Gerbils (Laboratory) - Pathology

Introduction

Gerbils are part of the sub-family Gerbillinae which is part of the Rodentia order. There are approximately 100 species of gerbil in 14 genera from all over the world but in laboratory testing, as well as for keeping as a pet, the mongolian gerbil is the most common. The mongolian gerbil or 'jird', Meriones unguichulatus, is commonly used for these purposes as they are known for having several characteristics that make them desirable over other species of gerbil. These include a very non-aggressive nature, a lower incidence than other species of spontaneous disease, a lack of odour and a monogamous mating behaviour. Some other species have been used for testing, although this is not common. The mongolian gerbil was introduced as a laboratory testing animal in 1963, although they only make up 0.5% of the total number of laboratory rodents used annually.

Strains and Stocks

The gerbil is not commonly used in laboratory testing and therefore when compared to the number of strains of other animals such as the rat, there are few specific strains in gerbils. The most common strain is predominantly agouti, but some black can be found within this strain also.

Physiology

The normal life span of a gerbil is between 2 to 3 years and the adult body weight is 50 - 90 grams. Males are slightly larger than females. Please find details of gerbil physiology within the WikiNormals section. Physiological information available includes “General”, “Biochemical”, “Haematological” and “Breed” parameters.

General Behavioural Characteristics

Gerbils do not have cheek pouches like hamsters but they do exhibit very pronounced hoarding characteristics and it has been found that even under controlled temperature, humidity and day-length conditions gerbils will engage in extensive hoarding behaviours. In their natural habitat, gerbils dig out burrows that can be up to 170cm deep and can extend horizontally for up to 8 metres. These underground complexes often contain nests, hoarding chambers and connecting tunnels. Therefore laboratory housing for gerbils should take both the hoarding and burrowing behaviours into consideration.

Some studies have found that under laboratory housing conditions, gerbils may develop a stereotypic digging behaviour. An example of this would be digging at the corner of the plastic housing container despite there being no material to move from the plastic. Changes in the housing parameters such as increasing the housing size or the provision of natural substrates such as sand or soil has been shown to prevent this behaviour. This behaviour often develops in juveniles. It has been shown that the provision of housing with burrows or a tiered tunnel structure can also help to reduce this behaviour.

Gerbils are a social species and in the wild group sizes range from 2 to 15. There is usually only one actively reproductive male and female per group. The formation of new groups can be problematic when introducing adults to one-another due to issues of dominance and other animals are much better tolerated when introducing new juveniles to one-another. Adults will display strong xenophobic reactions towards unfamiliar animals including grinding their teeth and lashing their tails.

Gerbils have a strong desire to scent mark their territory by depositing sebum from their ventral scent gland, urine and faeces.

Integument System

Rough Hair Coat

A poor hair coat is a very common problem in laboratory gerbils and is usually an indicator of excessive humidity in the environment. If the relative humidity of the environment is higher than 50% then this will usually result in unkempt or matted hair. Appropriate ventilation to the housing areas is important.

A poor quality or rough looking coat is the most common finding on clinical examination of gerbils that have some type of active disease. Therefore a rough hair coat may also indicate other disease processes.

Nasal Dermatitis (Sore Nose)

Nasal dermatitis is a relatively common problem in gerbils, with the incidence higher in younger animals. Although the etiology of this disease is unknown, it has been associated with trauma, stress or accumulation of secretions within the 'Harderian gland'. Bacterial infections with Staphylococcus species have also been linked.

Clinical signs for nasal dermatitis include frequent scabbing and/or ulcer formation, alopecia, erythema and dermatitis. The perinasal area is usually most affected which the periocular region becoming involved in more chronic cases. A very well established dermatitis can spread to the remainder of the head, forelimbs, chest and abdomen. Clinical signs are usually diagnostic. This disease is relatively self-limiting if the underlying causative factor such as overcrowding is removed.

Macroscopic findings will include numerous pruritic skin lesions. There may also be mild acanthosis, hyperkeratosis and accumulations of large numbers of neutrophils and macrophages in the adjacent dermis. Some gerbils may have concurrent hyperplasia of the Harderian gland.

Staphylococcal Dermatitis

This is an endemic disease that usually occurs in weanling gerbils. The gerbil will develop a localised alopecia and erythemia around the external nares which then rapidly spreads to the feet result in in moist dermatitis of the feet, legs and ventral body. Morbidity is between 75-100% and mortality is 25%. High humidity, fighting and stress associated with blood samples have all been shown to be causative factors.

Macroscopic pathologic findings will include lesions that are acute and suppurative. Some gerbils may have micro-abcesses in the liver. Microscopic findings will vary depending on the severity of the lesions but will include minimal neutrophilic infiltrates in the epidermis with acanthosis and hyperkeratosis to severe suppurative epidermitis, dermatitis and folliculitis. In very severe lesions there may be chronic ulcerative dermatitis.

Parasites

Parasites rarely cause disease in gerbils. The most common skin parasite is the hamster demodectic mange mites, Demodex aurati or Demodex criceti. Usual clinical signs are alopecia.

Tail Lesions

The most common type of tail lesion is called 'barbering' of the tail and is most common in cages that are overcrowded. Hair is lost from the tip of the tail to up to a third of its length. The hair often grows back once the population is reduced.

Reproductive System

Cystic Ovaries

Geriatric problems in gerbils commonly include cystic ovaries. Cystic ovaries occur in 50% of geriatric females and can result in decreased fertility.

