Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Cocker Spaniel

Cocker Spaniel (English)

Anal Sac Adenocarcinoma

Related terms: canine anal sac gland carcinoma (AGSC)

VeNom term:  Neoplasm – perianal gland (VeNom code: 1452).

Outline: Anal sac adenocarcinoma is a cancerous tumour arising from the apocrine glands in the walls of the anal sacs. The tumour is invasive with a high mortality rate. The cancerous cells may invade regional lymph nodes and travel to the spine, lungs, liver and spleen via the bloodstream.

The tumour may go unnoticed in the early stages and an abnormal mass or swelling in the region of the anus (perineal area) may only be recognised when the growth is large. Other signs of anal sac adenocarcinoma include problems with defecation, local pain or irritation, excessive drinking and urination, hind limb weakness and lethargy. The tumour cells produce a protein, parathyroid hormone related protein, which causes increased calcium levels in the blood, and may lead to hypercalcaemia. Dogs with hypercalcaemia may suffer from weakness and dizziness, anxiety, bone pain, abdominal pain and nausea. Severe hypercalcemia can cause coma and cardiac arrest.

English Cocker spaniels are predisposed to anal sac tumours, but the genetic basis for this predisposition is yet to be fully identified. An association between anal sac tumours and dog leucocyte antigen DQB1 has been demonstrated in this breed, but the causal mechanisms are not known. Routine examination is advised to aid early detection and improve prognosis. Treatment options include surgery, radiation, chemotherapy, or a combination of treatments. Monitoring affected dogs after treatment is necessary as reoccurrence is common.


Summary of Information

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1. Brief description

Anal glands are sebaceous glands found within the anal sacs of dogs. These are located beneath the tail and near the anus in dogs and are involved in communication of social, sexual and territorial information. Anal sac adenocarcinomas are cancerous tumours that arise from these glands.

The tumour may go unnoticed in the early stages and an abnormal mass or swelling in the region of the anus (the perineal area) may only be recognised when the growth is large. Other signs of anal sac adenocarcinoma include local pain or irritation, excessive drinking and urination, hind limb weakness and lethargy. Dogs may have difficulties defecating, with constipation or soft stools, and they may frequently attempt to evacuate the bowels with little or no stool passed. Early on in the growth of the tumour, the cancerous cells may invade the regional lymph nodes or the bloodstream, and travel to the spine, lungs, liver and spleen.

The cells of the tumour produce a protein, parathyroid hormone related protein (PTHrP), which increases calcium concentrations in the blood, leading to hypercalcaemia. Hypercalcaemia may cause muscle weakness and fatigue. Not all dogs with hypercalcaemia will show outward signs but dogs may suffer complications such as kidney dysfunction, gastrointestinal motility disturbances or heart rhythm abnormalities. Severe hypercalcaemia can induce coma and cardiac arrest.

2. Intensity of welfare impact    

Anal sac tumour is an aggressive cancer and may cause discomfort, pain, weakness and lethargy, especially when the growth is large. The tumours may impinge on the rectum, causing discomfort and pain on defecation. If the cancer spreads, dogs may experience pain or neurological abnormalities, such as sensory and movement disturbances or seizures, depending on the location of the spread.

Dogs with hypercalcemia (ie elevated calcium in the blood) may experience weakness and dizziness, anxiety, bone pain, abdominal pain and nausea. Severe hypercalcemia can lead to coma and cardiac arrest.

3. Duration of welfare impact

Tumours generally occur in older animals, with a typical onset at 8 to 12 years of age. One study found that the average (median) survival time for treated dogs was 1 year 25 weeks (range: 0 weeks to 5 years; Williams et al 2003). Dogs with larger tumours, hypercalcemia and pulmonary metastasis had significantly shorter survival times.

Treatment options include surgery to remove the tumour and radiation therapy or chemotherapy to kill any remaining cancerous cells, although reoccurrence is common.

4. Number of animals affected

English Cocker spaniels are at a significantly greater risk of developing anal sac tumours than other dog breeds and cross-breeds (Polton et al 2006).

Males and females are equally at risk of developing tumours but there is an increased risk in neutered males compared to entire males (Polton et al 2006; Williams et al 2003).

5. Diagnosis

Small anal gland tumours are often only found by chance during a routine physical examination, and therefore the tumour may be missed in the early stages of development. In advanced stages, the tumour becomes more noticeable by a mass and/or swelling in the perineal area. A biopsy of tissue is necessary for definitive diagnosis.

6. Genetics

English Cocker spaniels with anal sac tumours have been found to have a significantly higher frequency of a specific allele of a gene that is involved in regulating the body’s immune response, compared with healthy animals. However, the mode of inheritance or the causative mechanisms have not yet been determined.

