Gerbils are also known as jirds or sand rats. The pet and laboratory gerbil is Meriones unguiculates, commonly known as the Mongolian gerbil. There are 14 species in the genus Meriones.
Externally, gerbils are quite ratlike. The head and body length is 95–180 mm, and tail length is 100–193 mm. The average weight is 50–55 g for females and 60 g for males. The covering of fur on the tail is short near the base and progressively longer toward the tip so that it is slightly bushy. Coloration of upper parts varies from pale, clear yellowish through sandy and gray. The sides of the body are generally lighter than the back.
Wild Mongolian gerbils are found in Mongolia, adjacent parts of southern Siberia and northern China, and Manchuria. Gerbils inhabit clay and sandy deserts, bush country, and arid steppes. They have a high degree of resistance to heat stress and dehydration. They are terrestrial, and wild Mongolian gerbils construct simple burrows (2–3 ft long) in soft soil where they spend most of their time.
Mongolian gerbils have several coat colors. The wild coat color is agouti and is controlled by an autosomal dominant gene. Sandy gerbils have a recessive color gene and show a yellow to ginger color on the dorsum and the typical creamy white belly of a wild-type Mongolian gerbil. The dorsal yellow hairs have short black tips and a light olive green base. A clear demarcation line between dorsal and ventral color is present. Black gerbils have an autosomal recessive gene; white albino gerbils with red eyes also have an autosomal recessive gene.
Gerbils have a large, ventral abdominal marking gland that is androgen dependent. It attains greater size in males and develops at an earlier age. The gland is used for territorial marking. Females mark their territory after parturition and become more aggressive.
The adrenal cortex produces nearly equal amounts of corticosterone and 19-hydroxycorticosterone. When the gland weight is compared with body weight, the adrenal gland is ~3 times larger in gerbils than in rats. Gerbils have a high proportion of RBCs with polychromasia, basophilic stippling, and reticulocytosis.
Male gerbils attain sexual maturity by 70–84 days. Vaginal opening in females occurs between 40–60 days, followed by 30 days before sexual maturity occurs. Gerbils tend to pair bond, and when older females lose their mate, getting them to accept another is often impossible. Early-maturing females are more likely to breed successfully on first pairing, and the lifetime fecundity of early-maturing females is more than twice that of their late-maturing littermates. Two-thirds of the early-maturing females that do not reproduce after a first pairing will become pregnant after a second pairing, but only 10% of late-maturing females do so.
The gestation period of nonlactating gerbils is 24–26 days, but lactating females always have a prolonged gestation of 27 days. If females are bred in the postpartum period, implantation is delayed, and gestation can be as long as 48 days. Mean litter size ranges from 3 to 7 animals. Young gerbils suckle for ~21 days and begin to eat solid foods at 16 days. In general, day 25 is considered suitable for weaning. The normal life span of a gerbil is 2–3 yr.
The diet of wild Mongolian gerbils consists of green vegetation, roots, bulb seeds, cereals, fruits, and insects. They hoard food and are not normally coprophagic, unless diets lack adequate nutrient value. Gerbils thrive on commercially available pelleted rodent diets with 18%–20% protein but may have deficiency problems when fed primarily homemade diets, sunflower seeds, or table scraps, which lack specific nutrients. Sunflower seeds are high in fat and low in calcium. Pelleted chow (5 g/day) has been recommended to avoid obesity. Gerbils will develop high blood cholesterol concentrations on diets containing >4% fat. This is manifest as lipemia and is more pronounced in males.
Gerbils excrete little urine, and fecal pellets are hard and dry. Their cages require less frequent cleaning than other pet and laboratory rodents. Gerbils adapt to a wide range of ambient temperatures. Because they have a propensity to develop nasal dermatitis at relative humidities >50%, a low humidity is advisable.
Gerbils require sandbathing to keep their coats from becoming oily. The lipids have two sources: Harderian gland nasal excretions spread by autogrooming and sebaceous exudates from the skin. The consequences of lipid removal by sandbathing are multiple; it not only cleans and grooms the pelage but also deposits lipids on the substrate that act as olfactory signals. Sandbathing is usually completed within 5 min. Additionally, it has homeostatic consequences. Hair color lightens in Mongolian gerbils allowed to sandbathe. When sandbathing is prevented, accumulating hair lipids mat the pelage, and behavior changes. Sandbath-deprived gerbils increase their frequency of “sand rolls” (rolling onto their side or back and returning to their feet within 1 second), decrease grooming, and increase territorial marking (especially males).
