American Fancy Rat & Mouse Association

This article is from the May/June 1991 AFRMA Rat & Mouse Tales news-magazine.


Malocclusion and Environmental Conditions Contributing to Respiratory Disease

By Carmen Jane Booth


Rats and mice have a normal dentition of 2(I 1/1 C 0/0 P 0/0 M 3/3) = 16, where I = incisors, C = canine, P = premolars, and M = molars. The incisor teeth are open rooted and grow continuously. The molars are closed rooted and do not grow continuously (just as in people). Because rat and mouse upper and lower incisors never stop growing, there is the potential for problems. The two situations of concern are: 1. Where the upper and lower incisors do not meet (line up with) each other and wear down at the same rate, 2. Where the food is too soft and the incisors grow too long from lack of wear. The result is the same in both cases: the teeth are too long and the animal cannot open its mouth and eat or the teeth grow into the roof or floor of the mouth, causing pain and infection. Usually it is the lower incisors that cause the problem.

Skull (a) and mouth (b) of a 10·week old rabbit showing edge-to-edge occlusion of the primary incisors. (c) & (d) of a 3 month old rabbit showing extreme mandibular prognathism.

Malocclusion (the upper and lower teeth do not meet properly) is a heritable condition. If two animals with the problem have crooked teeth, their young will have crooked teeth. The easiest way to prevent the problem is not to breed severely affected animals. If the problem is caused by soft food, harder food, wood chew blocks (pet store), etc., can be given to affected animals. In cases where there is curling of the lower teeth, drooling, or inability to open the mouth and eat, the teeth can be clipped and filed. I have used dog nail clippers or human fingernail clippers, depending on the size of the teeth involved. It is important that the tongue and surrounding tissues are not cut. If the clipped tooth has a jagged edge, the surface can be smoothed with an emery board. Rodents normally have long lower incisors and all their teeth have the normal characteristic brown color. Restraint is important during the clipping and it is best to have someone clip the teeth who can restrain the animal and clip the teeth without hurting the animal.


Rats and mice are susceptible to a number of viruses and bacteria that can cause disease of the upper and/or lower respiratory track (nose to lungs). Many of these organisms are commensals: that is, they live in the animal and do not cause any harm unless the animal’s health status is compromised from other causes such as stress (defined as the emotional or biological responses to new or threatening stimuli). Some of the more common stresses that we impose on our little friends are: lack of food or water; unclean, drafty, poorly ventilated microenvironment (cage); high humidity and temperature extremes, as well as isolation or overcrowding.

One of the principal agents causing adverse effects on the respiratory tract of rodents is ammonia. Ammonia is produced by the action of urease positive bacteria on urine and feces. The enzyme urease converts the urea in urine to ammonia. The concentration of ammonia increases with increased numbers of animals per cage, decreased air flow and increased temperature or humidity. Cage design, type of bedding, and frequency of cage cleaning also influence the amount of ammonia present. (I will discuss this in a future newsletter.)

Ammonia is an irritant gas that adversely affects the immune system and causes pulmonary (lung) lesions. This is especially a problem in rodents with the bacteria Mycoplasma pulmonis. Rats with M. pulmonis exposed to 25–250 parts per million (ppm) ammonia for 4–6 weeks had more severely affected lungs than those who were not exposed. To give you an idea of how you can estimate how high the ammonia is in a cage, here are some criteria:

15 ppm - Smell detectable by nose
25 ppm - Eye irritation
50 ppm - Respiratory effects
500 ppm - Pulmonary edema and death

Essentially it is important to keep our rodents as clean as possible in a stress free environment. A room temperature between 72° and 76°F and a relative humidity between 40% and 60% with 14 room air exchanges per hour is recommended. I realize that these conditions are impossible to achieve in many situations. Cleaning the cages at least one to two times a week is beneficial and the easiest factor to control.

Next issue: 1. The pros and cons of different bedding material and 2. Everything you never wanted to know about Mycoplasma pulmonis.

Susan Ramsay, Lebanon, OR
QI am a new member, having very little experience with rats. Our first, Alicia, died from my own stupidity. I do not intend to let that happen again, so I joined AFRMA. What causes the “bloody tears”: does it mean poor health? Alicia had this, and it kept getting worse. Our local vet did not have the experience with these tiny creatures, and so did not advise us correctly. However, I can work with her, so if I learn more, I can advise her!

ARegarding the red colored ocular discharge known as chromodacryorrhea, it is secreted from the Harderian gland. The Harderian gland is a large pigmented lacrimal gland located behind the rat’s eyeballs and secrets a lipid (fat) and porphyrin-rich (protoporphyrin IX and coproporphyrin III) secretion that lubricates the eyes. This is the same function as human tears—keep the eyes moist. With some stressful situations or acute illnesses the secretions may overflow and stain the face and nose. When this dries it appears like dried blood. There is little or no blood contained in these secretions. You may also find the red staining on the medial sides of the front paws of rats that have chronic runny eyes from sialodacryoadenitis (SDAV) virus are common causes of increased Harderian gland secretion. There is no therapy for SDAV, but recovered rats are free of virus, are immune, and there is no carrier state. If stress or bacterial infection is the cause, remove the stress and treat with an appropriate antibiotic for the causative organism.

(Anyone suspecting their animal(s) of having a medical problem should seek the care of their local veterinarian.) *

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July 5, 2014