Occupational Mite Allergy and Asthma: An EAACI Task Force Report

ABSTRACT

Mite sensitization is notable in several occupational settings. Elevated house dust mite concentrations are primarily detected in workplaces where people congregate and are active. Allergy to storage mites and spider mites has commonly been reported in agricultural and various food processing occupations. Rapid expansion of biological pest control has resulted in increased exposure to predatory mites causing sensitization of greenhouse workers. Globally, mite populations in workplaces are likely to change due to climate change. Occupational relevant mites produce a variety of allergens and adjuvants that trigger both innate and adaptive immune responses. Cross‐reactivity between allergens occurs due to shared IgE‐binding epitopes to different allergens. Occupational allergy to mites typically causes rhinitis and asthma. Challenges of distinguishing the role of occupational exposure to allergens, also present in non‐occupational environments, complicate the diagnosis of occupational mite allergy and asthma. Nevertheless, preventive measures to reduce exposure to mite allergens in workplaces are essential in mitigating occupational hazards. Further research is needed to better understand the incidence of occupational mite allergy and asthma. It is essential to identify the risk factors in different occupational settings, assess the impact of climate change on exposure, and determine the relevant allergens and their potential cross‐reactivity.

Abbreviations

BAT

basophil activation test

EDC

electrostatic dust collector

FEIA

fluorescence enzyme immunoassay

HDM

house dust mite

IL

interleukin

MARIA

multiplex array for indoor allergens

MD‐2

myeloid differentiation factor‐2

OA

occupational asthma

OR

occupational rhinitis

PEF

peak expiratory flow

RAST

radioallergosorbent test

sIgE

specific Immunoglobulin E

SIT

specific allergen immunotherapy

SPT

skin prick test

Th

T‐helper cell

TSLP

thymic stromal lymphopoietin

1. Introduction

Domestic mites are a major source of indoor allergens [1]. They include both house dust mites (HDMs), which feed mainly on dander, as well as storage mites, which feed on food or animal feed. In addition to homes, mites are also encountered in work environments, commonly related to farming and food production worldwide [2]. In recent decades, apart from domestic mites, spider mites, living on fruit leaves in gardens, greenhouses, and orchards [1], and predatory mites, widely used for biological pest control, represent an additional source of exposure [3].

An increasing number of studies have demonstrated a high prevalence of mite sensitization among workers in agricultural and food processing occupations [4, 5, 6]. These sensitized workers are at increased risk of airway symptoms. Several species have been identified to cause occupational rhinitis (OR) and/or occupational asthma (OA) [2]. Mite sensitization in these contexts constitutes a major occupational hazard that can result in significant clinical impairment among affected workers, including the ability to continue working in their current jobs.

The aims of the Occupational Mite Allergy and Asthma (OMAA) Task Force were to:

1. Develop an up‐to‐date position paper to summarize the current scientific evidence on the risks associated with mite exposure in occupational environments, with specific reference to OA and OR.

2. Conduct a comprehensive scoping narrative review on allergen sources, high‐risk exposures in workplaces, identified allergens, and available diagnostic methods.

3. Provide a guidance note for the assessment, management, and prevention of OA and OR associated with mites in clinical practice.

4. Identify areas for future research.

2. Methods

This document was prepared by the OMAA Task Force expert panel of allergologists, pneumologists, occupational medicine physicians, and scientists from several countries. The publication is the result of expert consensus opinion following a thorough review of the available literature.

2.1Data Sources, Search Strategy, and Study Selection

A literature search was conducted using Pubmed and Scopus. Key search items included (Mites OR acari) AND (Occupational allergy OR Occupational allergies OR work‐related allergies OR Occupational asthma OR Work‐related asthma OR Occupational Rhinitis OR Work‐related Rhinitis OR Occupational conjunctivitis OR work‐related conjunctivitis). No specific period was included in the search, but it was completed during the period 17–25 October 2023. There was a combined total of 684 articles, and after removing duplicates, a total of 552 articles were identified. Subsequently, seven articles/abstracts listed in the review by Baur and Bakehe (2014) [7] and 33 recommended by the Task Force team focusing mainly on exposure assessment studies were added, resulting in 592 articles. Articles were sorted by date (from the most recent) and were accepted for review if they were available in English. The articles were selected in a qualitative manner using the Population, Concept, and Context (PCC) framework. Articles were screened by title and abstract. Studies of occupational settings in epidemiological or clinical settings were included. Key concepts that formed the basis for inclusion were epidemiology, occupational causes (systematic reviews and meta‐analysis, epidemiological studies, case reports, case series), specific exposure sources, exposure assessment, occupational allergens, mechanisms, clinical diagnosis, clinical management, and prevention. Non‐occupational studies were excluded. This resulted in 215 articles that were identified as being suitable, which formed the basis for this scoping narrative review and expert opinion.

3. Taxonomical Classification of Mites

More than 50,000 species of mites have been described. Mites belong to the Phylum Arthropoda (Subphylum Chelicerata), which is characterised by an exoskeleton (Figure 1). In contrast to insects, mites belong to the class Arachnida, characterised by four pairs of legs [8]. Mite species that cause occupational mite sensitisation and asthma belong to different orders and families. HDMs and storage mites belong to the order Astigmata. HDMs (Family: Pyroglyphidae) are mainly present in house dust, whereas storage mites, additionally found in workplaces related to farming and food production, belong to different families (Glycyphagidae, Acaridae, Echymyopodidae) [9]. However, the grouping according to the occurrence of mites is not consistent with the taxonomic classification. Predatory mites (Families: Phytoseiidae, Laelapidae) and poultry mites (Macronyssidae) belong to the order Mesostigmata. Predatory mites feed on spider mites and are often used for pest control, whereas poultry mites (Genus: Ornithonyssus) feed on blood, feathers, and skin of poultry [10]. Spider mites belong to the order Trombidiformes and the family Tetranychoidea (Genus: Tetranychus, Panonychus) and can be found on fruit leaves in greenhouses [11].

