All you need to know about the house dust mite
For the Curious

House dust mite droppings hold a secret

The net or membrane that holds a house dust mite dropping together is a semi-permeable network of fibres that contains proteins and a tough protective material called chitin. It is created in the hind gut of the mite by a valve that binds several clumps of various matter together into one 'poop'. Within this mass are enzymes that hold an ancient secret.
To describe the house dust mite's dropping as a simple 'poop' would be to ignore the fact that:

1. House dust mites are scavengers that consider their droppings as future nourishment.

2. The nourishment comes from digestive enzymes that break down hard-to-digest food the mite has eaten.

3. The membrane dissolves on contact with moisture to release its contents including spilling out active digestive enzymes capable of dissolving delicate cell structures.

The transformation of hard-to-digest food such as discarded skin, nail clippings, or dead mites into nourishment is the job of specialised enzymes created in the gut of the mite. The mite's exquisite sense of smell will recognise when the transformation is complete and food awaits.

Several of the house dust mite's enzymes are a reflection of an ancient and different way of life when the mite was a parasite of warm-blooded animals. As a parasite the mite didn't bite or burrow into skin, but chose to live on the top hiding under fur or feathers. Like other ectoparasites (living on the surface) the mite probably used enzymes in its dropping to cause sores by dissolving delicate cell structures on the skin surface of its host. Products from the weeping sores then fed the mite and its colony.

Today, the sheep scab mite, which causes acute allergic dermatitis in goats and sheep, lives in this manner and is a serious veterinary problem for farmers worldwide. In ancient times the house dust mite and sheep scab mite were close relatives. However, at some stage the house dust mite changed its way of life to become a scavenger of debris found in the nest sites of former hosts. This was a smart move for the mite because it no longer had to rely upon the health of its host for life. Even If the nest site failed to be dark, warm, and damp with plenty of food the mite just moved on to colonise elsewhere.

When considering the two mites together (sheep and house) a similarity occurs in the activity of dissolving enzymes in their droppings. Although house dust mites no longer rely upon the activity from the droppings for food, the enzymes may hold the secret to its former parasitic way of life; for it is the mite's dissolving enzymes that make millions of people react by itching, wheezing, coughing or sneezing.


References

'The Peritrophic Membrane of the Mite, Dermatophagoides Farinae: Acariformes' J. Parasitology, Wharton GW, Brody AR, 1972, p 801-804 'Ecology of House dust mite, 'Dermatophagoides pteronyssius' (Trouessart) 1991, David B Hay, Linacre College, Oxford University. British Library Document 170040 'Allergens and their role in the allergic immune response', TAE Platts-Mills, JA Woodfolk, 2011, Vol: 242 p 51-68 'Physiology and internal Anatomy', Dust Mites, Dr Matthew J. Colloff, 2009, CSIRO Publishing, ISBN 978-90-481-2223-3

'Is Permanent Parasitism Reversible? - Critical Evidence from Early Evolution of House Dust Mites\', Pavel B Klimov, Barry O\'Connor, Oxford University Press on behalf of the Society of Systemic Biologists (2013) doi: 10.1093/sysbio/syt008, Feb. 15 2013

'Sheep Scab, Psoroptes ovis Infestation', 'The Center for Food Security and Public Health' Iowa State University, psor_a2009 on-line