Essential pet advice from vet Jo Gourlay

After a brief interruption from our hopping, croaking friends last week, we now complete the series looking at skin tumours in dogs and cats. Having initially discussed the general methods of diagnosis and treatment, and followed this last week with the topic of common benign tumours, we now focus on a few specific malignant ones.

In contrast to benign skin masses, malignant ones are usually what we call poorly circumscribed. This means they do not have a distinct margin or edge. They are more likely to be locally invasive and have a tendency to metastasise (spread elsewhere in the body, generally to the lymph nodes or organs).

The first type we will mention are called squamous cell carcinomas. These tend to occur in older animals on areas of skin that are less hairy or lacking pigment (areas of pink skin as opposed to black or dark brown) or they can be found in the nail beds. Cats with white fur on their ears and nose area can be prone to this condition, especially if they enjoy sunbathing and have had episodes of sunburn and ulceration. The nose itself and eyelids on pink skinned cats can also be affected as these areas lack the protective hair covering. If your cat is a sun-worshipper and will tolerate it, speak to your vet about applying sun cream to protect them.

The next type of tumours, called fibrosarcomas, are not hugely common but are worth mentioning because there has been much hype regarding them being a side-effect of vaccination. In very rare cases (an incidence of 0.04 cases per 10,000 doses of vaccination, according to the Feline Advisory Bureau), vaccines, especially those with a substance called an adjuvant added to improve their efficiency, have been linked to causing this locally invasive very firm malignant mass.

In these rare incidents a reaction occurs post-vaccination that does not disappear as you would expect within three to four weeks, and the chronic inflammation then develops into a fibrosarcoma (it’s not uncommon to get a mild transient swelling post-vaccination that normally spontaneously self-resolves).

You can also have fibrosarcomas that are unrelated to vaccinations. It’s worth remembering that the benefits of protecting your pet against potentially fatal diseases by vaccinating them vastly outweigh this and other rare side-effects.

The next skin tumours, called mast cell tumours, are common and can occur at any age in both dogs and cats. Their behaviour, however, is variable between the species. In dogs, they are normally malignant, aggressive and highly variable in appearance. They are also likely to cause secondary problems such as gastro-intestinal ulceration because of the chemicals these tumours release. In contrast, in cats, they are usually benign and appear as solitary nodules.

The final malignant skin tumours we will mention today are carcinomas of the anal sac apocrine (sweat) gland. They most commonly occur in older spayed female bitches and arise within the anal sac (a small sac that sits just inside and below the anus). They can cause secondary problems by releasing a hormone that causes excess calcium to be found in the blood. These types of tumours frequently metastasise to lymph nodes and the lungs.

In summary, skin tumours are common in both dogs and cats, so much so that, according to the Animal Cancer Trust, the skin is the most likely place to be affected by a tumour in a dog and second most likely place in a cat. However, whereas in dogs the majority of these tumours are benign, in cats the majority are malignant. As stated in previous articles, although certain characteristics of tumours can give indications as to whether or not they are likely to be of concern, it’s always worth getting any skin lump checked by your vet and take further action as advised.