Identification and Treatment of Fur Mites (Radfordia lemnina) in California Deer Mice (Peromyscus californicus) Using Selamectin
Peromyscus species have been used in research for decades, yet there are no specific reports of mite infestations in the laboratory setting despite many reports of various mite species found in wild Peromyscus. This study documents P. californicus infestation with Radfordia lemnina in an academic research setting. During the colony quarantine period, deer mice tested positive on a general mite PCR but negative on all species-specific mite PCR assays. Tape tests were performed on a subset of cages, and 21% were positive for adult mites or viable eggs. Mites were sent for sequencing and identified as R. lemnina, for which the natural host is Microtus pennsylvanicus. The entire colony was treated with selamectin, applied topically to the nape of the neck, and repeated one month later. All deer mice were successfully treated using a novel method of restraint, and no gross adverse reactions to selamectin treatment were noted. Tape tests were performed weekly to biweekly on a subset of deer mice, and PCR was used to confirm negative tape test results. PCR was positive at 14 wk posttreatment, and tape tests were intermittently positive for egg casings for 27 wk, indicating continued presence of genetic material but not necessarily an active infection. Weaned offspring were tape test and/or PCR negative at 12 and 21 wk posttreatment, providing further support for successful treatment. At 31 wk, 2 rounds of tape tests and PCR were both negative. This report documents a safe and effective treatment method for mites in P. californicus.
Introduction
Fur mites have historically been a frustrating problem in laboratory mice due to time-consuming treatments and limitations of follow-up testing. The mite species of most concern in laboratory mice include Myocoptes musculinus, Myobia musculi, and Radfordia affinis, although others such as Ornithonyssus bacoti, Demodex musculi, and Psorogates simplex have been reported.3,5,10,14,20,23 Peromyscus species have been used in research for decades yet there are no specific reports in the literature of mite infestations in the laboratory setting.11 In field studies, many species of mites have been described in various Peromyscus species, including M. musculinus, M. musculi, O. bacoti, R. affinis, R. ensifera, R. subuliger, and R. lemnina.9,12,21,25 However, the one report of R. lemnina was found in P. maniculatus; to the authors’ knowledge it has never been reported in P. californicus.12 R. lemnina is more commonly found in voles and has been reported in Microtus spp. (including M. pennsylvanicus), Clethrionomys spp., and Chionomys spp.2,7,24 While the R. lemnina mite itself has been morphologically well characterized, little is known about its life cycle, transmission, and control. Based on previous reports and our experience, this mite can be detected via pelt examination, tape testing, and PCR; no reports have explicitly stated that it can be recovered via fur pluck or skin scrape.12 The current report documents infestation of P. californicus with R. lemnina, successful treatment using selamectin, and posttreatment testing. Furthermore, we describe a novel method of deer mouse restraint allowing accurate and consistent topical dosing of selamectin.
Case Report
A colony of 109 2-mo to 1-y-old male and female P. californicus was imported to the Ohio State University. At the previous institution, the animals had been the only mice housed in the facility, but purpose-bred, acute-use rats were occasionally purchased from a commercial vendor and housed in an adjacent room. No sentinel or health monitoring program was in place for these deer mice. The facility veterinarian indicated that they had no health concerns in the past 5 y for this closed colony, but health reports were not provided. Risks and potential costs associated with importing these animals were discussed with the Principal Investigator before shipment. Upon arrival, all animals appeared normal and were housed in static microisolation caging in same-sex pairs or trios within a single room in a dedicated quarantine facility.
