Rats Friendly, But Not High on Pet List

By Robert J. Russell and Jim A. Stunkard.

Pet and laboratory rats are derived from the wild brown or Norway rat (Rattus norvegicus). Rats are not one of the more popular pets, probably because of their social/historical impact on man than for any other reason.

Most rats become very friendly, are quick to learn, and respond positively to handling and attention. They are generally friendlier than mice, but they will bite if provoked.

Rats with brown hair coats are usually less friendly than the white-haired varieties. All tend to lose their trust if handled roughly on a recurring basis or if several injectable treatments are required. Feed and housing systems are similar to those used for gerbils, hamsters, and mice, although larger in size.

Diseases

Respiratory disease is the most common disease seen in rats. Micoplasma pulmonis is the primary cause and is responsible for a chronic disease that occurs in almost all rats held outside barrier facilities.

Affected animals produce a snuffling sound when they breathe, rub their eyes and nose, develop nasal discharge, have rough hair coats, become lethargic, and lose weight. Some animals develop middle ear infections that cause circling or head tilt.

Acute Pneumonia. A variety of secondary invaders for example, Streptococcus pneumoniae, Pasteurella, Bordetella, Pseudomonas aeruginosa can cause acute pneumonia in animals also infected with Mycoplasma. Increased respiratory difficulties and sudden deaths can occur.

Streptococcus pneumoniae is a particularly frequent secondary invader. A sudden high death rate is seen, and the majority of animals develop rapid shallow breathing and have eye and nasal discharges. Animals have rough hair coats and become rapidly debilitated.

Systemic antibiotics may help control clinical signs, but many animals develop a fibrinous pneumonia that results in either permanent lung damage or continued death losses. Isolation of the causative organism is required for definitive diagnosis, although clinical signs and gross and histopathologic lesions often are characteristic.

Treatment of respiratory disease with systemic antibiotics may be helpful. Infected animals should be isolated. However, eliminating infected colonies followed by repopulation with clean animals is often the only way to eliminate the problem.

High ammonia levels have been shown to exacerbate respiratory disease; therefore, good sanitation and ventilation are extremely important. Barrier systems are used in the laboratory environment to control the disease effectively.

Sialodacryoadenitis virus,a coronavirus, causes inflammation of the conjunctiva, discharge from the eyes, inflammation of the lacrimal (tear) gland with excretion of a dark to red pigment around the eye and the nose, swelling of the salivary glands in the ventral neck area, and weight loss. Affected animals usually recover spontaneously within 7 to 10 days, except that some may have permanent eye damage.

Susceptible animals should not be brought into infected colonies for at least several months after the last clinical case, to prevent continued outbreaks of the disease in incoming animals.

Skin, Tail Problems

Staphylococcus aureus has been responsible for a variety of skin infections including ulcerative dermatitis and pododermatitis (foot infections). Toenail clipping has been successful in some cases. Ringworm lesions are similar to those seen in rabbits and are caused by Trichophyton and Microsporum. Local and systemic treatments are effective.

Ringtail, usually seen only in young rats, is observed as constrictions of the tail and even loss of the tail distal (posterior) to the constriction. Usually the condition is seen in winter months in periods of low humidity (20 to 30 percent) when the heating system is in use. The relative humidity should be maintained around 50 percent in indoor rodent housing areas.

Lice (Polyplax) and, particularly, mites (Dermanyssus, Notoedres, Radfordia ensifera) can cause skin irritation and hair loss. Local treatment is generally effective except for Dermanyssus, which spend a portion of their life cycle away from the rat. In these cases the entire room must be treated. Dermanyssus mites occasionally will attack humans.

Mammary gland tumors are the most common tumors seen in the rat.

Progressive kidney disease is a frequent cause of death in old rats. Affected animals become lethargic, go off feed, and become severely debilitated. No treatment is available.

Healthy rats commonly carry the bacterium Streptobacillus moniliformis in their mouths and throats. This organism can be transmitted to humans and is responsible for the disease commonly known as rat-bite fever. Another organism, Spirillum minor, occasionally carried by both rats and mice, also can cause rat-bite fever in humans.

Robert J. Russell is Director, Laboratory Animal Sciences Program, Program Resources Inc., Frederick, Md. Jim A. Stunkard is Director of the Bowie Animal Hospital, Bowie, Md.