Dermatophagoides pteronyssinus is a tiny nest dwelling mite that scavenges on discarded skin scales in damp dark environments. It is a potent and major cause of human allergy and allergic disease worldwide.
House dust mites have learned to colonize damp human homes or offices. Four active enzymes found in its dropping can enter the body to trigger symptoms of asthma, rhinitis (hay fever), eczema and conjunctivitis, common disease of childhood. Fossil studies show that mites have been on earth for over 400 million years. This ancient species has been around longer than the dinosaurs! It is estimated that today there may be up to 100 million different species of mites in existence, from the depths of the ocean to the most remote desert. The house dust mite (HDM), evolved 23 million years ago as a scavenger living in birds’ nests or similar habitats. Approximately 10,000 years ago this mite found its way into the human environment. It is a close relative of scabies living mainly on fungi and rotting skin scales but as a scavenger it will eat what is available.(1,2)
The mite´s seemingly inefficient digestive system creates up to 20 dung-pellets a day. (3) The droppings, which are devoid of moisture and wrapped in a special film, contain scraps of food, debris and powerful enzymes. The enzymes, created by the mite, will continue to break down any remaining food particles; thus ensuring nourishment for the mite later.(4) In other words they can eat their own droppings up to three times over. The mite´s microscopic dung pellets, if disturbed in an unventilated room, can remain suspended in still air for 20 minutes.(5) It is a scavenger with a preference for discarded old skin scales, but will eat pollen grains, insect scales, bacteria and plant fibers.
The house dust mite has eight legs, each with a sucker and hooks. This ensures easy travel on clothing, blankets, soft toys and old furniture to colonise and infest suitable nest sites if the conditions are right. (6) The conditions needed are warm, dark and damp. Optimal growth for a mite colony occurs at a temperature of 25°C and 75% relative humidity. Below 54% humidity a mite colony will not thrive. Mites are unable to regulate their body temperature, they have no eyes, never drink and have no organised respiratory systems, yet their body weight is up to 75% water. Maturity from egg to adult may take up to 30 days in a life-span of approximately 3 months depending upon living conditions and sex of the mite. From eggs to adult the mite will pass through six stages of life. (2,3,7)
There are 16 known allergens of the house dust mite. (8) The powerful enzymes of the mite have been recorded by scientists as causing an allergic-like reaction in the absence of true allergic response.(9) Indeed, the major allergen Der p1 is so invasive it has been found in fetal amniotic fluid at 16 to 17 week gestation and in the cord blood of some babies at birth. Scientists are investigating the possibility of this phenomenon as a link to allergy, possibly through the immature gut. (10)
Nesting sites for the mite include carpets, padded sofas and chairs, soft toys and especially bedding. Modern homes with high temperature, high humidity and lack of ventilation have been shown to be ideal breeding grounds.(11) In a study in Oxfordshire, a collection of house dust, including bedding, has been found to contain micro-organisms and fungi in abundance as well as colonies of mites in various stages of life.(12,13) Some of these micro-organisms are known risk factors for disease. (14,15,16)
Genetically predisposed individuals (atopic-family, history of allergy) are most vulnerable to the mite. (17,18) A pathway towards sensitisation, or allergy, has recently been demonstrated by doctors in St George´s Hospital Medical School. The doctors found that the major allergen Der p1, once inhaled and dissolved, causes the breakdown of the adhesion molecules binding the epithelial cells together leading to permeability of the lung and a breach of its defense system.(19,20) This phenomenon initiates apoptosis (cell death) leaving cells exposed to the possibility of further invasion by inhaled allergens or other micro-organisms such as fungi and bacteria. (21) Potentially dangerous bacterial micro-organisms have been found living in the airways of adults with chronic asthma.(22) Fungi have been found as viable entities in the mite´s droppings. (23) Recent studies describe how the major mite allergen Der p1 can reduce lung defenses to the advantage of common pathogens.(24)
Once in the lung´s epithelium the enzymes in mite droppings continue to break down specific molecules. This may lead to exacerbation of harmful allergic response as the immune system reacts to misguided signals resulting in an over production of non-specific IgE antibodies.(25) It is interesting to note that the same over-production of non-specific IgE is a ploy used by certain parasites to avoid detection by the immune system. (26)
The immune response to this invasion has been well studied and is considered by scientists to be a cause of allergic asthma and the exacerbation of symptoms. (7) We must consider the house dust mite Dermatophagoides spp., a high risk determinant in allergic asthma and related diseases. These diseases are allergic eczema, allergic rhinitis and conjunctivitis; therefore the mite is unacceptable in the social environment of the vulnerable. (27,28)
Nell Nockles © August 2004
´Evidence suggests that allergen-specific helper T (th) cell memory, which underlines expression of atopy in adulthood, is programmed during very early life. Predisposition to primary allergic sensitization per se may be related to the function of discrete gene(s) that regulate responses to microbial stimuli, particularly during early infancy, whereas a second set of genes may control tissue expression of atopic disease.´
Primary Sensitization to Inhaled Allergens; Am J Respir. Crit. Care Med. 2000; 162 : 591-594
For more informaiton see Time Line 1995 (11)
References: References:
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5.´Who´s been sleeping in your pillow´ (Video) University of Southampton, Teaching and Support Media Services. Dr Jill Warner, 1997
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