New Building, Old Parasite: Mesostigmatid Mites—An Ever-Present Threat to Barrier Rodent Facilities

Abstract

Mesostigmatid mites are blood-sucking parasitic mites found in wild rodent populations. Periodically they can also become a problem for laboratory rodent colonies, particularly when building construction or renovations disturb colonies of commensal (building) rodents that had been acting as hosts. Mesostigmatid mites infest both rats and mice and, unlike the more common rodent fur mites (Myobia, Myocoptes, and Radfordia sp.), can survive for long periods in the environment and travel considerable distances in search of new hosts. They easily penetrate barrier caging systems, including individually ventilated cages, thus circumventing the usual precautions to protect rodents from infection. The two mites reported in laboratory rodent colonies, Ornithonyssus bacoti and Laelaps echidnina, also bite humans and have the potential to transmit zoonotic diseases. Once the mites gain access to a colony, eradication requires elimination of commensal rodent reservoirs in addition to insecticide treatment of both the laboratory rodents and the environment. In view of the undesirability of insecticide use in the animal facility, it is advisable to investigate the effectiveness of preventive treatments, such as environmental application of insect growth regulators or silica-based products. This article summarizes available information on mesostigmatid mites and their laboratory incursions, and provides suggestions for diagnosis, treatment, and control based on the author’s experience with several outbreaks at a large academic institution.

Introduction

The overall health status of laboratory rodent colonies has improved over the last 20 years such that organisms prevalent in the past—such as Sendai virus, mouse cytomegalovirus, mouse adenovirus 1 and 2, reovirus 3, polyomavirus, and pneumonia virus of mice—are increasingly rare in modern barrier facilities (Clifford 2007a,b; Jacoby 1998; Livingston 2003; Schoondermarkvan de Ven et al. 2006; Zenner and Regnault 2000). Pathogen-free rodents are readily available from vendors, and widespread use of barrier caging systems in concert with sterilized equipment, high-efficiency particulate air (HEPA)– filtered air cabinets, protective clothing, and disinfection help keep pathogens out (Brielmeier et al. 2006; Dillehay et al. 1990; Lipman et al. 1993; NRC 1991; Otto and Tolwani 2002; Stakutis 2003). Regular health surveillance (Livingston 2003) helps to detect excluded organisms, and should there be an outbreak, mites and pinworms are variously susceptible to treatment with parasiticides such as avermectins, pyrethroids, and benzimidazoles (Baumans et al. 1988; Bean-Knudsen et al. 1986; Bornstein et al. 2006; Coghlan et al. 1993; Flynn et al. 1989; Hill et al. 2005, 2006; Huerkamp et al. 2000, 2004, 2005; Klement et al. 1996; Pullium et al. 2005; Santora et al. 2002; Sueta et al. 2002). Also in the event of an outbreak, the rederivation of valuable genetic lines is possible through embryo transfer (Baker 1988; Reetz et al. 1988; Van Keuren and Saunders 2004), cesarean rederivation (Nagy et al. 2003), and neonatal transfer (Hickman and Thompson 2004; Huerkamp et al. 2005; Singletary et al. 2003; Truett et al. 2000; Watson et al. 2005).

But despite the technological and therapeutic tools available, it would be a mistake to become complacent. Mice carry pathogens that may remain undetected in barrier housing due to low prevalence and inefficient transmission to bedding sentinels (Besselsen et al. 2000, 2007; Compton et al. 2004b; Dillehay et al. 1990; Gaertner 2004; Smith et al. 2007; Thigpen et al. 1989; Whary et al. 2000). In addition, there is the threat of contamination from wild rodent populations, which harbor a wide variety of rodent pathogens (Becker et al. 2007; Easterbrook et al. 2007, 2008; Morita et al. 1996; Singleton et al. 1993, 2000; Smith et al. 1993; Van Vuuren et al. 1990), including some that may not be recognized because they are infrequently encountered in laboratory rodent facilities.

In this article I review published information on mesostigmatid mites, which have a wild rodent reservoir and cause sporadic problems in laboratory animal facilities, particularly during building renovations. I also provide additional comments and suggestions based on personal experience with several mesostigmatid mite outbreaks at a large academic institution.