7. How do you know if an animal is a carrier or likely to become affected?

Since the mode of inheritance for anal sac carcinomas is unknown, we are currently not able to identify specific individuals at risk. However, we do know that English Cocker spaniels are predisposed to developing anal sac carcinomas.

8. Methods and prospects for elimination of the problem

As a routine, veterinarians should check the anal sacs and perianal region as a routine part of physical examinations and health checks; this may aid the early diagnosis of anal sac tumours in dogs, which gives a better likely outcome and improved treatment options. It is advisable to avoid breeding between affected dogs or from dogs with affected relatives, including grandparents, siblings, previous offspring and siblings of parents.


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1. Clinical and pathological effects

Anal sac adenocarcinomas are cancerous tumours arising from the apocrine glands of the anal sac. Canine anal glands or anal sacs are small sebaceous glands found near the anus in mammals. There are two sacs located on either side of the anus, and they produce a substance involved in scent recognition. In dogs, these glands are involved in communication of social, sexual and territorial information.

The cells of the tumour produce a protein, parathyroid hormone related protein (PTHrP), that behaves in a biologically similar way to the parathyroid hormone involved in regulating calcium levels in the body, and which increases calcium concentrations in the blood (Meuten et al 1981). Increased concentrations of calcium lead to a condition called hypercalcaemia, with between 25-40% of dogs with anal sac tumours developing this condition (Bennett et al 2002; Ross et al 1991). Calcium interacts with the neuromuscular system, by blocking sodium channels and inhibiting depolarization of nerve and muscle fibres. Therefore increasing levels of calcium raise the threshold for nerve-muscle reflexes and cause sluggish responses, muscle weakness and fatigue. Not all dogs with hypercalcaemia show outward signs but dogs may suffer complications such as kidney failure, diarrhoea and other gastrointestinal disturbances or abnormal and/or irregular heart rates. Anal-sac tumours are the second most frequent malignant cause of hypercalcemia in dogs, next to lymphoma.

Metastasis i.e. the secondary spread of cancer, may occur and the cancerous cells may invade the iliac lymph nodes (which serve the pelvis area) or the bloodstream, and travel to the spine, lungs, liver and spleen. Evidence suggests that metastasis to regional lymph nodes occurs early with this tumour. This increases the chance of further complications and early death.

The common sign of tumours are perianal swelling, but polyuria (increased urine production), polydipsia (increased drinking), hind limb weakness and lethargy are also signs. Dogs may also have gastrointestinal disturbance including constipation or diarrhoea, and may frequently attempt to evacuate their bowels with little or no stool passed.

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2. Intensity of welfare impact

Anal sac tumour is an aggressive cancer and may cause discomfort and pain, especially when the growth is large. The tumours may protrude into the rectum, causing discomfort and pain on defecation. Affected dogs may be weak, lethargic and show lack of appetite as a result of the cancerous cell growth. If the cancer spreads, dogs may experience pain or neurological abnormalities, such as impaired cognition, sensory and movement disturbances or seizures.

Dogs with hypercalcemia (ie elevated calcium in the blood) may experience weakness and dizziness, anxiety, bone pain, abdominal pain and nausea, and they may also have excessive urination, and abnormal heart rhythms as a result of increased blood calcium levels. Severe hypercalcemia is considered a medical emergency and can lead to coma and cardiac arrest.

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3. Duration of welfare impact

Tumours generally occur in older animals, with a typical onset at 8 to 12 years of age, but some cases have been recorded in dogs as young as 5 years of age. Tumour growth may be slow so it may be some time before clinical signs develop.

In a retrospective study of 113 dogs with anal sac tumours (Williams et al., 2003), the average (median) survival time (MST) for treated dogs was 1 year 25 weeks (range: 0 weeks to 5 years), and dogs with larger tumours (greater than or equal to 10 cm2) had significantly shorter survival time than those with smaller tumours. Shorter survival times were also observed in dogs with hypercalcemia (MST 37 weeks) and those in which the tumour had spread to the lungs (pulmonary metastasis; MST 31 weeks).

Treatment options include surgery to remove the tumour and radiation therapy or chemotherapy to kill any remaining cancerous cells. The location of the tumour in the anal sac makes is difficult to remove and because of the aggressive nature of this form of tumour, reoccurrence after surgery is common. Veterinarians therefore will often recommend multi-modal treatments.

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4. Number of animals affected

Anal sac tumours may occur in dogs of any breed, but are a significant problem in English Cocker spaniels. In a study of records from multiple veterinary centres and multiple reference populations in the UK, the English Cocker spaniel were 7.3 times more likely to develop tumours than other dog breeds and cross-breeds (Polton et al 2006). 

