House Dust Mites and Mange Mites: Evidence of a Parasitic Ancestry in HDM

Evolutionary studies place the house dust mite (HDM) and the mange mite as close parasitic relatives. For the HDM, the change to a scavenger lifestyle happened about 80 million years ago, but in making the change it retained its old parasitic-like characteristics and an enzyme similar to the enzyme made by the mange mite. Perhaps this close similarity is why so many people are sensitive or allergic to the HDM, as their alert immune systems may mix the mites together. Many immune reactions indicate this is the case.

The Parasitic Lifestyle of the Mange Mite (Psoroptes ovis)

The mange mite discussed in this review lives in colonies under the scabbed sores of sheep, enjoying the high humidity conditions of the wool. The colony feeds on the liquid produced by sores that are the result of a progressive and intense allergic dermatitis. Once it was thought that the mite pierced the skin of its host with its mouth in order to feed, but that thinking has now changed. Research has found that enzymes in the mite's dropping cause an intense irritation on the animal's skin resulting in inflammation, scabs, discolouration of thickened skin, wounds open to bacteria, and loss of wool. Sores, if left unattended, can cause a progressive and intense allergic dermatitis and threaten the health of the animal. In perfect breeding conditions a female mange mite can produce 50 to 100 eggs in its lifetime. The egg will progress to adulthood in approximately 11 to 19 days, with adults living for up to 40 days. Unless disturbed, the mite will spend its entire life feeding on one animal.

Shared Characteristics and Evolutionary Shift

The house dust mite shares many of its core characteristics with the sheep mange mite. It lives in colonies, with a complete life expectancy of approximately 3 months. Both mites produce digestive enzymes in order to feed. Many of these are transferred into their droppings and are known to cause irritation or allergic reactions. Crucially, both the house dust mite and sheep scab mites need warm, humid conditions to thrive.

Here's the difference: The house dust mite has evolved to become a scavenger, a clever move because it no longer had to rely on one host for food. As a scavenger, the mite feeds on organic debris found in dust, including decaying old skin scales covered in yeasts, bacteria, and fungi. It will also eat wheat and corn flour, dead mites, plant fibres, pollen, and dog food, among other items. In order to digest this varied diet, the mite has developed an array of enzymes to tackle the job.

The Shared Enzyme: Der p 1 and Pso o 1

One of the most powerful of the house dust mite’s enzymes has an action that has been described as similar to a meat tenderiser used in the food industry. It acts by dissolving the 'glue' that binds delicate epithelial cells together, most notably in the respiratory system. This HDM enzyme, Der p 1, shares its characteristics and action with a homologous enzyme from the sheep mange mite, Pso o 1. Based on this, and the similarity in the mites’ lifestyles, it is not unreasonable to draw the conclusion that house dust mites were once mange mites, but in making the move to scavenger, retained its old parasitic lifestyle and parasitic-like enzyme.

Note on Allergens:

• *Dermatophagoides pteronyssinus* - HDM and its enzyme/allergen Der p 1.
• *Psoroptes ovis* - P. ovis and its enzyme/allergen Pso o 1.