Acarus siro

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Code: d70
Latin name: Acarus siro
Source material: Whole body culture
Family: Acaridae
Common names: Storage mite, Flour mite, Grain mite

Allergen Exposure

Geographical distribution
See common geographical background to mites in our Scientific Document (link to the right).
Acarus siro is an important agricultural pest and environmental allergen. This Storage mite is 0.2- 0.5 mm in length, with a shiny, soft, cream-white body. The mandibles and the legs are coloured yellow to brown, depending on what the mite eats. The mite’s predominant food source is flour, other cereal products, cheese, hay, and dried fruit. Some authors report that fungi growing in the feed are also consumed by these mites. Development from egg to adult occurs in 10 days at normal room temperature. Adults live for 30-50 days.

Though morphologically similar to other mites, A. Siro does have some distinct characteristics: on the back of the body is an incision between the 2nd and 3rd pair of legs. The males possess tarsal and anal suckers as well as a clearly expressed hook-like extension at the thighs of the first leg pair. The females possess a claw at the end of each foot. Both sexes possess 4 long, dragging hairs on the back end. This set of features is important in that, whereas many mites, collected from both outdoor and stored product habitats, are described in the literature as A. siro, many may in fact belong to a sibling species, A. farris or A. immobilis. The 3 species are difficult to separate morphologically, gene exchange between some of them is possible, and although each species displays environmental preferences, they occur together in some environments (1).

See common environmental background to mites in our Scientific Documents (link to the right).

A. siro and other Storage mites should be considered as possible causes of allergic disorders among farming populations even in northern climates such as Finland, where they have been found in byres and hay and grain storage facilities on farms (2). Storage mites can also cause allergies in individuals in urban homes, even in more-arid environments such as Brunei (in southeast Asia), where this mite was present in house dust, resulting in positive skin reactions in 35% of asthmatics. This demonstrated persuasively that Storage mites are significant allergens in such climates (3).

Unexpected exposure

Storage mites may contaminate processed food. Seven categories of cereal-based food products purchased at food retail outlets in the UK were examined for the presence of mites. Mites were found in 21% of 571 samples examined soon after purchase, and in 38% of 421 samples examined after 6 weeks of storage. Although most of the samples where mites were detected had fewer than 5 mites, a few samples contained more than 20 mites. A single sample contained 428 mites. The most common species recovered were Acarus siro, Tyrophagus putrescentiae, Lepidoglyphus destructor and Glycyphagus domesticus (4). Although the clinical significance of these findings was not evaluated, A. siro and Blomia kulagini present on cheese rind have been reported to result in occupational allergy (5).

See also under Other reactions for intestinal and urinary acariasis.


This mite’s extracts contain a number of allergens, which range in size from 10 kDa to 95 kDa. The 15 and 17 kDa allergens are important, but of these only the 15 kDa allergen has been characterised (6-7). Two allergens have been characterised to date: Aca s 2, a 15 kDa protein, a Group 2 mite allergen (8). Aca s 13, a 14.2 kDa protein, a fatty acid binding protein (FABP) (7,9). rAca s 13, a recombinant allergen (7). rAca s 13, a 14.2 kDa recombinant allergen, corresponds to the similar native 15 kDa protein. rAca s 13 was shown to have 64% sequence identity with Blo t 13 from Blomia tropicalis, as well as homology with several other fatty acid binding proteins from different organisms. rAca s 13 was recognised decisively by 3 of 13 (23%) subjects investigated (7).

Two allergens have been characterised to date:

Aca s 2, a 15 kDa protein, a Group 2 mite allergen (8).

Aca s 13, a 14.2 kDa protein, a fatty acid binding protein (FABP) (7,9).

rAca s 13, a recombinant allergen (7).

rAca s 13, a 14.2 kDa recombinant allergen, corresponds to the similar native 15 kDa protein. rAca s 13 was shown to have 64% sequence identity with Blo t 13 from Blomia tropicalis, as well as homology with several other fatty acid binding proteins from different organisms. rAca s 13 was recognised decisively by 3 of 13 (23%) subjects investigated (7).

Potential cross-reactivity

Stronger cross-reactivity has been reported among L. destructor, G. domesticus, and T. putrescentiae than between these storage mites and A. siro (10-11). Other studies have reported a high degree of cross-reactivity between T. putrescentiae and Acarus siro (12).

Although cross-reactivity of A. siro with other storage and with House dust mites has been shown (10-13), the cross-reactivity between Storage and House dust mites appears to be limited. Even though molecular cloning has demonstrated that the Group 2 allergens from Storage mites (Lep d 2 and Tyr p 2) have more than 40% sequence identity with the Group 2 allergens from Dermatophagoides species, 14 studies have concluded that there is no or poor cross-reactivity between D. peteronyssinus and nonpyroglyphid mites (11,13,15).

