The biology of the house dust mite

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.

The healthy mite's, seemingly inefficient, digestive system creates up to 20 dung-pellets a day. 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. In other words they can eat their own droppings up to three times over before it is 'spent'. The mite's microscopic dung pellets, if disturbed in an unventilated room, can remain suspended in still air for 20 minutes. It is a scavenger with a preference for discarded old skin scales, but will eat pollen grains, insect scales, bacteria and plant fibres.

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. The conditions needed are warm, dark and damp. Optimal growth for a mite colony occurs at a temperature of 25C 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. A female house dust mite may lay 1 to 3 eggs a day. After 6 to 12 days the egg will hatch producing a six-legged larva nymphal which will feed then pass through 2 nymphal stages before emerging as an adult.

There are 20 known allergens of the house dust mite. The powerful enzymes from the mite have been recorded by scientists as causing an allergic-like reaction in the absence of true allergic response. Indeed, the major allergen Der p1 is so invasive it has been found in foetal amniotic fluid at 16 to 17 week gestation and in the cord blood of some babies at birth.

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. 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. Some of these micro-organisms are known risk factors for disease.

People, with a family history of allergy, can be vulnerable to the mite and its droppings. 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 defence system. 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. Potentially dangerous bacterial micro-organisms have been found living in the airways of adults with chronic asthma. Selected fungi have been found as viable entities in the mite's droppings. Two of the most common species are 'Aspergillus peicilloides' and W sebi.

Allergists consider the house dust mite Dermatophagoides spp., a high risk determinant for allergic disease. These diseases are allergic asthma, eczema, rhinitis (hayfever) and conjunctivitis; therefore excessive exposure to the mite is unacceptable in the social environment of the vulnerable.