American Fancy Rat & Mouse Association

This article is from the Spring 1998 AFRMA Rat & Mouse Tales news-magazine.


Caved-In Appearance; Colostrum; Frizzie Rot

by Carmen Jane Booth, D.V.M.

Caved-In Appearance (update: Summer I ’98)

Pat Bromberek, Portage, WI
Q I hope you can answer the following question I am about to write because I know other rat owners have rats (or had) with this problem.

What causes rats to get a “caved-in” appearance around the area by their haunches? This is usually accompanied by rapid breathing. My rats get this way when they have respiratory problems. Treatment with various antibiotics doesn’t help even though they (antibiotics) will help out with any wheezing, etc., but the “caved-in” look never goes away! Does the rapid breathing have anything to do with the “caved-in” look, and if it does, is there anything that can be given to the rat to be rid of this problem? The antibiotics that were administered to my rats with respiratory distress have been Baytril, Panmycin, amoxicillin, and Clavamox.

A I have seen what you describe as the “caved-in” appearance in some rats that have had respiratory problems. Even though the antibiotic may have removed the symptom of wheezing, the underlying disease is probably Murine Respiratory Mycoplasmosis (MRM) caused by mycoplasma pulmonis in combination with possibly any number of the other rodent respiratory diseases. This disease (MRM) is incurable, and the damage to the lung is frequently so severe that the animal remains compromised after antibiotic therapy has alleviated the symptoms of wheezing. The altered body conformation is probably due to weight loss and the resultant loss of body condition. The body condition does not return because the underlying disease is still there and the animal is still being compromised even though it is not exhibiting obvious clinical symptoms. Rats that are having difficulty breathing do not eat properly and have a hunched posture which contributes to the “caved-in” appearance. Because of the minimal energy that caged rats need to expend to find food, they can live fairly comfortably with severely damaged lungs and decreased body condition. In the past when I have had rats with MRM, I have ended-up keeping them on lifelong antibiotics in order to keep them free of clinical symptoms and in reasonable body condition.


Q How long does a mother rat/mouse produce colostrum for her newborns. If there are two mothers in a cage and the #1 mom has her babies and #2 doesn’t have hers for a couple of days but starts to nurse the newborns from the #1 mom, will the newborns take all the colostrum intended for the unborn babies?

A Mice continue to produce antibodies in their milk throughout lactation according to Laboratory Animal Medicine, by Fox, Cohen, and Loew. Although colostrum is highly concentrated in antibodies, the pups should still be adequately protected as long as they nurse after birth.

Another interesting fact about rodents and reproduction. Most rodent species have the same number of tissue layers present in their placenta as humans (3) as compared to all hoofed mammals that have 6 layers. With only 3 layers, there is some transplacental protection prior to birth, in contrast to animals with 6 layers where it is crucial that they receive colostrum within 12 hours after birth.

Frizzie Rot

Q “Frizzie Rot,” as termed by the folks back East for the skin problems the Frizzie mice get, we have found to be genetic. Only by eliminating those with it can we get Frizzie mice that don’t get this problem (sores and scratching on the shoulders and head area, slowly eating away the mouse’s ears, eventually the eyes have major problems because the sores get so bad the whole face is affected). Is there a real medical term for this and what is it? Is it a parasite that just likes Frizzie mice (Frizzies can be housed with other mice without those mice getting affected), or is it genetic like we think (similar to blue dogs with a genetic skin problem)?

A If you want to get some idea of what is going on, there are some diagnostic tests that could be performed to help narrow the possibilities.

  1. Culture the lesions on the affected animals and determine if there is a bacterial component.
  2. If a pathogenic strain of bacteria is isolated, treat with the appropriate antibiotics to see the lesions disappear. There are differences in susceptibility to disease between different mouse strains and these Frizzy mice may be more susceptible than the other strains.
  3. Have samples taken of affected and unaffected areas and have the samples read by an experienced rodent pathologist.

One problem is that conventional rodents can have such a wide range of diseases that can adversely affect the ability to determine if there is an underlying genetic component. *

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Updated April 1, 2014