When heat leaves its mark

Breast and ovarian tumours beyond chance

Mammary tumours are one of the most common types of cancer in female dogs and cats, and represent one of the main clinical concerns in unspayed adult animals. The most worrying aspect is not only their frequency, but also their aggressiveness, especially in cats, where most of these tumours are malignant and have a high rate of metastasis.

What many owners do not know is that a simple surgical procedure, sterilisation, can drastically reduce this risk. If performed before the first heat, the probability of developing mammary tumours falls to less than 1%. Even if done after the first heat, it is still highly preventive. The longer the decision is delayed, the greater the cumulative risk.

This direct link between sex hormones and breast cancer is well documented and is one of the strongest arguments in favour of early sterilisation. That is why today we want to explain clearly how these tumours originate, what role the ovaries play and how a timely decision can save lives.

Hormones in charge: the biological ‘switch’ that explains the problem

Mammary tumours are the most common neoplasm in intact female dogs, accounting for between 42% and 70% of all tumours diagnosed in bitches and up to 82% of tumours of the reproductive system in this species. The probability of an intact female dog developing a mammary tumour during her lifetime is between 23% and 34%, figures comparable to or even higher than the prevalence of breast cancer in human women. These figures reflect that, while not all female dogs will develop a tumour, the risk is high enough that it should not be ignored or normalised.

The main cause of this high risk is the cumulative effect of ovarian hormones, which repeatedly stimulate cell proliferation in the mammary glands. Experimental studies have shown that progesterone stimulates the proliferation of the mammary epithelium and promotes the development of pre-neoplastic lesions, explaining why repeated oestrus increases the risk even in the absence of pregnancy. Each reproductive cycle exposes the tissue to oestrogen and progesterone, and the more cell divisions that occur throughout life, the greater the chance of mutations arising that could trigger a tumour.

Clinical studies have shown very clear differences depending on the timing of sterilisation: a female dog sterilised before her first heat has only about a 0.5% risk of developing a mammary tumour, which increases to 8% if sterilisation is performed after the first heat and up to 26% after the second heat. In cats, although the percentages may vary depending on the source, the risk reduction after sterilisation before the first heat exceeds 90%, and decreases dramatically if delayed, with very minor reductions in protection after two or more heats.

This pattern has a profound practical implication: it is not just “something that can happen,” but a quantifiable and predictable problem based on decisions made early in the animal’s life.

This phenomenon is accentuated when a female goes through multiple cycles without pregnancy: the hormonal stimulus is repeated chronically and “retrains” the mammary tissue over and over again. Therefore, if you live with an intact female, a monthly routine of gently palpating the mammary chain—from the chest to the groin—can allow for the early detection of nodules. Recording the date, position, and approximate size of any lump (a photo with a coin next to it is an excellent reference) greatly facilitates clinical follow-up and therapeutic decision-making.

At the same time, the ovaries can develop tumours of their own, although these are relatively rare—accounting for only 0.5% to 6% of all canine tumours—but when they do, they can cause hormonal changes that disrupt cycles and confuse the owner if not interpreted correctly.

Mammary and ovarian tumours: signs, behaviour and why time matters

Mammary tumours: common, silent and not always benign

Most mammary tumours in bitches and queens appear to be associated with sex hormones. In dogs, approximately half of mammary tumours are malignant, although the proportion may vary depending on studies and breeds. Several studies agree that between 85% and 90% of feline mammary tumours are malignant, with a high rate of early lung metastasis. Therefore, in cats, a small mammary nodule should never be considered “insignificant”.

They tend to appear more frequently after maturity (average age of onset between 6 and 10 years in bitches), although they can be diagnosed earlier. The typical signs that usually alert the owner are lumps in the mammary chain (sometimes small and firm, sometimes larger), skin changes such as redness, warmth to the touch, ulceration or discharge, and, in more advanced cases, systemic signs such as weight loss or apathy.

Did you know?

The caudal mammary glands (those closest to the groin) are the ones that develop tumours most frequently, probably because they have more glandular tissue and greater hormonal stimulation.

A critical point: the size of the lump does not define its severity. Small tumours can be aggressive, and large tumours can be benign or slower growing. Even so, retrospective studies have shown that size is associated with prognosis: small masses (e.g., below ~3 cm) tend to be associated with better survival times than large ones (e.g., above ~5 cm), reinforcing the practical value of early detection and action. Therefore, when faced with any breast mass, “waiting to see if it grows” is rarely the best strategy. Limiting manipulation and avoiding applying ointments without veterinary advice is important so as not to mask signs or infect the lesion.