Standard quarantine practices were initiated, which included PCR testing for rodent infectious agents 2 wk after arrival. Initial testing was performed on a subset of the animals, which included mice from each of the original 12 transport crates. Feces, oral swabs, and fur swabs were collected from 37% (20/54) of all cages. Two pooled samples, with specimens from 10 cages in each pooled sample, were screened via PCR for a standard list of viral, bacterial, and parasitic agents that included ectromelia virus, Theiler murine encephalomyelitis virus/GDVII virus, lymphocytic choriomeningitis virus, mouse adenovirus, mouse hepatitis virus, minute virus of mice, mouse norovirus, mouse parvovirus, mouse rotavirus, murine chapparvovirus, pneumonia virus of mice, reovirus 3, Sendai virus, Bordetella spp., Campylobacter spp., Citrobacter rodentium, Clostridium piliforme, Corynebacterium bovis, Corynebacterium kutscheri, Filobacterium rodentium, Helicobacter spp., Klebsiella oxytoca, Klebsiella pneumoniae, Mycoplasma pulmonis, Pneumocystis murina, Proteus mirabilis, Pseudomonas aeruginosa, Rodentibacter heylii, Rodentibacter pneumotropicus, Salmonella spp., Staphylococcus aureus, Streptobacillus moniliformis, beta-hemolytic Streptococcus spp., Streptococcus pneumoniae, Cryptosporidium, Demodex, Entamoeba, Giardia, fur mites, pinworms, Spironucleus muris, and Tritrichomonas spp. Both samples were positive for Helicobacter genus, Entamoeba, Tritrichomonas genus, and mites. Sample 2 was in addition positive for Giardia. The Helicobacter species-specific assay for common rodent species detected H. ganmani in sample 2 only but was negative for H. bilis, H. hepaticus, H. mastomyrinus, H. rodentium, and H. typhlonius. The mite species-specific assays for M. musculi, M. musculinus, R. affinis, and R. ensifera were negative. At our institution, we exclude the agents in bold above from mice at this health status. Confirmatory mite testing was performed 2 wk later using fur swabs for PCR. Ten swabs, pooled together as one sample, were collected from 19% of all cages, including 8 new cages not previously tested. The sample was again positive for mites but negative on all species-specific assays. The general mite PCR detects mites in the family Myobiidae, which includes the common rodent genera Myobia and Radfordia. No clinical abnormalities were noted in any of the animals. A decision was made to transfer mice to their final destination, a vivarium housing primarily USDA species, but animals remained quarantined at the room level. Personnel protective equipment (disposable gown, gloves, mask, and hair bonnet) was donned before entering the room and was removed on exit. No cage movement in or out of the room occurred, and there was limited access to animals by animal care and research team personnel.
To identify positive cages, tape tests were performed on all animals in cages tested for mites via PCR (n = 51 animals, 27 cages). Deer mice were restrained by holding at the base of the tail and clear tape was adhered to the hair along the dorsum of mice from neck to rump. A cage was considered positive if either eggs or mites were identified on any animal within the cage. Four cages were positive. Additional mice (n = 12 animals, 6 cages) were tape tested if they shared a shipping crate with any positive animals. Of these, 3 cages were positive, and we were able to trace positives to 4 of 12 shipping crates. Overall, 58% (63/109) of the colony animals were tested with 11% (7/63) positive on tape test, which is presented in Figure 1. The total number of positive cages was 21% (7/33) based on testing of 61% (33/54) of cages in the colony. All positive animals were male and 15 to 40 wk old at the time of initial testing. For reference, 64% of colony animals were male and 73% of tape-tested animals were male. Adult mites were only identified in one cage holding a singly housed male. Since we were unable to identify the mite based on morphology alone, it was sent for sequencing.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Materials and Methods
Animals and husbandry.
Animals were housed in static microisolation cages (Ancare, Bellmore, NY) on stainless steel racks. Cages were bedded with corncob bedding (1/8-in., The Andersons, Maumee, OH), and animals were provided a cotton square nestlet (Ancare), reverse osmosis water in water bottles, and an irradiated rodent diet (Teklad 7912, Envigo, Indianapolis, IN) provided ad libitum. All cage bottoms and water bottles were changed every 7 d on a workbench. Cage wires were changed monthly, and lids were changed every 6 mo. The housing room was maintained on a 12:12-h light:dark cycle on arrival and was shifted to a 14:10-h light:dark cycle when breeding pairs were set up. Humidity remained between 30% and 70% and temperatures remained within 68 to 79 °F. The Ohio State University’s Animal Care and Use Program is accredited by AAALAC International. Animals were imported into the institution on an IACUC-approved protocol and all animal activities were performed as part of standard veterinary care.