FIGURE 1.

Taxonomical classification of mites.

4. Epidemiology and Risk Factors

4.1. Epidemiology

A recent overview of systematic reviews indicated that there is moderate quality of evidence that the main group of mites and specifically the predatory‐, spider‐, and storage mites cause occupational mite sensitization, rhinitis, and asthma (Table 1) [1, 2]. Earlier reviews reported limited or contradictory evidence for HDMs and poultry mites [7]. Dalboge et al. also concluded that there was moderate evidence for OA associated with mite exposures in farming and bakery occupations [2].

TABLE 1.

List of mite species causing occupational allergy, rhinitis and asthma.

Mites (Acarina) species	Occupational exposure
Spider mites (Tetranychidae)
Red spider mite (Tetranychus urticae)	Tomato greenhouse worker [12] Carnation greenhouse worker [13, 14] Greenhouse workers [15, 16] Greenhouse workers and open‐field farmers [17, 18] Flower cultivator [19] Table grape farm workers [11] Citrus farmers [20] Apple‐cultivating farmers [21]
MacDaniel spider mite (Tetranychus macdanieli)	Vine growers [22]
European red mite (Panonychus ulmi)	Fruit tree workers [23]
	Apple‐cultivating farmers [21, 24]
Citrus red mite (Panonychus citri)	Fruit tree workers [25]
	Citrus farmers [20, 26, 27]
Storage mites (Acaridae, Glycyphagidae)
Lepidoglyphus destructor	Farmers [9, 28, 29, 30, 31, 32, 33] Arable farmworkers [34] Grain elevator workers [35] Grain‐store workers [36] Grain workers [37, 38] Grain miller [39] Bakers [40, 41, 42] Bakers, pastry factory workers, grain store workers and farmers [43] Thatcher [44] Grocery store workers [6] Dairy farmers [45, 46] Poultry workers [47] Ham production workers [48] Laboratory animal workers [49]
Acarus siro	Farmers [9, 19, 32] Arable farmworkers [34] Grain elevator workers [35] Grain‐store workers [36] Bakers [40, 41, 42] Thatcher [44] Grocery store workers [6] Dairy farmers [45] Food processing workers (cheese, chorizo, ham) [50] Ham production workers [48] Laboratory animal workers [49]
Acarus farris	Farm workers [32, 33] Arable farmworkers [34] Grain‐store workers [36]
Tyrophagus putrescentiae	Apple cultivating farmers [21] Farmers [9, 28, 31, 32] Arable farmworkers [34] Grain elevator workers [35] Grain‐store workers [36] Bakers [40, 41, 42] Thatcher [44] Grocery store workers [6] Dairy farmers [45, 46] Food processing workers (cheese, chorizo, ham) [50] Ham production workers [48] Ham transport driver [51] Laboratory animal workers [49]
Tyrophagus longior	Farm workers [33]
Glycyphagus domesticus	Farmers [9, 28, 32] Arable farmworkers [34] Grain‐store workers [36] Bakers [40, 42] Thatcher [44] Dairy farmers [45]
Thyreophagus entomophagus, Cheyletus eruditus	Farmers [32]
Storage mites (unspecified)	Cattle farmers [52]
Predatory mites (Phytoseiidae)
Thrips mite (Amblyseius cucumeris)	Greenhouse workers [53] Gardener [54]
Amblyseius swirskii, Suidasia medanensis, Neoseiulus cucumeris	Greenhouse workers [15, 55, 56]
Phytoseiulus persimilis, Hypoaspis miles	Greenhouse workers [55, 57, 58]
House dust mites (Pyroglyphidae)
Dermatophagoides pteronyssinus	Office workers [59] Poultry workers [47] Ham production workers [48] Farmers [19] Thatcher [44]
Dermatophagoides farinae	Office workers [60] Poultry workers [47] Ham production workers [48] Thatcher [44]
Euroglyphus maynei	Food processing workers (cheese, chorizo, ham) [50]
Blomia tropicalis, Blomia tijbodas	Farmers [32]
Blomia kulagini	Farmers [32] Food processing workers (cheese, chorizo, ham) [50]
House dust mites (unspecified)	Office/Lab workers [61]
Poultry mites (Macronyssidae)
Northern fowl mite (Ornithonyssus sylviarum)	Poultry workers [62]

Note: Updated from Baur et al. 2014 [7].

4.1.1. House Dust Mites

There is increasing evidence that HDMs (Dermatophagoides spp. and Blomia spp.) can cause OA/OR. Several studies have reported sensitized workers and high levels of domestic mite allergens in occupational settings, such as schools and day care centers, but clinically relevant exposures can also be found in other workplaces, including poultry farms and office environments [4, 63]. Due to their wide‐scale preponderance, HDMs have been identified in various geographical locations globally and increasingly associated with certain workplace settings (Table 1).

4.1.2. Storage Mites

The main species of storage mites that have been implicated in OA/OR are Lepidoglyphus destructor, Acarus siro/farris, Tyrophagus putrescentiae, and Glycyphagus domesticus (Table 1). Allergy to storage mites has been reported in various contexts, including occupations where hay and grain are handled, stored, or processed, such as agricultural workers, farmers, millers, and laboratory animal workers. Work in the food processing industry, such as bakers, ham, poultry, and dairy workers, has also been connected to OA/OR due to storage mites [40, 64, 65]. A certain degree of moisture in the stored food (> 15%) plays a decisive role in the presence of storage mites and the sensitization of employees [34]. However, sensitization to storage mites has also been reported without occupational exposure in rural environments [65]. In some Northern European populations, sensitization to storage mite was as frequent as HDM sensitization [66]. While general urban populations have a typically lower prevalence of sensitization (< 10%) [66], levels are higher in certain occupational settings. Danish farmers had a sensitization prevalence of 17% [67], while another study reported changes over time, with sensitization increasing from 5% to 13% over a decade [68]. In studies of European bakers and grain handlers, sensitization varied between 11% and 33% [35, 66].

