Breathing Problems in Mice & Rats

Respiratory illness among rats and mice is very common, but can become quite serious. What might start out as a little sniffling, sneezing, or wheezing (also called snuffles), can often lead to a chronic condition with secondary infections such as sinusitis, middle ear infections, bronchitis, emphysema, or pneumonia.

Rats and mice are particularly susceptible to respiratory infections for several reasons. To begin with, they live, eat, and sleep directly on or within a few inches of their bedding. If it is not changed regularly, or if it contains irritating chemicals, the animals have little chance to get a breath of fresh air.

Ammonia is a chemical found in urine, and it produces a strong-smelling odour in rodent bedding that has not been changed recently. Owners may notice a smell from several feet away; rats and mice must live with their noses directly above the source. As the amount of ammonia rises, it irritates the eyes and lungs, and can reach toxic concentrations. Long-term exposure to ammonia causes respiratory distress, depresses the immune system, and predisposes the animal to infections.

Cedar or pine bedding is another source of lung irritation to rats and mice. Although phenols, the scent-producing chemicals in soft woods, smell quite nice, they are also toxic to the lungs and other organs. They are the same chemicals found in the cleaning agents Lysol and Pine-Sol Many animals kept on cedar or pine bedding develop liver failure over time, as the phenols are absorbed through the lungs into the bloodstream. Shredded aspen, corn cob, and commercial pellet beddings are non-toxic and much kinder on animals’ lungs.

Rats and mice are also at an increased risk of contracting respiratory infections during interactions with other pets and other pet owners. Although cats and dogs are often socialized and taken to pet and breeding shows, there are many vaccines available to them that greatly reduce the risk of infection with common respiratory diseases. Rats and mice are also socialized and taken to shows, but there are currently no vaccines available to the average owner. As a result, respiratory illness outbreaks are often the result of a recent show or a new individual being brought into a colony.

Some of the most common infectious illnesses picked up by mice and rats are Mycoplasma pulmonis, Sialodacryoadenitis (SDA) virus, and Sendai virus. Several of these infections may occur at once, or in conjunction with inappropriate or dirty bedding. An animal that is young, old, stressed, or already sick is more likely to contract an infection.

M. pulmonis, which can cause MRM (murine respiratory mycoplasmosis) or CMP (chronic murine pneumonia), is carried by most rats and mice worldwide. Not all strains are highly virulent, but whether or not disease develops is mostly dependant on the health and genetics of the infected rat or mouse. Some strains of rodents are more resistant to infection, and educated breeders will not breed rats or mice with chronic respiratory illness. Unfortunately, pet store rats and mice do not usually have a guarantee of good breeding.

Although M. pulmonis can be treated with antibiotics, it is often only controlled in the long term rather than eliminated. Once an animal has developed clinical signs, it is likely to have permanent damage to its lungs. Mice are more susceptible and less likely to survive if they become seriously ill.

SDA virus is highly contagious among rats and very young mice, and respiratory signs occur in conjunction with eye rubbing, squinting, and swelling around the face and neck. The eyes may become so irritated that the animal scratches or gouges at them, and red-brown tears often stain the face. Although most rats and mice recover on their own from this virus, damage to the eyes may be permanent, and the animal must be monitored closely for secondary bacterial infections.

Sendai virus causes severe respiratory illness usually among sick or infant animals, and is especially serious in mice. It is often a complication of pre-existing CMP. Sendai virus and SDA virus are both illnesses that must come from contact with an infected mouse or rat; however, not all infected rodents will show clinical signs. Carriers may appear healthy. Any new introduction to a colony should be purchased from a reputable source and quarantined before coming into contact with the other residents.

So if you happen to own a pet rat or mouse, be aware of the potential for respiratory disease in your critters and take precautions to prevent these problems before they occur.

By Jennifer Perret – Pets.ca writer