This article is from the July/Aug. 1991 AFRMA Rat & Mouse Tales news-magazine.

By Carmen Jane Booth, vet student

The selection of animal bedding should be given special consideration. Bedding serves a number of basic purposes: absorb moisture, insulate, and provide comfort and nesting material. Bedding can be contact (animal housed directly on it) or non-contact (wire mesh bottom cages). Non-contact bedding is usually more absorbent. Some different types of bedding include: paper, wood chips, shavings, ground corn cobs, pelleted alfalfa, sawdust, ground peanut hulls, or diatomaceous earth (cat litter). Cat litter may dehydrate newborn animals. The material chosen should be free of dust, insects, toxins, pesticides, pathogenic organisms, and droppings from feral animals. In addition, the bedding should be non-edible and free of natural volatile chemicals. In general, soft woods such as white pine, yellow pine, and red ceder should be avoided because they release volatile hydrocarbons that can stimulate hepatic (liver) microsomal enzymes. These liver enzymes are in a large part responsible for eliminating foreign compounds (chemical, etc.) from the body. When these processes are abnormally activated, liver neoplasia (cancer) may result. These soft wood beddings have been shown to increase respiratory infections and may decrease reproductive productivity and act as carcinogens. Beddings made of hard woods such as maple, birch, or beech as well as aspen have been used without detectable adverse side effects.

Mycoplasma pulmonis is responsible for causing a serious respiratory disease in rodents known as Murine Respiratory Mycoplasmosis (MRM). MRM is common in rodents and can cause reproductive as well as respiratory problems. Rats and mice are the principal hosts of M. pulmonis, but rabbits, guinea pigs and other rodents may carry the agent although they are not clinically affected. Mycoplasma are bacteria that are pleomorphic and do not contain a cell wall. They have a diameter between 0.2 and 1.0 µm. Mycoplasma infections are extremely infectious and can be transmitted by respiratory aerosol over small distances, direct animal contact, sexual transfer, and in utero, but there is no evidence to support transmission through fomites (caging, equipment or clothing). Poor environmental conditions, high ammonia levels and concurrent infection with other disease agents can damage the respiratory tract and predispose the animal to mycoplasma infection.

The clinical signs of M. pulmonis are usually subclinical but they can be apparent. The three foci of infection are upper respiratory, bronchopulmonary, and genital. The upper respiratory signs involve sniffling, sneezing, rough hair coat, squinting, a serous or mucopurulent eye and/or nasal discharge, and in some cases a head tilt. The bronchopulmonary signs occur 3–6 months post infection and include: lethargy, rough hair coat, hunched posture, chattering, weight loss, dyspnea (difficulty breathing), congestion, and death. The reproductive form of the disease includes an ascending genital infection resulting in reduced fertility and fetal deaths. Infection of the uterus (pyometra) and oviduct often occurs frequently in rodents with respiratory mycoplasmosis.

A tentative diagnosis of mycoplasmosis is based on clinical signs and confirmed by gross and microscopic lesions and isolation of the agent from the nasal pharynx, tympanic bullae, trachea, uterus, or lungs. Treatment for mycoplasmosis is somewhat discouraging. For all practical purposes, elimination of mycoplasmal infection from affected mice and rats is impossible. Antimicrobials placed in the drinking water can suppress the clinical signs and infection, but do not clear the agent from the animal. As soon as the antibiotics are removed the disease resumes. Tap water contains salts that precipitate the drugs, especially tetracyclines, causing them to be inactivated. Tetracyclines as well as other drugs are inactivated by light and can be completely ineffective within an hour or so.

Treatment should not be advocated as a method to remove mycoplasma from a colony of rodents. Mycoplasma pulmonis does not affect humans although they can carry it in their nasal passages. Some of the palliative therapies follow:

• Tetracycline hydrochloride at 2 to 5 mg/ml given fresh daily in deionized drinking water for 5 days
• Sulfamerazine at 0.02% in the drinking water or 1 mg/4 g of feed
• Tylosin at 66 mg/L (2.5 g/10 gal) for 21 days
• Chloramphenicol 30 mg/kg for 5 days
• Baytril 5 mg/kg injectable

There are many other different therapies. All therapies are published except for the Baytril. This treatment was given courtesy of a laboratory animal veterinarian at the University of Washington. This has been an effective therapy for rabbits infected with Pasteurella multocida.

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