4.1.3. Spider Mites

Spider mites are outdoor phytophagous mites living on fruit leaves. Commonly found in gardens, greenhouses, and orchards, these mites feed on a wide range of fruit trees, vines, berries, vegetables, and ornamental plants [1]. Multiple case reports and cross‐sectional surveys have identified spider mites as significant allergens causing rhinitis and asthma among fruit farmers and greenhouse workers (Table 1) [7, 69]. The two‐spotted spider mite (Tetranychus urticae) is prevalent in pear farms, greenhouses, and among herbaceous plants, while the European red mite (Panonychus ulmi) frequently infests apple orchards, and the citrus red mite (Panonychus citri) is typically found on citrus farms and orange groves [1, 21, 26]. T. urticae is also a notable outdoor allergen in table grape farm workers [11]. Furthermore, there appears to be a correlation between increased allergy risk to spider mites and high pesticide exposure among crop sprayers [11].

A meta‐analysis and systematic review of twenty‐three epidemiological studies of agricultural farming populations, assessing spider mite sensitization based on skin prick tests (SPT) or specific Immunoglobulin E (sIgE) measurements revealed a pooled sensitization prevalence of 22.9% (95% CI 19%–26.8%), and was as high as 43.9% (95% CI 35.1%–52.9%) when the analysis was restricted to symptomatic patient populations [5]. The study reported a global average monosensitization prevalence of 7% (95% CI 5%–9%), underlining the unique allergenic nature of spider mites. Pooled estimates indicate moderate prevalence of spider mite sensitization in agricultural populations, with subgroup prevalences of 27% (95% CI 20.5%–33.5%) for T. urticae and 18.2% (95% CI 12.4%–24.0%) for P. citri [5]. Therefore, agricultural workers, particularly those in fruit orchards and greenhouses, along with the surrounding rural population, are at increased risk of developing sensitization to spider mites.

4.1.4. Predatory Mites

Predatory “beneficial” mites are increasingly being used for biological control in horticulture. The use of biological pest control has increased by 15% yearly in the past decade [3]. In contemporary times, predatory mites are also used to control plant pests in offices and other indoor environments. Predatory mites are mainly used in protected vegetable and ornamental cultivation systems to control phytophagous mites, thrips, and whiteflies. Amblyseius swirskii, Phytoseiulus persimilis, Neoseiulus/Amblyseius cucumeris, and Neoseiulus/Amblyseius californicus are the most commonly encountered species, accounting for approximately 60% of the entire arthropod biocontrol agent market. The sensitization prevalence to predatory mites among greenhouse workers varies between 15% and 52% (Table 1) [45, 53, 55, 57].

4.2. Risk Factors

The risk factors associated with occupational allergy, rhinitis, and/or asthma due to mite exposures are presented in Table 2. Aside from host factors such as atopy, occupational risk factors such as high‐risk jobs, as well as elevated, frequent, and prolonged mite exposures are the major determinants of risk. Inadequate ventilation and high humidity environments contribute to elevated mite allergen levels [64, 67, 70, 71, 72].

TABLE 2.

Risk factors for occupational allergy, rhinitis, and asthma associated with mites.

Occupational

Individual (host associated)

High‐risk jobs involving exposure to organic dust, mouldy material stored under damp conditions, decomposing organic feed, enclosed agricultural settings with infested plants Exposure to high levels of mite allergens present in dusty environments or use of predatory mites in enclosed greenhouses Prolonged duration or repeated exposure to mites contributing to increasing cumulative exposures to mite allergens Work environments with high humidity levels and poor ventilation Inadequate or improper use of appropriate respiratory protective equipment for certain tasks (e.g., biological pest control operators)

Atopic or pre‐existing sensitisation to mites (e.g., domestic exposures) Pre‐existing bronchial hyperresponsiveness Pre‐existing occupational rhinitis with ongoing exposures increase OA risk Pre‐existing sensitisation to tropomyosin or other cross‐reactive allergens from other sources (e.g., shellfish)

4.3. Emerging Challenge of Climate Change

In the context of climate change and increasing environmental influences, it is likely that mite populations and their favored habitats will change. Global warming increases humidity, which may affect the growth and survival of mites, as the survival of domestic mites in particular is strongly dependent on humidity and temperature [73]. B. tropicalis needs high humidity levels (74%–80%), followed by D. pteronyssinus (60%–65%), and D. farinae (47%–50%). Lower winter temperatures associated with heated homes reduce D. pteronyssinus levels more than D. farinae, since the latter is more resistant to lower humidity and can survive periods of drought. Therefore, D. farinae is more common in homes in the northeastern regions of North America, northern Europe, and Korea. In addition to the geographical distribution and local dominance, mite metabolism could be influenced by climate change, possibly resulting in changes in the proportion and frequency of the proteins/allergens produced. Due to climate change trends, northward expansion and increasing spread of the phytophagous spider mite, Tetranychus evansi (tomato red spider mite) over time [74], should also be considered as an increase in allergic sensitization risk for agricultural workers.

