You Never Sleep Alone

Mites as bed-mates and forensic indicators

When I moved into a wonderful flat on Hachijojima in Japan, I didn’t expect that some hungry bed bug had been waiting for a new victim! Terribly itchy red bites, some three in a row on my belly, made me suspect I was not alone in my bed. A few days later I saw the bug, known as Cimex lectularius. I moved from one tatami-room to the next, hoping the pest would not move with me. I was lucky  – until I had a visitor with her pussy cat. The latter had some fleas, which tasted my blood and once again left terribly itchy marks on my legs. Luckily, cat fleas don’t adapt to human hosts and mine eventually disappeared.

However, what almost certainly remained (and is actually present in virtually all beds, mattresses and beddings all over the world) must have been thousands of mites and various microscopic fungi. It has been estimated that a typical used mattress can be home to 100,000 to several millions of mites. In a British study published in 1995 a whole ‘zoo’ of mites was identified from a mattress. The dominant species was the so-called house dust mite with 95%; other species included Euroglyphus maynei (3%), Cheyletus tenuipilis (2%), Tarsonemus sp., Lepidoglyphus destructor, Acarus siro and the follicle mite Demodex folliculorum. In North America the house dust mite Dermatophagoides farina replaces the European Dermatophagoides pternoyssinus and in the tropics the Blomia tropicalis dust mite occurs.

The comforting fact is that the house dust mites in our beds and sheets are non-parasitic and do not transmit diseases. They can, however, cause allergies and (in genetically susceptible people) asthma attacks. Unfortunately, house dust mite sensitizations are a worldwide increasing phenomenon. After sensitization, re-exposures to mite allergens like the mites’ droppings, can initiate an immunoglobulin-dependent inflammatory antibody cascade that involves small proteins produced by white blood cells. What supports mite populations apart from an ample supply of dead human (or animal) skin cells, is warm and moist surroundings and a high co-occurrence with bed fungi, especially Aspergillus niger. Other common bed fungal genera noted were Penicillium, Rhizopus, Cladosporium, Alternaria, yeasts, and Rhodotorula. A dry, non-sweating body and an abstention to eat and drink while in bed should help to reduce fungal and mite bed co-occupant populations. But species and species compositions vary on different beds, linen, and blankets and can therefore indicate to a forensic expert which bed a person has slept in, if s/he has picked up some of that bed’s species. Just like insects, whose forensic potential I have highlighted in an earlier blog (The insect crown witness) mites can therefore help criminalistic sleuths. Sherlock Holmes, did he not know that?

The Turkish professors Rana Akyazı and Ayla Tüzün, however (as well as others elsewhere in the world), knew this very well and explain in their papers that each of the five stages of a decomposing corpse (fresh, bloat, active-, advanced- and dry decay) attracts different insect and mite species to the dead body. Mites occurring on humans like Demodex folliculorum and Sarcoptes scabieihominis survive for one or two weeks, respectively, on a dead human and can therefore sometimes provide a clue on the time of death. Other mite species arrive on the dead body together with the succession of flies or beetles that colonize the corpse. Since such succession waves have been studied since 1894 when P. Mégnin’s “La Faune des Cadavres” appeared, forensic acarologists (that is what mite experts are called) can reasonably accurately determine since when a corpse has been exposed to the environment. Since indoor mites have other preferences from outdoor mites, finding them on a dead body can provide important clues as to where a crime took place. Even a corpse submerged in water may not be totally free of mite ‘informants’, because aquatic mites could contaminate the body and impart some useful hints. If only Sir Arthur Conan Doyle and Sherlock Holmes’ Dr Watson had known all this!

© Dr V.B. Meyer-Rochow and, 2021.
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