Treatment.
Treatment using selamectin was extrapolated from current literature in laboratory mice.3,13,18,19 Due to the unknown effects on deer mice, a pilot treatment was initiated using 6 cages containing 11 male mice. These mice were treated with 0.1 mL of 5 mg/mL selamectin (Selarid 120 mg/mL; VETone, Boise, ID) diluted in 70% ethanol (Volu-Sol, Salt Lake City, UT) applied topically to the nape of the neck once. Mice in the pilot treatment were monitored for 3 d for evidence of skin irritation, neurologic signs, or other systemic signs of illness. No adverse effects were noted, so the remainder of the colony was treated with the same selamectin regimen 3 d later. The target dose was 10 mg/kg, and mice weighed approximately 36 to 50 g, so the actual dose per mouse ranged from 10 to 13.9 mg/kg. The selamectin treatment was repeated one month later in all mice at which time treatment was considered complete. Cage changes occurred weekly and were performed the day after treatments.
Since deer mice have a thick fur coat and selamectin needs to be applied to the skin to be efficacious, we felt that scruffing would not be the best restraint method as the operator’s hands would obscure the treatment area. In addition, deer mice can be difficult to handle due to their quickness and jumping ability. Therefore, we created a novel method of restraint using a rat DecapiCone (Braintree Scientific, Braintree, MA) to dose selamectin. This was done by shortening the length of the DecapiCone as well as cutting a window for treatment proximal to the breathing hole as shown in Figure 2. Proper deer mouse restraint in the device allowed for parting of the fur and direct access to the skin (Figure 3). In addition, the use of the novel device allowed for simultaneous animal restraint and application of selamectin by a single individual (Figure 4). The restraint device was disinfected with a peracetic acid and hydrogen peroxide disinfectant (Spor-Klenz; STERIS, Mentor, OH) and dried when visibly soiled and at the end of the treatment session.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Posttreatment testing.
Animals in the 7 positive cages were targeted for posttreatment testing. All 13 mice in these 7 cages were tape tested, as previously described, 5 d before the completion of treatment and then weekly for 15 wk after completion. Once 2 consecutive weeks of tape tests were negative, which was confirmed by 2 observers, pooled PCR testing of fur swabs was performed, and tape test frequency was reduced to every other week until the end of the study when fur swab PCR was negative. Fur swabs for PCR were collected along the dorsum of all 13 mice in the 7 cages using adhesive swabs (Puritan, Guilford, ME). Each swab was vigorously swept through the dorsal pelage of all mice in a cage, and swabs from the 7 cages were pooled into one sample for PCR. At the time of the second treatment, the research team was permitted to establish breeding cages within the quarantined room. Clear documentation and tracking of parents and offspring were required. Subsequently, offspring were used for testing to determine if mites were viable and could be transmitted to offspring. Offspring (male and female) were tested at 2 different time points. Twelve weeks after completion of the parent treatment, the first group of offspring, 4 cages containing 9 animals 5 to 8 wk of age, was tape tested as previously described. At 21 wk after treatment, 10 of the 20 available cages of offspring, which contained 21 animals aged 6 to 17 wk (including all offspring previously tested,) were tape tested and fur swabbed as previously described. These 10 cages were representative of all the productive breeding pairs in the room, which included animals known to be positive before treatment. Tape tests were collected from all offspring before swabbing each mouse with a sticky swab for PCR testing. Sticky swabs were collected and pooled as previously described (1 swab for all mice in a cage, 10 swabs per sample). Ongoing health monitoring using sentinel-free soiled bedding sampling (PathogenBinder; Charles River Laboratories, Wilmington, MA) was performed quarterly starting at the time of treatment completion. A timeline of posttreatment testing is shown in Figure 5.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Mite identification.