5. Allergen Sources and Exposure Assessment

There has been an increasing interest in evaluating exposure to mite allergens to initiate preventive measures following the 1964 discovery that mites in house dust were causing asthma. While the enumeration of microscopically identified mite species was used initially, the first EIAs (enzyme immunoassays) to estimate mite allergen exposure became available only in 1987. The most commonly used EIAs, even in workplaces, were based on monoclonal antibodies to major allergens (Der p 1, Der f 1, Der p 2 and Der f 2) of the HDMs D. pteronyssinus and D. farinae [47, 59, 75, 76, 77, 78, 79, 80]. The samples collected were mainly reservoir dust obtained through vacuuming surfaces such as mattresses or floors. Single allergen specific EIAs of airborne dust samples were often not sensitive enough for allergen detection [81]. This was due to the fact that allergen‐bearing particles from decomposing mite bodies or their faecal pellets, being relatively large, settle rapidly to the ground in a room where there is no activity and therefore do not remain airborne for long periods of time. Furthermore, since more than 30 mite allergens per species and several major allergens have been identified, single‐allergen‐specific EIAs can only detect a small percentage of mite allergens in dust, which is generally below the detection limit.

In the past two decades, more sensitive assays have been developed that are based on the same monoclonal antibodies for HDM allergens using amplification by polymeric enzyme‐conjugates and detection by fluorescence (FEIA, fluorescence enzyme immunoassay). Fluorescence is also used for detection of indoor allergens (MARIA, Multiplex array for indoor allergens) in the multiplex array, since it detects several different individual allergens simultaneously [82, 83, 84]. However, this improved sensitivity was still unable to determine mite allergens using passive samplers in schools [85]. As an alternative and supplement to the monoclonal antibody‐based EIAs, radioallergosorbent test (RAST) inhibition or polyclonal rabbit antibodies to mite extracts have also been used for assay development [81, 86, 87, 88]. Since numerous allergens and antigens are recognized, detection is often successful even for airborne dust samples. In addition, development of these assays is simpler and faster. For some mite species especially relevant for the workplace, EIAs are not yet available, and detection is still done by counting the microscopically identified mites [86, 89, 90, 91].

Prior to quantification by microscopic or immunological methods, on‐site sampling is required, ranging from collection of reservoir dusts directly or by vacuuming to collect dust samples [88]. An intermediate position is occupied by passive collection of settled dust using electrostatic dust collectors (EDC). Table 3 summarizes the original studies from the last 25 years in which mites or allergen concentrations were determined in dust samples from various workplaces. Due to differences in sampling and quantification methods, comparison of results between studies is challenging. A comparison of the mite allergen load between the workplace and the living area enables a crude estimation of exposure. While the HDM load tends to be rather low in offices and schools, it can reach relatively high levels in day care centers and home sleeping areas. Most floor dusts from US schools were below the detection limit for Der p 1 and Der f 1 in MARIA, containing a maximum of 0.78 μg/g and 1.64 μg/g respectively [83, 84]. EDC samples from Dutch schools also remained below the detection limit for these individual allergens in MARIA [85]. Domestic mite allergens in these samples reached a geometric mean of 133 ng/m²/week, while samples from German day‐care centers had a median of 367 ng/m²/week, which was even higher than samples collected from households at the same time (248 ng/m²/week) [95].

TABLE 3.

Studies of mite allergen exposure assessment conducted in occupational settings during the past two decades.

Sampling location	Method for allergen or species quantification	Samples/material	Sample numbers	Publication year
Bakeries, farms, silos, stables, Spain	Microscopic counting: Dermatophagoides pteronyssinus, Dermatophagoides farinae, Lepidoglyphus destructor, Tyrophagus putrescentiae, Blomia tropicalis, Blomia kulagini, Acarus siro, Corthoglyphus arquatus, Glycyphagus domesticus, Euroglyphus maynei	Reservoir: settled floor dust	10	1997 [89]
Cowsheds, pigsties, barns, chaff‐cutter buildings, poultry houses, Poland	Microscopic counting: Acarus siro, Glycyphagus domesticus, Acarus farris, Lepidoglyphus destructor, Acarus immobilis, Tyrophagus putrescentiae, Lepidoglyphus michaeli, Chortoglyphus arcuatus, Cheyletidae	Reservoir: debris and litter	60	2019 [90]
Pig farms, grain mills, Germany	EIA: Lep d 2, Der p 1, Der f 1, Der 2	Reservoir: settled dust, vacuumed floor	approx. 500	2000 [92]
Cattle stables, Germany	EIAs: Acarus siro, Lepidoglyphus destructor, Tyrophagus putrescentiae, Dermatophagoides pteronyssinus, Der p 1	Passive sampling: EDC	24	2011 [87]
Poultry farms, Croatia	EIA: Der p 1	Reservoir: floor and surface dust	17	2010 [47]
Textile recycling, feather bed filling and cleaning, carpet cleaning, schools	FEIA: Domestic mite, EIA: Der f 1, Der p 1	Reservoir: vacuumed floor dust	147	2012 [81]
Textile recycling, cattle stables, feather bed filling, carpet cleaning, slaughterhouse, Germany		Airborne dust	76
Textile recycling, Germany	FEIA: Domestic mite	Airborne dust	26	2019 [93]
Day care centres, US	Microscopic counting: Dermatophagoides pteronyssinus, Dermatophagoides farinae, Blomia tropicalis, Cheyletus malaccensis, Mesostigarnata, Oribatida	Reservoir: settled dust, vacuumed floor	20	2001 [86]
	EIA: Der p 1, Der f 1	Reservoir: settled dust, vacuumed floor	20
	RAST inhibition, reference extract of Dermatophagoides pteronyssinus	Airborne dust	40
Day care centres, US	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor	86	2005 [78]
Day care centres, Germany	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor and furniture	143	2002 [76]
Day care centres, Germany	FEIA: Domestic mite	Reservoir: vacuumed floor dust, upholstered furniture	1340	2016 [94]
Day care centres, Germany	FEIA: Domestic mite	Passive sampling: EDC	620	2018 [95]
Child care centres, Singapore	MARIA: Der p 1, EIA: Blo t 5	Reservoir: vacuumed floor	123	2008 [82]
Day care centres, Schools, Norway	EIA: Der p 1	Reservoir: vacuumed floor and furniture	> 81, > 155	2005 [96]
Schools, day care centres, US	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor and furniture	204	2009 [80]
Schools, Sweden	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor and furniture, clothes	48	1998 [75]
Schools, Sweden	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor and furniture	46	2007 [79]
Schools, Korea		Passive sampling: Petri dishes	68
Schools, US	MARIA: Der p 1, Der f 1, Der 2	Reservoir: vacuumed floor and furniture	443	2017 [84]
Schools, Netherlands	FEIA: Domestic mite, MARIA: Der f 1, Der p 1	Passive sampling: EDC	117	2014 [85]
Schools, US	MARIA: Der p 1, Der f 1, Der 2	Reservoir: vacuumed floor and furniture	229	2012 [83]
		Airborne dust	118
Schools, offices, Italy	EIA: Der p 1, Der f 1, Der 2	Reservoir: vacuumed floor and furniture	161	2009 [61]
Offices, US	EIA: Der p 1, Der f 1	Reservoir: settled dust, vacuumed floor	251	2005 [77]
Offices, archives, Italy	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor and chairs	160	2004 [97]
Offices, Canada	EIA: Der p 1, Der f 1	Reservoir: vacuumed floor, sieved dust	214	1998 [59]
Offices, Germany	FEIA: Domestic mite	Passive sampling: EDC	436	2022 [98]
		Reservoir: vacuumed floor dust	437
Laboratory, offices, Italy	EIA: Der p 1, Der f 1, Der 2	Reservoir: vacuumed floor and furniture	60	2009 [99]
Offices, day‐care centres, schools, hospitals, shop, bakery, canteen, bus, archive, cowshed, Finland	Microscopic counting: Acaridae, Tarsonemidae Tydeidae	Reservoir: water damaged surfaces	50	2007 [91]

