Mycoplasma Pulmonis

For anyone who has ever owned a pet rat, they will be familiar with common bacteria which all conventionally bred rats carry. Mycoplasma Pulmonis is a small organism, a unique bacterium that lacks a cell wall and is often referred to as MRM, Murine Respiratory Mycoplasmosis. Clinically silent, but slowly progressive, it affects the nasal passages in the nose to the lungs. Although primarily it affects the lungs and airways, it can also cause genital infections, sterility and arthritis in rats.

Mycoplasma Pulmonis is highly infectious and passes generally through a rat sneezing, by aerosol droplets, but it can also pass through the womb into the kitten before birth. Once inside the rat, it lives in the nose and middle ear but causes no immediate disease. The first symptoms you will see include sneezing, crusty and runny nose with a reddish discharge, called porphyrin staining. Over time, this can lead to wheezing, difficult breathing, loss of appetite and hunched posture.

Mycoplasmas are more complex than viruses. They never cause sudden (acute) disease. Because they utilize primarily RNA instead of DNA to reproduce, they are only killed by the protein-synthesis inhibiting antibiotics from the Macrolide family of drugs. These include Clarithromycin and azithromycin (Zithromax).

It is the surface proteins that most interest researchers. These are the proteins that give the mycoplasma stealth, suppress the rat’s immune system and prevent it from killing the bacteria. Some strains of mycoplasma are more virulent than others and you won’t know which strains are present, unless a culture and sensitivity test is performed to determine the most appropriate choice of antibiotic to use. This is why the elimination of mycoplasma can be very difficult. The bacterium is resistant to most antibiotics that work by preventing growing bacteria from making a new call wall. So Penicillin and other antibiotics that work in this way are useless against it. The mycoplasma is 100% resistant.

Although you cannot treat mycoplasma effectively and cure the rat, using an antibiotic from either the Tetracycline or Macrolide family will work to reduce the amount of mycoplasma bacteria in the rat, thus limiting the time you give antibiotics to times when the rat is showing clinical signs. The organisms are tough to eradicate.  The infection is chronic. While there may be an early acute stage, death does not usually occur until the animal is much older. Acute death is usually the result of a secondary bacterial infection and not mycoplasma alone. Elimination of a mycoplasma infection in rats is for all practical purposes impossible. It can only be suppressed.

Stresses that can cause symptoms to worsen are other freeloading infections including viruses, poor nutrition, overcrowding, high ammonia levels in the cage and poor hygiene. (Urine quickly turns into ammonia so must not be allowed to build up).

Of all the pathogens occurring in laboratory rats, mycoplasma pulmonis has had the greatest negative impact on studies.

Zithromax is a human medication and as such your vet may be unfamiliar with it.
Recently I had a girl who was no longer responding to baytril and doxycycline so a combination of zithromax and doxy and millophyline was tried.
Because the practice had no other clients using zithromax it needed ordered in (next day delivery), and I had to purchase the whole pack (about £12).
It requires refridgeration and to be mixed fresh every 5 -7 days.
There is information in the IVIS files (link on left) regarding dosing, and also on ratguide.com - it may be worthwhile printing some things for your vet.

Mana Rattery March 2005 (with many thanks to April).