Monday, December 17, 2018

Rodent mites: They're a Thing

Several years ago, a nice lady left a “bug” sample for me to identify. It consisted of an electric alarm clock in a Ziploc bag. The accompanying note read: “Help! We’re being bitten by tiny bugs that come out of this clock when it’s plugged in.”

It’s not uncommon for our office to be visited by folks who for a variety of reasons are convinced they are being attacked by invisible [often non-existent] bugs. The descriptions often range into the fantastical, but most often there are simply no bugs. I figured this sample would fall into the imaginary bug category. For that reason, the sample sat on my desk for a few days before I reluctantly examined it. The clock was ordinary enough--no insects in sight. To be fair, I figured I would humor my client and plug in the clock.

A minute later I gazed at the clock. To my surprise I saw a half dozen (what turned out to be) rodent mites exiting and entering the holes in the back of the clock. The homeowner, I later learned, had just had the home cleared of roof rats by a pest control company. Upon losing their hosts, the now-lonely rat mites had apparently discovered the next best thing to a warm rodent nest was a warm electric clock.

This experience taught me two things. Rodent mites are attracted to warm places, and I should take every sample (goofy or not) seriously.

Rodent mites are one of the most poorly known biting arthropods. Yet as rodents grab headlines from New York City and Chicago, even the media is beginning to take notice of these tiny pests. One recent New York periodical recently proclaimed: "Rat mites. They're a thing, and they're worse than bed bugs." Like bed bugs, rodent mites are active at night and can leave you with an itch. But their tiny size and painful bites make them scarier than bed bugs to many.

Still, many PMPs get caught off-guard when asked by a customer about mites; or when customers complain about mystery bites after a successful rodent control job. It pays, therefore, to know something about these tiny pests.

Not insects

This is what a  rat mite caught in a sticky trap will  look like
with a hand lens. In size they will be little larger than a period
at the end of a sentence.
Mites are arachnids, more closely akin to spiders than insects. They belong to the taxonomically-complex sub-Class Acari, which includes mites and ticks. Like all arachnids, adult mites have four pairs of legs. They mainly differ from ticks in being smaller (almost microscopic), often having long body hairs and mouth-parts without teeth.

Mites include species that feed on stored products like flour and cheese, species that are predatory, species that are parasites, plant feeders and scavengers. In all, over 250 species of mites cause problems for humans including allergies, dermatitis, serving as bridge hosts for parasites. Some species even infest skin and transmit disease. If mites are so bad, why don't we talk more about them?

The answer may have something to do with size. Mites are crazy small! Their tiny size means that they often get overlooked, and few people feel compelled to study them (yes, even scientists are sometimes drawn to larger, easier subjects of study).

Secret lives of mites 

So what do we know about the secret lives of mites? All rodent mites are ectoparasites, meaning they live externally on their hosts, not internally like a tapeworm. They reproduce quickly, usually going through their life cycle (egg to egg) in about two weeks. There are several species of rodent mites, including the tropical rat mite, Ornithonyssus bacoti, the spiny rat mite, Laelaps echidnina, and the house mouse mite, Liponyssoides sanguineus. All of these mites, if you take time to look, can be found in rodent nests or in the fur of their hosts. According to rodentologist Dr. Bobby Corrigan, one of the few people I know who literally combs dead rats for science, mites can be found on most Norway and roof rats. Less is known about the prevalence of house mouse mite on house mice.

In structures, rodent mites will usually be found in walls or attics close to their hosts’ nests. Baby rodents are likely a favored blood meal during breeding season. When they become too numerous for the nest, or when the baby and adult rodents die or leave, rodent mites will wander in search of another host. These homeless mites are most likely to bite people; but luckily, we humans make poor hosts. Our lack of fur combined with fastidious grooming (think scratching) means that rat and mouse mites don’t last long when people are the only hosts around. Even dogs and cats do not appear to be suitable hosts for rodent mites.