The major allergen Der p 1 also reaches relatively high levels in poultry houses in Croatia (mean 0.78 μg/g) as well as in homes in the same region. Special workplaces, such as textile recycling, are more significantly contaminated by domestic mite allergens [93]. Several storage mite species of the Acaridae, Glycyphagidae, and Chortoglyphidae families have been identified in samples from barns, pigsties, cowsheds, poultry houses, and farm buildings [89, 90]. On average, 3–30 Acarus siro mites, 1–13 Lepidoglyphus destructor mites, and 2–8 Glycyphagus domesticus mites per gram were counted in samples from barns and stables in Poland [90].

6. Mechanisms of Mite Allergy and Asthma

Unique attributes of mites have allowed them to colonise indoor and outdoor environments, producing an unparalleled diversity of allergens and adjuvants, perfectly complemented to elicit both innate and adaptive immune reactions [100]. For example, mites contain several proteases, contained primarily in mite feacal particles but also in shed mite exoskeletons and decaying mite body fragments, which can affect epithelial membrane integrity including the breaching of tight junctions directly or activate protease‐activated receptors [101] and has homology with the lipopolysaccharide‐binding component of the Toll‐like receptor 4 [102]. These intrinsic activities of some mite allergens stimulate key innate immune responses in the skin or airway epithelium leading to the release of proinflammatory cytokines and innate alarmins. Mite allergens (including the three major serodominant allergens Der p 1, 2 and 23) are found in both faecal pellets and mites' bodies so that continuous exposure to these allergens triggers the development of allergic reactions, commonly immediate type‐I hypersensitivity reactions [1, 102, 103].

7. Clinical Manifestations

The clinical manifestations of occupational mite allergy are primarily OR [104] and OA [105, 106], which have the potential to impair workers' quality of life and work ability. In addition, mite allergy may manifest with ocular symptoms and present as a conjunctivitis. Mites have not been documented as triggers of anaphylaxis in the European Anaphylaxis Registry (personal communication Prof. Margaritta Worm).

Typically, the natural progression after sensitization begins with rhinitis, characterised by nasal congestion, sneezing, and itching (sometimes accompanied by conjunctival symptoms), which is a strong predictor of subsequent asthma [107]. OA manifests as wheezing, breathlessness, and chest tightness symptoms that often worsen with continued exposure to allergens.

As is the case with other sensitizers, a period of repeated allergen exposure before the onset of symptoms is required. Nevertheless, given that mite exposure may occur in multiple exposure contexts, and since workers are not typically evaluated for mite sensitization prior to commencing employment, it is possible that pre‐existing IgE sensitization may exist, resulting in a reduced latency period for manifestation of work‐related symptoms.

With regard to OA caused by mites, making the diagnosis can prove challenging due to the potential presence of mites in other locations. With the exception of predator mites and spider mites, other mite species are not principally workplace sensitizers, and patients can become symptomatic due to prolonged exposure at home. However, if mite allergens are found in higher concentrations in occupational rather than domestic settings, patients are likely to experience worsening of their respiratory symptoms in the workplace [1].

8. Diagnostic Approaches

The optimal diagnostic approach for occupational allergy and asthma is to integrate the comprehensive clinical history with objective diagnostic tests [108, 109]. The latter should include evidence of work‐related airway responses, including peak expiratory flow (PEF), non‐specific bronchial hyperresponsiveness, and fractional exhaled nitric oxide (FeNO), where appropriate. Furthermore, evidence of specific sensitization, including SPT, sIgE, and possibly basophil activation test (BAT), is an important test to identify causes of the allergic symptoms. These elements are further supported by specific nasal [110] and conjunctival allergen provocation tests [111], and specific inhalation challenge, which is considered the reference standard for OA [112].