Aca s 13 has a 64% sequence identity with Blo t 13 from Blomia tropicalis, as well as homology with several other fatty acid binding proteins (FABPs) from other organisms (7).

Nevertheless, co-sensitisation to Storage mites is a frequent finding in patients sensitised to D. pteronyssinus, and the immunologic responses to the different mite species may be complex (12,16).

A study has reported cross-reactivity among several allergens in Anisakis simplex, A. siro, L. destructor, T. putrescentiae, and D. pteronyssinus (17).

Clinical Experience

IgE-mediated reactions
Acarus siro may commonly induce symptoms of occupational allergy in farmers and bakers (2,13,18-23), but sensitisation has also been found in non-farming populations, as described below (7). Furthermore, a study in Denmark concluded that in humid and temperate regions of Europe, allergy to Storage mites in farmers is not caused exclusively by occupational exposure; damp housing conditions and indoor domestic exposure to Storage mites may also be important (20). In a study of the prevalence of Storage mite allergy in 440 farmers with respiratory symptoms on Gotland, an island in the Baltic Sea, allergy to at least 1 of 4 storage mites was found in 52 of the subjects (12%). The corresponding prevalence among farmers with hypersensitivity symptoms was 15.4%, and among those with possibly IgE-mediated symptoms, it was as high as 37.8% (18).

A study of Wisconsin dairy farmers found that the most prevalent allergies were to House dust mites (21.6%), Storage mites (11.2%), grain smuts (11.2%), Cladosporium (7.5%), Aspergillus (6.0%), and Cattle (5.2%). Among Storage mites, the most prevalent sensitisation was to L. destructor (7 of 8), followed by T. putrescentiae (6 of 8), G. domesticus (5 of 8), Chortoglyphus arcuatus (5 of 8), and A. siro (2 of 8) (21). Serum samples from 600 people, randomly selected from a 1-day submission of approximately 3,000 samples from a southwestern Ohio population to a clinical diagnostic laboratory, were screened for IgE to allergens of L. destructor and A. siro. Thirty-two (5.3%) of the 600 serum samples screened had IgE to allergens from at least 1 of the 2 mite species; 14 (2.3%) and 20 (3.3%) had serum IgE to proteins of the mites A. siro and L. destructor, respectively (24).

Among 136 eastern Polish farming students, skin reactivity to L. destructor was found in 18.4%, to T. putrescentiae in 15.4%, to D. pteronyssinus in 14.0%, and to A. siro in 13.2% (25). In a study of 14 Polish farmers, of whom 19.2% complained of work-related skin symptoms, mostly related to working with Hops (11%), grain (5.6%), hay (5.5%) and straw (4.1%), sensitisation to A. siro was found in 9.6%, to L. destructor in 17.8%, and to T. putrescentiae in 13.7% (26).

The prevalence of mite sensitisation among 4,379 patients residing in a cereal industrial region of Spain was found to be 18.96%. Sensitisation to Storage mites among mite-sensitive patients was 11.88%. In 50 selected patients, the most frequent sensitisation was to D. pteronyssinus (58%), followed by D. farinae (48%), L. destructor and T. putrescentiae (38%), B. kulagini (34%), and A. siro and C. arcuatus (24%). Importantly, 22% of the patients not sensitised to Dermatophagoides species were found to be sensitised to Storage mites (27).

Forty-four percent of 139 workers in 4 grain elevator stores in Aalborg, Denmark, reported pulmonary symptoms; 31% complained of work-related respiratory symptoms. Although only 6.4% were diagnosed with respiratory Storage mite allergy, 15.9% were sensitised to Storage mites. Examination of samples of grain and dust revealed Storage mites (in particular A. siro, L. destructor and T. putrescentiae) in 73% of grain samples, while all dust samples contained mites (18).

In a study of bakery workers and salt packing workers in the United Kingdom, 42% of both groups were found to be atopic, and 33% had skin-specific IgE sensitisation to at least 1 of 4 storage mites (L. destructor, G. domesticus, T. putrescentiae and A. siro) (28).

In an investigation of the prevalence of various domestic mite species in Kutahya, Turkey, an 18.05% prevalence of domestic mites was found. The following species were identified: Tyrophagus putrescentiae (43.96%), Dermatophagoides pteronyssinus (31.03%), Acarus siro (13.79%), Lepidoglyphus destructor (1.72%), and Glycyphagus domesticus (2.58%). A very high rate of A. siro was found in July (29).

In a study of bakers, IgE antibodies to flour was related to IgE antibodies to A. siro and L. destructor, but not to T. putrescentiae. Almost all bakers sensitised to flour were also sensitised to Storage mites (6 of 7). The authors suggested that if a baker became sensitised to flour, he would be more prone to develop IgE antibodies to Storage mites too (18).