Macroscopic pathologic findings will include moderate to large cysts on the surface of the ovaries together with metritis, endometrial hyperplasia and myometrial mineralisation.

Urinary System

Lead Toxicity

Depending on the type of housing, there is some potential for the development of lead toxicity due to excessive gnawing. Due to the concentrating ability of the kidneys in the gerbil, they may become emaciated.

Macroscopic lesions may include a shrunken liver which will be darkly pigmented. The kidneys will also be small and pitted. Microscopic findings may include acid-fast inclusions within the proximal collecting ducts, tubules and within hepatocytes.

Glomerulonephritis

Glomerulonephritis develops with concurrent polyuria and polydipsia together with progressive weight loss.

Macroscopic pathology will show neoplastic lesions within the kidneys which will be shrunken, fibrotic and pitted.

Nervous System

Aural Cholesteatomas

Spontaneous aural cholesteatomas occur in 50% of gerbils over the age of two. The tumor displaces the tympanum of the ear into the middle ear. Clinical signs include a head tilt.

Macroscopic pathologic findings will include compression within the inner and middle ear. There is often secondary infection in the ear which results in some bone necrosis and inner ear destruction.

Cardiovascular System

Myocardial Necrosis

Older male gerbils have been shown to be at increased risk from the development of focal myocardial necrosis. Where this occurs there is often a concurrent fibrosis and formation of ischemic lesions.

Digestive System

Aminoglycoside Toxicity

Aminoglycosides are toxic to gerbils. Due to this antibiotic ointments that contain aminoglycosides can cause mortality due to ingestion of the drug. Clinical signs may include neuromuscular paralysis due to dysfunction within the acetylcholine system.

Endoparasites

Nematodes

There are two main types of endoparasite that are found within laboratory gerbils. These include the mouse pinworm, Syphacia obvelata which is found in the cecum and/or the small intestine. The gerbil pinworm 'Dentostomella translucida can also be found in the same areas of the digestive tract. Transmission is via the faecal-oral route. Mild enteritis may occur with a heavy infection but no clinical signs have been reported in gerbils. Both types of worm can be identified by a fecal flotation test.

Protozoa

Flagellated protozoa such as Giardia and Tritrichomonas are commonly found colonising the intestines of the gerbil. However, these protozoa are commensals and naturally occurring and therefore are usually not associated with any clinical signs. Overgrowth of these protozoa can lead to disease. Identification of these protozoa can be performed by faecal examination for cysts.

Tapeworm

The tapeworms Rodentolepis nana (dwarf tapeworm) or Hymenolepis diminuta can cause dehydration and diarrhoea. R. nana has a direct life cycle and has the potential to infect humans if ingested.

Enteritis

Salmonellosis

The bacteria Salmonella enterica has been shown to cause enteritis in gerbils along with food deprivation. Clinical signs include moderate to severe diarrhoea, a rough coat, weight loss, depression and dehydration. Morbidity and mortality are higher in younger animals. Transmission is associated with food or bedding contaminated by insects and/or other gerbils.

Macroscopic pathological findings will include a congested liver and distension within the gastrointestinal tract. The liver may display multifocal necrogranulomatous hepatitis. Some of the larger hepatic granulomas may display mineralisation of the necrotic centres. There may also be a fibrinosuppurative or fibrinopurulent peritonitis. The spleen may contain patchy pulmonary haemorrhage. Microscopic findings will include a positive culture of Salmonella species.

Listeriosis

Listeriosis in gerbils is characterised by acute death without clinical signs.

Macroscopic lesions include necrosuppurative hepatitis, enterocolitis, splenitis and hepatic and mesenteric lymphadenitis. There may also be diffuse acute pneumonitis.

Starvation/Water Deprivation

These findings will often be associated with gerbils that have been kept in overcrowded conditions or after shipping. Weanlings are also particularly susceptible if they are unable to reach the food container or water bottle.

Post mortem will show no gross lesions but an empty stomach and loss of body fat in adults would be common.

Tyzzer's Disease

Tyzzer's disease is caused by Clostridium piliforme which is a gram-negative, pleomorphic, obligate intracellular bacteria. The disease results in an acute enterohepatic disease in gerbils. The incidence of Tyzzer's disease is relatively low in gerbils. Infection is via contact with infected animals via the fecal-oral route. Gerbils are fairly susceptible to this disease and can catch it without any other concurrent diseases. High morbidity and mortality rates can occur, especially in young and pregnant females. Clinical signs will include depression, an unthrifty appearence and watery diarrhoea.

Macroscopic pathology will include multiple white foci of necrosis within the liver with associated mild to moderate enteritis. There may also be serosal oedema together with hyperaemia of the intestines. Intestinal lesions will be most pronounced in the distal ileum and cecum but can also occur in the duodenum, jejunum and colon. The intestines may also contain foamy, yellow faecal contents. There is also a potential for brain and cardiac involvement. A resultant septicaemia may lead to myocardial lesions and encephalitis. Microscopic pathology will include coagulative liver necrosis with only minimal inflammatory cell infiltrate. Definitive diagnosis is by demonstration of the organism within hepatocytes surrounding necrotic foci of the liver.

Other

Neoplasia

Older gerbils often develop spontaneous neoplasms. These most commonly affect the skin, adrenal glands, kidneys, spleen, intestine and the female reproductive tract. The most commonly encounteredt type of neoplasms include leiomyomas, subcutaneous fibrosarcomas, sebaceous gland adenomas, adenocarcinomas, duodenal adenocarcinomas, adrenal adenomas and malignant melanomas.