A glass slide containing an adult mite and eggs was sent to Charles River for amplification and sequencing. Isolated DNA was first evaluated with proprietary generic rodent fur mite real-time PCR assays that broadly detect rodent fur mite sequences including Myobia spp., Radfordia spp., and Myocoptes spp. Samples positive for the initial screening assays were further evaluated with specific mouse and rat fur mite PCR assays specific for M. musculi, R. affinis, R. ensifera, and M. musculinus. All genus screening assays were positive, and all species-specific fur mite assays were negative indicating the presence of a fur mite not detectable by the species-specific assays. To further the investigation, Sanger sequencing was performed by amplifying a portion of the 18S fur mite sequence (620 bp) using proprietary sequencing primers. PCR products were evaluated by agarose electrophoresis and a single band consistent with the expected product size was observed for each sample. Individual DNA bands illuminated by UV and GelRed staining were excised from the gel. The excised PCR products were purified (MinElute; Qiagen, Redwood City, CA), quantified (260/280 spectrometry analysis), and submitted to Tufts University Core Facility for DNA sequencing. The sequences obtained were analyzed by using Geneious software (Dotmatics, Boston, MA) and evaluated by BLAST (NCBI BLAST, National Library of Medicine).
Results
All mice were successfully treated and no gross adverse reactions to selamectin treatment were noted in any mice. We found that deer mice tolerated restraint in the plastic sleeve well with minimal vocalization and struggling once restrained. The technique was also easily learned by novice operators with minimal practice. These restraint devices were rapid and inexpensive to construct, and they were easily sanitized for reuse and disposed of when worn. Multiple sleeves were used for simultaneous restraint of cage mates, ensuring all mice in a cage were treated.
No adult mites were detected after the first treatment and egg casing counts generally decreased over time. It took 13 wk posttreatment to obtain a completely negative round of tape tests from the 13 mice being tracked. Two consecutive rounds of negative tape tests were achieved at week 14, but fur swab PCR remained positive at that time point. Tape tests continued to be intermittently positive for egg casings from at least 1 cage at weeks 17, 19, 23, and 27 posttreatment. Two consecutive rounds of negative tape tests were again achieved at week 31 at which point fur swab PCR was also negative. All offspring tested had negative tape tests at weeks 12 and 21, and negative fur swab PCR at week 21. Results for posttreatment tape tests are presented in Table 1. Egg casings posttreatment appeared shriveled, were poorly defined in some instances, and were more often broken as compared with eggs pretreatment which appeared plump and well-defined. A comparison of eggs pre- and posttreatment is presented in Figure 6. Routine health surveillance in the room using sentinel-free soiled bedding sampling was positive for the mite screen at 12 wk posttreatment and negative for the mite screen at 25 wk posttreatment.
