A New Insecticide Delivery Method for Control
of Fur Mite Infestations in Laboratory Mice

By Thomas N. Mather and Niels C.G. Lausen tigators placed treated cotton in captured Peromyscus leucopus cages, they found the animals readily accepted Thomas N. Mather is affiliated with the Department of Population Sci- the cotton and would create fluffy nests with the material ences, Harvard School of Public Health. Niels C.G. Lausen is affiliated with the Animal Resources Center, Harvard Medical School. We thought that permethrin might also kill mites in- festing laboratory mice. To test the effectiveness of per-methrin for that purpose, we supplied laboratory mice THE FUR MITES, Myobia musculi, Radfordia affinis with cotton balls impregnated with three concentrations and Myocoptes musculinus (Acari: Myobiidae), are important ectoparasites of laboratory mice. These mites can infest whole colonies of laboratory rodents and may Materials and Methods
adversely affect the general health of mice1. Mite infesta-tions often cause hypersensitivity-induced skin lesions2 We conducted the study in a colony containing over that can promote secondary bacterial infection. Infested 1200 transgenic mice (including about 20 strains). All mice may have altered immune statuses3, and epider- the mice were backcrossed to CD-1 or Balb/C mice. The mis and hair regrowth may be affected in some mouse colony had been in existence for about 24 months and strains4. A mouse experiencing a severe or prolonged was infested with fur mites. We housed all the mice in mite infestation may permanently disfigure itself by vig- standard shoebox cages, each holding 1-5 mice. We kept the cages on five open stainless steel racks in one animal The toxicity of mitacidal agents, both to animals and room. We provided the mice with water and food ad li- caretakers, and low treatment cost-effectiveness have bitum. An animal technologist cleaned the cages weekly hampered the control of fur mites in laboratory mice. and lined the cage bottoms with fresh wood chip bedding Though scientists have assessed many methods of fur (Alphachip, Northeast Wood Products).
mite control2,5-7, most methods do not work in the long- To begin the study, we randomly selected and marked run. M. musculi infestations in laboratory mouse colo- eight cages from each rack. We looked for mites and mite nies can be effectively curbed with the permethrin, an eggs on the heads and backs of 50% of the mice from acaricide, as recent evaluations have shown8. Permethrin, each of the marked cages. To perform this step, we used the investigators discovered, provided satisfactory short- the scotch tape impression method with microscopic in- term reductions in mite infestations. When the investiga- terpretation: we thoroughly rubbed each mouse sampled tors applied permethrin directly onto the mice or mixed with clear adhesive tape, affixed the hair-covered tapes to it in powder form with wood shavings, it did not produce glass microscope slides, and examined them at 40X mag- any noticeable effect in the mice. Their histopathologic nification. We have found this method to be as reliable as findings in the mouse livers, lungs and kidneys were nor- scraping or post-mortem examination for diagnosing fur mal. Presumably, scientists would use permethrin as an mite infestations. We examined mice from each marked acaricide unless it was difficult to administer, or if it was cage using the procedure described above both before, and at one-, two-, and four-week intervals after the per- Investigators found that permethrin-impregnated cotton, intended as rodent nest material, effectively kept Following the initial examination, we gave the animal immature ticks from infesting natural populations of the technologist bags of cotton balls. We had saturated some white-footed mouse, Peromyscus leucopus9. The investi- cotton balls with a range of permethrin and left some un- gators distributed treated cotton in dispensing tubes and treated. We then instructed the technologist to provide all placed them at regular intervals throughout the wooded the cages on a rack either with cotton balls containing and brush-covered habitat frequented by this mouse. Cot- the same concentration of permethrin or untreated cotton. ton disappeared from the dispensing tubes (presumably To prepare the cotton balls, we soaked them in aqueous into mouse nests)-as it did, the investigators captured solutions9 containing 0.16, 1.6 and 2.4% of commercial- increasing numbers of tick-free mice. When the inves- ly formulated permethrin (Permanone 40 Mfg-Fairfield New Insecticide Delivery Method for Control of Fur Mites
American Corp., Frenchtown, NJ) to produce treated cot- this treatment, the mite infestations persisted. The num- ton containing 0.5, 5.0 or 7.4% active permethrin (w/w) ber of mite-infested mice after four weeks was close to when dried. The volume of liquid we used just saturated the number of infested mice in cages receiving untreated the cotton mass (about 10 times the weight of the cotton). One must take into account the absorbent properties of the dosing material to arrive at the permethrin concentra- Discussion
After soaking the cotton, we air-dried it. We con- Investigators have found that several formulations of firmed the concentration of permethrin in the various permethrin are effective acaricides against ectoparasites batches of cotton by examining acetone extracts from of rodents and other animals, in the wild9, or in labora- treated cotton balls using reverse phase high pressure liq- tory8 and commercial settings10,11. Many of the features uid chromatography. For this step, we used a C18 column of this acaricide, including its low degree of toxicity to with the sample diluted in 80% methanol in water. We the animals and their caretakers, make it more advanta- monitored the effluent from the column at 280 nm and geous than other products and methods for controlling fur used a computer to calculate the area of each peak.
