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Tumours in Mice

Mice, rats and other rodents have a high incidence of tumours – indeed it is one of the major health problems that they suffer from, along with respiratory problems and mites. This article is about my experiences of tumours and related problems in mice, their treatment and resolutions.

Reidun (10 week old doe)

On returning from holiday a few years ago I discovered that one of my favourite ‘babies’ (she was from a recent litter) had a tumour and the person looking after her had not noticed. The lump itself was near one of her legs on her side. It was immediately noticeable – on entering the room I exclaimed from a distance “Reidun’s got a lump!” and my fiancé, who is not noted for his mouse-related observance, also saw it straight away. Now, tumours on mice come up literally overnight in many cases, so I cannot estimate precisely how long she had had the lump. I do know that it was 6 days or less, since my housemate had seen her the day after we left and there was no lump then.

I booked a vet’s appointment immediately for her and managed to take her in a couple of hours later. The vet came to the same conclusion as me – it was not attached and what is termed as ‘free-floating’ – and therefore likely to be easily removed. Reidun was taken in overnight and operated on the next morning. I rang the vet to find out what had happened and she informed me that it was a lymphoma but had been successfully removed. I collected Reidun that afternoon and isolated her with 3 other calm does in a smaller tank so she could not climb about and hurt herself. Regardless of this she had pulled her stitches out before we even got home… typical!

The wound was quite large and I was worried it wouldn’t close up after she had pulled the stitches out. However, she healed faster than I could have imagined and within no time she hardly even had a mark there. Her fur started to grow back and in the end she didn’t even have a scar! There was no re-growth of the tumour whatsoever and Reidun lived a long happy life with her sister, mother and friends.

Reidun after her operation with bald patch and wound

Whiskas (adult doe)

This tumour appeared overnight and was a bit of a shock. Whiskas was an old mouse, but a real trooper and still very active. With other health problems to worry about, we had been informed that she could not be operated on and survive. This meant that we only had the choice of whether to put her to sleep or not. I decided to let her go on for a while until the tumour began to affect her.

In the beginning it really was the case that she didn’t notice it and behaved perfectly normally, not even slowing down. The tumour stayed the same size for a week or two and then suddenly increased quite dramatically in size. For a start it was just Whiskas’s walking that was affected, as the tumour was now between her front legs too and she began to waddle a little.

Photo of Whiskas with her tumour when it was at its worst

As the days went by, she slowed down a little – but still proceeded to climb up both my arm and the toys, managing feats that some of the young healthy mice could not. However, we began to notice that she was gradually declining with the tumour’s progress and I knew that the time had come for her to be put to sleep peacefully, before she began to suffer. We took her to the vet’s soon afterwards. It was very peaceful and very sad, but we knew we had done the right thing for her. To let her go on in pain would not have been fair, especially when there were no possible good outcomes left.

Whiskas front and side with her tumour

The Nature of Tumours

Whenever I see a lump on one of my mice the first thing I do is to get the mouse out and have a good look. I have never had a mouse have any problem with me examining the lump, no matter where on the body it was. I can only assume that these lumps do not hurt the mice as they are always perfectly calm and happy to have them poked (gently!) and felt to try to establish what type of growth they have. Obviously if the lump was sore or ulcerated this would not be the case, but you should notice a lump before it gets to this stage. After all they can reach huge proportions within days or even hours.

I look at the size and shape of the lump (tumours are always roundish in my experience) and the location (which effects the ease or even the possibility of removal). I then feel around the lump to try and establish whether it is attached to something internally, which might make it difficult or impossible to remove, or whether it is free-floating under the skin. Those in soft tissue are easier to remove as you can imagine and are less likely to affect other organs etc in the body. Since the mouse can’t have all the various therapies we have as humans it is a complete removal of cancerous cells or nothing really….

But of course we have to remember that not all lumps are cancerous! Many could be fatty tissue or cysts and will not harm the mouse if left. The problem is that we do not know because we cannot do biopsies on mice as we can humans. Therefore the options are either to leave the lump where it is or remove it. There are of course several situations and courses of action, such as:

- A tumour which cannot be removed because it is too invasive or the mouse would not withstand anaesthetic
- A tumour which can be removed (some people will pay to have it removed by a vet and some will not – not necessarily because of the money issue, they may just feel that the mouse is better off living out its life until it is pain rather than going through the stress when the tumour may just regrow)
- A fatty lump/cyst which can be removed (there is no reason to do so unless this will affect the mouse, or if it grows/changes etc)
- A fatty lump/cyst which cannot be removed (this may not be dangerous unless it continues to grow)

There are different attitudes to tumours in rodents and whether they should be removed. Personally I have had tumours removed from mice before (see Reidun’s story above) with success, but this is sadly not always the outcome. Many will re-grow (as in humans) after a period of time. It is up to the individual to balance up the benefits and drawbacks for the mouse (and not themselves) of having the tumour removed. It is a hard decision to leave a mouse with a tumour to live out their life (while not in distress) but it can be the best course of action in some circumstances – some older animals would not cope well with anaesthetic and would also not receive an extension of their natural life.

Having lumps removed can be expensive and this often puts people off. When I had Reidun’s lump removed my vet charged £30 (UK) but I have had friends in the USA who have been charged $130! This may be all very well if you only have a couple of mice but if you have other pets (personally around 150 mice alone), are on a budget or cannot find a good vet nearby the costs do tend to escalate ;) I would always recommend to anyone needing vet treatment for any animal to ring around local vets and ask for quotations for a consultation. I did this when I first had mice and had vastly varying answers, from £5 to £15… that’s an extra 200% on top, so you can see it’s worth the trouble. Also make sure they have treated mice before and are confident with them – there’s no point choosing the cheapest but getting inadequate care for your mice.

If It’s Not A Tumour… What Is It?


This is basically a blocked milk duct (teat) in a nursing doe. I have only experienced this once and according to my books it is rare. In my case the sufferer was a nursing doe called Domino who had had 2 previous successfully reared litters. At first I thought the lump was a tumour but because of the location suspected mastitis and told the vet when I took Domino in. It grew in size rapidly, which only gave more weight to the mastitis hypothesis, as the milk built up behind the blockage.

The vet took Domino (and babies) in overnight, separating her from the 2-week-old babies only briefly for the operation the next morning. She had to attempt to remove the blockage and flush the area; informing me that there is no instrument small enough to actually ‘push down’ the tube to remove the blockage as is possible in other animals.

Domino’s mastitis at its biggest before her operation

When I collected Domino she was fine, with no lump and back to feeding the babies. The vet told me that the milk might build up again but should be reabsorbed after the babies stopped nursing. She had given Domino an antibiotic injection to prevent infection of the site and predicted a full recovery. Domino went on to raise those babies successfully.


A prolapse is a displacement of a body part such as an organ, caused through straining in many cases. It can happen to many different parts of the body, but displacement of an internal organ will look like a lump to the untrained eye that could be a tumour. Prolapses can be treated surgically by replacing the organ in its correct position and sewing up the hole. Therefore it is important to establish that a lump is not a prolapse or similar – the vet may be able to tell this by a physical examination.

The only experience I have personally had with a prolapse is when one of my males managed to prolapse his penis… the vet took him in overnight to operate the next day but it had ‘righted’ itself by then. Her diagnosis was that he got a little over-excited and basically it ‘got stuck out’! Strange considering that he was living with only other males, lol.

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