Neoplasia in Gerbils
Tumours and Cysts in Gerbils
What is Neoplasia?
Neoplasia is the process of abnormal and uncontrolled cellular growth, which in turn is then termed a neoplasm or a tumour.
Both male and female gerbils possess a scent gland, which is a hairless, tan/skin coloured, oval patch of skin that is located in the central belly/abdominal region along the mid-ventral line of the gerbil. The male scent gland is slightly larger and he uses it more often, usually to mark his territory. A common problem with elderly gerbils is tumours of the scent gland and they most often occur in elderly males, however they are not unknown in females.
The tumour itself usually starts off as a small rough and scabby spot on the scent gland. Often it is sore and can irritate the gerbil, and many gerbils attempt to bite it off as a result, this can lead to secondary bacterial infections. The tumour can be both slow or fast growing, and is often benign (Adenoma) but malignant tumours are not unknown (Adenocarcinoma). It can also bleed excessively if the gerbil attempts to remove it itself. If left unchecked it could have the potential to invade locally or metastasize to other regions such as the lung or lymph nodes. For these reasons it is in the gerbils best interests to have the entire gland removed at the earliest possible opportunity, and should never be left until it grows larger or becomes a problem. If the tumour itself is just removed it can often re grow and the problem will persist. The operation is a simple procedure with a very high success rate. Because the gerbil has such a fast metabolism, gas anaesthetic is best used throughout the operation. Controlling the dosage is easier, so overdosing is minimised, which effectively means that the recovery time is shorter, and the side effects are vastly reduced. The stitches used are the dissolving type, and if your gerbil decides to pick at these, healing is often underway and very little harm can occur. However it is still wise to keep the gerbil under close observation for several days and the gerbil will be sore in this region so will attempt to investigate the area.
The most common types are known as either squamous cell carcinomas or melanomas. Melanomas have a tendency to develop around the ear, nose, feet or more uncommonly, the base of the tail. These can be problematic if it irritates the gerbil and as a result the gerbil is constantly scratching at them. This can often cause profuse bleeding and sometimes ulceration or even a secondary infection can result. Vets can perform surgical removal of these types of tumours and it will present virtually no problems in a strong healthy gerbil, however in elderly gerbils care must be taken as the removal of these tumours can result in excessive bleeding. Often after the operation it is necessary to put plasters or something similar over the rear feet of the gerbil to prevent it scratching at the wound until it heals properly.
These are small benign tumours with or without a stalk, and may occur in many locations but most often around the anus or genitals. The causes and implications are different in these different locations.
Aural Cholesteatoma This is a cyst in the ear and occurs mainly in elderly gerbils of 2 years and over, and the most obvious symptom is a pronounced head tilt. The growth itself tends to displace the tympanum (eardrum) into the middle ear, and is often followed by chronic secondary infections. The cyst effectively sheds layers of old skin that then builds up inside the ear. If left untreated the cyst grows in size and destroys the surrounding bones of the middle ear. In humans a cholesteatoma mainly occurs because of poor eustachian tube function, and repeated infections in the middle ear. However in gerbils the eustachian tubes have a vertical orientation so they rarely get primary ear infections. Ear pressure is normally equalized by eustachian tube function, which conveys air from the back of the nose into the middle ear. When these tubes function poorly, perhaps due to allergies say from pine shavings or a mild respiratory bacterial infection, the body absorbs the air in the middle ear and this then causes a partial vacuum. The pressure from this vacuum sucks in a pouch or sac by effectively stretching the eardrum, especially around areas weakened by previous infection. It is this sac that often becomes a Cholesteatoma.
Although the cyst itself is untreatable, the chronic phase of this condition that arises from these cysts can be severely reduced by the administration of an anti-inflammatory injection administered by a competent vet, followed by a course of antibiotics. In most of these cases the head tilt will still remain but it is reduced and the gerbil will quickly adapt to this. It must be remembered though that it can reoccur because the cyst itself is untreatable.
