This article is from the WSSF 2005 AFRMA Rat & Mouse Tales news-magazine.

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

Lynn Lehman, Racine, WI
Q I’m writing to tell you that my precious little bunny mouse has passed on only a few days ago. She was approximately 2 years old. I found her in her wheel, on her side with her bright eyes still open. The problem is that when I picked her up with a paper towel, there were very many tiny red bugs scampering off her body onto the paper towel! When she was alive, I never saw any bugs on her or saw her scratching at all, so where did these come from? I was just horrified at how many there were! I disinfected her cage very well and moved the new family of four babies and their mother into her cage, and there is no sign of these terrible red bugs. I’m so afraid that they will reappear though! I haven’t brought any new things or new mice in, so it can’t be from anything I can think of? I have bought some spray, just in case. All the other mice and cages so far seem to be free from any of these pests.

I always kiss my mice and hold them close to my face; is there any concern the mites may bother me? Now I’m afraid to kiss them and I feel bad (I know the mice could care less about being kissed, but I still feel bad!).

UPDATE: My poor critters are severely infested with the mites. They also get onto myself and they cause terrible itching and sores. They are very small, some are grey, and some are reddish. I think they must have come from a bad bag of bedding as I’ve never had this problem in all the years of having critters (shortly before this infestation occurred, I ran out of my usual kind of bedding and had to buy some real crummy stuff because it was the only stuff available at the time; it’s very hard to find good bedding around here).I have been dusting the mice/rats and their cages with flea and tick powder for kittens, and they just won’t give up! Some of the poor mice have little sores from itching so much. One of the girls has a very large raw sore on her back the size of a nickel and the tips of her ears are gone. I feel so bad and so helpless! As I said, they even affect me; I have sores all over my legs from them crawling on me when I go by the mice and rats. So, I know just how horrible it is to have these things driving you crazy with itching!!!

I was hoping the rats were not affected, but of course they are (these two girls are the ones that are always hiding and never come out). Poor Jasmine has fur missing near her face, but Jade looks just fine. There was a rare moment that they both were sitting out in the open in the corner of their cage, so I got to pet Jasmine and feed her a treat since Jade won’t accept one.

The pet shop said to use kitten flea powder, so I did but it didn’t seem to do much. But I went to another pet shop to get another brand of power and the bugs are gone now. I’m just afraid they will come back! Did they come from the bedding?

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

Ectoparasite Infestations in Rodents¹

One of the scourges of pet mice are fur mite infestations which are referred to as acariasis. Mice can be infested with fur mites, follicle mites, or lice (pediculosis). In most cases, lice and mites are species specific and do not bother humans or other species. The exception to this is the Tropical rat mite (Ornithonyssus bacoti) which has a nonselective host range and will bite humans, dogs, cats, etc. O. bacoti only stays on the rat (or human) while feeding and then seeks refuge in the surrounding environment. The bites from this mite are very itchy (pruritic).

Mice with acariasis are frequently infested with mixed populations of fur mites including Myobia musculi, Radfordia affinis, and Myocoptes musculinis. The most clinically significant mite in mice is Myobia musculi. At the death of the host, these mites leave the host in droves searching for a new host.

The life cycle of M. musculi follows:
a. Eggs are laid on the hair shafts close to the skin
b. Larvae hatch in 7–8 days
c. Larvae mature into egg-laying adults as early as 16 days after the eggs are laid

This species of mite feeds on the skin secretions but not on blood. Transmission is by direct transfer of adult mites to sucklings. Newborn mice are only susceptible after the hair grows. Adverse effects of mites are highly variable and can range from hair loss (alopecia), thickened skin (epidermal hyperplasia), scaley/flaky skin (hyperkeratosis).

Ivermectin is the drug of choice for eradication of mites in rodents. This drug crosses the placenta and blood brain barrier in mice fetuses and neonatal mice and can be lethal. In general, Ivermectin is safe for all mice and rats post weaning. I prefer the equine version of Ivermectin (Eqvalan, Merial, USA) since it can be diluted with water for appropriately dosing rodents.

Rats: A 1% solution of ivermectin (2 mg/kg of body weight) is applied to the skin on the dorsal aspect of the shoulder. Three treatments applied at approximately 2-week intervals can eliminate infestations of R. ensifera².

Mice: Topical, oral, or subcutaneous administration of ivermectin at 200 micrograms/kg of body weight 2 weeks apart is effective at elimination of fur mites in mice^{3 4}.

It is important to disinfect all caging, etc.

Since O. bacoti spend so little time on the host, eradication can be difficult and may require the services of a professional exterminator for the environment. No report of topical treatment for this mite has been reported.

References:

1. Percy, DH, and Barthold, SW. Pathology of Laboratory Rodents and Rabbits, Second Edition. Iowa State University Press, Ames. 2001
2. Kondo S, Taylor A, Chun S. Elimination of an Infestation of Rat Fur Mites (Radfordia ensifera) from a Colony of Long Evans Rats, Using the Micro-dot Technique for Topical Administration of 1% Ivermectin. Contemp Top Lab Anim Sci. 1998 Jan;37(1):58–61.
3. Wing SR, Courtney CH, Young MD. Effect of ivermectin on murine mites. J Am Vet Med Assoc. 1985 Dec 1;187(11):1191–2
4. Booth, CJ Personal experience.

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