| Cage number | Number of weeks post final treatment | |||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| −1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 17 | 19 | 21 | 23 | 25 | 27 | 29 | 31 | |
| 1 | + | − | − | + | + | − | − | − | + | − | + | + | + | − | − | − | − | − | − | − | − | − | − | − |
| 2 | + | + | + | + | + | + | − | + | + | + | + | − | + | − | − | − | + | + | − | + | − | − | − | − |
| 3 | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | + | − | − |
| 4 | − | − | − | + | − | + | − | + | − | − | + | − | + | − | − | − | − | − | − | + | − | − | − | − |
| 5 | − | − | − | − | − | − | − | + | − | + | − | − | − | − | − | − | − | − | − | − | − | − | − | − |
| 6 | + | − | − | − | + | + | + | + | − | − | + | + | − | − | − | − | − | − | − | + | − | + | − | − |
| 7 | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − | − |
| PCR | nt | nt | nt | nt | nt | nt | nt | nt | nt | nt | nt | nt | nt | nt | + | nt | nt | nt | nt | nt | nt | nt | nt | − |
Each cage contained 1 to 2 mice for a total of 13 mice. The cage was considered positive if at least one animal in the cage demonstrated egg casings. Weeks with completely negative tape tests are shaded gray. Fur swab PCR results from these 7 cages are shown at the bottom; nt, weeks where no PCR testing was performed.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
An adult mite discovered on a tape test is shown in Figure 7. The Sanger sequencing data indicated that the mite detected was most closely related to R. lemnina, with 99.45% homology. The natural host species for this mite is Microtus pennsylvanicus.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-055
Discussion
This study describes successful treatment of a colony of P. californicus using selamectin to eliminate an infestation of R. lemnina. While the treatment was likely successful shortly after application, limitations of diagnostic testing did not enable us to confidently declare the colony mite-free until many months after treatment. Tape tests demonstrated egg casings for 27 wk posttreatment although egg morphology was greatly altered, indicating ovicidal activity of selamectin for R. lemnina. At 14 wk posttreatment, we obtained 2 negative rounds of tape tests, yet fur swab PCR remained positive, supporting the likely continued presence of genetic material but not necessarily an active infection. Negative tape tests from offspring generated from the colony at 12 and 21 wk posttreatment and the subsequent PCR at 21 wk were used to validate treatment success. These results were used as criteria to lift the quarantine 22 wk after treatment completion. We continued to perform tape tests every other week on the target cages and obtained a negative fur swab PCR result at 31 wk following 2 sequential negative rounds of tape tests.
Various diagnostic methods for fur mites have been employed in laboratory mice, including fur pluck, skin scrape, pelt examination under a microscope, gross examination for clinical signs, fecal float, PCR, and tape testing.4,19 However, the literature is inconsistent regarding which method is most sensitive. While pelt examination was historically generally regarded as a gold standard for diagnosis, several studies found skin scrapes or fur plucks to be more sensitive.1,17 PCR is a newer diagnostic test but has been shown to be extremely sensitive, especially when the fur rather than the cage is swabbed.19,23 Tape testing ranks among the more sensitive tests, but it has a higher risk of false negatives when mite burden is low.4,13 Diagnostic test sensitivity has also been linked to mite species with Myocoptes more likely to be detected via tape testing than Myobia due to their morphology and mobility.4,23 While predilection sites for R. lemnina in Peromyscus have not been reported, we attempted both tape tests of the ventrum and dorsum early on. We found that dorsal tape tests produced more positives and eggs from the same animal, so the nape of the neck and dorsum were targeted for future tape tests. We also attempted fur plucks and skin scrapes from known positive animals without success. Older adult laboratory mice chronically infested with mites have been shown to be more likely to test negative via fur pluck so this may explain why our adult Peromyscus did not test positive via fur pluck.19
The persistent identification of egg casings for months after treatment is not surprising as eggs are firmly adhered to the hair shaft.17 Previous studies have demonstrated eggs or egg casings up to 6 mo after treatment.1,3,13,17,23 In theory, tape tests can demonstrate mite or egg pieces and parts until the haircoat has been entirely shed after successful treatment. In laboratory mice, the haircoat can take at least 8 mo to shed and is strain dependent.17 Peromyscus are reported to molt once per year.8,22 Another challenge with tape testing is differentiating viable eggs, nonviable eggs, and debris. One study noted that it can be extremely difficult to distinguish viable compared with nonviable eggs and even implemented a 3-mo moratorium on follow-up testing to avoid confounding results.17 Based on previous descriptions of viable compared with nonviable M. musculinus and M. musculi eggs in laboratory mice, we determined that all R. lemnina egg casings seen posttreatment were nonviable.4,16 While the total number of egg casings detected generally decreased over time, samples were intermittently positive despite only finding one mouse with live mites before treatment. Weeks with negative tape tests likely reflect the known risk of false negatives with low mite burdens. While we use the terminology ‘positive’ to denote detection of mite egg casings after treatment, we did not feel these were viable. Institutions need to define what a positive tape test means to them and how that information will be used. Other studies have also seen intermittent positives with tape tests and fur plucks, highlighting the importance of not relying on one negative tape test result to declare an infestation cleared.1,13,16,19
