American Fancy Rat & Mouse Association

This article is from the WSSF 2007 AFRMA Rat & Mouse Tales news-magazine.



By Carmen Jane Booth, D.V.M.

Pneumonia in Rats

Kathryn L. Lane, Taylor, MI
Q Why is pneumonia curable in people, but not curable in rats? Aren’t they the main test subjects in labs for these kinds of things? Is it a totally different type of illness? Are there not enough people trying to find a cure?

I’ve lost two babies already in the past 2 years. Each barely lived to see a year of age. Why can’t it be stopped? Does children’s bubble gum Amoxicillin help? A small animal vet prescribed it but my baby died before I could get him home to give it to him. It took him out in only one day.

At this time I own four rats (three are 1 year old and one is 1½ months). Each has a separate, very clean cage, but I still want to be sure of what I need to have on hand if another should get this illness. What helps?

Answer by Carmen Jane Booth, D.V.M., Ph.D.


Pneumonia is the medical term for an infection of the lung.


Bacteria are the most common causes of pneumonia; however, virus and other microbial organisms can also cause pneumonia. In many cases it is often impossible to identify the specific bacteria.

Bacteria are categorized by the Gram-staining procedure used to visualize them under a microscope. The stains determine if they are gram-negative or gram-positive bacteria. Gram-positive bacteria appear blue with the stain, and Gram-negative appear red or pink.

The most common causes of pneumonia in humans are the Gram-positive bacterias, Streptococcus pneumoniae (also called pneumococcal pneumonia) and Staphylococcus aureus. Staph. aureus is one of the main causes of pneumonia that occurs in the hospital (nosocomial pneumonia). It is uncommon in healthy adults but can develop about five days after viral influenza, usually in susceptible individuals, such as people with weakened immune systems, very young children, hospitalized patients, and drug abusers who use needles.

Gram-negative bacteria are common infectious agents in hospitalized or nursing home patients, children with cystic fibrosis, and people with chronic lung conditions. Streptococcus pyogenes or Group A Streptococcus can cause pneumonia. The most common Gram-negative species causing pneumonia is Haemophilus influenzae (generally occurring in patients with chronic lung disease, older patients, and alcoholics).

Klebsiella pneumoniae may be responsible for pneumonia in alcoholics and in other people who are physically debilitated.

Pseudomonas aeruginosa is a major cause of pneumonia that occurs in the hospital (nosocomial pneumonia). It is common in pneumonia patients with chronic or severe lung disease.

Moraxella catarrhalis is found in everyone’s nasal and oral passages. Experts have identified this bacteria as a cause of certain pneumonias, particularly in people with lung problems, such as asthma or emphysema.

Other Gram-negative bacteria that cause pneumonia include E. coli (a cause in newborns), Proteus (found in severely damaged lung tissue), and Enterobacter.

Atypical Pneumonia

Atypical pneumonias are generally caused by tiny organisms called Mycoplasma or Chlamydia pneumoniae and produce mild symptoms with a dry cough. Hospitalization is uncommon with pneumonia from these organisms.

Mycoplasma pneumoniae is the most common cause of atypical pneumonia. Mycoplasma is a very small organism that lacks a cell wall. It spreads from prolonged, close contact and is most often found in school-aged children and young adults. The condition in humans is usually mild and is commonly known as walking pneumonia.

Another small non-bacterial organism, Chlamydia pneumoniae, is now thought to cause 10% of all community acquired cases of pneumonia. It is most common in young adults and children, where it is usually mild. In one study, it was the cause of 14% of cases in a group of children with pneumonia. While less common in the elderly, it can be very severe in this population.


Viruses that can cause or lead to pneumonia include influenza, respiratory syncytial virus (RSV), herpes simplex virus, varicella-zoster (the cause of chicken pox), and adenovirus. Outbreaks usually occur between January and April.

Influenza is associated with pneumonia directly or by altering the mucociliary-apparatus in the lung and thus, making a person susceptible to bacterial pneumonia.

Respiratory syncytial virus (RSV) is a major cause of pneumonia in infants and people with damaged immune systems. Studies indicate that RSV pneumonia may also be more common than previously thought in adults, especially the elderly.

Adenoviruses have been implicated in about 10% of childhood pneumonia. In adults, herpes simplex virus, adenoviruses, and varicella-zoster (the cause of chicken pox) are generally causes of pneumonia only in people with impaired immune systems.

[There are also fungi that can cause pneumonia but they are uncommon unless a person is immune compromised.]


The most common cause of pneumonia in rats is Mycoplasma pulmonis. Bacteria causes include: Corynebacterium kutscheri, Bordetella bronchiseptica, Cilia-associated Respiratory Bacillus infection (CAR Bacillus), Pasturella pneumotropica, Streptococcus pneumoniae, and the agent of Rat Bite Fever in humans, Streptobacillus moniliformis.

Viral causes of pneumonia in pet rats include: the Rat corona virus (Sialodacryadenits Virus), and the rat paramyxoviridae family virus (parainfluenza 1, or Sendi Virus). Wild rats can also harbor Hantavirus (Bunyaviridae family), a zoonotic disease that can be fatal in humans.

From here I will digress from the text. In general, most rodent viruses are species specific (Hantavirus being one of the exceptions) and do not infect people and the same is true for human viruses leaving rats alone. However, when it comes to bacteria, the situation is different. Although many bacteria are species specific, many are not and can and do cause infections in multiple species. This is in part because they are ubiquitous in the environment.

In reality, humans with refractory pneumonia are used in clinical trials to study treating humans with new drugs against pneumonia. There have been studies of rodents used as mandated by the Food and Drug Administration to determine the way a drug is metabolized or if there are unforeseen side effects, before being used on people.

So why can’t we cure rats with pneumonia? In reality, we can cure some causes of pneumonia in rats. However, many rats have multiple problems at the same time such as endemic Mycoplasmosis with other problematic bacteria, or viruses. Once you have Mycoplasma pulmonis in a pet rat environment, it is next to impossible to get rid of it. The rats even if treated, continually become re-infected. Most rats do okay until they are stressed from other reasons (pregnancy, old age, cancer, etc.) In addition, the damage caused by M. pulmonis is exacerbated by the conversion of urea in the urine to ammonia by the bacteria in the feces. For reasons that are not completely understood, M. pulmonis does not respond as well to treatment in rats as M. pneumonia does in humans. Treating Mycoplasma infections in people is also challenging, and my personal experience in treating cats with mycoplasma wound infections has also been very difficult to get complete resolution. *


  1. Adapted from
  2. The Pathology of Laboratory Rodents and Rabbits, 2001 by Percy and Barthold.

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Updated April 2, 2014