i203 Mediterranean flour moth

The Mediterranean flour moth, Indian flour moth or mill moth is found around the world and is a common pest of dry plant produce, especially cereals. It is found in households as well as in mills, warehouses, and processing plants. The Mediterranean flour moth is a major pest of flour mills in particular. Its main habitats are flour and grout mills, corn milling plants, bakeries and any other place used for processing grains or preparing flour products. Mill moth larvae prefer wheat flour, but will also feed on all sorts of grains, cereals, seeds, macaroni, dried fruits, cocoa, nuts and almonds. E. kuehniella occurs in most of the temperate and sub-tropical parts of the world, where average temperatures are around 20°C - 25°C.

The adult moth is pale grey and up to 12 mm long, with dark bands and a wingspan of 16-20 mm. The life cycle takes three to four months under optimal circumstances. Eggs are laid near the products where they feed. A single female may lay up to 562 eggs; when hatched, the larvae become pupae in the food. The larva (caterpillar) is off-white with a darker head, and is about 12 mm long when mature. The caterpillars are often found feeding on flour, cereals, baked goods and other dry grain products in food storage areas. Less often, dried fruits or mushrooms and even peat or rotting wood may be eaten. Larvae feed and then produce silk, creating webs which entwine all the material on which the larvae feed, resulting in solid lumps of food particles, faeces and larval exuviae. The silk may form compact masses (webbing) that may obstruct tubes and chutes in wheat mills, and may even interfere with the normal operations of machinery such as flour sieves. (1)

Adults live for about 14 days, do not feed, and usually fly near the roof of the mill. They fly more actively in the early morning and late afternoon.

The product on which the moth feeds acquires an unpleasant smell and a grey/brown colour from the faeces. The moths grow completely and form pupae within the same products that they infest.

Allergen-exposure can occur at establishments where moths are bred (reared) for pest control, (2, 3, 4) and as a contaminating factor in flour and cereals. (5)

Allergen Description

No allergens have been characterised to date.

Although no allergens have yet been characterised, a number of possible proteins have been identified. The serum of a 36-year-old male baker ­– with IgE-mediated occupational respiratory allergy to Mediterranean flour moth, positive skin-prick test, and the presence of serum-specific IgE to this moth – was shown to have significant specific-IgE binding to at least seven proteins (of 22, 35, 43, 53, 65, 77 and >86 kDa) in an extract of flour moth in immunoblot studies. Less significant proteins of 25, 27, 28, and 31 kDa were also identified. (6)

A study of mixed night-flying moths of Ephestia spp. reported the identification of proteins of around 30 kDa as the most important allergens. (7) Tropomyosin, a protein around 34-36 kDa, has been identified as an important cross-reacting allergen among invertebrates, and the authors speculated whether Tropomyosin may be a relevant allergen in E. kuehniella.

Studies have utilised whole body extract for determining sensitisation. However, the origin of the Lepidoptera allergens has not been clarified and authors have proposed that some important allergens may derive from the powder of the wing. (8)

Potential Cross Reactivity

There seems to be very strong cross-reactivity between the various Lepidoptera species (moths and butterflies), and to some degree also with other insects. Both common and species-specific allergens exist, and one common allergen appears to be the actin-binding protein tropomyosin. (7)

Few studies have explored the specific cross-reactivity of E. kuehniella. Inhibition studies in a cereal worker showed cross-reactivity between Anisakis simplex, Eurygaster austriaca and E. kuehniella. The clinical significance has not been elucidated. (9)

Cross-reactivity has been demonstrated between E. kuehniella and mites in inhibition studies. (6)

Clinical Experience

IgE-mediated reactions

Sensitisation to flour moth allergens has been shown to be quite common in occupational settings, and IgE-mediated reactions can induce both asthma and allergic rhinitis. (5, 10, 11, 12, 13, 14, 15, 16) A number of reports illustrate clinical and laboratory finding in individuals sensitised or allergic to flour moth. A 36-year-old male baker who had worked in a bakery for 16 years was evaluated for the development (over the two previous years, at work) of rhinitis, conjunctivitis, and shortness of breath. These symptoms had become more intense with time. Skin-prick test for flour moth was positive, and an elevated serum-specific IgE for flour moth was measured (1.9 PRU/ml). A number of allergens, described above, were detected in the patient’s serum. Allergen cross-reactivity with mites was demonstrated in inhibition studies. A nasal challenge confirmed an immediate-type allergy to flour moth, and daily variability of respiratory PEF values was observed during workplace exposure. (6) Fifteen asthmatic workers, exposed to cereal dust but in whom sensitisation to cereal allergens was not clear, were evaluated for allergen sensitisation. All 15 were shown to be sensitised to Eurygaster and Ephestia. Although only 2 had detectable levels of serum IgE to these pests, 13 were shown to have a positive skin-prick test and serum-specific IgE to Anisakis simplex, and were asymptomatic after eating fish. Bronchial challenges were positive to Eurygaster in 7 patients, and to Ephestia in 2 patients. Twelve patients had positive skin-prick tests to parasitised flour, but not to pure flour. A patient who had experienced anaphylaxis after ingesting cereal was shown to be sensitised to both flours but not to Anisakis. (17) In a subsequent report, the authors elaborated on a cereal worker who experienced IgE-mediated occupational asthma from contact with Eurygaster austriaca and E. kuehniella. The patient was not sensitised to cereal flour or mites, but skin-prick tests were positive for A. simplex – although he was asymptomatic after eating fish. Skin-prick tests were positive for Eurygaster, Ephestia and A. simplex. Serum-specific IgE was present for all three parasites. Allergenic cross-reactivity between A. simplex, Eurygaster and Ephestia was demonstrated. (9) Moths, including E. kuehniella, are bred for biological pest control purposes, and allergen exposure resulting in sensitisation can occur at these centres. (3, 4) Seven cases of asthma and allergic rhinitis were described in 13 workers engaged in the production of these beneficial arthropods. The subjects experienced asthma and rhinitis when coming into contact with eggs and scales or waste of Chrysoperla carnea, Leptinotarsa decemlineata, Ostrinia nubilalis and E. kuehniella. Onset of symptoms developed after an average exposure period of 18 months. Serum-specific IgE to these various arthropods was positive in all symptomatic subjects (7 of 13), and negative in the others. (3) In a prevalence study of 178 French bakers and pastry workers from small businesses in western France who experienced symptoms of allergy, 65 were evaluated using skin-prick tests and serum-specific IgE: in order of frequency: Dermatophagoides pteronyssinus 36 (57%); Alpha amylase 23 (35%); wheat flour 17 (26%); Saccharomyces cerevisiae 16 (25%); Ephestia 15 (24%). (5) In an early study of patients with a diagnosis of inhalant allergy to insects, based on case histories and skin tests, sera were tested for the presence of specific IgE to antigens from seven insects, including a closely-related genus member, the warehouse moth (Ephestia cautella). The other insects were house fly (Musca domestica), blowfly (Calliphora stygia), common clothes moth (Tineola bisselliella), cockroach (Blattella germanica), carpet beetle (Anthrenus verbasci), and silverfish (Ctenolepisma longicaudata). Approximately one-third of the sera were shown to have specific IgE to extracts from all seven species. More than 50% of the sera were shown to have specific IgE to extracts from 4 insects, and only 3 sera had no specific IgE to any of the insect extracts. The authors suggested that a 'pan-allergy' to insects may occur in subjects who have been sensitised to one or a few insects, and allergenic similarities may extend to at least some other non-insect members of the phylum Arthropoda. (14).

Other reactions

Nil reported.

Last reviewed: June 2022.