Latin name: Tenebrio mollito
Family: Coleoptera (Insecta)
Common names: Mealworm, Yellow mealworm, Yellow mealworm beetle, Golden grub, Darkling beetle, Live fish bait
Mealworms fall into the following categories:
- Tenebrio molitor – Yellow mealworm
- Tenebrio obscurus – Dark mealworm
- Zophobas morio – Superworm
- Alphitobius diaperinus – Lesser mealworm
An occupational allergen, which may result in allergy symptoms in sensitised individuals.
Allergen ExposureGeographical distribution
Tenebrio molitor is one of the most common pests found in stored grain products. The adults are approximately 13-20 mm in length, and larvae can measure up to 32 mm in length. Mealworm beetles are long, ovoid and slightly flat, and reddish black to black. The head is wide and rounded. Their elytra have many fine lengthwise crenulations. The larvae are yellow to pale reddish brown (1).
The main difference between the Yellow mealworm (Tenebrio molitor) and the Dark mealworm (Tenebrio obscurus) is that the adults of the latter species are somewhat darker and duller in colour.
The Mealworm is not a worm. It is the larval stage (grub) of the Yellow mealworm beetle. Like all insects, this beetle has a hard exoskeleton, 6 jointed legs, 2 antennae, compound eyes, and a body divided into 3 parts (head, thorax, and abdomen). Although the grub looks something like a worm, it has 6 small legs. Larvae may spend up to 2 years in this stage, depending on the environment. Larvae are born in the spring, and most mature by autumn and pupate the next spring. There is thus a single generation per year at most. Individuals may wander far from the food source to pupate, causing homeowners difficulty in locating the source of infestation. The pupa is white or cream-colored, with a large head and a pointed tail, and darkens as it grows.
This species probably evolved in Eurasia, but has been introduced around the world by human activity. Tenebrio molitor thrives in temperate climates and is found in areas where cereal is stored, such as on farms and in the cupboards of homes. Tenebrio molitor is an abundant stored-grain pest in the northern United States (2).
Mealworm adults and larvae are scavengers, and pests of flour, meal and other grain products, especially in the northern United States (3). They also eat decaying animals and plants. Both the larva and the beetle are nocturnal, but they are also active during the day; they attack rugs, clothing, skins, dried plants, and taxidermy mounts. Mealworms live in closets and cupboards, in rotting wood, termite and ant nests, and even desert areas. They prefer dark, cool, moist places – under rocks and logs, for example. The adult beetles can serve as hosts for several species of parasites, including Hymenolepis spp.
Mealworms are used for research and as fish bait. Both the larvae and the beetles are bred as food for animals such as reptiles, amphibians, primates, fish, birds and bats.
Occupational exposure to Mealworms may occur in schoolteachers, mill and grain storage workers, live bait farmers, laboratory personnel, pet store owners, workers in the fishing industry, and individuals feeding Mealworms to their own pets.
More than 15 protein bands have been detected, but they vary according to the life stage of the Mealworm: 6 different IgE binding proteins were identified in the larval extract, 5 in the pupal extract, and 7 in the adult extract; similar IgE binding patterns were noted in the larval and adult extracts (2).
The allergen For t 1, from F. taiwana (biting midge), was shown to have significant sequence identity with triosephosphate isomerase of Anopheles merus, Tenebrio molitor, and Ochlerotatus togoi. The clinical significance of this relationship was not evaluated (4).
RAST inhibition studies have demonstrated immunologic cross-reactivity among the life stages of T. molitor, and also between T. molitor and Alphitobius diaperinus (Lesser mealworm), as well as a slight cross-reactivity with blowfly (2).
