Lumps and bumps on or under the skin are relatively common in rats and mice. Sometimes these masses can occur suddenly and grow quite quickly and other times they grow slowly over time.

In any case, it is recommended to book in for a vet check as soon as you notice a lump on your pet mouse or rat.

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What causes lumps?

Lumps in rats and mice are most commonly caused by one of two things:

    • Abscesses

      These lumps are generally the result of a bacterial infection that becomes walled off by the body and fills with pus. There are many different species of bacteria that can cause infection and the initial infection can be due to bite wounds from cage mates, self-trauma (when an animal licks or bites at themselves, often due to pain or itchiness), or any other accidental injury that subsequently becomes infected.

      • Neoplasia (Cancer)

        Cancerous lumps in rats and mice are usually of mammary origin (i.e. breast cancer) but they can appear anywhere on the body, including on the neck, shoulders, belly, flanks, tail base, groin, and armpits.

Neoplasia in Rats

In rats, these masses are often not malignant, meaning they do not tend to spread to other areas of the body. However, in some cases malignant tumours can also occur.

The tumours most commonly appear in older female rats that have not been desexed, but can also sometimes be found in males. Tumours tend to occur more frequently in obese rats. While the tumours do not usually spread to other tissues, they can grow very quickly, interfere with the rat’s movement, and occasionally become ulcerated or infected.

Mary rat and mouse
Rats & Mice

Neoplasia in Mice

Tumours in mice are also most commonly found in older females, but unlike rats these tumours are usually malignant and often spread to the lungs, causing fatal disease. Mammary tumours in mice can be associated with a virus that is passed to baby mice from their mother.

Diagnostics - How do we find out what is causing the lump on my pet?

In order to find out what is causing the lump, your vet will need to take a sample of the tissue. One sampling method, called a fine needle aspirate, is performed by putting a needle into the mass and pulling out some cells to be examined under a microscope.

This may not always provide a representative sample of the mass, so further testing via an excisional biopsy may be required. An excisional biopsy involves removing the mass and sending the tissue to a laboratory for histopathology to determine the type of cells that the mass is made up of.

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What is the treatment for lumps?

The recommended treatment of masses will depend on what is causing the mass, however, a general guide is below:

  • Abscesses

    To treat an abscess your vet will likely prescribe antibiotics to treat the infection and anti-inflammatories to treat and pain and inflammation associated with the mass. In some cases, it may also be necessary to surgically lance and drain the pus from the abscess or even remove it completely.

  • Neoplasia (cancer)


Treatment of Neoplasia in Rats

The best course of action for masses in rats is usually to surgically remove the mass before it begins to limit the rat’s movement or becomes ulcerated or infected. As these masses are usually benign, surgical removal can be curative.

Predisposing factors such as obesity and hormonal factors may also need to be addressed to reduce the likelihood that the mass will return.

Treatment of Neoplasia in Mice

Unfortunately, there is often no effective long-term treatment for cancerous tumours in mice. Surgery is often challenging due to the structure and blood supply of the masses, however it can be performed in some cases.

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What else can I do to help my rat or mouse?

Early detection is important, so as soon as you notice a lump on your pet you should have them checked by a vet. High calorie diets and obesity can contribute to tumour formation, so ensuring that you pet is on a healthy, balanced diet can help to decrease the likelihood of tumours developing.

Talk to your local vet about the diet that is best suited for your pet. There is evidence that desexing your rat before they are 6 months old can reduce the likelihood of tumour formation as well.