Gerbils often stand erect on their hindlimbs, so it is important that cages have a solid bottom and that the floor-to-lid height is tall enough to allow for this behavior.
Pet gerbils kept in inferior cages painted with lead paint or that use alloys containing lead have a high potential to develop chronic lead toxicosis because of their gnawing behavior and the urine-concentrating ability of their kidneys. Chronically, gerbils become emaciated, livers are small and pigmented, and kidneys are small and pitted. Microscopically, acid-fast inclusions are noted in the proximal collecting tubules and hepatocytes.
The gerbil’s overall appearance and behavior, particularly in relation to its cagemates, should be noted. Sick animals are often isolated from others and may demonstrate weight loss, hunched posture, lethargy, rough fur, labored breathing, and a loss of exploratory behavior. Early signs of illness involve changes in the color, consistency, odor, and amount of urine and feces. The perineal area should be checked for fecal or urine stains or discharges from the vulva in females. Fecal samples may be taken for parasite detection and bacterial culture. The fur and skin should be examined for alopecia, fight wounds or other trauma, ectoparasites, and elasticity for evidence of dehydration. The oral cavity should be checked for overgrown teeth. Ears and eyes should be examined for discharges or inflammation. Feet should be examined for sores and overgrown or broken nails. The abdomen should be palpated for masses. Normal body temperature is 98°–102°F (37°–39°C). Respiratory rate or signs of labored breathing should be noted. The thorax can be auscultated with a pediatric stethoscope.
Gerbil tails are fragile, and only the base of the tail should be grasped during handling to avoid injury.
“Facial eczema,” “sore nose,” and nasal dermatitis all describe a common skin condition seen in gerbils. Clinical lesions next to the external nares appear erythematous initially, progress to localized alopecia, and develop into an extensive moist dermatitis. The cause is believed to be increased Harderian gland secretion of porphyrins (similar to chromodacryorrhea in rats), which act as a primary skin irritant. Experimental Harderian gland–adenectomized gerbils do not develop nasal or facial lesions. Various staphylococcal species (Staphylococcus aureus and S xylosus) may act synergistically to produce the dermatitis. Stress factors such as environmental humidity >50% or overcrowding cause excessive Harderian gland secretion. Nasal dermatitis infection may extend to the maxillary sinuses. Affected gerbils develop anorexia, stop drinking water, lose weight, and die. The distribution and nature of the lesions are useful in diagnosis. Accumulated porphyrins will fluoresce under ultraviolet light (Wood’s lamp). Routine bacteriology yields isolation of pathogenic staphylococci. Treatment includes carefully cleaning the skin lesions and use of topical (chloramphenicol 1% ophthalmic ointment, tid) or parenteral antibiotics (except streptomycin, which is fatal in gerbils). Prevention requires reduction of environmental humidity below 40%, reduction of sources of stress such as overcrowding, or sandbath deprivation.
Naturally occurring Tyzzer disease, an enterohepatic disease caused by the obligately intracellular bacterium Clostridium piliforme, is the most frequently described fatal infectious disease of gerbils. Common clinical and pathologic findings are sudden death or death after a short period of disease, and the presence of multiple foci of hepatic necrosis. Diarrhea and necrotic lesions in the intestinal tracts are variably present. The probable route of infection in naturally occurring infection is by mouth, because gerbils exposed to infected bedding will contract Tyzzer disease. Supportive fluids and prophylactic treatment with doxycycline (5 mg/kg, PO, bid for 7–10 days) or metronidazole (20 mg/kg, PO, bid for 7–10 days) are recommended to reduce mortality in cagemates. Because the bacteria form spores, the housing environment should be thoroughly sanitized and disinfected.
The Mongolian gerbil is susceptible to infection by Helicobacter pylori, which causes severe gastritis, gastric ulceration, and intestinal metaplasia. Gastric adenocarcinoma develops in approximately one-third of infected gerbils >15 mo old. Clostridium difficile–associated fatal enterotoxemia has been associated with treatment using nutritionally balanced triple-antibiotic wafers (containing amoxicillin, metronidazole, and bismuth) to eliminate naturally occurring Helicobacter infections. Affected animals are reported to die within 7 days of antibiotic treatment.
Syphacia obvelata, the mouse pinworm, and Dentostomella translucida, an oxyurid, are found in Mongolian gerbils. Pet store gerbils often are infected with mouse pinworms. D translucida is commonly found in the small intestine of both research and pet gerbils. There is an average of four parasites per animal, but no clinical manifestations of disease are associated with the infection.
