By way of comparison, urethral sphincter mechanism incompetence is uncommon in intact bitches (0.2–0.3%). None of the 49 bitches with urinary incontinence were relinquished to a shelter or another owner and the rate of euthanasia was not higher than the overall rate, probably due to older age of onset and strong bonding established. However, urinary incontinence was a cause of disharmony in 10–20% of affected households (de Bleser et al. 2011).

Noise phobias and sexual behaviours were increased (low HR), whereas obesity, separation anxiety, escaping behaviours, inappropriate elimination when frightened, and relinquishment for any reason were decreased.

The authors concluded that for male dogs, EAN was beneficial, but that for bitches the best time to neuter was more difficult to determine. Because urinary incontinence was affected by age, it is reasonable to conclude that female dogs should not be gonadectomised until at least 3 to 4 months of age.

A protective effect of desexing on the risk of malignant mammary neoplasia in bitches has been accepted since the 1960s. However, a recent systematic review by an epidemiological group at the Royal Veterinary College (UK) concluded that there was only weak evidence of such a protective effect, and it was not consistent/strong enough to be the basis for recommendations (Beauvais et al. 2011a). The same group also looked at the evidence that desexing and especially EAN increase the risk of urinary incontinence. They assessed the evidence as weak and at moderate risk of bias and again felt that recommendations as to age of neutering require further research (Beauvais et al. 2011b).

Recommendations: My View

Male and Female Cats

There are no significant negative effects of EAN, and as long as anaesthetic differences are taken into account, male and female cats can be neutered from 8 weeks of age.

Male Dogs

There are no significant negative effects of EAN, and as long as anaesthetic differences are taken into account, male dogs can be neutered from 8 weeks of age. However, the risk of CCLR should be considered in large and giant breeds and desexing delayed till puberty.

Female Dogs

EAN is appropriate for shelters with excess puppies, where population control is paramount, EAN before adoption should be prioritised regardless of age.

For shelters attempting to lower relinquishment rates of adoptees, waiting till after 3–4 months of age should decrease incidence of urinary incontinence.

For veterinarians in private practice, the ideal time for gonadectomy in bitches is after the completion of vaccination and at least 4 months of age but still prepubertal. The dog's breed can be taken into consideration as large breeds mature later and are more susceptible to urinary incontinence and CCLR.

References

1.  Barnes K, Baltzer W. Incidence of cranial cruciate ligament rupture in dogs ovariohysterectomised at less than 6 months of age. 39th Annual Conference of the Veterinary Orthopedic Society, Crested Butte, CO; 2012.

2.  Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs - a systematic review. Journal of Small Animal Practice. 2012a;53:314–322.

3.  Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of urinary incontinence in bitches - a systematic review. Journal of Small Animal Practice. 2012b;53:198–204.

4.  Duerr FM, et al. Risk factors for excessive tibial plateau angle in large-breed dogs with cranial cruciate ligament disease. Journal of the American Veterinary Medical Association. 2007;231:1688–1691.

5.  Howe LM. Short-term results and complications of pre-pubertal gonadectomy in cats and dogs. Journal of the American Veterinary Medical Association. 1997;211:57–62.

6.  Howe LM, Slater MR, Boothe HW, Hobson HP, Fossum TW. RSPCA Research Report. Early-age desexing of dogs and cats, 2010. http://www.rspca.org.au/assets/files/Science/EADResearchReportAug2010.pdf. (VIN editor: This link was modified on 2/15/13).

7.  Spann AC, Wilkiw WS. Long-term outcome of gonadectomy performed at an early age or traditional age in cats. Journal of the American Veterinary Medical Association. 2000;271:1661–1665.

8.  Salmeri KR, Olson PN, Bloomberg MS. Elective gonadectomy in dogs: a review. Journal of the American Veterinary Medical Association. 1991;198:1183–1192.

9.  Spain C, Scarlett JM, Houpt KA. Long term risks and benefits of early age gonadectomy in cats. Journal of the American Veterinary Medical Association. 2004;224:372–379.

10. Spain C, Scarlett JM, Houpt KA. Long term risks and benefits of early age gonadectomy in dogs. Journal of the American Veterinary Medical Association. 2004;224:380–387.

11. Walsh VP, Worth AJ. Early age neutering. NZVA Conference 2008, VetLearn, NZ.