Almost all cat owners will be startled when they notice a sudden change in their pets. If this change refers to a large lump that seems to have appeared overnight; It is understandable that your fear is even greater.
Lumps on the body usually don’t bring anything good – both in humans and in our pets. Of course, the most important thing in these situations is to seek veterinary help immediately.
But it is very important that you know what are the potential causes of lumps in cats. Let’s look at the 6 most common reasons for this skin phenomenon.
1. Insect Bites
If your cat is allowed to go outside, there’s a chance any kind of insect might bite her, causing a lump to appear on her body overnight.
Cats are very curious, and love to hunt; This is why many of them might kill or even eat bugs.
PetMD explains how fleas, ticks, mites, and mosquitoes are insects that are most likely to bite cats and cause several symptoms in them.
The most common signs of insect bites in felines are irritated, itchy skin, and a sudden appearance of a lump on their bodies.
Some insect bites can even be fatal for cats; If the insect is poisonous or if it transmits an infectious disease to a cat.
You should consult your veterinarian before using any kind of bug bite treatment on your kitten. Also, you should check your cat’s fur whenever she goes outside.
If a large lump appeared on your cat overnight, she might also be dealing with an abscess.
An abscess is a purulent ulcer caused by the dissolution of cells in the inflammatory process. This kind of inflammation in cats is caused by bacterial infections.
Purulent ulcers can develop in cats in any organ. Abscesses most often enter the cat’s body through open wounds, or occur in cats with dental problems.
The appearance of a lump is one of the typical symptoms of an abscess, along with redness and swelling of the affected tissue.
Cats will feel severe pain, which will probably be manifested by their refusal to eat, fatigue, or even aggressive behavior. Many cats will also develop a high fever.
The abscess treatment involves removing the pus – either by draining and flushing it, or surgically – according to VCA Animal Hospitals. Your veterinarian will prescribe the appropriate antibiotic therapy, too.
3. Feline Acne
A lump on your cat can also mean she has feline acne. If you’re unsure how they look, the video below shows cats with these skin issues.
Danny Scott and William Miller  point out how several factors can increase the risk of feline acne, such as stress, viral infections, allergies, poor grooming habits, and abnormal sebum production.
You shouldn’t treat feline acne on your own, since these can only worsen the issue, and even cause more pain in your cat.
Rather, you should consult the veterinarian on the treatment, which usually includes topical shampoo and wipes, as well as injectable antibiotics.
Hives are another potential explanation for a sudden large lump on your cat’s skin. Stephen White explains that hives appear in the form of red, swollen, and itchy lumps on the cat’s skin.
This skin condition usually appears as a result of allergic reactions in felines. Hives can be seen on any part of the cat’s body, and this happens a few minutes or a few hours after the cat has been exposed to the causative agent.
Some cats with hives can also develop fever, poor appetite, and lethargy. In some cases, hives will disappear without treatment a couple of hours after they appear.
However, some cats will require treatment, and it will include corticosteroids. Here it’s most important to detect what exactly caused the hives (such as a food or environmental allergen) to be able to prevent them from appearing again in the future.
5. Sebaceous Cyst
There are a couple of types of cysts in felines; and one of them, the sebaceous cyst, could be a reason for a large lump appearing in your cat.
Sebaceous cysts appear as lumps filled with fluid. If the cat is injured, fluid can even leak out of these cysts.
This skin problem in cats occurs due to clogging of the hair follicle with sebum, or some other material. The cyst is formed because the cat’s immune system tries to isolate the damaged follicle.
These lumps can appear on any part of the cat’s body – wherever there are hair follicles.
It’s necessary to take the cat to the vet clinic to confirm that she indeed has a sebaceous cyst. This skin issue can be harmful to cats if it continues to grow and becomes infected.
In some cases, the vet might suggest surgical removal of the cyst, but this is still considered minor surgery, according to Hospital Veterinari Glories.
I have left the worst scenario for the end; Unfortunately, a large lump on your cat could indicate a tumor in her.
There are several types of feline tumors. Read more about the two most common ones in the following paragraphs.
Mast Cell Tumor
A mast cell is a malignant tumor that arises from the mast cells of the skin tissue. It occurs more often in older cats; Any cat can suffer from this disease, but it is most common in the Siamese breed.
Skin changes in the form of lumps usually appear on the cat’s neck and head with mast cell tumors. They are also usually itchy, and cats could display some additional symptoms such as loss of appetite, vomiting, and weight loss.
The treatment involves surgical removal of the tumor. Depending on the diagnosis, the vet might also suggest chemotherapy or radiation therapy.
A large lamp in a female cat could indicate a mammary tumor. Any cat can develop it, while Siamese and Domestic Shorthairs have the highest incidence rates.
Intact females are mostly affected by mammary tumors; Even if your cat has had her first heat, and isn’t so young anymore, remember that you can still sterilize her; It’s never too late to spay a cat.
The most feline mammary tumors are malignant; approximately around 85% of them, according to Ohio State University.
Most widely used treatment for this type of tumor is surgery. The recovery rate depends on the tumor size, as well as histologic grading.
Of course, the most important thing is to diagnose and treat this disease on time; With this, the cat will have a higher rate of survival and recovery.
A large lump appeared on cat overnight; Should I panic?
I don’t want to scare you even more, but a lump on your cat’s body is not good news. It can mean many things, such as insect bites, that usually can be treated easily, but it can also be a sign of a tumor that can even be life-threatening for your kitten.
Some lumps might go on their own, like the ones caused by hives. But, this doesn’t mean you should ignore them and just wait for them to disappear.
As soon as you notice this skin change in your cat, you should call your veterinarian. You should also explain to him exactly when the lump showed up, and whether you have noticed any additional symptoms in your cat, such as fever, lethargy, appetite loss, etc.
Remember that, no matter what has caused this lump, your veterinarian is the only place where you should seek help.
 Scott, DW, Miller, WH. Feline Acne: A Retrospective Study of 74 Cases (1988–2003) 2010 Volume 16 Issue 4 Pages 203-209. DOI, Retrieved May 31, 2023.