My cat has an almond-sized lump near the mouth, which I guess it is an abscess due to a bite wound.
I'm wondering whether it can heal by itself? If so, usually how long does it take? And if not what are the risks of leaving it untreated?
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Untreated abscesses can heal on their own, but it's almost always a long, messy and painful process. Usually they heal without complications, but in bad cases they may lead to death, for various reasons.
Abscesses are encapsulations of foreign objects or bacteria and pus. This encapsulation prevents the inflammation from infecting surrounding tissue, but it also keeps immune cells from fighting the bacteria. (Source: Wikipedia)
The natural way an abscess heals is by breaking open and draining the pus out of the body. A doctor (or vet in this case) would do the same thing, but with a sterilized scalpel in a clean environment.
If the inflammation is well contained and doesn't affect the animal too much, the abscess may stay intact for a long time, possibly till the natural end of life.
If this lump is actually an abscess, it's filled with pus and probably inflamed. This means physical stress on the body because the immune system is constantly active and fighting the inflammation, often resulting in fever.
In most cases the swelling is painful and very sensitive to touch. If the abscess is located near the mouth, it can hurt while eating or drinking, causing malnourishment in the animal. This further weakens the body.
Another consequence may be hair loss around the area, because the pain and itching induces scratching the skin. This is usually how the abscess is opened and drained in a natural way. Be aware that excessive scratching is a sure sign of pain and discomfort in a cat.
Depending on what caused the abscess and which bacterial infect it, the inflammation can spread and cause more abscesses in the same area or, even worse, can reach the blood stream and cause sepsis (blood poisoning).
In bad cases, extensive regional tissue death (gangrene) may occur.
If the abscess breaks open and the wound is untreated, consequent contamination may lead to further infections.
This is what might happen.
Your cat gets sepsis (blood poisoning) and dies.
This is Lillepus, a feral cat I had a couple of years ago. He got lost for two weeks. This picture was taken the day after he returned. The abscess ruptured during the night.
I did visit the vet for treatment. He did not need antibiotics, only cleaning and flushing the wound.
The abscess ruptures, but the cat's immune system is not able to fight the infection. See #1.
So the short answer is take your cat to the vet for treatment NOW.
If the abscess ruptures or gets drained by your vet and there are no complications, it will heal in a couple of weeks, but please do not take any chances.