David Crowther

Reproduction and Development of Laboratory and Wild House Dust Mites (Acari: Pyroglyphidae) and Their Relationship to the Natural Dust Ecosystem

Abstract Life histories of “wild” house dust mites, Dermatophagoides pteronyssinus (Trouessart) (Acari: Pyroglyphidae), were compared with laboratory cultures by using a diet consisting of skin and dust or a laboratory diet consisting of dried liver and yeast. Under constant conditions of 25°C and 75% RH, fecundity and rate of reproduction were higher in laboratory cultures on both diets compared with wild mites. There were also trends for a shorter prereproductive period and more rapid egg development of laboratory mites compared with wild mites. Overall, there was little effect of diet on either strain of mites at 75% RH. At low RH (64%), fecundity was significantly lower (for both strains on both diets), and there were also trends for longer prereproductive period, reduced rate of reproduction, reduced adult survival, prolonged egg and juvenile development, or a combination compared with 75% RH. Additionally egg and juvenile mortality were significantly higher on the liver and yeast diet. Overall, the skin and dust diet favored both strains of mites at 64% RH. On the liver and yeast diet at 64% RH, wild mite adults performed significantly better than laboratory mites, and egg mortality was lower. These results suggest that laboratory mites have stronger reproduction and development than wild mites, except when under environmental stress and that diet is a significant factor, particularly in suboptimal conditions. This could have important implications for predictive models of house dust mite populations in their natural habitat. Ideally, such models should be developed using data from wild dust mite populations reared on a natural diet.

Application of a transient hygrothermal population model for house dust mites in beds: assessment of control strategies in UK buildings

This article discusses the capabilities and the application of an innovative combined hygrothermal and population model to assess the impact of building design and occupant behaviour on house dust mite populations in a mattress. The combined model is the first of its kind able to predict the impact of hourly transient hygrothermal conditions within a 3-dimensional mattress on a population of ‘wild’ Dermatophagoides pteronyssinus mites. The modelling shows that the current drive for energy efficiency in buildings might lead to an increase in house dust mite infestations in UK dwellings. Further research is needed to accurately determine the size of these effects and to adequately evaluate any trade-offs between energy efficiency measures and health outcomes.

A steady-state model for predicting hygrothermal conditions in beds in relation to house dust mite requirements

This paper describes the development, testing and validation of a simple steady-state hygrothermal bed model (BED) which predicts conditions of temperature and relative humidity within the bed core (the occupied space between mattress and covering), given the temperature and relative humidity of the bedroom. BED is the second of three simple steady-state models that in combination allow the impact of modifying bedroom hygrothermal conditions on dust mite populations to be assessed. The first of the trio is Condensation Targeter II, an existing validated model that predicts average monthly conditions of temperature and relative humidity within the bedroom. These conditions are then used as boundary conditions for the BED model which predicts hygrothermal conditions within the bed core. Finally, these outputs are in turn used as inputs to a simple Mite Population Index (MPI) model (to be described elsewhere) that predicts their likely effect on house dust mite population growth in the bed. As reported here, BED has been validated using monitored bedroom and bed data for a full year in three dwellings and the results show that the steady state model predicts monthly bed hygrothermal conditions with a reasonable degree of accuracy. Using Condensation Targeter II and BED in combination, a sensitivity study has been carried out to assess the impact of changes in input parameters of both models on hygrothermal conditions in the bed core. This highlights the importance that the design of the fabric and services of the building has on the hygrothermal conditions in a bed. The impact of climate change has also been assessed using future climate change scenarios. Practical application: This paper describes in detail a simple steady-state model, (BED) which is used to predict the monthly average temperature and relative humidity within a bed, given the ambient conditions within the bedroon. The input parameters, output parameters and the model formulae are provided so that the model can be easily implemented. BED is the second of three simple models that are used to predict, first the bedroon conditions (Condensation Targeter II), second the bed conditions (BED) and finally the likely effect on house dust mite population growth using a simple Mite Population Index (MPI).

The psychrometric control of house dust mites: a pilot study

This paper describes a pilot intervention study on the effectiveness of house dust mite allergen avoidance for 12 asthmatic children (two being controls). In addition to mite allergen removal, the study included tailored advice aimed at reducing mite population growth via changes in moisture production, heating and ventilation habits. This paper focuses on the effects of this advice on household behaviour, hygrothermal conditions and mite populations. The efficacy of monitoring and modelling techniques is also discussed. The study highlighted a number of interrelated confounding factors which have to be addressed in future similar larger scale studies, but the results are promising with regards to the effectiveness of such studies. Practical application: This study suggests that in temperate climates tailored advice on moisture production, heating and ventilation habits can lead to valuable changes in hygrothermal conditions, which in turn can result in reduced mite populations. However, pre-existing adverse building conditions may hinder such changes, and the effectiveness of tailored advice and of hygrothermal modifications is often difficult to assess. It is therefore recommended that any similar larger intervention study measures ventilation rates and adequately controls for a number of confounding variables — including the effect of changes in outdoor conditions and of the removal of existing mite populations. In this respect, hygrothermal population models can play a very useful role in the assessment of study effectiveness.