House Dust Mites

A major source of human allergens in the home is house dust and its associated mite fauna. Where humidity is sufficiently high, fungi tend to thrive in accumulated dust, providing food for a variety of house-infesting mites that are primarily saprophages or fiingivores. Many of these mites are the same species that infest stored products, nests of rodents and birds, and animal litter. When their populations reach high levels in the home, they can cause acute or chronic allergic reactions commonly known as house-dust allergy. The principal allergenic components in house dust are mites and mite feces, rather than the dust material itself.

As many as 10 families and 19 species of mites have been recovered from house dust in a single urban community (Tandon et al., 1988), reflecting the diversity of mites that occur in that microhabitat. The most important taxa that cause human allergy are members of the Pyroglyphidae, notably those belonging to the genera Dermatophagoides and Euroglyphus. These mites typically comprise 90% or more of the mites found in house dust. The other families of mites commonly associated with house dust are the Acaridae, Glycyphagidae, and Cheyletidae, represented by many of the same species that infest stored products. Four of the more common storage mites found in house dust are Acarus siro, T. putrescentiae, L. destructor, and Glycyphagus domesticus (Fig. 23.20) (Wraith et al, 1979).

The most widespread pyroglyphid species that causes house-dust allergy is the European house-dust mite (D. pteronyssinus), which thrives in floor dust and the surface dust of mattresses. It is regarded as the most frequently encountered house-dust mite, occurring especially in humid coastal areas of western Europe and North America. This was the first mite to be identified as a cause of house-dust allergy in 1966, shortly after the genus Dermatophagoides was first linked to house dust and bronchial asthma. The American house-dust mite,

Carpoglyphus Lactis
FIGURE 23.20 Glycyphagus domcsticus (Glycyphagidae), female with four large, ovoid eggs, ventral view. (From Hughes, 1976.)

D.farinae (Fig. 23.21), tends to be common in drier regions than D. pteronyssinus, such as the more continental-type climates of central Europe and the central United States. It is a frequent inhabitant of dried animal meal (e.g., dog biscuits, poultry feed) and coarsely ground wheat. The common name reflects its more common and widespread occurrence in the United States than in Europe and other parts of the world. The third most common mite known to cause house-dust allergy is Eu-roglyphus maynei (Fig. 23.22). This is a cosmopolitan species frequently implicated in human allergy cases in Europe and Japan. It typically occurs in damper habitats than that of D. pteronyssinus, with which it is often associated.

House-dust mites thrive in environments with relative humidities above 65—70%. These mites are dependent on water vapor as their primary source of water, which they extract from the air. They cannot actively survive more than 6—11 days at relative humidities below 50%. They can, however, endure prolonged dry periods by forming desiccation-resistant protonymphs which can survive for months below the critical humidity for the active stages. Their feeding activity, reproductive rate, and amount of fecal material generated are all directly related to humidity levels (Arlian and Hart, 1992). Populations tend to increase beginning in early summer, reach their highest

FIGURE 23.21 American house-dust mite, Dermatophagoidesfarinae (Pyroglyphidae), female, ventral view. (From Gorham, 1991; courtesy of the US Department of Agriculture.)

levels in early fall, and remain relatively constant during the winter months, reflecting indoor humidity.

The developmental times of Dermatophagoides and Eu-roglyphus species vary with temperature and humidity. Under favorable conditions at room temperature and a relative humidity of 75%, they typically complete a generation in about 30 days. Females do not lay eggs unless the eggs are fertilized and commonly experience multiple matings. They lay one or two eggs per day during their adult life, which usually lasts 30 or more days. Half or more of the mites in dust samples may be represented by eggs, and, if overlooked, this often leads to underestimates of population sizes when only the nymphs and adults are counted (Colloff and Hart, 1992).