Since PCR is very sensitive and will remain positive if any mite DNA persists in the haircoat, we did not submit any samples for PCR until we were reasonably confident no egg casings were present, which we defined as 2 completely negative rounds of tape tests in the animals we were tracking. Unfortunately, even with 2 consecutive weeks of negative tape tests at 14 wk posttreatment, fur swab PCR was still positive at that time. This was not entirely unsurprising as PCR has been shown to be positive up to 16 wk after selamectin treatment in laboratory mice infested with Myocoptes.19 Another study also found fur swab PCR to be positive for at least 12 wk in laboratory mice coinfested with Myocoptes, Myobia, and Radfordia after treatment with permethrin.23 This same study also demonstrated false negative PCR results based on positive tape test results at 12 wk. Routine health monitoring in the room using nonanimal-based soiled bedding PCR testing was positive for the mite screen at 12 wk posttreatment, which was expected and used as a positive control. The next round of health monitoring testing was negative for the mite screen at 25 wk posttreatment. However, egg casings were found at weeks 17, 19, and 23 (when the PathogenBinder was in use), and one additional round of tape testing was positive at 27 wk posttreatment. This suggests the week 25 health monitoring PCR test may have been negative due to dilution of mite DNA. The positive health monitoring PCR result at 12 wk posttreatment had a low copy number of 3 per reaction and offspring cages were continuously being added to the room, which likely contributed additional mite-free bedding to the sample. All subsequent health monitoring PCR tests since week 25 posttreatment have been negative for mites. Taken together, PCR should be used with caution in the first few months after treatment as egg casings and mite parts are likely still present on the haircoat, and therefore PCR will be positive even in the absence of an active infection.
We also used offspring produced from treated cages to verify treatment success. At our institution, pups are weaned at 21 d ensuring ample time for the pups to become infested once they grow a haircoat. In addition, laboratory mice less than 6 wk of age are more likely to have higher mite burdens and test positive via fur pluck than those 8 wk or older.19 Since all pups at 2 time points tested negative via tape test with or without PCR, we felt this was a strong indicator of successful treatment. This finding is consistent with another study that used this approach in laboratory mice infested with M. musculi where all offspring from treated cages tested negative via fur pluck and fur swab PCR.19 One limitation with this approach is that only some of the pups tested were from known positive cages.
The only reported treatment for mites in Peromyscus is 5% Sevin insecticide dust.9,11 Given this product is not marketed for use on animals and veterinary acaracides are readily available, we chose to treat them with selamectin. Selamectin treatment was extrapolated from previous reports in laboratory mice. The most common dose is 10 mg/kg (undiluted or diluted in 70% ethanol), but one study used up to about 20 mg/kg diluted 1:27 in 70% ethanol.3,13,18,19 Treatment is often repeated at least once 4 wk later. We chose to dilute the selamectin 1:24 to increase the dose volume to 0.1 mL for better accuracy without overdiluting the active drug. While 2 studies found selamectin to be effective for infestations of M. musculi and/or M. musculinus,18,19 one study was less certain of its efficacy against M. musculinus due to egg casings found 6 mo after treatment.13 Another study found selamectin, when combined with other topical treatments, to be efficacious for R. affinis.3 Many other treatment options for fur mites have been described for laboratory mice and could be attempted, including fipronil, which has also been used to treat Ixodes scapularis in wild P. leucopus.6
Unlike treating relatively more docile laboratory mice, Peromyscus are notoriously difficult to handle due to their quickness and jumping ability. Anecdotally, we found this to be especially true of weanlings and juveniles compared with adults who became more accustomed to routine handling. At our institution, animal care staff may use plastic tubs as secondary containment to minimize the opportunity for escape when changing cages. In addition, deer mice have a thick fur coat that needs to be parted to effectively apply topical treatment. Although scruffing is an effective and safe restraint method for deer mice, it was not suitable for topical treatment at the nape of the neck as it would obscure the treatment area. Restraint in our modified plastic sleeve allowed us to quickly and easily dose all deer mice, and a single operator could simultaneously restrain and dose an individual mouse. Since inaccurate dosing and missed doses have been implicated as possible causes of treatment failure, we created multiple sleeves so that one mouse could remain in the sleeve and be released only after its cage mate was safely contained in a separate sleeve. The low cost of creation and ease of sanitation make this modified plastic sleeve a viable option for clinical topical treatments of other small rodents or a restraint method for additional procedures.