Ovarian Tumours and cysts
Ovarian tumours can often occur in female gerbils, and the usual signs are an early cessation of reproductive activities, a severe drop in litter sizes (if in a breeding situation) or a prominent or distended abdomen. Ovarian tumours can be varied and include three major types, these are epithelial, sex chord and germ cell. Surface epithelial-stromal* tumours can be either benign or malignant and are usually derived from the ovarian surface epithelium or from ectopic endometrial tissue. Granulosa cell tumours are probably the commonest type in gerbils, and are regarded as sex chord-stromal tumours. (The sex chords are structures that develop from the genital ridge and after sexual differentiation become either the testis cords in males, or the secondary or cortical chords in females) Granulosa cell tumours can be either benign or malignant and develop from the granular layer of the Graafian follicle and can often secrete oestrogen, they are sometimes referred to as folliculomas. These tend to be commonest in elderly female gerbils. Another type of sex-chord-stromal tumour reported in gerbils is a thecal cell tumour. These can also form considerable quantities of estrogens and are derived from ovarian mesenchyme and consist mainly of spindle-shaped cells that contain small droplets of fats. Dysgerminomas have also been reported in gerbils. These are classed as germ cell tumours. Although relatively rare they can affect young gerbils, and signs can be early infertility in the animal. Normally germ cells are encapsulated at birth within the primordial follicle, normally if they escape this encapsulation, cell death occurs. However if the germ cell manages to survive, a rapid growth is then seen mainly because no cellular contexts can provide contact inhibition, which then leads to Germ cell tumour formation. All dysgerminomas are considered as malignant, but not all behave aggressively. Also reported in gerbils was the formation of Luteal cell tumours. These are the cells that are normally present in the corpus luteum, which is a yellowish glandular mass in the ovary formed by an ovarian follicle that has matured and discharged its ovum. If the ovum has been impregnated the corpus luteum persists and increases in size, if impregnation doesn't occur it degenerates and shrinks. The corpus luteum itself secretes the hormone progesterone.
*Stroma- is the supportive tissue around or beneath an epithelium. For e.g. Skin is an epithelium supported by the fibrous fatty stroma beneath it.
The signs mentioned above, that of early cessation of reproductive activities, a severe drop in litter sizes (if in a breeding situation) or a prominent or distended abdomen can also just as easily indicate ovarian cysts being present. A cyst is a lump that is filled with either fluid or a soft material and may occur in any organ or tissue. They are usually harmless unless its presence disrupts the function of an organ or tissue.
A follicular cyst of the ovary ~ Is a fluid-filled sac present in the ovary and is often the most common type of ovarian cyst. The follicle itself is the normal fluid filled sac that contains an egg. A cyst results from the follicle growing larger than normal, which in turn doesn't rupture to release the egg. Problems from ovarian cysts from can be caused from rupture of the cyst, the rapid growth of the cyst and stretching, bleeding into the cyst, or twisting of the cyst around its blood supply. Haemorrhagic or endometrioid cysts are cysts containing blood from injury or through the leakage of tiny blood vessels into the egg sac. On occasions the tissues of the ovary can start to develop abnormally to form other body tissues such as hair and teeth, these are known as Dermoid cysts. Usually ovarian cysts can resolve themselves over time.
Uterine Adenocarcinoma ~ Is a tumour of the glandular lining of the uterus
Endometrial Hyperplasia ~ This is a condition where the endometrium (Uterus lining) grows abnormally in size. It is usually related to too much oestrogen or an oestrogen/progesterone imbalance. The glands can become enlarged and irregular but the condition isn't regarded as pre-cancerous unless it contains atypical cells (not entirely normal)
Leiomyomas ~ these are benign smooth muscle growths that are not pre-malignant and can occur in any organ but are most common in the oesophagus or the uterus. Uterine fibroids are known as leiomyomata (plural) and are benign, but may lead to infertility in the gerbil.
Related problems in elderly female gerbils
- Metritis ~ inflammation of the lining of the uterus
- Dystocia ~ Complications of the birthing process or a difficult birth.
- Myometrial Mineralisation ~ The smooth wall of muscle in the uterus becomes mineralised
Related problems in male gerbils
- Benign testicular hyperplasia
Studies on young gerbils (12-30 weeks of age) with infertility revealed that often the cause was found to be spontaneous hyperplasia in both seminiferous and epididymal tubules. This high incidence of hyperplasia in young gerbils is an indication of possible congenital lesions and has led to the possibility of using the gerbil as a model for the further study of male infertility in humans. Hyperplasia is a general term for an increase in the cell number of an organ or tissue causing it to increase in size. Various causes such as an increase in demand, a chronic inflammatory response, hormonal dysfunction or neoplasia can be the cause of this
Related problems in elderly male gerbils
- Orchitis ~ This is an acute inflammatory reaction of the testis usually secondary to bacterial infection. It has been noticed that suppurative (the formation of pus) orchitis was present in gerbils after salmonella infections.