Characteristics

Mesostigmatid mites are a large order of highly mobile mites, class Arachnida, subclass Acari, order Mesostigmata. Most are predators of other mites, but three genera are ectoparasites of mammals and birds: Liponyssoides, Laelaps (syn. Haemogamasus, Echinolaelaps), and Ornithonyssus (Baker 1999; Flynn 1973). Among laboratory rodents, they usually parasitize the Norwegian rat (Rattus norvegicus) and black rat (Rattus rattus), but they also readily infest mice.

Wild rodents are hosts to Liponyssoides sanguineus (house mouse mite), Laelaps echidnina (spiny rat mite), Laelaps pontiger (syn. Haemogamasus pontiger), and Ornithonyssus bacoti (tropical rat mite). Rodents may also be accidental hosts to O. sylviarum (northern fowl mite), O. bursa (tropical fowl mite), and Androlaelaps pontiger (poultry litter mite). O. bacoti (syn. Bdellonyssys bacoti, Liponyssys bacoti) (Flynn 1973) has the widest host range, parasitizing a variety of domestic and wild mammals and birds, and is the most commonly reported mesostigmatid mite both in laboratory rodent colonies (Chu and Couto 2005; Cole et al. 2005; Coons 2005; Fox 1982; Harris and Stockton 1960; Hill et al. 2005; Keefe et al. 1964; Kelaher et al. 2005; Ram et al. 1986) and in human rat mite dermatitis cases (Baumstark et al. 2007; Beck and Pfister 2004, 2006; Creel et al. 2003; Fishman 1988; Fox et al. 2004; Pizzi et al. 2004; Skirnisson 2001; Theis et al. 1981).

Mites are distinguishable from insects as the adults have eight legs and never have wings. All mesostigmatid mites are dorsoventrally flattened in the body (idiosome) and head (gnathostome), and have four pairs of jointed legs attached to the anterior half of the body. Both males and females have a single dorsal shield (thickened chitinous area); males usually have one ventral shield and females three: sternal, genital, and anal. Females are readily identifiable by the characteristic arrangement of their ventral shields together with the number and arrangement of setae (hair-like extensions) (Figure 1).

Figure 1.

Ventral aspect of female Ornithonyssus bacoti (top) and Laelaps echidnina (bottom). The three chitinized shields—sternal (1), genital (2), and anal (3)—and setae (4) are often used for identification. Diagram reproduced, with modifications, from Baker EW, Evans TM, Gould DJ, Hull WB, Keegan HL. 1956. A Manual of Parasitic Mites of Medical and Economic Importance, with permission from the publisher, National Pest Management Association, formerly the National Pest Control Association, NY.

Laelaps spp. are easiest to distinguish because they are large (0.75 to 1 mm) and heavily sclerotized with sucker-like ambulacra (foot pads) on all tarsae (legs). The female L. echidnina has an unmistakable ventral shield-shaped genital plate with a rounded posterior concavity that perfectly matches the closely opposed anterior convex margin of the anal shield.

Liponyssoides sp. and Ornithonyssus sp. are smaller in the unengorged state (∼0.5 mm), although Ornithonyssus females can reach 1.0 mm when engorged. They are only weakly sclerotized and semitransparent, appearing white when unengorged and red-brown when engorged. Compared with Ornithonyssus, Liponyssoides has much sparser and shorter setae and its genital shield is broader and rounder than the narrow tapering genital shield of Ornithonyssus sp. (Baker et al. 1956; Baker 1999; Flynn 1973; Green and Baker 1996; Hirst 1913; Sudd 1952). O. bacoti is distinguishable from O. sylviarum and O. bursa because its dorsal shield setae are as long as or slightly longer than those on the surrounding cuticle (Baker 1998).

The mesostigmatid mite life cycle consists of an egg, six-legged larva, and two eight-legged nymph stages before adulthood. Only the first nymphal and adult stages feed on blood and serum (Laelaps sp. also feed on lacrimal secretions). Under laboratory conditions (65-75% relative humidity and 25°C) the life cycle is completed in as little as 6 to 8 days (Baker 1999; Flynn 1973).

