There are a variety of terms used to describe neutering, including de-sexing, castration (males), and spay/ovarian hysterectomy (females). Surgical desexing involves the removal of part of the reproductive system whilst under a general anaesthetic. In females, this is the removal of the uterus and ovaries. In males, this is the removal of both testicles.
For dogs, neutering (spaying) females before their first heat almost totally eliminates their chance of developing mammary tumors; once they have had three seasons, this advantage is lost. In addition, neutering stops the mess associated with oestrus, and the attention of male dogs.
Another advantage is that as they get older, dogs that are not spayed are at risk of developing pyometra, which is the name for a pus-filled infected uterus. Toxins and bacteria leak across the uterine walls and into the bloodstream, causing life-threatening toxic effects; even with emergency surgery, about 1 in 20 unfortunately do not survive.
For male dogs, the prostate gland can gradually enlarge over the course of their life due to the influence of testosterone. Over time, this is likely to become uncomfortable and can interfere with defecation. This can lead to infection, which is difficult to treat without neutering. On top of this, male dogs are at most risk when they sense a bitch on heat, and will pursue them tirelessly as it is their instinct to do so. This is often when dogs get hit by cars.
In the case of cats, the main benefits of desexing are:
When neutering can be carried out
Large surveys in USA and Australia found that neutering can be performed at any age over 8 weeks, but as everything is very small at that stage it is often done at about six months of age.
It is recommended to desex cats around 4-6 months of age. If male cats are left until about 18 months, they can start spraying around the house, and after this has started, castration may not help.
Rabbits can have their first heat at 10-12 weeks of age, and will keep coming on heat continually until mated, so it is advisable to have female rabbits neutered at around two months of age.
There is no upper age limit for neutering; indeed it has to be done all too often to treat pyometra or other ailments that could have been prevented by neutering earlier. It could be argued that leaving an animal to see if it develops problems and then neutering if needed is all that needs to be done, but many of the problems and illnesses caused by not neutering are serious and can be life threatening, which is why we recommend early neutering of pets.