Mycoplasma pulmonis is a tiny wall-less bacterium that colonizes the upper airway of many pet rats, especially rats from pet-store or rescue origins. (Established breeder lines like ours are often clean or controlled, so it is NOT the case that every pet rat is mycoplasmal.) When the bacterium gets the chance, usually because of stress, ammonia buildup in the cage, or another respiratory infection running interference, it can ascend the Eustachian tube into the middle ear and cause otitis media (and from there, otitis interna with head tilt). Or it can settle into the lower lungs and cause chronic bronchopneumonia.[1]
⚠ Roof rats are not as resistant to this as Norway rats, and the difference can be fatal. The breeder, from his own account, 2021: “Probably because they have been bred in captivity in large colonies for so long, pet Norway rats have developed partial resistance to chronic respiratory infections like Mycoplasma. However, they are carriers even when they are still asymptomatic. Unfortunately, Roof Rats are not as resistant, and they can sometimes become seriously ill, even die, when exposed to infected Norway Rats.”
He does not say never mix them, and the caveat is his too: “If you are sure that your rats are healthy, you may (cautiously) allow Norway Rats and Roof Rats to play together. We have found that Male Roof Rats can usually, safely play with Female Norway Rats, and there is no danger of pregnancy as they are different species.” The two statements are a pair and only make sense together: an apparently well Norway rat may still be carrying it.
Treating a rat? Work out the dose with the Roof Rat Medication Dosing Calculator.
How you would know
The classic signs: sneezing, snuffling, discharge around the eyes and nose, sometimes a clicking or rattling breath sound. Those classic signs are written for Norway rats, where the staining is rust-red porphyrin. In a roof rat, watch for CLEAR discharge, not red. “I’ve never seen any roof rat get porphyrin. Their discharge is clear, and if you see it, it is usually a sign of serious infection.” (the breeder, from his own account) If the infection reaches the inner ear, you may see head tilt, falling, or rolling. Sustained weight loss is common in chronic cases. Any of these signs needs an exotics vet. Don’t wait, especially if breathing sounds wet or the tilt is severe.
How it’s treated
Treatment is suppressive, not curative.[1] Mycoplasma is rarely fully eradicated. But it CAN be driven down to where the rat’s own immune system controls it.
The standard veterinary combination: doxycycline (a tetracycline that blocks the bacterial ribosome) paired with enrofloxacin (a fluoroquinolone that shreds bacterial DNA gyrase). The two together hit the bacterium at two completely different molecular targets at the same time. A single mutation can’t protect against both, which keeps resistance from breeding.[1]
Supportive care that matters: clean cage with low ammonia (one of the biggest single factors in mycoplasma stability), soft food while she recovers, hydration, sometimes meclizine for vestibular nausea if she’s tilting. Important: meclizine is NOT a Rat Guide drug. That dose comes from an exotic-animal formulary, extrapolated across species with no rat-specific efficacy trial, so treat it as comfort care a vet may or may not choose, not an established rat protocol. Some vets add azithromycin for an extra week or two to reach intracellular bacterial reservoirs. The rat will hopefully start improving within a few days, but treat for at least 2 weeks to make sure the infection is gone. The Rat Guide’s own courses run 10 to 30 days, and a chronic case often needs 6 to 8 weeks and then a reduced maintenance dose.[1]
Why penicillin or amoxicillin do nothing
Mycoplasma has no cell wall. Beta-lactam antibiotics (penicillin, amoxicillin, amoxicillin-clavulanate) all attack the bacterial cell wall. On Mycoplasma they have nothing to bind to.[2] Not resistance: a biochemical null operation. A rat with mycoplasma otitis given amoxicillin LOOKS untreated, because she IS untreated. The infection keeps progressing while everyone thinks the drug is doing its job. See the dedicated FAQ entry on this for the full pharmacology.
Walk through it interactively
Our JUSTICE LEAGUE vs MYCOPLASMA text adventure puts you in command of a Justice League of antibiotic and supportive heroes against a Mycoplasma horde inside a rat’s lungs. You learn the team coordination through play: clear the mucus terrain first, hold the infection still with doxycycline, then strike with enrofloxacin, and don’t kill the rat by over-treating. And don’t bother deploying penicillin.