Disease prevention, daily care and experimental procedures for rats are vital for the well-being and health of rats. Today, we will introduce some non-infectious diseases that affect rats, across the categories of genetic predisposition to disease, nutritional disorders, colony management-related disorders, and iatrogenic (injury-related) illnesses.
Genetic Predisposition to Disease
For decades, each population and strain of experimental rats has been carefully selected to possess specific genetic traits, such as albinism and characteristics associated with certain tumors. However, these traits are not universally desirable for all researchers.
Besides the known and induced genetic variations in rats with specific traits, spontaneously occurring genetic drift inevitably occurs in isolated breeding populations of rats. While genetic drift can be monitored in inbred rat strains through molecular techniques, it is challenging to assess the extent of its occurrence in closed populations, where individual variations may mask differences at the population level.
Nevertheless, any two populations originating from the same source will gradually change over time unless there is sufficient and consistent breeding exchange between the populations.
1. Eosinophilic Granulomatous Pulmonary Inflammation
● Brown Norway rats (BN rats) have a high incidence of eosinophilic granulomatous pulmonary inflammation. At the age of 3-4 months, nearly 100% of both male and female rats will develop this disease.
● This disease has been found in Brown Norway rats distributed worldwide, including those kept in isolated colonies.
● Affected populations are negative for all known pathogens, and rats of other strains housed with Brown Norway rats do not develop pulmonary lesions.
● The pulmonary lesions are characterized by scattered granulomas in soft tissue, composed of Langhans giant cells, macrophages, and eosinophilic granulocytes.
● No foreign bodies, fungi, or bacteria were found upon routine examination, polarization microscopy, and special staining.
● Due to their inherently sensitive lungs, these inbred rats have been used in research on allergies and asthma. It has been hypothesized that the eosinophilic inflammatory lesions seen in this syndrome may be an allergic or reactive response to environmental damage.
2. Nodular Polyarteritis
● Nodular polyarteritis (panarteritis) is a vascular disease that has been observed in Sprague-Dawley and spontaneously hypertensive rats.
● Major arteries in the mesentery and visceral organs exhibit macroscopically visible alterations, such as arterial enlargement and bending.
● The histological changes in the arterial wall include features like fibrinoid necrosis and muscular hypertrophy.
● It is typically considered an incidental lesion, but in research involving inbred rats with the ACI background, affected arteries rupturing have been identified as a potential cause of incidental mortality.
Nutritional Disorders
● High-pressure sterilization of feed reduces the vitamin levels, particularly lysine, vitamin A, vitamin E, riboflavin, and thiamine. Prolonged storage has a similar effect.
● Additionally, maintenance diets designed for adult rodents may have protein and fat levels that are too low to support the rapid growth and successful reproduction of juvenile mice.
● Clinical manifestations of inadequate feed nutrition may include lighter body weight, decreased reproductive performance, reduced litter size, poor growth, and sparse fur.
● Symptoms of severe deficiency of specific vitamins are rare. These include squamous epithelial metaplasia of the salivary ducts with vitamin A deficiency, diffuse hemorrhage with vitamin K deficiency, embryonic death and testicular degeneration with vitamin E deficiency.
● Nutritional deficiencies can also alter susceptibility to diseases and their severity.
● In addition to the total calorie level and specific nutritional components of the diet, it is important to consider the quality of feed, including chemical contaminants, microorganisms, and the size and hardness of feed particles. For instance, powdered feed can increase the occurrence of dental malocclusion.
Colony Management-Related Diseases
● Poor husbandry practices can lead to a wide range of health issues, including those related to experimental procedures.
● Maintaining the cleanliness of animal cages, bedding, water and food, as well as laboratory equipment, is crucial.
● High moisture content in bedding can lead to rapid bacterial growth, increasing the incidence of urinary tract infections and potentially causing mammary gland inflammation and skin lesions. Bacterial urease production in the saturated area of bedding can elevate ammonia levels.
● High moisture content in feed can potentially increase environmental fungal spore contamination through the proliferation of molds. Considerations for air quality may also include factors such as ammonia levels, carbon dioxide levels, dust, residual disinfectants, and pollutants.
● Rats are highly sensitive to factors such as temperature, humidity, noise, vibration, light, indoor activity, and other perceivable environmental changes. Tail necrosis symptoms are associated with relative humidity levels below 40%, but other factors may also be involved. Tail necrosis is primarily seen in young rats, typically nursing pups, characterized by the formation of distinct ring-like constrictions on the tail and toes.
● Rats, especially albino rats lacking protective iris pigment deposition, are prone to retinal degeneration when exposed to environmental light levels above a certain threshold. This threshold depends on light intensity, the duration of the light-dark cycle, and the previous exposure of the animals to light levels. In addition to pure retinal damage, exposure to high-intensity light at 1600 lux for 12 hours daily for 8 days can lead to Harderian gland necrosis, which may be a consequence of the photosensitivity of porphyrin secretions.
Injuries and Iatrogenic Diseases
● Traumatic injuries are not common in rats. Compared to mice, group-housed rats are much less likely to sustain injuries from fighting.
● There have been reports of ulcerative dermatitis in rats, associated with Staphylococcus aureus and self-inflicted scratching.
● Paralytic ileus can occur following intraperitoneal injection of chloral hydrate and may sometimes lead to death. This condition shares similarities with the pathological changes caused by Tyzzer's disease in young rats, making them prone to confusion. Clinical symptoms typically manifest a few days after anesthesia, including lethargy, anorexia, and abdominal distension. The most pronounced dilation occurs in the jejunum, ileum, and cecum.
Reference:
[1]Fox J G, Anderson L C, Otto G, et al. Laboratory Animal Medicine:Third Edition[M]. 2015.
