Dirty cages aggravate Mycoplasma

Mycoplasma is a respiratory disease caused by the organism Mycoplasma pulmonis, a nastly little pathogen carried by all pet rats and found in wild rats too. All rats carry it, but not all show symptoms. These symptoms can be treated, but M. pulmonis can never be completely eradicated from a rat's respiratory system by medical treatment. Mycoplasma is therefore of enormous concern to rat enthusiasts.

What predisposes a rat to developing an outbreak of mycoplasma symptoms? There are many factors involved in triggering or worsening an outbreak of mycoplasma symptoms, and one of these factors is urine odor (ammonia) in the cage.

The common wisdom of rat owners states that urine odors make mycoplasma worse. Rats sneeze more when their cages are dirty. Clean the cage, sneezing declines. Why should this be the case? What is going on in the rat's respiratory system?

First, we'll look at how M. pulmonis and urine odors affect the rat's respiratory system when considered one at a time, then we'll look at their combination:

Mycoplasma alone: myco-infected rats in clean cages

Rats infected with Mycoplasma pulmonis and kept in scrupulously clean cages show typical mycoplasma symptoms like snuffling and head shaking.

Inflammations of the nasal mucus membranes (rhinitis), the middle ear (otitis media), and the trachea are very common (Broderson et al. 1976).

A few rats develop partial or complete collapse of the lungs (atelectasis) and fluid in the lungs (consolidation). A small number develop dilated and distorted lungs or lung abscesses. Lung lesions may occur, always associated with upper respiratory disease (Broderson et al. 1976).

Therefore, under normal circumstances, M. pulmonis inhabits the upper respiratory tract. It produces lung diseases as a secondary effect.

Urine odors alone: myco-free rats in dirty cages

Laboratories have produced mycoplasma-free rats (called SPF, or specific pathogen free rats) by delivering litters by c-section. M. pulmonis is passed from mother to offspring in the birth canal.

What happens if you take mycoplasma-free rats and keep them in dirty cages?

Mycoplasma-free rats exposed to ammonia develop lesions in their noses. Most of the lesions were at the front of their nasal passages, and the skin of their nasal and respiratory passages grows three or four times thicker than normal. This damaged area becomes swollen and has dilated vessels, and some of the submucosal glands are replaced by collagen. In contrast, myco-free rats kept in clean cages develop no lesions at all (Broderson et al. 1976).

So, ammonia alone causes lesions in the nasal passages.

Mycoplasma and dirty cages: a bad combination

Rats infected with M. pulmonis and exposed ammonia for many weeks tended to have more severe mycoplasma symptoms (snuffling, head shaking) than myco-infected rats kept on clean bedding. A few developed labored breathing and panting, which were not seen in clean-caged rats (Broderson et al. 1976).

Myco-infected rats exposed to ammonia develop the same lung conditions as myco-infected rats on clean bedding -- partial lung collapse, fluid in the lungs, lung lesions -- but these conditions were much more common and more severe in rats exposed to ammonia than in rats kept in clean cages. Some of the myco-infected rats exposed to ammonia developed dilated and distorted lungs (bronchiectasis), a condition rarely seen in myco-infected rats kept on clean bedding. The prevalence of lung lesions was associated with ammonia levels: the higher the ammonia concentration, the more rats had lung lesions (Broderson et al. 1976). Lesions in the nasal passages were more severe in rats exposed to high levels of ammonia than in rats exposed to no ammonia (Pinson et al. 1986).

Ammonia promotes the growth of M. pulmonis in the respiratory passages (Saito et al. 1982, Schoeb et al. 1982, Pinson et al. 1988). In fact, Saito et al. (1982) found that mice infected with M. pulmonis and exposed to high ammonia had 1000 times more bacteria than mice infected with M. pulmonis that were not exposed to ammonia.

Therefore, chronic exposure to ammonia, perhaps through injury to the nasal mucosa, enhances the growth of M. pulmonis in the upper respiratory tract, producing more bacteria which subsequently invade the lungs.


Under normal circumstances, M. pulmonis infects the upper respiratory tract of rats and only produces lung diseases secondarily. In mycoplasma-free rats, chronic exposure to ammonia produces nasal lesions.

When M. pulmonis and chronic ammonia exposure are combined, by keeping rats on dirty bedding, mycoplasma infections are drastically enhanced. Ammonia promotes the growth of M. pulmonis bacteria in the upper respiratory tract, and these large numbers of bacteria invade the lung and cause lung damage.

Therefore, keeping rats in dirty cages dramatically enhances mycoplasma in rats.

Further reading: for the curious, here's a summary of the 1976 research article on environmental ammonia and mycoplasma.

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