8.1. Skin Prick Tests and Specific IgE Using Mite Extracts

A few manufacturers have developed standardized extracts for SPTs to detect sensitization to common house dust and storage mites. Unfortunately, these tests are not available for several mite species implicated in occupational allergy (Table 1). Therefore, in‐house generated extracts for both SPTs and sIgE determinations would be required to test for most cases of occupational allergy. To our knowledge, D. pteronyssinus and farinae, Euroglyphus maynei, A. siro, L. destructor, B. tropicalis, T. putrescentiae, and Chortoglyphus arcuatus extracts are currently available in Europe, but not in all countries, and the ongoing regulatory process and economic reasons could hinder their production in the future.

Although many companies offer extracts for SPTs for Dermatophagoides species, considerable batch‐to‐batch variations have been detected. Besides, some important allergens may be underrepresented or completely lacking [113]. Additionally, extracts contain a mixture of species‐specific and cross‐reactive allergens, which makes it difficult to determine the disease‐eliciting allergen source. It may often be impossible to distinguish between sensitization caused by mite exposure at work and sensitization at home.

The same applies to sIgE determination of Dermatophagoides species, A. siro, B. tropicalis, L. destructor, T. putrescentiae, G. domesticus, and E. maynei, which can be measured using the ImmunoCAP platform (ThermoFisher Scientific) and Immulite 3G Allergy (Siemens), while Hycor and EuroImmun only offer sIgE measurements for Dermatophagoides species and MacroArray Diagnostics for A. siro and T. putrescentiae.

8.2. Molecular Diagnosis and Allergens Identified

Several mite allergens have been identified and characterized (www.allergen.org), the majority derived from D. farinae and pteronyssinus, B. tropicalis, T. putrescientiae, and L. destructor (Table 4). During the past decade, the use of individual natural purified or recombinant allergens for the diagnosis of mite allergy has increased tremendously. Table 4 provides a complete list of registered allergens and categorizes them based on their biochemical function. Some of these mite allergens are used in singleplex IgE assays or in multiplex assays (macroarrays) that contain a variety of individual allergens from different allergen sources [114, 115]. As for the molecular diagnosis of allergen components, only Der p/f 1, Der p/f 2, Der p 10, Der p 23, Lep d 2, and Blo t 5 can be determined on the ISAC platform. ImmunoCAP can be used for Der p 1, Der p 2, Der p 10, Der p 23, Lep d 2, and Blo t 5. The ALEX platform can be used for Gly d 2, Lep d 2, Tyr p 2, Blo t 5, Blo t 10, Blo t 21, and a more complete panel for Dermatophagoides species (1, 2, 5, 7 10, 11, 20, 21 and 23). Other platforms do not have sIgE reactants for molecular diagnosis. Individual allergens are only available for storage mites and HDMs of the species Dermatophagoides [116].

TABLE 4.

Mite allergens—categorized according to their biochemical function (www.allergen.org).

Allergen group	Biochemical name	Allergen	MW (kDa)
1	Cysteine protease	Blo t 1, Der f 1, Der m 1, Der p 1, Eur m 1, Tyr p 1	25–39
2	NPC2 family; MD‐2‐related lipid recognition (ML) domain containing protein	Blo t 2, Der f 2, Der m 2, Der p 2, Eur m 2, Gly d 2, Lep d 2, Tyr p 2	14.5–16
3	Trypsin‐like serin protease	Blo t 3, Der f 3, Der p 3, Eur m 3, Tyr p 3	23.8–26
4	Alpha‐amylase	Blo t 4, Der f 4, Der p 4, Eur m 4, Tyr p 4	56–58
5	Lipid binding protein	Blo t 5, Der f 5, Der p 5, Lep d 5	~12.5–14
6	Chymotrypsin‐like serine protease	Blo t 6, Der f 6, Der p 6	25
7	Lipid binding protein, Bactericidal permeability‐increasing like protein	Blo t 7, Der f 7, Der p 7, Lep d 7, Tyr p 7	22–25
8	Glutathione S‐transferase	Blo t 8, Der f 8, Der p 8, Tyr p 8	26–27
9	Trypsin‐like serine protease, Collagenase like serine protease	Blo t 9, Der f 9, Der p 9	27
10	Tropomyosin	Blo t 10, Cho a 10, Der f 10, Der p 10, Lep d 10, Tyr p 10	33–42
11	Paramyosin	Blo t 11, Der f 11, Der p 11, Tyr p 11	98–110
12	Peritrophin‐A‐like domain containing protein	Blo t 12	14
13	Cytosolic fatty acid‐binding protein	Aca s 13, Blo t 13, Der f 13, Der p 13, Lep d 13, Tyr p 13	14, 6–15
14	Apolipophorin	Eur m 14	177
15	Chitinase	Der f 15, Der p 15	58.8, 61.4
16	Gelsolin/villin	Blo t 16, Der f 16, Der p 16	53–55
17	Calcium binding protein	Der f 17, Der p 17	53
18	Chitinase‐like protein (contains a C‐terminal peritrophin‐A‐like domain)	Blo t 18, Der f 18, Der p 18	49–60
19	Anti‐microbial peptide homologue	Blo t 19	7
20	Arginine kinase	Der f 20, Der p 20, Tyr p 20	40
21	Lipid binding protein	Blo t 21, Der f 21, Der p 21	13–15
22	NPC2 family; MD‐2‐related lipid recognition (ML) domain containing protein	Der f 22	14.7
23	Peritrophin‐A‐like protein	Der f 23, Der p 23	8, 19
24	Ubiquinol‐cytochrome c reductase binding protein; Cytochrome b‐c1 complex subunit 7	Blo t 24, Der f 24, Der p 24	13–14
25	Triosephosphate isomerase	Der f 25, Der p 25	27–34
26	Myosin light chain	Blo t 26, Der f 26, Der p 26	14–22
27	Serpin serine protease inhibitor	Blo t 27, Der f 27, Der p 27	44–48
28	Heat shock protein Hsp70	Blo t 28, Der f 28, Der p 28, Tyr p 28	45–76
29	Peptidyl‐prolyl cis‐trans isomerase (cyclophilin)	Der f 29, Der p 29	15–28
30	Ferritin	Blo t 30, Der f 30, Der p 30	12–20
31	Cofilin	Blo t 31; Der f 31, Der p 31, Tyr p 31	15–17
32	Inorganic pyrophosphatase	Blo t 32, Der f 32, Der p 32, Tyr p 32	33–35
33	Alpha‐tubulin	Der f 33, Der p 33	44–53
34	Rid‐like protein; enamine/imine deaminase Troponin C	Der f 34 Tyr p 34	16 18
35	NPC2 family; MD‐2‐related lipid recognition (ML) domain containing protein Aldehyde dehydrogenase	Der f 15 Tyr p 35	14 4 52
36	C2 domain‐containing protein Profilin	Der f 36, Der p 36 Tyr p 36	23 14
37	Chitin binding protein (contains 2 peritrophin‐A‐like domains)	Blo t 37, Der f 37, Der p 37	19
38	Bacteriolytic enzyme	Der f 38	15
39	Troponin C	Der f 39	18
40	Thioredoxin like protein	Der f 40	12
41	Putative chitin‐binding protein (contains a peritrophin‐A‐like domain)	Blo t 41	14
42	Na/K‐exchanging ATPase beta‐subunit	Der f 42	36
43	Peroxiredoxin 1	Der f 43	27
44	Peroxiredoxin 2	Der f 44	25