Case studies 

Rodent mites can also infest the cages of captive rodents, and are occasional
pests in animal rearing facilities. From Beck and Folster-Holst, 2009.
In the absence of their preferred hosts, rodent mite infestations generally go away naturally within one or two weeks. One European case study (Beck 2008) involved three children with rat mite-caused dermatitis. The children lived in the same room with a pair of mite-infested gerbils. Once the gerbils were treated (selamectin) and cages cleaned and treated (permethrin + pyriproxyfen), the dermatitis ceased within seven days.

In another case a German medical student lived above a bakery with a rat infestation. The student was bitten repeatedly at night by mites that had apparently moved upstairs through the walls from the infested bakery. One week after professional treatment with permethrin, the hundreds of mites present in the home had disappeared and dermatitis ceased (Beck 2008, Beck and Folster-Holst 2009).

In Houston, a 60-year-old woman living in a dilapidated home infested with rats went to her physician complaining of itching and red bumps scattered on her upper body and arms (Hetherington et al 1971). Rats in the house were trapped and examined and found to be infested with the mites. The rashes and bites stopped when the woman left the house but resumed when she returned. Unlike bed bugs, rodent mites are not hitchhikers and not likely to be carried from one place to another by human transport.

The well-known urban entomologist, Walter Ebeling (1960), reported successful treatment of rodent mites with fluoridated silica aerogel, a type of desiccant dust, applied to the attics and bedding of severely infested homes. He reported that bites stopped immediately after treatment.

In another case involving an old house and six medical students (Engel et al. 1998), rodent mites were found to be the cause of severe itching and red bumps. Noises in the attic suggested an active rat problem, so a professional was called. During extermination the bites became so severe that the students had to leave the house temporarily. After the rooms were sprayed repeatedly with a miticide (benzyl benzoate + tannic acid) the students returned in a few days with no more cases of dermatitis or sighting of mites. This case demonstrated how rodent mite infestations can become worse after rodents are eliminated, as the mites seek new hosts.

In all cases, the use of creams and medications without addressing the rodent problem was ineffective. Once rodents were eliminated, and appropriate miticides applied, bites stopped within a week, or immediately in some cases.

Rodent mites and mystery bugs 

Because of their small size, rodent mites can be difficult to diagnose. Many cases of supposed mite bites turn out to have a medical or psychological explanation. For this reason, inspections are important. Use your flashlight to look for tiny arthropods little larger the the period at the end of this sentence. Look on solid colored surfaces in areas where your customer is reporting bites. To see a short video of rodent mites crawling on a person's hand, click here

Sticky cards are effective mite collecting devices. Place them in suspected hot spots. If your customer claims they are getting bites everywhere in the house, or even in their car, or place of work, rodent mites may not be the problem.

If you cannot find mites during an inspection, instruct your customer how to collect a sample. A piece of Scotch tape, or a hobby paint brush moistened with a bit of gel hand sanitizer or rubbing alcohol is an easy way to pick up small arthropods like mites. Bring the sample back to the office and have it checked by a certified entomologist or Extension entomology specialist.

Wrapping up

Some key points to communicate to your rodent control customers:
  • Rodent mites may appear during or after rodent extermination. 
  • If customer thinks they have mites, ask them to collect a sample so you can confirm the problem. Mites should be large enough for most people to see--they are not invisible.
  • Rodent mites can bite us; but they do not infest people, dogs or cats. 
  • Pet rodents can become infested with rodent mites. If so, a veterinarian should treat the pet and the customer clean and (optionally) treat the pet cage. 
  • Bird and rodent mites do not hitchhike on people (though “mouser” cats may serve as temporary transport for rodent mice to people) 
  • Rodent mite bites almost never cause disease (house mouse mites may rarely carry rickettsial pox)
Rodent mites may be the smallest pest your technicians are likely to ever encounter, but their impact can be big. Anticipate problems with mites when doing rodent control, especially in offices or residential accounts. Use sticky cards and consider treating any discovered rodent nests or room perimeters with a proven insecticide like permethrin or bifenthrin. Void areas can be treated with a desiccant dust like diatomaceous earth or silica aerogel.

Rodent mite infestations are not uncommon. You owe it to your customer to take all reports of biting insects seriously, even when they are difficult to see. And if your customer hands you a clock, try plugging it in.  

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