Dermatophagoides and Euroglyphus species are sapro-phages which, in the home, feed primarily on fungi growing on the organic components of floor and mattress dust. A number of ingested fungi have been identified in the alimentary tract of house-dust mites, including xerophylic species in the genera Aspergillus, Eurotium, and Wallemia (Hay etal., 1992). Evidence indicates that the mites feed selectively on different fungi and that species differ in

Euroglyphus Maynei
FIGURE 23.22 European house-dust mite Euroglypbtts maynei (Py-roglyphidae), female, ventral view, (From Gorham, 1991; courtesy of the US Department of Agriculture.)

their nutritional value. Some fungi, such as Aspergillus penicillmdes, actually may be detrimental to mite growth and reproduction (Hay etal, 1993). Sloughed skin scales from humans and household pets serve as nutritional substrates for fungi. Mattresses thus provide particularly favorable sites for mites to develop. Human semen associated with sleeping quarters has been shown to be a dietary supplement for house-dust mites and can significantly increase the number of eggs a female produces (Colloff etal., 1989).

House-dust mites occur in greatest numbers in the more humid living quarters of homes frequented by the occupants, notably wherever dust accumulates in bedrooms and living rooms. Mattresses are especially suitable, apparently due to the accumulation of human squamal cells and other skin debris. Under optimal conditions, as many as 5000 mites have been recovered per gram of mattress dust. The type of floor can influence the species and number of mites. In damp houses, carpeted floors contribute to high populations, whereas in drier homes there may be litde difference in the numbers of mites in carpeted and noncarpeted flooring. When humidity levels are high, even floors covered with linoleum and other plastic materials will support relatively high numbers of house-dust mites. In general, however, drier floors and carpets support higher populations of D.farinae than either D, pteronysinnus or E. maynei. E. maynei has the highest humidity requirements, occurs primarily in mattresses and bedding, and is the least likely to be found in carpets. Although some studies have shown that wool carpets have higher numbers of mites than carpets made from synthetic fibers (e.g., nylon), other studies have shown no significant differences between them.

House-dust mites also may occur in fairly large numbers in other situations in the home. In a survey of household fabrics in Germany, 18% of the mites recovered were found in clothing (e.g., suits) hanging in closets (Elixmann et al., 1991). The same mites also may infest improperly stored food products. In one such case in Alabama (USA), an individual experienced sneezing, intense ocular pruritus, and facial edema within minutes of inhaling a puff of dry pizza-dough mix heavily infested with D.farinae (Skoda-Smith et al., 1996).

House-dust mites are now recognized as the primary source of allergens that cause house-dust allergy, especially in children and young adults. The most common clinical manifestation is bronchial asthma, characterized by difficulty breathing, inflammation of the nasal passages, and conjunctivitis. This may be accompanied by atopic eczema in some sensitized individuals. Asthmatic attacks tend to occur at night, especially in poorly ventilated bedrooms with old bedclothes and accumulated mattress and floor dust. Occurrence of symptoms is usually seasonal, reflecting the size of the mite populations. The acuteness of the allergic attacks is directly correlated with the number of mites present.

D. pteronyssinus generally produces the most potent house-dust allergens. However, a portion of individuals of other species, including D.farinae, E. maynei, and certain storage mites, can elicit allergic responses as great as that of D. pteronyssinus. Each species of mite appears to have its own species-specific antigens and allergens, with differences between those associated with the mite body and feces. There is significant cross-reactivity among the antigens of different species. This makes it difficult to determine which mite is involved in individual cases, thereby complicating clinical diagnosis and treatment. Diagnostic tests for mite-induced house-dust allergy include skin tests and bronchoprovocation using commercial extracts of individual mite species. Enzyme-linked immunosorbent assays and radioallergosorbent tests have been developed to help in the diagnosis of house-dust mite allergy cases. However, they tend to be less effective than the traditional skin-prick test in identifying people who are only mildly sensitized to mite allergens in house dust.

Several sampling techniques have been developed to determine if house-dust mites are present in the home and, if so, what species they are and their relative numbers. Most of the techniques entail collecting samples of mattress and floor dust with a vacuum device and examining the samples microscopically for the presence of mites. Various flotation and staining methods can be used to facilitate the process. Another approach is the use of a guanine test as an indirect means of determining the number of mites present. Guanine is excreted in mite feces and serves as a quantitative index of mite numbers, irrespective of the species. The amount of guanine can be measured using high-performance liquid chromatography, providing a simple, rapid method for determining the amount of mite activity in different parts of the home (Quoix etal., 1993).

