Desexing is a very commonly performed elective veterinary surgery. Desexing has traditionally been performed at 5-6 months of age in all breeds of dogs. As more information has become available about the pros and cons of desexing at certain ages, this decision has become more complex. There are many factors which need to be considered. We do not have any studies that provide unequivocal evidence on appropriate timing of desexing, so the decision needs to be based on the individual patient, their owner, and the data we have available.

Some important considerations include:

• Hormones seem to have a protective effect on common orthopaedic problems such as cruciate ligament disease, hip dysplasia and elbow dysplasia in some breeds of dog. There is a higher incidence of these diseases in dogs desexed before skeletal maturity. There is a consideration to delay desexing until around 1 year of age in dogs that will be >20kg adult weight.
• Mammary cancers are common in entire females (accounting for 50-70% of all cancers in populations with large numbers of entire females). There appears to be a significant reduction in risk of mammary cancers when females are desexed prior to their first heat, a lesser, but still significant reduction in risk when desexed between the 1st and 2nd heat, and no reduction in risk if desexed after the second heat. For most smaller breed female dogs (<20kg adult weight) this is a reason to desex before their first heat (approximately 6 months of age). For larger breed this needs to be weighed against the increased risk of orthopaedic disease and so desexing between the 1st and 2nd heat (approximately 3 months after the first heat) should be considered.
• As far as behaviour goes there is a reduction in roaming, hormonal inter-dog aggression and urine marking in dogs desexed at any age. The most serious bite injuries seen in human hospitals involve sexually intact dogs. However, in puppies that have shown signs of aggression prior to desexing, there may be an increased risk of aggression towards family members after desexing and this risk is reduced when they are desexed older.
• Castration of male dogs reduces the risk of testicular tumours, benign prostatic hyperplasia, prostatitis, perianal hernias, and perianal adenomas.
• Ovariohysterectomy (spey) of females reduces the risk of ovarian, uterine, and vaginal tumours. It prevents diseases involving the uterus, and problems associated with pregnancy and parturition. Pyometra, a potentially life-threatening infection in the uterus, is prevented by desexing. Approximately 25% of intact dogs develop pyometra by 10 years of age. It is most common around 4 years of age but can occur as young as 9 months of age.
• There appears to be an increased risk of some cancers after desexing in certain pure breed dogs (but not in mixed breeds). This risk is sometimes reduced when desexing is performed beyond a certain age. For example, Rottweilers have an increased risk of osteosarcoma (an aggressive malignant bone tumour) and desexing further increases this risk.It appears that the earlier they are desexed the higher the risk. However German Shepherds, Labradors and Vizslas do not have an increased risk of osteosarcomas if desexed.
• There is an increased risk of urinary incontinence in desexed females in some breeds. Those desexed before 3 months of age appear to have the highest risk. Most cases can be managed medically.
• There is an increased risk of obesity in desexed pets. This can be managed with appropriate diet and exercise but changes to their diet need usually need to be made after they are desexed, especially in animals that are desexed when they are adults.
• There is an overall increase in lifespan in desexed dogs.
• In entire dogs there is a high rate of unwanted pregnancies and animals ending up in shelters. If dog owners are not able to make sure their dog does not roam and that females are appropriately isolated while on heat, then desexing at 6 months of age is recommended. Another consideration, especially for female dogs, is that desexing is faster and easier when they are younger. Desexing when they are older will usually cost more, and there may be some additional risks, especially if they are overweight.

Ther pros and cons of desexing can vary by breed and some breeds have been studied in more detail than others. A good summary for risk factors for various diseases can be found by accessing the following article: https://www.frontiersin.org/articles/10.3389/fvets.2020.00388/full.

For male dogs there is also an alternative to surgical desexing which provides the benefits of surgical desexing without the anaesthetic and surgery. This is an implant that is placed under the skin which lasts 6 or 12 months (both options available) after which time you have the choice of repeating the implant, surgical desexing, or allowing the effects of the implant to wear off.

As you can see the decision of when to desex your pet is not always a simple one! Speak to your vet about the specific considerations for your pet.

As a general guide consider:

• For most breeds of dogs that are <20kg adult weight – Desex at 6 months of age.
• For large breed >20kg adult weight males – Castrate at skeletal maturity (around 12 months).
• For large breed females >20kg adult weight females – Spey 3 months after their first heat (usually 9-12 months). 

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