Deciphering the Current Neuter Debate
Neutering (the general term for removing the reproductive organs, whether male or female) is now very commonplace for cats and dogs in the United States. However, this wasn’t always the case. Neutering only became available to US families in the 1930s and did not gain traction until the 1970s. In its early days, neutering was part of a movement to reduce unwanted pets and euthanasia rates. For example, in 1971, shelters in Los Angeles, California euthanized an astonishing 110,835 unwanted dogs and cats. By 2007, this number had been reduced to 15,009, an 86% decrease. This decrease was attributed in large part to widespread neutering efforts.
This success continued to grow as veterinarians nationwide began recommending neutering for virtually all canine and feline patients in the US between 6 and 9 months of age. This not only aided in population control by reducing unwanted litters, but in many cases also improved pet quality of life and strengthened the bonds between owners and pets . For example, eliminating heat cycles and unwanted discharge allowed female dogs to spend more time in the home. Neutering male cats helped reduce in-home urine spraying, while neutering male dogs kept them from wandering so much.
In the 1990s, shelters adopted a new standard of having all cats/dogs neutered BEFORE adopting them out to new homes. This began the practice of “pediatric neutering” or neutering between 6 and 14 weeks of age. Collectively, the number of neutered pets in the US skyrocketed. Now, the prevalence of neutered companion animals in the US is so high, it is difficult to even study unaltered pets or the diseases that affect them, because they are so uncommon.
However, in recent years, new research has started to arise about the potential risks of neutering. It seems that we have effectively solved a number of societal concerns with widespread neutering, but new issues for the individual pet may be surfacing. Some of these are fairly well known, such as the risk of certain cancers, urinary disorders, or orthopedic/bone abnormalities. Questions about when/if to neuter cats and dogs are growing. We hope to address a few, more prominent, of those questions here.
The controversy surrounding the benefits and risks of neutering cats and dogs is intricate to say the least. The concerns being raised are diverse, ranging from bone development to behavior to urinary incontinence to a myriad of cancers. Many of these have little, no, or conflicting research backing them. Here we will address several of the most pressing issues, but also those with the most scientific support. As for the rest, the jury is still out.
The Benefits of Neutering
- Neutering can be expected to greatly improve “sexually dimorphic behaviors”. Sexually dimorphic behaviors are those behaviors most commonly displayed by one sex, such as mounting and urine spraying. One common misconception is that neutering will resolve aggressive behaviors. However, it will only improve aggression associated with the presence of females in heat, and even that is not a guarantee. Other forms of aggression (fear, resource guarding, territory) will likely be unaffected. The behaviors that will be helped, aside from aggression, include urine spraying, pet roaming for the purpose of mating, and likely reduction of mounting behaviors.
- Mammary Gland Cancers
- Mammary gland cancers are the most common tumors of female dogs and the third most common of female cats. Malignant mammary gland cancers (meaning those mammary gland cancers that are aggressive and tend to spread) are the most common malignant tumor of dogs. In 77% of dogs with malignant mammary gland neoplasms the tumor has already spread (usually to the lungs) by the time of diagnosis. The spread and aggressive nature of the tumors themselves can make treatment options limited and largely unsuccessful. We truly cannot overemphasize the serious and widespread nature of this cancer in unspayed females.
- The good news is that we can significantly improve this cancer risk by neutering. Being sexually intact raises the risk of mammary gland cancers sevenfold. Neutering not only reduces the general risk, but when appropriately timed, neutering can virtually eliminate the risk altogether. That is, compared to the incidence of mammary gland cancers in intact dogs, dogs neutered before their first heat cycle have only a 0.5% risk. After their first heat, neutering reduces their risk to 8%. After 2 heats, neutering reduces risk to 26%. As you can see, even just one heat cycle can make a significant, negative impact on cancer risk, hence the recommendation to neuter between 6 and 9 months of age, BEFORE any heat cycles.