The total cost of treatment was approximately $307, including 7 h of technician time and treatment supplies. The total cost of follow-up testing was approximately $1,786, including 40.5 h of technician time, tape-testing supplies, and 2 PCR sample submissions. In this study, we conducted tape tests weekly to biweekly because we wanted to know as soon as possible when the tape tests would be negative, and we could therefore submit a sample for PCR testing. This frequency of testing is not necessarily practical or reflective of real-life scenarios. Furthermore, our data demonstrate that very frequent tape testing, especially early after treatment, is potentially not necessary as these samples are likely to demonstrate egg casings and therefore be considered a positive result. While no commercial services for embryo or cesarean rederivation exist for Peromyscus, successful cross-foster rederivation has been reported for elimination of Helicobacter and may be useful for mite eradication.15 However, at the present time this has not been studied and topical treatment is likely the most time and cost-effective method of mite elimination from a colony.
Since it is unknown whether R. lemnina can infest laboratory mice and rats, deer mice and meadow voles are both used in research and can both be infested with R. lemnina, and given deer mice can harbor mites that are known to infest laboratory mice and rats, the authors recommend testing and treating Peromyscus colonies for mites or separating them physically and functionally from other rodent species. This case study also highlights the need for colony health surveillance programs for Peromyscus colonies, even if the colony is closed and there have been no health issues. While standard rodent surveillance panels are helpful for this purpose, it should be noted that Peromyscus may harbor agents or specific species not standardly included in these panels. A positive result at the genera level should not be dismissed simply because all species-specific assays are negative. Instead, additional diagnostic methods should be employed to confirm or deny a positive result. When considering importation of rodents without health monitoring information, we recommend having discussions with the Principal Investigator up-front about potential risks and costs.
Initial tape test screening results for a subset of colony deer mice (63 of 109 animals). Each box represents a shipping crate and each circle inside the box represents an individual deer mouse. Circles shaded white tested negative, circles shaded black tested positive, and circles shaded gray were not tested. At the time of testing, deer mice had been redistributed into 54 cages, 33 of which were tested.
Modification of a rat DecapiCone to create a restraint device for selamectin application in deer mice. The length was shortened by following the natural curve of the end of the sleeve and a window for treatment was created proximal to the breathing hole. Measurements for modification are shown in blue.
Proper restraint technique for topical administration with the deer mouse at the distal end of the modified plastic sleeve and the nape of the neck lined up with the treatment window. The nose is through the breathing hole and the mouse is restrained gently at the base of the tail with one hand. The other hand is available to part the hair at the neck and apply topical treatment.
Application of topical treatment to a restrained deer mouse using a micropipette. One hand is used to restrain the tail base while simultaneously parting the hair at the nape of the neck. The other hand is used to apply treatment.
Timeline of posttreatment testing. Tape tests are shown above the timeline, and PCR tests are shown below the timeline. Light gray triangles and ribbons denote adult tests, diamonds denote offspring tests, and dark triangles denote room-level tests. Results for individual time points are shown in parentheses.
Comparison of mite eggs found adhered to hair shafts on tape tests before (A) and after (B) selamectin treatment. Note that the pretreatment egg is plump and well-defined whereas the posttreatment egg casing is shriveled and poorly defined.
Adult mite found on a tape test from one deer mouse, 20×.
Contributor Notes