Note: Allergen names and species: Aca s, Acarus siro; Blo t, Blomia tropicalis; Cho a, Chortoglyphus arcuatus; Der f, Dermatophagoides farinae; Der m, Dermatophagoides microceras; Der p, Dermatophagoides pteronyssinus; Eur m, Euroglyphus maynei; Gly d, Glycyphagus domesticus; Lep d, Lepidoglyphus destructor; Try p, Tyrophagus putrescentiae.

Testing with individual allergens may distinguish between cross‐ and co‐sensitization. Furthermore, the use of species‐specific allergens may also enable the identification of the mite species responsible for OA/OR in a patient. A thorough understanding of the molecular sensitization profile could be useful in identifying markers of workplace exposure, but knowledge gaps in this area remain. Zakzuk et al. recently reported a higher prevalence of IgE sensitization to Blo t 21 and Blo t 5 in asthma patients than in non‐asthmatics in Colombia, while Blo t 2 was the most common sensitizer in exposed subjects [117]. This study did, however, not specifically focus on occupational allergy.

Cross‐reactivity between allergens occurs because of shared similar IgE‐binding epitopes between different allergens. These shared epitopes in close or distantly related mite species can lead to the development of allergic reactions. Cross‐reactivity between different mite species has been demonstrated, in particular for the well characterized group 10 and 20 allergens, with their tropomyosin and arginine kinase homologues, sometimes implicated with other arthropod sensitization as detailed below. In addition, several other cross‐reactivities have been partially confirmed [118]. The paramyosin allergen from B. tropicalis (Blo t 11) has been shown to cross‐react with sera of subjects infected with Ascaris lumbricoides [119]. Recent studies among the group 2 allergens [120] (NPC2 family) however identified limited cross‐reactivity of Blo t 2 with Der p 2 and Der f 2 [121]. The mature protein Blo t 2 has only 52% sequence identity to Der p 2 and up to 50% unique IgE‐epitopes but was clinically relevant as demonstrated in 34% binding in a large cohort of mite allergic patients from Singapore. While nine natural occurring isoforms of Blo t 2 were identified, each having less than 3 amino acid variations, only two demonstrated significant IgE binding. This limited cross‐reactivity between some mite species due to the major group 2 allergens, and possible species‐specific sensitization, may lead to underdiagnosis and ineffective management.

A series of allergens have been characterised from different storage mite species, but only the cross‐reactive tropomyosin has been characterised from the poultry mite, Ornithonyssus sylviarum (www.allergome.org). Being one of the most common cross‐reactive allergens, tropomyosin is found in invertebrates, including shrimp, molluscs, mites, mosquitoes, and helminths [122]. Cross‐reactivity has also been demonstrated between Blomia and food allergens from crustacean (Pen m 13) and oyster (Cra g 1) [122]. Furthermore, tropomyosin is a somatic antigen of the helminth Ascaris lumbricoides, which has been reported to be a primary cause of sensitization to HDM (Blomia spp.) in endemic tropical regions. Several IgE‐binding components have also been detected in extracts from spider mites ( T. urticae ) or citrus red mites (Panynychus citri), but these allergens have not been characterised any further [123, 124].

9. General Approach to Clinical Management and Prevention

9.1. Clinical Management at the Individual Level

The management of a patient with OA or OR associated with mite exposure includes environmental interventions based on the outcome of immunological tests of exposed workers. This is primarily aimed at avoiding or reducing exposure to the offending agent, timely pharmacological treatment, assessment of impairment, and optimizing rehabilitation and compensation. Making an early diagnosis is essential for a favorable outcome in the case of OA [125]. Removal from exposure, where possible, is the intervention of choice [126]. Where this is not possible, for example, when there is cross‐reactivity between mites or food products, exposure reduction is an alternative. Exposure avoidance for HDM when it is identified as the primary cause may be difficult given the ubiquity of HDM in domestic and certain workplace settings.