Mesostigmatid Mites in the Animal Facility

Infestations in laboratory animal colonies most likely originate with wild rodents that transmit their mite populations to laboratory mice via commensal mice living in the building. Pigeon or chicken colonies may also harbor Ornithonyssus bursa or O. sylviarum that can make their way to laboratory rodents.

At our institution the first report of an infestation was from a laboratory that housed experimental mice for short periods and that showed evidence of commensal rodents. The experimental mice transferred the infection back to their housing facility and, despite apparently successful treatment of the affected mice and facility, four other housing facilities were subsequently infested over a period of 2 years (JW personal observation).

Building demolition and renovations or removal of commensal rodents are particularly high-risk periods and can result in new laboratory rodent mite infestations as the mites travel in search of new hosts. Renovations therefore require coordinated treatment to prevent infestation of adjacent rodent facilities when sites harboring commensal rodents are demolished. Treatment of the environment for mites should precede vermin eradication, which should in turn be coordinated with demolition.

Limitations of Cage Barrier Systems

Cage barrier systems (NRC 1991) are commonly used in research institutions as a means to exclude unwanted pathogens. Unlike barriers at the room or facility level that depend on limiting access to a few highly trained individuals, barriers at the cage level allow facility access by multiple personnel while still protecting the laboratory rodents. Whether the institution uses individually ventilated or static caging with filter tops, the effect is the same: barrier cages protect the rodents inside from commonly encountered rodent pathogens while they remain closed (Brielmeier et al. 2006; Compton et al. 2004a,b; Dillehay et al. 1990; Kraft 1958; Lipman et al. 1993; Otto and Tolwani 2002; Sedlacek and Mason 1977; Wescott et al. 1976), and the use of aseptic procedures in biosafety cabinets can prevent infection when the cages are opened.

In the case of mesostigmatid mites, however, the cage barrier system breaks down. These blood-sucking mites live in the environment adjacent to their rodent hosts and are freely mobile (Baker 1999; Flynn 1973). Indeed, we have observed them leaving occupied, closed cages on individually ventilated racks after cage treatments (JW personal observation). Disturbances to existing rodent hosts (e.g., during vermin eradication efforts or building renovations) may cause the mites to travel long distances in search of new hosts (Baker 1999; Flynn 1973). Once established, they colonize the environment adjacent to the rodent and, due to their small size (0.55 to 1.0 mm) (Baker 1999; Green and Baker 1996), may escape detection until they reach large numbers or bite personnel who handle the laboratory rodents or their caging.

Prevalence

Reports of mesostigmatid mite infestations in laboratory rodent facilities have generally been uncommon in the last 2 decades. Recently, however, several institutions have reported infestations with O. bacoti (tropical rat mite) and Laelaps echidnina (spiny rat mite) (Chu and Couto 2005; Cole et al. 2005; Coons 2005; Hill et al. 2005; Kelaher et al. 2005). Both O. bacoti and L. echidnina have recently been found on wild urban Rattus norvegicus captured in alleys in Baltimore, Maryland (Easterbrook et al. 2007, 2008), and both mites were also identified during repeated outbreaks of mesostigmatid mites in the rodent colonies in my institution between 2005 and 2007 (JW personal observation).

When laboratory rodents are infested, personnel who have contact with materials or equipment used in the animal facility or who handle the infested animals may be bitten (Kelaher et al. 2005).