It is possible that hormones play a role in tumour development, and previous reports suggest female dogs are more susceptible to anal sac tumours (Ross et al., 1991). However, recent work suggested that males and females are equally at risk of developing tumours (Polton et al 2006; Williams et al 2003) but that there is an increased risk in neutered males compared with entire males (Polton et al 2006; Williams et al 2003).

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5. Diagnosis

Small anal gland tumours are often only found by chance during a routine physical examination, and therefore the tumour may be missed in the early stages of development. In advanced stages, the tumour becomes noticeable by a mass and/or swelling in the perineal area, although a mass may not be seen externally and veterinarians may need to carry out a through digital rectal examination to locate it. Histological examination of the lump tissue is necessary for definitive diagnosis; a tissue sample can be taken via a fine-needle aspiration procedure.

A fine-needle aspirate and cytological examination of an anal sac mass can differentiate adenocarcinoma from other tumour types that may arise in this area (ie adenoma, mast cell tumour, melanoma, lymphoma). Abdominal ultrasonography and thoracic radiography may be performed to determine the stage of tumour development.

Serum calcium concentrations should be evaluated in all dogs suspected of having anal sac tumours, because of the risk of affected individuals also having hypercalcemia.

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6. Genetics

The Major Histocompatibility Complex (MHC) is a cluster of genes important for the immune response, and it has been associated with various diseases in humans and dogs, including cancer and autoimmune diseases (Kennedy et al 2007).

English Cocker spaniels with anal sac tumours had a significantly higher frequency of a specific version of a particular gene, the Dog Leukocyte Antigen allele of the MHC (DLA-DQB1*00701), compared with healthy controls, suggesting that this allele is associated with tumour development. However, the mode of inheritance has not yet been determined, and it is unclear if the allele itself has a causative effect in tumour development, or whether it is simply located on part of the chromosome close to another gene, that is responsible for causing the tumour, and therefore both are likely to be inherited together.

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7. How do you know if an animal is a carrier or likely to become affected?

Although we know that there is an increased risk of anal sac tumours in English Cocker spaniels, the mode of inheritance is unknown and so there is currently no way of telling in advance which animals are likely to become affected.

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8. Methods and prospects for elimination of the problem

Since the exact mechanisms involved in the predisposition of English Cocker spaniels to developing anal sac tumours are unknown, there are few solutions to eliminate the problem. As a routine part of physical examinations and health checks, veterinarians should check the anal sacs and perianal region; as this may aid the early diagnosis of anal sac gland tumours, and which gives a better likely outcome and improved treatment options. Research is required to identify the genes and mode of inheritance of anal sac tumours in this breed, and to develop tools to identify animals which carry the genetic predisposition. With the current state of knowledge on the causal factors of anal sac tumours, the best advice that can be offered to prospective breeders is to avoid breeding between affected dogs or from dogs with affected relatives, including grandparents, siblings, previous offspring and siblings of parents.

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9. Acknowledgements

UFAW thanks Dr Emma Buckland (BSc, PhD), Dr David Brodbelt (MA VetMB PhD DVA DipECVAA MRCVS) and Dr Dan O’Neill (MVB BSc, MSc, PhD, MRCVS) for their work in compiling this section.

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10. References

Bennett PF, DeNicola DB, Bonney P, Glickman NW and Knapp DW (2002) Canine Anal Sac Adenocarcinomas: Clinical Presentation and Response to Therapy. Journal of Veterinary Internal Medicine 16: 100–104. doi:10.1111/j.1939-1676.2002.tb01613.x

Meuten DJ, Cooper BJ, Capen CC, Chew DJ and Kociba GJ (1981) Hypercalcemia associated with an adenocarcinoma derived from the apocrine glands of the anal sac. Veterinary Pathology 18: 454–71

Polton GA, Mowat V, Lee HC, Mckee KA and Scase TJ (2006) Breed, gender and neutering status of British dogs with anal sac gland carcinoma. Veterinary and Comparative Oncology 4: 125–131. doi:10.1111/j.1476-5829.2006.00100.x

Ross JT, Scavelli TD, Matthiesen DT and Patnaik AK (1991) Adenocarcinoma of the apocrine glands of the anal sac in dogs: a review of 32 cases. Adenocarcinoma of the apocrine glands of the anal sac in dogs: a review of 32 cases 27, 349–355

Williams LE, Gliatto JM, Dodge RK, Johnson JL, Gamblin RM, Thamm DH, Lana SE, Szymkowski M and Moore AS (2003) Carcinoma of the apocrine glands of the anal sac in dogs: 113 cases (1985-1995). Journal of the American Veterinary Medical Association 223: 825–31

© UFAW 2015


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