The technologist placed up to 25 cotton balls in each Acute, short-term, and long-term studies show that cage (about five cotton balls per mouse) and put fresh dermal or oral exposure of up to 2000-5000 mg per- cotton in all the cages at cleaning time. The animal tech- methrin per kg body weight (that is up to 125 mg per nologist, alone, knew the concentration of acaricide used 25 g mouse) is not toxic to mice12. We calculate that ef- for each rack. We performed all sampling, examinations fective application rates using our method – five cotton and evaluations without that knowledge.
balls containing either 5% or 7.4% permethrin per 25 g mouse – would deliver just 1200-1800 mg permethrin per kg body weight, or a maximum exposure to about 46 mg per 25 g mouse. Furthermore, impregnating cotton with Eighty-eight percent of the mice we initially ex- permethrin binds much of the toxicant (up to 80%) to the amined (n = 76) were infested with mite eggs and 40% cotton fiber so it is not readily available for absorption (B. were infested with nymphal or adult mites: Ninety per- McNally, personal communication). Most toxicological cent of the mice we originally examined were infested studies use application methods intended to maximally with either eggs or mites. We identified mites of the M. absorb solvents into the application material (i.e. oral and musculi, R. affinis, and M. musculinus species, but made dermal applications formulated in corn oil or solvents).
no attempt to separate these species in the actual counts. Even considering the focused method of our per- Heavy infestations more than 5 adult or nymphal mites or methrin application, we apparently needed a relatively more than 15 eggs per slide-occurred on about 20% of the high concentration of this acaricide to completely control mice. Only about 10% of the mice we sampled suffered fur mites infesting mice in our study. Reportedly, other skin lesions or other signs of severe mite infestation.
investigators have successfully controlled M. musculi The mice readily accepted the treated and untreated when directly applying as little as 0.5 mg permethrin per cotton and used it to construct nests. After the first treat- mouse either as a dust or liquid emulsion8. At our lowest ment, the number of mice infested with either adult or treatment level (cotton containing 0.5% permethrin, or nymphal mites decreased in cages with treated cotton about 120 mg per mouse) we failed to completely con- (See Table 1), although we observed no reduction in the
trol adult and nymphal mites, although fewer of the mice number of mite egg-infested mice. After two weeks of- were mite-infested after 4 weeks of treatment than before treatment, nearly all the mice nesting in cotton balls con- taining either 5.0 or 7.4% permethrin were free of adult Our treatment might require more permethrin due to and nymphal mites. Additionally, the number of animals the actual exposure degree of mites to the active ingredi- infested with mite eggs decreased. After four weeks of ent impregnated onto the treated fiber, the nature of the treatment, all mice nesting in cotton balls containing ei- impregnation process, or it may be due to different sus- ther 5.0 or 7.4% permethrin were completely free of adult ceptibilities of mite species or strains between studies. It and nymphal mites, and less than half the original num- may be that a combination of these three possibilities ac- counted for the seemingly higher concentration required In contrast, fewer of the mice treated with 0.5% per- methrin-soaked cotton were mite-free. After 4 weeks of Though our method seems to require more chemical New Insecticide Delivery Method for Control of Fur Mites
than others, it has several advantages: treated nest material is odor-free; since the toxicant is contained on the cotton fiber, problems such as Summary of actual counts of mite eggs, nymphs and overdosing, spillage or toxic dusts and fumes, adults infesting mice treated with permethrin-impreg- are reduced; animal technologists can routine- ly add nest material to cages when they clean them; both the cotton and the permethrin are Proportion of mice infested on
Week 0 Week 1 Week 2 Week 4
biodegradable so technologists can dispose of them safely with other refuse; the natural nest- building behavior of mice is satisfied by the cotton supply, perhaps making them more se- cure and less prone to other stresses.
In our study, mice in cages with 5%- and 7.4%-permethrin-soaked cotton balls were mite-free after four weeks, although a few em-bryonated and empty mite eggs remained on the fur of some mice. We observed that eggs present prior to the initial treatment and eggs on control mice could be distributed anywhere along the entire hair shaft. Most of the eggs we found on treated mice later in the study were usually nearer the distal end of the hair. Eggs that we ob- References
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were most often empty. Additionally, after three and four 2. Weisbroth, S.H. Friedman, S., and Scher S. The parasitic ecology of the weeks of treatment, we found eggs on fewer of the mice rodent mite Myobia musculi: III. Lesions in certain host strains. Lab.
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The authors appreciated the assistance of Jose M.C. 11. Taylor, S.M., et al. Efficacy of pyrethroid impregnated ear tags for Ribeiro and Rachael Wallace. Supported in part by a grant prophylaxis of tickborne diseases of cattle. Vet. Record; 114:454-455,
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name MiteArrest® and will be available commercially 13. Friedman, S., and Weisbroth, S.H. The parasitic ecology of the rodent pending registration by the Environmental Protection mite Myobia musculi: IV. Life cycle. Lab. Anim. Sci.; 27:34-37, 1977.
Agency. The product is manufactured by EcoHealth, Inc., 14. Levine, J.F., and Lage, A.L. House mouse mites infesting laboratory 33 Mt. Vernon Street, Boston, MA 02108.
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