House Dust Mites – why such a problem in New Zealand?
House dust mites (HDM) are found in most human habitats and are one of the most common triggers of allergies globally. Symptoms include asthma, atopic eczema, and allergic rhinitis. New Zealand appears to have a bigger problem with house dust mites than many other countries. This is because HDM thrive in our temperate and humid climate, and particularly in our damp homes.
HDM are tiny parasites - 8-legged arthropods - which live off the dead skin cells humans shed daily. They essentially convert these cells into tiny particles of dust, which is the airborne allergen that causes all the problems. Dust mites prefer temperatures around 18 -24ᵒC with humidity of 75 to 80 percent. HDM therefore live in houses where they can source food, as long as they have moisture to breed. They die when the humidity falls below 50 percent and are not usually found in dry climates.
Humidity is therefore a critical factor for HDM prevalence both inside and outside the home, with higher concentrations found in damp homes. Damp housing is known to a widespread problem in New Zealand. Cold air has a higher relative humidity and in the 2014 New Zealand General Social Survey (NZGSS) nearly half of New Zealand adults reported living in a cold house, with more than 1 in 5 individuals (21 %) feeling their homes were often or always cold. More than 3 in 10 (32 %) felt they had a problem with dampness, with 6 % reporting the problem was a major one.
Dust mite particles are often found in pillows, mattresses, carpeting and upholstered furniture. They float into the air when disturbed by anyone walking on or vacuuming carpet, sitting on furniture, or pulling back bedding. They settle back onto surfaces once the disturbance is over. Relative humidity increases quickly after a bed is occupied, so there is usually a concentration of HDMs found in beds.
Allergy to House Dust Mites
HDM are the most common allergen source in New Zealand particularly in coastal areas. Prevalence of HDM allergy is linked to exposure to the HDM allergen, with a cumulative increase in the development of allergy associated with increasing exposure to HDM allergens. Some studies suggest allergies will start to develop (patients become sensitized) at levels of 2 micrograms of allergen per gram of dust; above 10 micrograms per gram allergic patients are likely to have symptoms.
Another study of HDM sensitisation in children found sensitisation was low in the first year of life (about .05%) but increased to nearly 2% by the age of 3 years. There is some evidence to suggest that children who are sensitised to house dust mites at an early age may go on to develop asthma. It has been recommended that primary prevention of HDM allergy be focussed on preventing sensitisation in infancy.
There are a number of public health strategies to improve the standard of housing in New Zealand, including subsidised insulation and heating installations; and regulations for landlords as well as for new houses now require higher standards to ensure ‘healthy homes’. It is hoped over time, respiratory and other illness associated with poor housing, as well as HDM allergy causing allergic rhinitis, atopic eczema or asthma, will significantly reduce.
The symptoms of house dust mite allergy
Symptoms are generally year round and often worse at night, but may go away when the person is not exposed to HDM e.g. at work or school.
House dust mite allergy is known to cause significant disruption to sleep. This can lead to:
Managing HDM Allergy
There are a number of strategies those with HDM allergy and their families can take. There are three major categories of treatment:
As with any allergy, for most people symptoms can be controlled or significantly reduced with the right medications. See your GP to confirm a diagnosis of dust-mite allergy, and for medications for symptom control. These might include nasal sprays (corticosteroids) and antihistamine tablets. Ask your G.P. to give you an Allergic Rhinitis Treatment Plan (available from ASCIA on www.allergy.org.au). If you (or your child) also have asthma, ask for an Asthma Management Plan as well.
Environmental control measures can help minimise exposure to HDM in the home. Insulation, ventilation and heating systems which reduce humidity levels to be consistently below 50%, are likely to be the most effective. Remember if installing these retrospectively, even if mites are dead, their bodies and the dust they made can still cause allergic reactions. Thorough cleaning at the outset is still required.
If it is not possible to install these systems and get humidity levels below 50%, the next best step is to minimise exposure in and around the bed through the use of HDM allergen-barrier covers. These totally encase the mattress, pillow, and duvet, and act as a barrier to HDM and their dust from moving through beds and pillows. Some covers are made of vinyl or plastic but can be uncomfortable. Others use high-tech fabric with a very tight weave that HDM can’t penetrate, but still allow air-flow. These should be constructed so that seams and zips are well- sealed. A recent study found mite-proof covers may reduce the severity of asthma exacerbations in children allergic to dust-mites.
Bedding put over the dust-mite covers should be washed weekly, preferably at water temperature of 60ᵒC. Children’s soft toys can be put in the freezer over-night – this is effective in killing HDM. Toys should then be washed to remove any dust particles.It is recommended in many studies to remove carpet and have bare floors, this can be challenging in damp homes in New Zealand where carpet at least provides some insulation. However standard carpets and carpet underlays will encourage the growth of dust mites if they do not have built in anti-microbial protection. Look for products that feature anti-microbial protection with specific reference to inhibiting the growth of dust mites.
Vacuum regularly with a cleaner with a HEPA (high-efficiency particulate absorption) filter. These trap dust mites and dust in the exhaust and reduce the amount blown back into the room. Regularly vacuum soft-fabric furnishings, wash curtains and rugs, and clean and dust using a damp cloth. Sheepskin or woollen under-lays and rugs should be removed from the bedroom.
Allergen immunotherapy is a form of treatment which has been around for over 100 years and has been proven effective in providing long-term relief from symptoms. It aims to reduce the immune system’s over-active response to the allergen concerned – essentially to ‘desensitise’. There are two forms – subcutaneous (SCIT), which involves regular injections into the arm, and sublingual (SLIT), which is in form of drops or tablets dissolved under the tongue. There are two phases of treatment, the first is the ‘build-up’ phase where small and increasing amounts of the allergen are administered, then a maintenance phase where the dose level is maintained. Immunotherapy to house dust mite allergy usually takes at least three years to complete but can provide years of relief from symptoms and in need for medications.
However, neither form of immunotherapy for HDM allergy is funded in New Zealand and there is significant time and commitment required. For this reason, it is recommended patients be referred to an allergy specialist for assessment and recommendations prior to commencing this treatment.
This information is adapted from an article published in Allergy Today Winter Issue 2017 (Issue 161).