Pathogenesis and Clinical Effects

There is considerable evidence that rat mites carry and have the potential to transmit a number of pathogens. Experiments have shown that Ornithonyssus bacoti transmits Rickettsia akari (rickettsialpox; Phillip 1948), Francisella pestis (plague; Yamada 1932), Coxsackie virus (Schwab et al. 1952), Francisella tularensis (tularemia; Hopla 1951), and Trypanosoma cruzi (Chagas disease; Cortez-Jimenez and Aguilar 1994). Researchers have also documented O. bacoti specimens with Coxiella burnetii (Q fever; Zemskaya 1968), hantavirus (Zhuge et al. 1998), Borrelia sp. (Lopatina Iu et al. 1999), Bartonella sp. (Kim et al. 2005), and Rickettsia sp. (Reeves et al. 2007). L. echidnina specimens have been found carrying Junin virus (Argentine hemorrhagic fever; Parodi et al. 1959), L. sanguineus is a vector of Rickettsia akari (human rickettsialpox; Saini et al. 2004), and Coxiella burnetii (Q fever) has been isolated from L. sanguineus specimens feeding on laboratory animals (Zemskaya 1968). In addition, O. bacoti is the intermediate host of the rodent filarial worm Litosomoides carinii (Bertram et al. 1946; Renz and Wenk 1981), and L. echidnina transmits Hepatozoon muris (Weinrich 1949).

Rat mites thus can transmit several zoonotic diseases, and both O. bacoti and L. echidnina readily bite humans, but the status of rat mites as vectors in naturally occurring human infections has not been proven (Baker 1998). There are, however, sporadic but persistent reports of cases of rat bite dermatitis caused by O. bacoti from rodent-infested buildings and yards or from pet rodents (Baumstark et al. 2007; Beck and Pfister 2004, 2006; Creel et al. 2003; Fishman 1988; Fox et al. 2004; Pizzi et al. 2004; Skirnisson 2001; Theis et al. 1981).

Severe rat mite infestations cause debility and anemia in rodents (French 1987; Harris and Stockton 1960; Keefe et al. 1964; Olsen 1946), but in my laboratory we have observed no clinical effects in lightly infested cages (JW personal observation).

Detection, Eradication, and Prevention

Detection

The first sign of a mesostigmatid mite infestation is usually a complaint of bites among personnel who work with the laboratory rodents. Follow-up investigation reveals the presence of mites as tiny moving black dots on the animal or in the cage, most easily visible against a pale background, such as a cage filter; indeed, where cages have filter tops, their close examination proves to be a good way to detect an infestation.

In our institution, sticky insect traps were inefficient as a diagnostic tool; instead, in each of our outbreaks the diagnosis of mites came from animal handlers who had either been bitten or had observed the mites on themselves or on cage tops. Sticky traps placed on or adjacent to the racks remained negative even after confirmation of the presence of mites in the cages (presumably because the mites did not venture far from their rodent hosts). The traps proved more useful for determining the extent of an outbreak after the fact: trapping mites from previously undiagnosed positive cages after administering mite-repelling permethrin cage treatments (JW personal observation).

Eradication

Permanent elimination of mesostigmatid mite infestations requires a coordinated and sustained effort to eliminate both mites and rodent vermin reservoirs. Mites have been reported to survive 2 weeks to several months without feeding (Baker 1998), necessitating repeat treatments and/or residual environmental treatments to prevent reinfestation.

In our experience, although individual facility treatments were successful, further outbreaks subsequently occurred in other rodent facilities. These were likely due to the transfer of infested rodents before discovery of the outbreak, undiscovered commensal rodent reservoirs, or mites that escaped during early treatments before we learned to sequence our environmental and cage treatments (JW personal observation).

Cage Treatments

The use of insecticides is contraindicated in animal facilities, but the zoonotic potential of mesostigmatid mites requires extraordinary measures. Recent reports of successful eradication from laboratory vivaria describe the use of permethrin-impregnated cotton balls (MITEARREST®, Eco-Health, Inc., Boston, MA) in the mouse cages for 5 to 8 weeks together with environmental treatment with various sustained-release pyrethrin (Chu and Couto 2005; Cole et al. 2005; Hill et al. 2005) or dichlorvos (Coons 2005) products. We used MITEARREST for 8 weeks in combination with sustained-release deltamethrin 0.06% (Suspend®SC, Bayer Environmental Science, Montvale, NJ) fan-sprayed on floors and baseboards monthly for three treatments, timed to precede cage sanitation (JW personal observation).

