A canine insulinoma is a tumour affecting the endocrine pancreas. They are part of the pancreatic neuroendocrine group of tumours and are functional, which in this case means that they produce an excessive and sustained release of insulin. This in turn results in a decrease in blood glucose levels. Insulinomas can be benign or malignant. Although a minority of human patients have metastatic insulinomas, they are significantly more common in dogs.

In this article we will discuss insulinomas in dogs. Specifically, we look at the causes, symptoms and treatment of canine insulinomas.

What is a canine insulinoma?

To understand what canine insulinomas are, we need to explain the structure and function of the pancreas, the organ affected by this type of canine tumour. The pancreas is known as a mixed heterocrine gland, as it has both endocrine and exocrine functions:

  • Exocrine pancreas: it is related to the digestive system. It secretes pancreatic juice, which is necessary for the digestion of food.
  • Endocrine pancreas: contains pancreatic islets (also known as islets of Langerhans) which consist of alpha cells (secreting glucagon), beta cells (secreting insulin) and delta cells (secreting somatostatin). These cells produce some of the most important hormones in the canine body, in particular those that regulate blood glucose levels.

Now that we know more about the structure and function of the pancreas, here’s how tumours affect it. As an insulinoma affects the beta cells of the pancreas, it will affect insulin production. Insulin is a hormone that is released in response to glucose in the blood, allowing glucose to enter cells and be used for energy. When a dog has an insulinoma, these cells begin to secrete an excessive amount of insulin on a permanent basis, resulting in hypoglycaemia, i.e. low blood glucose levels in dogs.

Within insulinoma we can find benign (adenoma) and malignant (carcinoma) cell proliferations. Pancreatic carcinomas have a high mortality rate and often metastasise to the mesentery, liver, spleen and regional lymph nodes. Fortunately, the development of insulinoma tumours is rare in dogs.

Canine insulinoma usually occurs in dogs between the ages of 3 and 14 years, being more frequent from 9+ years of age. There is no sexual predisposition, but there is a breed-specific predisposition. There appears to be a higher incidence in German Shepherd, Golden Retriever, Poodle, Irish Setter, Fox terrier and Boxer breeds.

Symptoms of insulinoma in dogs

Clinical signs associated with canine insulinoma occur as a result of sustained hypoglycaemia and stimulation of the sympathoadrenal system. Specifically, insulinoma in dogs usually presents with:

  • Weakness and lethargy (due to central nervous system glucose deficit).
  • Abnormal behaviours (e.g. nervousness, irritability, etc.).
  • Hindlimb weakness, muscle cramps and twitching (contractions)
  • Collapse
  • Ataxia (lack of coordination)
  • Seizures

Other less common clinical signs that can be detected in canine patients with insulinoma include:

  • Polyuria (increased urination)
  • Polydipsia (excessive drinking)
  • Polyphagia (increased hunger)
  • Anorexia
  • Weight gain
  • Diarrhoea
  • Syncope (fainting)
  • Tilt of the head
  • Urinary incontinence
  • Blindness

Causes of insulinoma in dogs

The exact aetiology of insulinoma in dogs is unknown. Like all neoplasms, insulinoma is caused by a genetic alteration that results in disorganised cell proliferation. However, the specific cause that triggers this genetic alteration is not yet known to veterinary science.

Diagnosis of insulinoma in dogs

Diagnosis of canine insulinoma should be based on the following factors:

  • Clinical history and physical examination: as mentioned above, dogs with this pathology present with clinical signs mainly associated with sustained hypoglycaemia.
  • Blood tests (complete blood count and biochemistry): the most indicative parameter of canine insulinoma is the detection of fasting blood glucose (values below 60 mg/dl), which is due to excessive insulin production by tumour cells. However, a single measurement is not sufficient to confirm hypoglycaemia. It is necessary to plot the measurements every hour, during an 8-hour fasting period.
  • Histopathological analysis: this is carried out once the tumour has been removed and allows the diagnosis to be confirmed. Microscopically, pancreatic cell neoplasms are made up of well-differentiated cells with few mitoses, but very easy to metastasise.

Treatment of insulinoma in dogs

If your dog is diagnosed with an insulinoma, it is normal to want to know if it is curable. In some cases, surgery can completely remove it and leave the dog with a functioning pancreas. In other cases, metastasis and organ failure make the prognosis poor. There are two types of treatment for insulinoma in dogs.

Surgical treatment of canine insulinoma

The goal of surgery is to remove the pancreatic tumour, either partially or completely. The latter is always preferred. If the tumour has metastasised to tissues such as the mesentery, liver or lymph nodes, these cancerous tissues should also be removed.

Although surgery is the treatment of choice in all stable dogs, a number of postoperative complications may occur:

  • Pancreatitis: due to manipulation of the pancreas during surgery. Careful management of the pancreas during surgery, adequate fluid therapy before, during and after surgery and adequate postoperative nutrition should be provided to prevent the occurrence of pancreatitis.
    Diabetes mellitus: when the tumour is removed, the pancreas may not be able to synthesise enough insulin, as the remaining beta cells are atrophied. In these cases, exogenous insulin must be administered until the pancreas regains its functional capacity to produce insulin.
  • Sustained hypoglycaemia: occurs when there are metastases that continue to produce insulin. In these cases, additional medical treatment is required.

Veterinary treatment

Veterinary treatment will be necessary both in dogs in which surgery is not possible and in dogs in which part or all of the tumour is surgically removed. Medical treatment of insulinoma in dogs is usually one of two options:

  • Treatment of acute hypoglycaemic shock: this is an emergency situation in which animals go into convulsive shock or seizure. In these cases, guardians should be prepared and act quickly by rubbing a sugar solution (such as jam or honey) into the oral cavity. The buccal mucosa has the capacity to rapidly absorb the glucose contained in these foods, thus resolving the convulsive shock in about 30-120 seconds.
  • Treatment of chronic or sustained hypoglycaemia: the aim of this part of the treatment is to alleviate the symptoms of hypoglycaemia and prevent the occurrence of acute crises. Chemotherapy protocols for dogs should not be used as they all cause severe side effects. Medical treatment should only be aimed at increasing glucose absorption in the gut and decreasing insulin secretion.

Prognosis of insulinoma in dogs

Unfortunately, the prognosis of canine insulinoma varies greatly from case to case, as most of these tumours, when detected in dogs, are malignant.

The life expectancy of dogs with insulinoma depends on the treatment that is established:

  • In dogs receiving medical treatment only: life expectancy is 12 months.
  • In dogs undergoing surgical treatment: one third die from intra- or post-operative complications, another third live less than 6 months and the remaining third may have a life expectancy of 12 to 14 months.

Whether or not surgery is an option for your dog will depend on the severity and size of the tumour. If you suspect insulinoma in your dog, please contact us so that we can make a timely diagnosis for your four-legged companion.