One of the most serious problems which affect cats that aren't spayed is a condition called pyometra. This condition almost exclusively affects intact females who have gone through at least one heat cycle. Pyometra, when left unchecked and untreated, can lead to a long period of suffering and death. If you have an unspayed cat, then you should always be on the lookout for this condition.
What is pyometra?
Pyometra is a severe bacterial infection of a cat's uterus which is directly affected by hormone levels. When a cat goes through a heat period and does not get pregnant, her hormone levels continue to rise and thicken the uterus. When your cat is in heat, the cervix tends to relax to facilitate insemination, but this also increases the chance that bacteria will enter into the womb. Because your cat's progesterone levels are high, it is less likely that your cat will fight off the bacteria and, thus the infection starts. If your cat goes through multiple heat periods, then she is more at risk because the lining continues to thicken with each cycle.
What are the symptoms?
The most common symptom is that your cat will appear very sick and have a distended abdomen, especially if it's contained in the uterus by a closed cervix. She won't eat and seem very tired and the area may appear tender. If the cervix is open, then you could also notice a smelly discharge from her vulva. Some cats will vomit and appear to lose weight. Problems generally start about one to two months after their last heat cycle. If you don't get medical attention as soon as possible, then your cat could experience kidney and blood poisoning which will eventually lead to death.
How is pyometra treated?
It's nearly impossible to cure this problem without surgery. First, your veterinarian will run tests and run x-rays to rule out any other problems. Once it is determined that your cat definitely has pyometra, then your cat's health will be stabilized with fluids and antibiotics. She will then need a complete spay where the entire reproductive tract is removed. Home care involves basic monitoring of appetite, urination and bowel movements as well as making sure the incision heals properly.
The best way to reduce your cat's risk, or prevent it altogether, is to have her spayed early before she starts going into heat. Cats who have been partially spayed, such as having their ovaries removed but not the uterus or cervix, may also be at risk, but it's extremely rare. Talk to your veterinarian about what age is best to spay your cat or about the benefits and complications of different spaying and neutering procedures.