- Pyometra (uterine infection)
- In the United States, the incidence of pyometra is very low, largely because the majority of our dogs and cats are neutered. However, in countries where neutering is not so widespread, the incidence of pyometra is 15.2-24% with a significant mortality rate of up to 17% in dogs and 8% in cats. The risk of pyometra is higher in dogs and cats that have never had a litter and increases with age. Neutering is the treatment of choice, as well as the only effective form of prevention.
- Non-Cancerous Prostate Disease
- Benign Prostatic Hypertrophy (BPH) is extremely common in intact male dogs, affecting 63.4%. It can also develop very early in life, with 50% of unaltered male dogs having some sign of BPH by 2.4 years of age. BPH can be very uncomfortable, resulting in painful, bloody urination and difficulty defecating. Thankfully, BPH has a very low morbidity rate and is very effectively prevented or treated with neutering. To date, no medical treatments have proven as effective as neutering. BPH also increases the risk of prostatitis, so reducing BPH risk reduces prostatitis risk.
- Other Reproductive Tumors
- Although neutering protects against testicular, uterine, and ovarian tumors all of these tumor types are fairly uncommon and slow to spread. So this benefit is relatively negligible.
The Detriments of Neutering
- Bladder Tumors
- Transitional cell carcinoma is the most common urinary tract canine tumor, with an incidence of 1% in the general population. This risk is raised 2-4 times in neutered dogs depending on the report. Unfortunately, it’s not that cut-and-dried. There are also several strong breed predispositions. Airedale Terriers, Beagles, Collies, Scottish Terriers, Shetland Sheepdogs, West Highland White Terriers, and Wire Fox Terriers are all predisposed to these tumors. Therefore, being a neutered Beagle carries more risk for bladder cancer than being a neutered Labrador.
- Hemangiosarcoma is the most common cardiac (heart) tumor of dogs with a 0.2% incidence rate. In general, larger breed dogs are predisposed, such as Boxers, English Setters, German Shepherds, Golden Retrievers, Great Danes, Labrador Retrievers, Pointers, Poodles, and Siberian Huskies. In neutered female dogs, the risk of cardiac hemangiosarcoma increased five times. Hemangiosarcomas can also develop in the spleen and neutered female dogs have a 2.2 time increased risk for this. For males, being neutered increases the risk of both forms about 2.4 times. Even with aggressive therapies, hemangiosarcoma survival times are very poor - typically less than 1 year.
- Prostatic Neoplasms
- Prostatic neoplasms, typically malignant adenocarcinomas, are relatively uncommon (0.2-0.6% prevalence rate) but are more common in castrated dogs (2.4-4.3 times increased risk). It is hypothesized that the increased risk is due to a lack of sex hormones, but the exact relationship has yet to be determined.
- Obesity is widespread among altered cats and dogs and does seem to be correlated to neuter status. Obesity affects 24% of neutered males and 38% of females. This is likely related to reduced metabolism. Another hypothesis is that sex hormones may actually aid in satiety (feeling full), so a lack of them results in increased appetite/hunger.
- Urethral Sphincter Incontinence (urinary incontinence)
- There is a proven increase in risk for the development of urinary incontinence in neutered female dogs, more so when spayed before 3 months of age. As such, we traditionally neuter later (6-9 months of age) to try to mitigate this risk. If it does develop, urinary incontinence is easily treated and controlled in most cases and has no associated morbidity.
- Endocrine Disease
- A mild increase in the risk of hypothyroidism has been found in neutered cats and dogs (0.2% to 0.3% risk).
- Osteosarcoma is an aggressive bone cancer with 0.2% incidence in dogs. Similar to mammary gland tumors, these tumors are highly malignant and most have already spread to other areas of the body (usually the lungs) by the time the tumor is diagnosed in the bone. There are several clear breed predispositions as well - Doberman Pinscher, Great Dane, Irish Setter, Irish Wolfhound, Rottweiler, St. Bernard. Being neutered increases a dog’s osteosarcoma risk by 1.3-2 times and neutering at less than one year of age may increase this risk further. Unfortunately, the exact relationship is unclear. The true increase in risk may be related to obesity and body weight more than actual neuter status. In one study with Rottweilers, while being spayed significantly increased osteosarcoma risk, it also significantly increased lifespan (by over 2 years).