In individuals with IgE‐mediated OA, specific allergen immunotherapy (SIT) [127] may be a useful option when validated extracts are available, as is the case for HDM allergy. A short‐ and a long‐term beneficial effect of SIT in occupational mite allergy, rhinitis, and asthma while the patient remained at work has not been reported. SIT has not been approved for routine management of mite allergy due to storage mites, spider mites, nor predator mites. This is due to the lack of commercially validated extracts for the latter two and very little evidence of clinical efficacy for storage mites.

Patients with co‐existent arthropod or food‐related OA/OR should practice both dietary and environmental avoidance. Workers who have had a previous anaphylactic reaction should be subjected to strict environmental control and surveillance, as well as provided with a written emergency management plan, an adrenaline auto‐injector after being educated on its use.

9.2. Preventive Approaches at the Workplace Level

OA/OR due to specific mites in an occupational setting is preventable through adequate assessment and management of risk factors (Table 2). Workplace occupational health risk assessments are key and should include establishing the degree of exposure, prioritizing the risks, and evaluating the effectiveness of existing allergen control measures [63]. Primary prevention through elimination of exposure to known sensitizing agents may not be possible unless they are introduced in a controlled manner as part of the work process. Removal of mite allergen reservoirs (e.g., vacuuming for HDM) is an effective way to reduce mite allergen exposure. A Cochrane review found that there was little evidence of clinical benefit of using mechanical ventilation with dehumidifiers alone for dehumidification in domestic environments [128]. Hence, a combination of cleaning, dehumidification or air conditioning, and appropriate food storage in very damp climates is advised [1]. Where HDMs are implicated, allergen avoidance measures such as limiting or replacing soft furnishings or using non‐upholstered furnishings, adequate ventilation, and reducing the level of humidity could be used to reduce antigen concentrations [63]. In greenhouses, changes to work methods have been shown to reduce exposure to dust, which could reduce exposure to mite allergens [129]. Furthermore, environmental measurements could be used to evaluate whether controls are working. The utilization of chemical agents to reduce mite levels is generally not recommended.

Since specific exposure standards for various mites do not exist, keeping exposure as low as reasonably possible using appropriate risk management and exposure control strategies is advocated. Strategies for reducing exposure to the causal mite agent/s are specific to the industry and may include better organization of work, modification of food storage practices, working techniques that minimize dust production, mechanization or enclosure of processes, improving ventilation, short‐term use of personal protective devices (e.g., respirators) while performing work tasks with the highest exposures, and general improvement of workplace conditions.

High‐risk workers should undergo regular medical surveillance for early detection of OR/OA. This could include symptom questionnaires, lung function, and SPT/sIgE levels where appropriate. Medical surveillance, especially of workers exposed to specific mite allergens, should be prioritized in the first 2–3 years of exposure since OR/OA occurs more frequently during this period [130]. Atopic workers should not be excluded from occupations at risk due to the low positive predictive value of atopy for the development of OR/OA as well as other ethical considerations.

Finally, workers should be educated on various aspects including improved work practices to reduce exposure, the role of mite allergens in causing acute and chronic symptoms as well as compliance with treatment, where appropriate.

10. Conclusions

In this taskforce report, the current scientific evidence on the health risks associated with mite exposure in occupational environments was reviewed with specific reference to allergen sources, high‐risk exposures in workplaces, identified allergens, and available diagnostic methods. Furthermore, a guidance note for the assessment, management, and prevention of OA and OR in clinical practice is provided.

Further studies are needed to understand the incidence of occupational mite allergy and asthma, the risk factors for diverse occupational settings, climate change factors influencing exposure and health risk, as well as the identification of the major allergens of relevant mite species, their biological non‐IgE‐mediated activities, and their cross‐reactivity (Box 1).

BOX 1. Unmet needs and future research.

Future research that would enable a better understanding of occupational allergy and asthma to mites should focus on:

• Epidemiological studies to better understand the incidence of occupational mite allergy and asthma and their risk factors among workers in diverse occupational exposure settings.

• Understanding the association between pesticide exposure and increased risk of mite allergy, as well as the impact of mite allergens on different demographic groups, including rural residents.

• Quantifying storage mite allergens using standardised sampling methods in different workplaces and climatic regions and comparing the levels in residential environments from these areas.

• Epidemiological and experimental research to enhance the understanding of climate change factors on occupational mite exposure and allergy/asthma risk. This research can contribute to exposure prevention (including control measures) and medical surveillance programmes.

• Elucidating the mechanism of interactions between mite allergens and adjuvants to better understand the innate and adaptive immune responses.

• Understanding the cross‐reactivity patterns between mites found in occupational and domestic settings.

• Considering cross‐reactivity between inhaled mite allergens and ingested shellfish allergens.

• Characterising the molecular profiles in relation to clinical outcomes such as OR and OA.

• Identification and characterisation of major and minor species‐specific allergens causing occupational mite allergy is a prerequisite for ensuring the availability of allergen extracts for component‐resolved diagnostics.

• Understanding the progression from rhinitis to asthma associated with occupational mite exposure. The focus should be on both individual susceptibility and environmental factors, using job‐exposure matrices to estimate risks. Additionally, identifying risk factors and refining diagnostic criteria for occupational mite‐induced asthma.

• Studies aiming at development of immunotherapy for storage, spider and predatory mites.

Author Contributions

All authors contributed to the conceptualization of the work. H.S., M.J., M.R., I.S., S.Q., S.V., A.L.L., and C.V. contributed to writing the first draft of the manuscript. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The authors have nothing to report.