Permethrin is often chosen for use in laboratory animal facilities because of its exceptionally high (>1000) selectivity ratio (mammalian oral LD₅₀/insect topical LD₅₀) (Valentine 1990), poor skin absorption, and rapid inactivation in the body; the skin LD₅₀ for rats is >4000 mg/kg, and the oral LD₅₀ for rats is 430 to 4000 mg/kg and for mice, 540 to 2700 mg/kg (WHO 2006). Permethrin is a more stable synthetic analog of pyrethrum, which was originally derived from chrysanthemum cinerariaefolium flowers; it acts as a neurotoxin, prolonging sodium channel activation and causing repetitive firing of peripheral nerves and eventual paralysis in insects (Narahashi 1982). It is also an insect repellent (Mencke 2006) and is available impregnated into clothing for that purpose (Faulde et al. 2003).

Environmental Treatments

Any treatment plan should start with environmental treatments. As mentioned above, we have observed mites exiting ventilated rack cages after permethrin was placed inside to treat infested mice (JW personal observation), so treatment of cages without first treating the environment can result in the establishment of escaped mites on rodents outside the treated area.

Unlike animal treatments, environmental treatments that can result in personnel exposure usually require application by a licensed pest control professional. Operatives should work inward from the periphery of the risk area toward the center to prevent mites escaping to untreated areas. To avoid reinfestation, they should treat all areas occupied by rodents or rodent equipment, including support areas, investigator laboratories, and procedure areas. In addition, treatments should address potential locations for colonies of escaped rodents, such as inside poorly sealed animal facility doors, biosafety cabinets, and ventilation ducts.

Prevention

Prevention, particularly in laboratory housing areas or during high-risk periods such as building renovation, is unquestionably preferable to treatment. Given the undesirability of insecticides, alternatives include physical methods such as silica-based products and biological methods such as insect growth regulators.

Insect growth regulators fall into two categories, nonsteroidal ecdysteroid agonists that mimic the action of molting hormones and cause accelerated and incomplete molting, and juvenile hormone chemical analogs that inhibit maturation (Dhadialla et al. 1998). Juvenile hormone analogs are registered with the Environmental Protection Agency (EPA) as biopesticides and approved for indoor use (they are also effective against the German cockroach) (Atkinson et al. 1992). The EPA reports no evidence of toxicity in nontarget species (EPA 2001), but it is advisable to avoid exposure of Drosophila melanogaster colonies and mice or cell lines that use ecdysone-inducible gene expression systems (No et al. 1996).

There is no published information on the use of insect growth regulators (IGRs) to prevent mesostigmatid mite infestations in animal facilities. We chose to add them to our pest control program for a number of reasons, not least of which was our previous inability to prevent further infestations in distant areas despite apparently successful cage and environment insecticide treatments. Product literature suggested IGRs would have no impact on research and were effective against many insect and mite species, and applications could take place during regularly scheduled visits, which made them relatively inexpensive. We selected baseboard applications every 4 months of (S)-hydroprene (Gentrol® IGR Concentrate, Wellmark International, Bensenville, IL), a juvenile hormone analog, because it was already approved for animal facility use against cockroaches. In the 12 months since we added it to our pest control program, we have remained free of rat mites and have observed no untoward effects (JW personal observation).

Environmental application of silica aerogel has been effective for controlling rat mite infestations in homes (Ebeling 1971) and, as it is not likely to have a research impact, could also be a candidate for environmental control of insect and mobile mite pests in laboratory animal facilities.

Conclusion

Mesostigmatid mites are present on wild rodents and thus may migrate to commensal rodents. Laboratory rodent colonies are at increased risk of infestation if they are exposed to infested commensal rodents (e.g., in laboratories with inadequate pest control programs) or if the latter are disturbed (e.g., during renovations or aggressive vermin control efforts). Once an infestation is diagnosed in laboratory rodents, eradication requires removal of commensal rodent reservoirs and widespread treatment of both the environment and the mice. Pyrethrins appear to be safe and effective for both cage and environmental use. Failure to eliminate the commensal rodent reservoir or to prevent mite migration during treatments may result in repeated infestations or in new infestations at sites distant from the original problem. Noninsecticidal preventive methods, such as environmental application of insect growth regulators or silica-based sprays, may be appropriate under high-risk conditions such as during building renovations or ongoing mite infestations.