- Neutering before the growth plates close can elongate limbs, but has not been consistently proven to contribute to fractures or other orthopedic disorders. Growth plates in most dogs are closed by 1 year of age, perhaps closer to 18 months for giant breed dogs.
- Studies trying to link neutering with hip dysplasia have been unable to establish a clear correlation. What studies have shown, is that obesity increases risk, severity, and rate of progression of hip dysplasia. As discussed above, obesity does seem to be related to neutering.
- No studies have been able to identify a direct link between neutering and cranial cruciate ligament (CCL) rupture (like ACL tears in people). However, there is a reported increase in risk for neutered male and female dogs. Again, this may be due, at least in part, to body weight, body condition, and genetics (all proven factors in CCL disease).
- Neutered female dogs have an increased risk of urinary tract infections. However, this may be linked to obesity. Early neutering can lead to a recessed vulva which increases the risk of dermatitis and urinary tract infections. Again, how much of this is due to the actual neuter and lack of sex hormones rather than obesity or other factors is unclear.
- Some studies cite an increased risk for diabetes in neutered cats and dogs. Others claim being neutered reduces risk. Again, it is possible that the true connection is not between the neuter itself but rather other factors such as obesity or simply living longer.
- Neutering may increase the progression of geriatric cognitive dysfunction (dementia/alzheimer's like disease).
- Osteoporosis, a common problem in women with lowering estrogen levels, has not been shown as an issue in neutered males or females, dogs or cats.
- No correlation has been established between neutering and feline lower urinary tract disease or urinary obstruction in male cats.
We at the Clark Fork Veterinary Clinic understand that there is a lot of potentially confusing and overwhelming information out there regarding neutering. Our goal is to serve as a source of reliable information by reviewing the above topics. We also want to be a source for reliable counsel.
So what are our official recommendations? It all boils down to what we know for sure and what is more common:
- We know that mammary cancers have a tendency to be aggressive and malignant and are VERY common in unaltered female dogs. We know that spaying, especially before the first heat, can greatly reduce or essentially eliminate this risk.
- We know that the incidence of mammary gland neoplasia in unaltered dogs is higher than the incidence of hemangiosarcoma, osteosarcoma, and transitional cell carcinoma in altered dogs.
- We know that pyometra (uterine infection in unaltered cats/dogs) carries significant risk of death and that risk increases with age.
- We know having longer limbs is not of proven detriment and there’s no consistently proven connection between neutering and hip dysplasia or cruciate disease.
- We know that obesity is a common consequence of being neutered. However, obesity is preventable with proper diet and exercise.
- We know that urinary incontinence is another common consequence of neutering. However, it is easily treated (as opposed to untreatable, terminal malignant mammary cancer).
- We know we can mitigate many risks by neutering at appropriate times - we neuter at 6 months at the earliest, perhaps later in larger dogs.
- We know we can mitigate risks by making educated decisions based on breed.
In summary, let’s consider a case from the Journal of the American Veterinary Medical Association:
“As an example, consider a discussion between a veterinarian and the owner of an 8-week-old female Labrador Retriever that is not intended for breeding. This dog would benefit greatly from OHE (ovariohysterectomy or spay/neuter) before her first estrus as a means of preventing mammary gland tumors, which are extremely common and cause substantial morbidity. Because of her breed, detriments of OHE include an increased predisposition to CCL injury, hemangiosarcoma, and obesity. However, there is a low incidence of hemangiosarcoma, and obesity can be readily controlled with good husbandry, which leaves CCL injury as the most important possible detriment. Because the incidence of CCL rupture is lower than that of mammary gland neoplasia, a veterinarian may choose to recommend OHE and educate the owner about maintenance of optimal body condition and other management techniques that will minimize potential for CCL injury. An OHE should be performed before the dog’s first estrus. To minimize the potential for development of urinary incontinence, the veterinarian may choose to wait to perform the OHE until after the dog has reached 3 months of age.”
What does it all boil down to? We support the responsible neutering of healthy canine and feline patients with appropriate consideration for their risk factors. Please do not hesitate to contact us to chat with a doctor about any questions you may have!