Medical Corner - Mycoplasmosis in the Pet Rat

Mary Ann Isaksen
From the September/October 1997 Rat & Mouse Gazette

Mycoplasma pulmonis, the organism that almost all rats in the general pet population carry is the cause of the majority of respiratory and genital infections in rats. The organism is carried in the upper respiratory system and is transmitted by direct contact between mother and babies, by sexual transfer, through the air over short distances, and can even be passed to the babies while going through the birth canal if the mother is infected.


Stress or other illnesses can weaken your rat's immune system and cause your rat to break with an active mycoplasmal infection. If left untreated, this can lead to pneumonia. Therefore, it is very important to ensure a stress-free environment for your pet. This includes providing an uncrowded, well-ventilated, clean cage with a safe bedding product in which to live at all times. This will eliminate any chance for ammonia buildup from urine and feces, or phenols from cedar and pine, to damage the respiratory system's own protective capacity. It is also very important not to expose your pet to other rats who may be sick since a secondary bacterial or viral infection could allow the mycoplasmal infection to accelerate to pneumonia very quickly, possibly causing death.

Some of the bacteria that may accompany mycoplasma pulmonis during respiratory disease and may worsen symptoms include Pasteurella pneumotropica, Streptococcus pneumonia, Bordetella bronchiseptica, CAR bacillus (cilia-associated respiratory bacillus), and Corynebacterium kutscheri. Viral infections such as SDA Virus (Sialodacryoadenitis) and Sendai Virus in combination with mycoplasma may also quickly lead to death. SDA Virus is frequently seen in the pet rat population, so be especially careful about quarantining any new rats you bring into your home for a least two weeks.


The symptoms of the upper respiratory disease, which involve the nasal passages and middle ears, can include sneezing, sniffling, occasional squinting, rough hair coat, and porphyrin staining around the eyes and nose. If the inner ear becomes involved, head tilt (wry neck) and rolling may occur.

As the disease progresses, it will enter the lungs. If exacerbated by bacterial infections, viral infections, or ammonia, symptoms may include lethargy, rough hair coat, hunched posture, porphyrin staining, chattering, weight loss, labored breathing, and eventually death. If the mycoplasmal infection is not complicated by any other bacterial infection or viral infection, the terminal stage of the disease can last for weeks or even months.


Genital Mycoplasmosis is quite common in female rats. The infection is not usually apparent, but reduced fertility, fetal deaths, embryonic resorptions, and small litters may occur. Infection of the fallopian tubes and uterus frequently occurs in rats who have respiratory mycoplasmosis, but can also take place independently.


There is no cure or vaccine for mycoplasma pulmonis. However, infection and symptoms can be suppressed with antibiotics in the early stages of the disease, but will most likely become a chronic condition requiring periodic or even constant treatment. This disease progressively worsens, eventually causing consolidation of the lungs and abscesses in the lungs, leading to death.

Some antibiotics proven to be effective against mycoplasma are tetracycline, tylosin, gentamicin, amikacin, chloramphenicol, doxycycline, and enrofloxacin (Baytril). Doses are as follows:

Tetracycline orally at 5 to 10 mg per pound three times daily for at least 14 days
Tylosin orally at 4.5 mg per pound twice daily or 66 mg per litre of water for 14 to 30 days
Gentamicin (best used in combination with Cefadroxil) at 1 to 2 mg per pound injected twice daily for 7 to 14 days
Amikacin at 5 mg per pound injected twice daily for 7 to 14 days
Chloramphenicol at 15 to 25 mg per pound injected twice daily for 7 to 14 days
Doxycycline orally at 2.5 mg per pound twice daily for 14 to 30 days
Enrofloxacin (Baytril) orally at 5 mg per pound twice daily for 14 to 30 days.
Baytril and doxycycline also work extremely well in combination.

In the advanced stages of the disease, or in cases of wry neck and rolling, a corticosteroid such as Prednisone (given in addition to an antibiotic) may be helpful in reducing inflammation. By reducing inflammation in the infected lungs, the rat may breathe easier, resulting in less frequent gasping attacks, allowing the rat to live an extended life more comfortably. In the case of wry neck and rolling, reducing inflammation in the inner ear will stop the rolling within a few days. Prednisone dose: .25 to 1 mg per pound once daily. Normal use of prednisone would be only for a few days, but in terminal cases where comfort of a dying rat is the goal it can be used longer.

Another drug helpful in severe cases of pneumonia is Aminophylline. This drug relaxes and expands bronchial tubes, helping the rat to breathe easier. Dose: 2.5 to 5 mg per pound orally or injected twice or three times daily.

Good husbandry, breeding only from mycoplasma resistant rats, and early treatment are the only things we can do to fight this deadly disease at this time. Someday, maybe we'll be lucky enough to have a drug company develop an effective vaccine. We can only hope.