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

By Carmen Jane Booth, D.V.M., Ph.D.

Rachel Czurlanis, Clifton Park, NY
Q I am 12 years old and as of right now I have two rats (female) and one mouse (male). I keep the rats separated from the mouse. I keep the mouse in a 10-gallon tank.

I am worried about my mouse. He has large cuts on the back of his neck and back. I noticed it about last week, and just today, I realized it’s just getting worse. I’m very unsure about his background. I got him from my neighbor who is a janitor, who got him from one of the classrooms during the summer. I think he is an albino. He also has some dried blood around the inner part of his ears.

My friend has a rat who gets “hot spots” if she has cheese or seeds. I am wondering if it could be anything like that. I don’t know much about mice, but I do know that something is definitely wrong.

A The most likely cause is self-induced trauma from scratching. The most common cause of pruritis in pet mice is mites. The most effective treatment is Ivermectin—two doses exactly 14 days apart. This, in combination with cleaning the environment and getting rid of any shaving- type bedding, is usually successful. I stopped using commercial shaving bedding years ago since it was often contaminated with mite eggs from the wild rodents at the lumbar yard. Ivermectin can be harmful to the developing pups in utero and newborn pups until their blood brain barrier is fully developed, but is safe in weanling and adult mice.

The second most common cause of pruritis in laboratory mice is ulcerative dermatitis. This is seen in mice on a C57BL/6 background, is genetic based, and treatment is often not successful long term.

Emily, e-mail
Q I own a 1-year-old Hairless rat named Peka. Recently, she got two bumps on the lower part of her stomach. I was very worried and thought that they might be tumors so I decided that I would take her to the vet in a week. Then, one of the bumps apparently popped because it had a small scab on it and was shrinking. The other one is still there and seems to be staying the same. Along with this, she has lost a considerable amount of weight (and doesn’t eat too often) and she sleeps more and doesn’t play as much. She lives in the same aquarium as Kendra, a large 9-month-old Satin female, and Moca, a petite 5-month-old standard female. Neither of these rats have shown any symptoms and are doing fine.

A What you are probably seeing are keratin plugs in the skin due to the defect that results in the hairs breaking off and the rats appearing hairless. The plugs can be quite pronounced and when they rupture, are reminiscent of a pimple. Thus, what you are seeing is completely normal. However, the Hairless mutations are often associated with defects in the immune system and these rodents are not able to fight off infections and usually die within 1 year unless they have special housing.

Hairless rats and mice both do succumb to bacterial problems as they age. Only time will tell. But both do get the follicular plugging that makes them look bumpy.

Here is an excerpt from an article that describes the pathology in Hairless rats.
Panteleyev AA, Christiano AM.
The Charles River “hairless” rat mutation is distinct from the hairless mouse alleles.
Comp Med. 2001 Feb;51(1):49-55

“The postnatal homozygous rat skin was characterized by abnormal keratinization of the hair shaft and formation of a thick and dense layer of corneocytes in the lower portion of the epidermal stratum corneum. This layer prevented the improperly keratinized hair shaft from penetrating the skin surface. Starting from the latest stages of hair follicle (HF) development, obvious signs of HF degeneration were observed in homozygous skin. This process was extremely rapid, and by day 12, mainly atrophic HFs with abnormal or broken hairs were present in the skin. Therefore, the mutation in the CR rat abrogates cell proliferation in the hair matrix and affects keratinocyte differentiation in the HF and interfollicular epidermis, a phenotype that is completely distinct from hr/hr mutation in Hairless mice.”

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