This article is from the Jul.-Oct. 1992 AFRMA Rat & Mouse Tales news-magazine.

By Carmen Jane Booth, D.V.M.

Three books that contain medical information on rats and mice are: 1. The Biology and Medicine of Rabbits and Rodents by John E. Harkness and Joseph E. Wagner; 2. Clinical Laboratory Animal Medicine, An Introduction by Donald D. Homes; 3. Laboratory Animal Medicine edited by James G. Fox, Bennett J. Cohen, and Franklin M. Loew.

This article begins a series concerning the viral diseases of mice.

Mousepox Ectromelia

Ectromelia means imperfection or absence of limb. The only natural host for this virus is mice. Infection of other rodents only results in a brief noncontagious infection. This is a highly infectious disease which is relatively uncommon in the United States but is associated with high morbidity (number of animals affected) and mortality (number of animals that die). The disease is clinically severest in some inbred strains of mice while other inbred strains are comparably resistant and may be inapparent carriers of the virus and serve as a reservoir (source) of the disease for other mice in the colony. The ectromelia virus belongs to the family Poxviridae, subgroup Vaccinia, genus Orthopoxvirus, consists of double stranded DNA with a dumbbell-shaped nucleoid, and is found in the cytoplasm of infected cells. The virus is 175 x 290 nm and is oval to brick-shaped. The routes of transmission are respiratory aerosols, contact with skin debris, skin abrasions, and fecal contamination. In addition, urine, other animals, ectoparasites, and fomites, may also spread the virus. The virus is relatively stable in dry conditions, and has a latent period of about 10 days for infection to development of skin lesions and shedding of the virus into the environment. Asymptomatic carrier mice that are stressed because of environmental changes, handling, or shipping are predisposed to converting to a clinical disease state. Mice infected with ectromelia vary from asymptomatic to acute to chronic. The clinical signs of the disease are variable. In the acute form, hunched posture, rough hair coat, diarrhea, conjunctivitis, high mortality,and swelling of the face or extremities may be observed. The cutaneous rash is rarely seen in acute outbreaks. In the subacute to chronic or cutaneous form, there is a generalized papular rash that leads to swelling, ulceration, and amputation of appendages with variable morality. The skin lesions may look like bite wounds, Corynebacterium spp arthritis, or mite allergies. On necropsy, the gross lesions consist of hyperemia and edema of the viscera, splenomegaly, enlarged Peyer’s patches, and a peritoneal exudate. As the disease progresses there may be hemorrhage into the intestinal lumen, and focal necrosis of the liver, spleen, pancreas, lymph nodes, thymus, and other organs. Focal necrosis becomes more extensive in the subacute to chronic forms with the vesicular, crusted cutaneous pox lesions, and swelling and necrosis of the extremities. Widespread splenic and hepatic necrosis are characteristic histopathologic changes. In addition, there is necrosis of Peyer’s patches, lymph nodes, and the thymus. Eosinophilic type A cytoplasmic inclusion bodies can be found in infected epithelial cells at the sites of focal epidermal hyperplasia and erosion early in the rash. Basophilic type Bcytoplasmic inclusion bodies can be found in infected hepatocytes adjacent to the areas of hepatic necrosis. Diagnosis of mousepox is based on clinical signs, serology, gross lesions, and histopathology changes. Fluorescent antibody test on epithelial cells of the skin, small intestine, and pancreas can be used for demonstration of the intracytoplasmic inclusion bodies. Ahemagglutination inhibition test on sera from mice with subacute orchromic disease may also be used to detect the presence of the virus. There is no treatment for infectious mouse pox and humans are not susceptible to this disease. There is a vaccination (IHD-T strain)available for susceptible mice that does not interfere with subsequent HAI serological testing. Prevention of the disease may be accomplished by selecting ectromelia-free mice, careful husbandry and quarantine, serologic screening tests and the use of sentinel mice. (Anyone suspecting their animal(s) of having a medical problem should seek the care of their local veterinarian.)

Trish Romero (Burgert), Murray, UT
Q I have two girls, Nippy and Lucy, who are littermates; Starface is their half sister (same father, different mother). All had their second birthdays in early July. Starface and Nippy have had eight tumors between them. Lucy had one tiny tumor that was removed in May 1991. She subsequently developed pyometra which was described in an earlier issue of Rat & Mouse Tales (Sept./Oct. 1991). Because I don’t have animals “put to sleep” unless they are terminal and unable to be helped, I had Lucy spayed in December 1991. She has had no further tumors since her spay, while her sister and half- sister have been a mess. Lucy is now 25 months old and looks great! All of the tumors have grown from girl rat’s mammary tissue, and the vet thinks the tumors have all been noncancerous. I have not actually sent a specimen to a pathologist lab because I would only get more hysterical if I found out my rats had cancer. The vet was wondering if these tumors could be estrogen induced and this is why Lucy doesn’t seem to grow tumors. What do you think? I have had entire litters of rats where some girls grow smaller tumors than others but none so drastic as this. I would buy the estrogen theory.

Speaking of tumors, my vet has perfected anesthesia for tumor removal in rats. He gives 1.25 mg of Valium intramuscularly and numbs the area of incision with lidocaine. I hold the rat while he does this. Then we wait for the rat to become “comfortably numb” (10–15 minutes). It takes about five minutes from open to close for an uncomplicated tumor; 20–25 minutes for a troublesome one. To close, he uses staples—stitches are a delicacy for rats—yum-yum. The rats are up and walking (not a straight line) right after surgery; the staples come out in ten days. I’ve held my rats several times for this procedure and it always goes well. I’ve had too many bad experiences with general anesthesia on rats and there’s no need to use it anymore.

A A few years ago, one of our members was working with her vet on the theory of spayed rats not getting tumors. She never did write up any findings on what the outcome was, but from what she told us during the experiment was that the spayed rats did seem to not get tumors like the non-spayed ones. Your idea (and Lucy) seem to point in that direction. I am concerned that you have never sent in a tumor sample to get a diagnosis on what kind it is. Yes, it is scary to think what the verdict might be; but if your rats are cancer-prone, you can be aware of this and not breed from those lines that have this problem. Karen Robbins, Winnetka, CA

Excerpt from article in Live Animal Trade and Transport Magazine, September 1992

The Colorado Veterinary Medical Association reported that 4 oz of unsweetened baking chocolate can cause death in a 10-pound animal. Pets (dogs, cats, birds, and others) cannot tolerate the theobromine compound in chocolate, which is higher in baking chocolate than in milk chocolate; but that doesn’t mean milk chocolate can be fed to your pets.

Ed. Note: See the Sept./Oct. 1986 issue for more on chocolate and pets.

Jennette Meyers, Honolulu, HI
Q Can rats have allergies? Dude and Homer always sneeze around me, and when I put them back home their sneezing fits stop within 5–10minutes. My roommate thinks it might be because of the hair spray in my hair.

A Yes, it can be the hair spray, it could also be perfume. Rats have a very sensitive respiratory system and strongly scented soaps, perfumes, and aftershave can cause sneezing, wheezing, and bloody noses. See the July/Aug. 1988 “Medical” section for more. Karen Robbins, Winnetka, CA

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