1) What it is
Mammary tissue or mammary gland is essentially breast tissue. Mammary tissue is more developed in female rats but still exists in a limited state in males. Mammary masses are one of the most common types of tumour diagnosed in female rats- they can occur anywhere within mammary tissue which in rats covers from around the arm pit (axillary region) to their groin. Mammary masses can occur in young rats but are typically diagnosed in rats over 18 months old – sometimes, you may get a male rat that experiences it too.
You can have benign and malignant forms of mammary masses – for example they can be a benign Mammary fibroadenoma or a malignant and aggressive Mammary adenocarcinoma.
2) What causes it
In mammary masses in rats several factors could play a role in its development. This could include age of the rat, sex - with females being the most likely - genetics, endocrine involvement, environment, and even diet. Unfortunately, sex, old age, and bad luck are the most important factors.
The growth of mammary tumours is dependent on the hormones oestrogen and prolactin - benign mammary tumour growth is caused by an increase in prolactin.
3) What animals are at risk
Older entire female rats are more at risk, as are those fed on certain high fat diets.
4) The clinical signs/symptoms
In mammary masses you typically get a soft, round growth that can be anywhere from below the neck area to the groin area (anywhere along the region of mammary tissue). They can be movable on palpation or more firmly and attached to surrounding tissue. Remember it is also possible to get development of one or more growths along the mammary chain too.
They can be very fast growing and as they get larger (especially if under an armpit), you may find your rat experiences impaired mobility.
If left untreated the masses may also become ulcerated and you can get local tissue death in later stages. Depending on the tumour type and size you may also get general illness - poor appetite, weight loss, and lethargy could occur – especially if the rat is in discomfort or pain.
5) Diagnosis
To diagnose a mammary mass, you will need to pop to your local veterinary practice. The veterinary surgeon will palpate the mass and assess the skin for ulceration. They will assess your rat's weight and overall health as your rat may develop weight loss as the tumour grows.
Your veterinary team may suggest a ‘fine needle aspiration’ (FNA) of the lump before removal - this is done by microscopic examination of aspirated cells from the mass. This only gives a snapshot of the mass so the vet may opt to fully remove the lump or depending on the size debulk it – you can also send the sample off to have a histologic examination of tissue to find out if your rat’s mass is benign or malignant. This could help with knowing what to expect from future masses or possible complications with spread of the tumour.
6) Treatment or management
Surgical removal of the tumour is almost always recommended, as even benign tumours will grow indefinitely, eventually posing major problems for the rat (who may even end up being outweighed by “their” tumour).
If surgery for mammary mass removal is being performed on female rats it is worthwhile to discuss with the vet the possibility of also spaying if the rat’s general health status is good. As benign mammary and malignant mammary masses have a degree of hormone dependency, spaying females could reduce or remove those hormones that influence these masses.
7) Prevention
In most cases it may be hard to prevent – especially if a strong genetic component increases the risk. It's recommended to handle your rats regularly to palpate for any lumps or bumps – they can start off very small and are much easier to remove when small than when they get overly large!
Routine health checks with your vet could also aid in early detection of mammary masses -prompt treatment while the mass is still small can make surgery easier and less invasive and potentially improves outcome. Consideration should be given to spaying your female rats. Some studies support that ovariectomy prevents, or reduces frequency of, the development of spontaneous mammary masses.
One study suggested that high fat diets promote the development of mammary tumours induced in rats. In general, high dietary levels of unsaturated fats (e.g., corn oil, sunflower-seed oil) increases tumour forming risk more than high levels of saturated fats (e.g., coconut oil). Diets rich in certain fish oils are often the most inhibitory to this tumorigenic process. Some evidence suggests the major mammary tumour development is down to a caloric (energy) mechanism – offering a diet that is nutritiously low in fat and calories could be recommended.
8) Further reading
Dietary polyunsaturated fat in relation to mammary carcinogenesis in rats - PubMed
Mammary tumorigenesis in rats fed diets high in lard - PubMed
Dietary fat, calories, and mammary gland tumorigenesis - PubMed