Under certain exposure conditions, mites may enter natural body orifices, leading to cases of temporary internal acariasis. These cases commonly involve the ingestion of mites with food and the inhalation of airborne mites or mite-contaminated dust via oral or nasal routes. Mites that are swallowed or inhaled can lead to acariasis involving the alimentary tract, whereas mites that are inhaled also can invade the respiratory tract. Cases of mites infesting the urinary tract are rare. For a general discussion of the mites involved, see Ma and Wang (1992).

Pulmonary acariasis, in which mites invade the lungs, occurs most frequently among individuals exposed to mite-infested stored grains and dried herbs. This reportedly can be a serious problem among workers in grain-storage facilities and medicinal-herb warehouses in China (Chen et al., 1990; Li and Li, 1990). Clinical signs and symptoms include cough, expectorated phlegm and blood, difficulty breathing, chest pain, low-grade fever, restlessness, and marked eosinophilia. Pulmonary lesions have been documented on X-ray film as shadows and nodular opacities in lung tissues. The following five families and nine species of mites have been recovered from sputum of affected individuals: Acaridae (Acarus siro, Tyrophagus putrescentiae, Aleuroglyphus ovatus, Sancas-sania berlesei [reported as Caloglyphus berlesei], and what was probably Sancassania mycophaga [reported as Carpo-glyphus mycophagus]); Pyroglyphidae (Dermatophagoides farinae, D. pteronyssinus)\ Tarsonemidae (Tarsonemus granarius); and Cheyletidae (Cheyletus eruditus). It is not clear which of these mites cause the more serious problems. Some species, such as S. berlesei, can thrive in exceptionally damp food stores covered with a film of water and may be able to survive for some time in the lungs.

A Carpoglyphus species (believed to be Carpoglyphus lactis) was associated with a case of pulmonary acariasis in Spain (Taboada 1954), whereas a Tyrophagus species and other unidentified mites were recovered from sputum, bronchial washings, and needle-aspirated lung specimens in routine examinations of patients suffering from respiratory ailments (Farley et al., 1989). Mite eggs, larvae, and adults were found in cytology specimens in the latter study, with evidence of their being surrounded by acute inflammatory cells in several cases.

Human cases of enteric acariasis occasionally are reported in which mites are found in excreta, suggesting their presence in the digestive tract. In most cases they are acarid mites in the genera Acarus, Suidasia, or Tyrophagus. Suidasiapontifica (reported as S. medanensis) was recovered from feces of a woman and two infants in Mexico (Martinez Maranon and Hoffman, 1976), whereas various stages of an Acarus or Tyrophagus species, together with eggs, were recovered from bile of a Romanian patient with chronic cholecystitis (Pitariu et al., 1978). It was concluded that the woman probably ingested the mites with her food and that the mites simply aggravated her preexisting cholecystitis by causing inflammation of her digestive tract until the mites were eliminated with the bile. Other cases of enteric acariasis have been reported in children with chronic digestive disorders in Russia (Prisich et al., 1986).

A few cases of urinary acariasis have been reported, primarily involving acarid mites in the genus Tyrophagus. Two species allegedly recovered from the human urinary tract are Tyrophagus putrescentiae and T. longior (Harwood and James, 1979). Many, if not most, of these cases appear to be misleading and probably involve contamination of containers in which urine was collected or examined. A possible exception was the recovery of unidentified mites in urinary samples from several patients in Romania during acute attacks involving inflammation of the kidneys and urinary bladder (Pitariu et al., 1979). Numerous acarid mites and their eggs were observed in the urinary sediments; others were dead and encrusted with salts. Whether or not contamination of samples can be ruled out in these cases is unclear. Other cases of urinary acariasis have been reported in Japan (Harada and Sadaji, 1925) and China (Chen et al., 1992; Ma and Wang, 1992).

T. longior (Fig. 23.15) occurs primarily in cool, temperate regions of Europe where it infests stored grains, hay, and straw; hay stacks in open fields; cucumber plants, tomatoes, and beets; and poultry litter in broiler houses. Cases of digestive and urinary acariasis in humans involving T. longior have been reported (Harwood and James, 1979).

Stored-products mites in the genus Tarsonemus (family Tarsonemidae) have been reported to be associated

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