Do we need a paradigm shift in canine neutering?

Source:dvm360

The history of spay and neuter

In the 19th century, urbanization and increased pet ownership were catalysts for performing neuters. Owners found a pet’s heat cycle in their home to be inconvenient, so neuters were performed to eliminate this annoyance. In the 1950s, neutering became the solution to overpopulation of pets in New York City. By the 1960s, when compliance to neuter contracts was low, humane groups made neutering a requirement prior to pet adoption. In 1975, the Maryland Society for the Prevention of Cruelty to Animals recommended early neuter to get as many pets neutered before adoption as possible. Then in 1993, the American Veterinary Medical Association supported early neuter for the purpose of curbing overpopulation.

A standard of practice

Neutering young dogs between 6 and 9 months of age is now common practice. This practice in the US contrasts with many European countries, where neutering is quite uncommon. For example, in Norway, it’s illegal to neuter a pet unless there is a valid medical reason. With these opposing philosophies, veterinarians in the US have begun questioning the validity of the juvenile neuter approach. Practitioners are asking: What are the clinical effects of removing these hormones? How early is too early? Are there even benefits to spay and castration?

Spay and neuter benefits

Preventing mammary tumors is a well-known justification for spaying females before their first heat. Every vet student reads the classic paper, which reports only half a percent of female dogs developing mammary tumors if they are spayed prior to their first heat.¹ In contrast, 26% develop tumors if they are spayed after their second heat. Removing these hormones has also been shown to decrease or eliminate the development of reproductive tract tumors and pyometra, which affects nearly 1 in 4 intact females by age 10.² It also eliminates other things, such as bleeding in the house and unwanted pregnancies. Additionally, there is evidence that these pets may live longer.³

For males, castration has similar benefits in reducing reproductive tract disease. Castration removes the risk of testicular cancer, which affects 27% of intact males.⁴ Removing cryptorchid testicles is particularly important, as they have a significantly higher risk of developing tumors.⁵ There is also a reduced risk of prostate hyperplasia, perianal tumors, and perineal hernias with castration.^6,7 There is even indication that castration decreases unwanted behaviors, such as urine marking in the house, roaming (which can lead to injury and reduced lifespan via fights and getting hit by cars), and mounting.⁸

Unintended consequences

Spay and castration have a handful of negative effects worth noting. To start, it is a surgical procedure, which carries an inherent 6% risk of complications, including incisional infections and anesthetic events.⁹ It also increases the risk of urinary incontinence in females.^10-12 Fewer than 1% of intact females develop incontinence, but 4% to 20% of spayed females are at risk. This risk goes up when spayed before 3 months of age.¹² Another concern is leaving a pet with immature genitalia.¹³ Immature, recessed vulvas can lead to urinary tract infections and perivulvar dermatitis. Spay and castration have also been reported to increase the risk for hypothyroidism and vaccine reactions.^14-15 Further, spaying can predispose females to aggression toward family members.¹⁶ There may also be a faster progression of cognitive impairment in neutered pets, especially males.¹⁷ Finally, obesity, which has its own set of risks, is much more common in both spayed and castrated pets.¹²

In addition to the above medical concerns, there are also various cancers that have been shown to be more prevalent in neutered pets. Mast cell tumors, hemangiosarcoma, lymphoma, and osteosarcoma have been reported to occur more frequently in neutered pets.^18-21 A study of Rottweilers found that when spayed prior to 1 year of age, females had nearly 3 times the risk of developing osteosarcoma, and males had almost 4 times the risk.²² For neutered males, there is also about 4 times the risk of developing prostate cancers.²³

There are also orthopedic considerations for pets neutered at a young age. Early neuter has been shown to delay growth plate closure, resulting in taller pets.¹³ By altering bone growth, a pet’s conformation is affected and may predispose them to excessive tibial plateau angles. When the tibial plateau angle is excessive, there is a significantly greater risk of developing cranial cruciate ligament disease in 1 or both knees.²⁴ Hip dysplasia has also been found to be more common in altered pets, especially when neutered before 6 months of age.¹²

Predisposition considerations

Although identifying these unintended consequences is important, the impact of being affected by these diseases should also be considered. We must consider factors, such as the frequency of the disease. Even though an increased risk of developing a specific cancer is identified, this cancer may only occur in a very small percent of the population. For example, osteosarcoma affects less than 20% of the population.²⁵ We must then also consider the severity of the disease, availability of treatments, and how effective these treatments are for the diseases with an increased risk from neutering. Lastly, the impact of these diseases on specific breeds should also be individually evaluated. Luckily, multiple large studies—primarily out of the University of California, Davis—have recently attempted to tackle this complicated decision-making process for common breeds of dogs, including golden retrievers, Labrador retrievers, and German shepherds.^26-28 These studies have made specific recommendations for male and female pets of more than 30 different breeds and mixed breeds.²⁹

When to spay and neuter

If broad conclusions are made from all these studies, then the standard recommendation of spaying prior to the first heat cycle can be continued. Early neutering is also still warranted in shelters to help curb overpopulation. However, castration after 2 years of age (or full maturity) is likely a more appropriate recommendation for male dogs.

That said, based on the evidence presented in these more recent studies, shifting away from the 1-size-fits-all approach of neutering at 6 to 9 months of age is warranted. An open discussion of the pros and cons of neutering at various ages for different breeds should be discussed with every responsible owner. For example, female golden retrievers could be left intact because of their high risk of developing common cancers after being spayed.²⁹ In these unaltered pets, owners would then need to diligently monitor for signs of pyometra or mammary tumors to allow for prompt treatment.