Infections with dwarf tapeworms, Hymenolepis diminuta and Rodentolepis (formerly Hymenolepis) nana, are reported in pet gerbils. Dehydration and mucoid diarrhea are often presenting signs. R nana has a direct life cycle and may potentially infect people if ingested. Recommended treatment is niclosamide fed at 10 mg feed/100 g body wt for two 7-day periods separated by 1 wk. Also effective are thiabendazole (0.33% mixed in the feed for 7–14 days) or praziquantel (5–10 mg/kg, IM, SC, or PO, repeated in 10 days).
Historically, reports exist of pet gerbils infected with the tropical rat mite Ornithonyssus bacoti (see Mice and Rats as Pets).
Gerbils develop spontaneous, insidious periodontal disease after 6 mo on standard laboratory rodent diets. On the same diets, ~10% of the animals will become obese, and some will show decreased glucose tolerance, increased serum immunoreactive insulin, and diabetic changes in the pancreas and other organs.
Thin skin covers the tail of the gerbil. Unlike rats or mice, if a gerbil is picked up by the tip of its tail, the skin will often slip off, leaving a raw, exposed tail that eventually becomes necrotic and will shed. If the tail skin is lost, the bare tail must be surgically amputated where the skin ends.
A fatal syndrome of acute toxicity is produced in Mongolian gerbils after injection of penicillin-dihydrostreptomycin-procaine combination. The toxicity is due to the dihydrostreptomycin, and 50 mg will produce almost 100% mortality in adult gerbils. Approximately 20%–40% of gerbils develop reflex, stereotypic, epileptiform (clonic-tonic) seizures from ~2 mo of age. Animals seize in response to sensory stimulation and forced exploratory behavior, but the incidence and severity of their seizures are variable; the seizures generally pass in a few minutes, may be mild or severe, and have no lasting effects. Although the incidence and severity of seizures often decrease with age, certain subsets of adult gerbils do not improve with age but progressively become more severe. The susceptibility is seen in selectively bred lines but may occur in pet gerbils. Seizures can be suppressed in genetically predisposed gerbils if they are frequently stimulated by handling during the first 3 wk of life. Anticonvulsant therapy is unnecessary.
Cystic ovaries occur frequently in Mongolian gerbils. Cysts range in size from 1–50 mm in diameter. Removal of affected ovaries does not significantly affect reproductive performance.
Females with one ovary are slightly inferior in fertility compared with normal females; a general decline in fertility may be evident in older females.
Major surveys of spontaneous neoplasia in laboratory colonies of Mongolian gerbils have been reported. A 25%–40% incidence of neoplasia in gerbils usually occurs after 2–3 yr of age. Squamous cell carcinoma of the sebaceous ventral marking gland in males and ovarian granulosa cell tumor in females account for 80% of tumors seen in animals >3 yr old. The ventral marking gland tumors invade locally and can metastasize to lymph nodes and lung. Adrenocortical tumors, cutaneous squamous cell carcinoma, malignant melanoma, and renal and splenic hemangiomas were the next most commonly reported tumors. Numerous other tumors, including duodenal and cecal adenocarcinoma, hepatic lymphangioma, hemangioma and cholangiocarcinoma, splenic and renal hemangioma, uterine leiomyoma and hemangiopericytoma, ovarian teratoma, testicular teratoma, and malignant melanoma, were also reported. However, the total incidence of these tumors was <5%.
Case reports of spontaneously occurring tumors in pet gerbils include infiltrative craniopharyngioma, histiocytic sarcoma, systemic mastocytosis, malignant melanoma, and astrocytoma.
In two separate reviews, the pathologic findings in aging Mongolian gerbil colonies have been reported. Besides neoplasia, there was a high incidence of chronic glomerulonephropathy and focal myocardial degeneration and fibrosis, especially in older male gerbils.
Mongolian gerbils have a remarkable propensity for development of aural cholesteatoma. They occur in 50% of gerbils > 2 yr old. Cholesteatomas in the ear canal displace the tympanum into the middle ear. Compression and secondary infection result in bone necrosis and inner ear destruction. Clinical signs include head tilt.
There are no specific reports of zoonotic disease transmitted by pet Mongolian gerbils, although pet gerbils infested with Ornithonyssus sylviarum (the northern fowl mite) and Dermanyssus gallinae (the chicken mite) have been the source of avian mite dermatitis in children. Avian mite infestation is a rare cause of pruritic dermatoses in people. The mites spend most of their life cycle on the avian host but may be transmitted to people by direct or indirect contact. The scarcity of reports may be a true reflection of the absence of zoonotic disease in gerbils or may reflect their low popularity as a pet rodent.