Occupational allergy as a result of A. siro and Blomia kulagini present on cheese rind has been described (5).

Sensitisation to Storage mites may also occur among urban populations (3,12, 30-37).

In an early study in France, of 248 allergic urban children, 44 (21%) were moderately or strongly positive on skin-specific IgE tests to A. siro.38 In Brunei (in south eastern Asia), A. siro was found to be present in house dust and resulted in positive skin specific IgE reactions in 35% of asthmatics, demonstrating that Storage mites are significant allergens in this climate also (3).

Similarly, in 512 patients with rhinitis and/or asthma, living in urban or rural areas of central Germany, 103 (20.1%; 77 urban dwellers and 26 farmers) were found to be sensitised to at least 1 of the Storage mites. The study concluded that sensitisation to Storage mites in Germany was frequently connected with sensitivity to D. pteronyssinus, and that sensitisation to Storage mites was more prevalent in rural than in city dwellers (39).

In the warm, humid climate of Cuba, 148 Cuban asthmatic patients with a clinical history of asthma and possible mite allergy were evaluated for sensitisation. Skin reactivity was found to D. siboney in 88%, to D. pteronyssinus in 87%, to A. siro in 85%, to B. tropicalis in 85%, and to D. farinae in 83% (33).

In a study of 100 children living in Mexico City, who had a history of mild and moderate asthma, 7% were found to be sensitised to A. siro (34).

In Barcelona, Spain, 39 of 356 (11%) children were sensitised to Storage mites (Acarus siro, Lepidoglyphus destructor and Tyrophagus putrescentiae). However, only 3 children were sensitised exclusively to Storage mites, the rest being sensitised to House dust mites as well. The most prevalent Storage mite sensitisation was to L. destructor (35). In a second Spanish study, of 50 children with rhinitis and asthma, no skin sensitisation to Storage mite was found without concomitant sensitisation to House dust mites. A. siro sensitisation was found in 4% (40).

In Russia, in a group of patients with allergic disease and sensitisation to D. pteronyssinus, 80% were sensitised to D. farinae, 55% to E. maynei, 45% to A. siro, and 35% to L. destructor (41).

In a study of 196 individuals from an urban environment who were not occupationally exposed to Storage mites, IgE antibodies to D. pteronyssinus was found in 24%, and in 14% to at least 1 of 3 Storage mites: A. siro, L. destructor, and T. putrescentiae (37).

In a Bavarian study using sera of patients with known House dust mite allergy, a sensitisation prevalence of 7% was shown for A. siro, 17% for L. destructor, 3% for T. putrescentiae, and 13% for G. domesticus. The authors concluded that Storage mites are important allergens in allergic rhinitis and that routine testing of patients with perennial rhinitis should be undertaken (36).

In Valdivia, Chile, of 100 consecutive asthmatic paediatric patients evaluated, 80 were confirmed to have skin-specific IgE to at least 1 mite species. All patients with skin specific IgE for mites were positive to D. pteronyssinus, 99% to D. farinae, 92% to Euroglyphus maynei, 80% to Lepidoglyphus destructor, 73% to Tyrophagus putrescientae, 72% to Blomia tropicalis, 70% to A. siro and 68% to Chortoglyphus arcuatus. All of the patients with severe persistent asthma, 85% of those in the moderate group, and 73% of those in the mild group had skin-specific IgE to mites. Ninety-five percent of patients with asthma and allergic rhinitis were shown to have skin-specific IgE to mites, along with 92% of patients with asthma and eczema, and 100% of patients with asthma, allergic rhinitis and eczema (42).

Other reactions
Intestinal and urinary acariasis has been described. In a study of 1,994 individuals, skin-specific IgE tests were positive in over 9%. Mite was positive in 4.61% of stool samples, 1.86% of urine samples, and 1.60 % of both. The mites from stool and urine samples comprised a number of species, including A. siro. The species of mites in stool and urine samples were consistent with those separated from the working environment. The frequency of mite infection in individuals correlated with occupation among medicinal herb storehouse workers (15.9%), rice storehouse and mill workers (13%), miners (3.3%), railway workers (2.5%), pupils (5%), and teachers (2.6%) (43).

Systemic anaphylaxis can occur after the ingestion of heated or unheated mitecontaminated foods. This problem may be more prevalent in tropical and subtropical countries than previously recognised. The most common symptoms following the ingestion of mite-contaminated flour were breathlessness, angioedema, wheezing, and rhinorrhoea, and these started between 10 and 240 minutes after eating (44).

Compiled by Dr Harris Steinman,


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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.