Mammary Tumours

Mammary tumours (MTs) are one of the more common ailments seen in rats, though more often in does than bucks. They tend to occur under the armpit, but can be found anywhere where mammary tissue is found (see the below diagram, Hvid et al., 2011). They start off at around the size of a pea and can cause an owner's heart to sink when found, but they're quite simple to deal with if an experienced vet sees your rat and are usually benign (non-cancerous).

As with all illnesses, prevention is always the better route where possible. One of the main factors is genetics - rats from rescue and other poor backgrounds are more likely to develop mammary tumours than well-bred rats, so ensure that you save accordingly. Diet is also important. Some foods are carcinogenic (they have the potential to cause cancer) and the main food group to be concerned about is highly processed meats. A lot of cheap rat foods have highly processed meats, and can therefore contribute towards a higher risk of tumours. Finally, being overweight or obese can also contribute towards the development of tumours as fat tissue can produce oestrogen (excess oestrogen can lead to tumours) among other pathways.

Not every lump will be a mammary tumour and surgery isn't an emergency, so it can be worth observing for a week or two to see if it's an abscess or cyst (good), and if it stays the same size (good) or grows (bad). Failing that, you can have a vet do a fine needle aspiration (FNA). If pus comes out then it is an abscess/cyst. If the results are inconclusive or yield only blood, it is likely a tumour.

A rat who is otherwise fit and well is a good surgery candidate, and the survival chances are good if they are operated on by a vet who is experienced with rats. You may choose to delay surgery on a rat who has an illness that can be resolved with treatment beforehand, or to take time to ensure that the rat is of ideal weight. If the rat is in poor health, then management then euthanasia would be the kindest course of action.

Some people opt against operating on rats over 1.5 - 2 years as a preference, instead choosing to euthanise when the time comes. This is because, with good management and a bit of luck, a rat may live for another ~6 months with a tumour and therefore surgery may not extend the rat's lifespan. Another common circumstance for opting against tumour removal is when the tumour is located around the groin area. These are trickier to remove, so surgery may not be in the best interests of the rat.

If you choose not to operate, you will need to have the rat euthanised before the tumour ulcerates, which is incredibly painful for the rat. The tumour will darken in colour, lose its fur and the skin will thin before ulceration. You may choose to euthanise sooner if the rat's movement and quality of life is impaired. Also bear in mind that there is a risk of the tumour snagging on something and rupturing, which would then be either an emergency removal or euthanasia.

To manage the growth of the tumour, keep them slightly underweight so that the tumour has fewer resources to draw on and grows more slowly. If they become too underweight, you can supplement them, but bear in mind that their weight loss is because of the tumour taking more and more of their resources. If they start to noticeably lose weight and the tumour is rapidly growing, earlier euthanasia may be worth considering. There are cases where rats have been prescribed cabergoline/galastop to slow the growth of the tumour as well.

If you are choosing to remove the tumour, you can also consider spaying the rat at the same time (in the case of does). Spaying reduces the rat's hormone levels, which can then reduce the risk of further tumours developing. However, the benefits lessen the older the rat is. If you decide to spay, bear in mind that the vet may opt against spaying during the surgery if the rat isn't particularly stable/there have been complications/the rat is getting cold.

You will be given the option to send the tumour off to the lab for analysis to see if it is cancerous, but many rat owners opt out of this. Histologies are expensive, the vast majority of the time the tumour is benign, the results are unlikely to change the treatment plan, and you'd soon know if the tumour was cancerous.

Surgery preparation and post-op care is as normal for lumpectomies, but with a greater focus on discouraging climbing to allow the wound to heal both externally and internally. If the tumour was large, keep the rat in the hospital cage with a companion for about a week before reintroducing them back to the rest of the group.

Moving forward, it's important to bear in mind that there is a good chance of reoccurrence and you will need to consider whether successive removals are worthwhile or fair to the rat. If the tumour was caused by a hormonal imbalance, then keep an eye out for other maladies that are linked to high oestrogen or prolactin levels such as pituitary tumours.