Your vet advises

If you feel a lump in the mammary gland, do not try to “monitor” it by pressing it every week. Excessive manipulation can inflame the tissue and confuse the clinical evolution. It is better to observe, take notes and consult your vet.

Ovarian tumours: less visible, but not harmless

Ovarian tumours are not as common as breast tumours, but they can be insidious because they often do not produce obvious symptoms at first. When they do manifest, the signs may include changes in sexual cycles, irregular or prolonged heat cycles, or signs that do not fit the typical pattern.

Given that by the time they are detected they have often already reached a noticeable size or are associated with other hormonal changes, careful observation of the reproductive cycle—dates, duration, associated signs—is a practical tool for anticipating problems and assessing the need for early diagnostic intervention, such as an abdominal ultrasound.

Prevention and diagnosis: how to move from ‘hoping for the best’ to smart preventive medicine

The most powerful prevention is sterilisation, especially when it involves the removal of the ovaries. The first study linking sterilisation and reduction of mammary tumours in female dogs was published in the 1960s, long before preventive veterinary medicine was a common concept. In fact, it is not about “removing something”: it is about removing the hormonal fuel! By eliminating the main source of oestrogen and progesterone, the likelihood of hormone-dependent mammary tumours is significantly reduced and the development of ovarian tumours is prevented.

In practical terms, even when performed later in life, sterilisation can still provide a protective effect compared to not sterilising, although the benefit is clearly less than that of early sterilisation. A systematic review confirmed that sterilisation in adulthood still reduces the risk of mammary tumours compared to not sterilising, although the protective effect is clearly less than that obtained with early sterilisation. Prevention is not all or nothing, but the benefit diminishes over time. Therefore, when a guardian is unsure, it often helps to think in terms of real balance: it is not just a matter of avoiding litters, but of reducing future health risks which, if they arise, are often more expensive, complex and emotionally difficult to manage.

The figures that prove this are clear: a classic study showed that sterilisation before the first heat reduces the risk of mammary tumours in female dogs to around 0.5%, while after the second heat the risk exceeds 25%. It is not the surgery itself that is decisive, but the hormonal timing of the procedure. In cats, the risk reduction after sterilisation before the first heat is over 90%, and although the benefit decreases with age and repeated cycles, it is still significantly greater than not intervening.

What many owners do not expect

In female cats, mammary tumours can grow and metastasise so quickly that it can take only a few months to go from ‘nothing’ to ‘already spread’. That is why, in felines, surveillance and prevention are twice as important.

When a lump is already present or suspected, it is important to stop guessing and start finding out. The veterinary clinical approach usually includes a complete physical examination, cytological tests in some cases, and surgery with histopathological analysis of the tissue, which confirms the type of tumour and its actual behaviour. If malignancy is suspected, staging is planned to assess possible metastasis and design a comprehensive treatment plan.

From a practical point of view, preparing for the veterinary visit with a clear chronology of domestic findings speeds up decision-making: dates of onset, variations in size, presence of ulceration, general signs, history of heat or previous hormonal treatments. This “field” information provided by guardians often decisively enriches the clinical history and facilitates faster and more accurate management.

It is also useful to review some enduring ideas that often delay effective preventive decisions. The belief that “going into heat or having a litter prevents future problems” has no solid scientific basis and, from the point of view of mammary tumour risk, only adds to hormonal exposure. The calm nature of the animal or indoor living also does not reduce the risk of hormonal tumours: biology does not negotiate with habits.

In short, there is no perfect universal decision, but there is a well-informed decision, tailored to the age, breed, health status, and lifestyle of each animal. And if you notice any mammary lumps, skin changes, or atypical heat cycles, early screening is not alarmism: it is common sense, evidence-based medicine.

Mammary Tumours in Dogs.

MSD Veterinary Manual (Merck Sharp & Dohme Veterinary Manual).

Authors: Misdorp W., Else R.W., Hellmén E., Lipscomb T.P.

Description: Clinical and pathological review of canine mammary tumours, including hormonal factors and prognosis.

https://www.msdvetmanual.com/reproductive-system/mammary-tumors-in-dogs/mammary-tumors-in-dogs

Mammary Tumours in Dogs and Cats.

Journal of Veterinary Science / review in PubMed Central.

Authors: Salas Y., Márquez A., Díaz D., Romero L.

Description: Review article on the epidemiology, malignancy and biological behaviour of mammary tumours in small animals.

https://pmc.ncbi.nlm.nih.gov/articles/PMC9257276/

Prognostic Factors in Canine Mammary Tumours.

Journal of Veterinary Internal Medicine.