- Prostatitis ~ Is inflammation of the prostate gland, which is below the bladder and produces components of semen. It is often due to bacterial infection.
- Testicular mineralisation ~ Testicular degeneration with mineralisation
Other tumours reported in gerbils
The elderly gerbil over two years can be prone to a range of tumours. These tumours can be either external or internal and some types are more common than others. Whenever possible, surgical intervention is the usual treatment of choice. However with elderly gerbils surgical intervention can be problematic, and when there is no obvious treatment the owner has to make a decision on the quality of life of the gerbil. Often slow growing tumours present little problems for gerbils and they can often lead a normal life for quite a long time after diagnosis.
It has been noted in gerbils that there is a very low incidence of mammary, pulmonary and pituitary tumours. Other notable tumours recorded in gerbils include,
- Haemangiomas ~ This is an abnormal proliferation of blood cells that may occur in any vascular tissue. There has been much debating as to whether these type of lesions are neoplasms, hamartomas* or just vascular malformations. In the gerbil there have been reports of both Renal (Kidney) and splenic (Spleen) hemangiomas.
- *Hamartomas ~ Are a benign malformed growth usually resulting from some form of faulty development in an organ. They are composed of an abnormal mixture of tissues that develop at the same rate normal tissue but aren't likely to compress adjacent tissue,
- Haemangiosarcoma ~ This is an aggressive type of tumour that arises from the blood vessels. This type of tumour can occur anywhere in the body, but are most often found in the liver, spleen, heart and skin. This type of tumour is often metastatic, i.e. it can spread to other areas in the animal.
Fibrosarcomas ~ Are a malignant type of tumour, which are the result of fibroblasts, which are the cells that produce connective tissue such as collagen. It can be found in soft tissues such as muscles, blood vessels, joints and fat. It can also occur in bone.
Pancreatic cell adenoma ~
Thymoma ~ Is a tumour of the thymus gland that is a lymphoid organ located behind the sternum in front of the great vessels. These surrounding vascular and neural structures may be invaded during the spread of thymoma.
Caecal adenocarcinoma ~ Is a malignant tumour of the Caecum. The caecum marks the beginning of the large intestine and is basically a pouch that contains the waste material from the small intestine
Osteosarcoma ~ Is a disease in which cancerous cells are found in the bone. It is usually found located at the growing ends of bones, and like other sarcomas has the ability to spread to other parts of the body.
Hodgkin's like lymphoma ~ This is a type of lymphoma very similar to the human form of Hodgkins lymphoma, and is characterized by the spread of the disease from one lymph node group to another. It has been reported in several animal species as well as the gerbil.
Exorbital lacrimal gland adenoma ~ This is a tumour of the lacrimal gland. These are paired glands located close to each eye
Adrenal cortical tumours ~ Adenomas and carcinomas the adrenal glands are small glands that sit above each of the kidneys. The gland itself is made up of two parts. The outer part is known as the cortex and this is where the tumours develop. There are two main types of tumours of the adrenal cortex, but the vast majority are benign and are called adenomas. Most of these adenomas are non-functional, i.e. the metabolic pathways that produce steroid hormones are non-functioning and the cells involved don't produce any steroid hormones to cause any problems. However adenomas can produce too many steroid hormones, which in turn could possibly lead to other problems such as high blood pressure.
Carcinomas develop in the cortex of the adrenal gland and produce excessive hormones that cause noticeable changes in the body of the gerbil, such as temporary weight gain and fluid retention. These tumours can eventually become very large and cause other symptoms by pressing on other organs in the abdomen. This can cause pain and weight loss because the animal may not be able to eat very much.
Article by Eddie Cope
Thanks to Aed Ni Bhroin for photos 1 - 4 and 6 - 8, Isabel Saldana for photos 2a, 5, 9 - 11 and 15 - 18, Liz Arblaster for photos 12 - 14, Gill Colling for photo 18.Symphoonic for pics 19-21