Clinical ExperienceIgE-mediated reactions
Mealworm may commonly induce symptoms of occupational allergy in sensitised individuals; the symptoms include asthma, rhinoconjunctivitis, and urticaria. The organism affects fishermen, grain handlers, and pet shop workers (3, 5-8). As the larvae are used as live fish bait, they are an especially common source of allergy for anglers, and for persons with similar, but occupational, exposure (9). The inhaled particulates derived from the exoskeletons of Mealworm larvae are potent sensitisers and can induce IgE-mediated occupational asthma in fish handlers. The same source reports anaphylaxis following the ingestion and inhalation of Mealworm (7).
However, the main group of workers exposed to allergens from Mealworm are workers on live bait farms and resellers of live bait. Immediate-onset asthma, rhinitis, and/or contact urticaria have been reported in 4 of 5 bait handlers, from unwrapping and packaging the larvae in a warehouse (5). The presence of skin-specific IgE to Mealworm extract was demonstrated in all 4 symptomatic workers. Serum-specific IgE against Mealworm antigens was demonstrated in the 2 workers with asthma. Bronchial provocation challenge with Mealworm performed on the 2 workers with asthma resulted in an immediate fall in FEV1 of more than 20% (6).
In a study of 50 live fish bait farm workers, 8 retailers and 18 laboratory workers exposed to live fish bait, sensitisation to live fish bait was found in 24 workers (31.6%), and 7 (9.2%) reported work-related symptoms (3 reported asthma, 5 rhinoconjunctivitis, and 1 contact urticaria). Five were skin-specific IgE test-positive and found to have serum-specific IgE to one or more live fish bait extracts. One was also exposed to live fish bait while fishing. Serum-specific IgE was raised to bluebottle in 3, to bee moth in 3, to Mealworm in 1, and to gusano rojo in 2 (9).
In a study of 14 subjects with respiratory symptoms associated with exposure to live fish bait, skin- and serum-specific IgE tests with Lucilia caesar, Galleria mellonella, and Tenebrio molitor extracts were conducted. Thirteen subjects had asthma, all 14 had rhinoconjunctivitis, and 3 had contact urticaria. Eleven subjects had respiratory symptoms after fishing with live bait, and 3 subjects had symptoms during their work in a commercial fish bait farm. The presence of skin- or serum-specific IgE to L. caesar extract was demonstrated in 13 subjects, to G. mellonella extract in 4 subjects, and to T. molitor extract in 3 subjects. After exposure to live fish bait, 2 subjects had an early asthmatic response, 3 had a late asthmatic response, and 2 had no asthmatic response. Late asthmatic response was associated with a long-lasting increase in bronchial responsiveness (8).
A 24 year-old female employed in a pet shop developed occupationally related asthma, rhinoconjunctivitis and contact urticaria following exposure to Yellow mealworm (Tenebrio molitor Linnaeus), which was being sold as food for birds and reptiles. The presence of Mealworm skin-specific IgE was demonstrated, and a histamine release test was positive to Mealworm wholebody extract (3).
A 26-year-old woman who worked as an animal handler, which included feeding the animals Mealworm beetles that were raised in wheat bran, experienced work-related rhinoconjunctivitis on exposure to this insect. Skin-specific IgE tests with extracts prepared from the larval, pupal, and adult-life stages were positive for the patient and for another individual with allergy to a closely related species of beetle, Alphitobius diaperinus (Lesser Mealworm). The presence of serum-specific IgE to Mealworm was demonstrated (2).
Occupational allergic rhinitis and asthma following exposure to bats in a 31-year-old scientist was reported and found to be caused by sensitisation to the Mealworms that she was feeding them (10).
In an area in which cereal production is the most important industry and where Storage mites, tenebroids, and Cockroaches are frequently found as stored-grain pests, an epidemiological analysis of sensitisation to these stored-grain pests was performed on 4,379 patients. Of these, a group of 50 grain workers were evaluated for sensitisation to 9 genera of mites, Tenebrio molitor and Blatta orientalis. Sensitisation was high for Dermatophagoides pteronyssinus (58%), Dermatophagoides farinae (48%), Lepidoglyphus destructor and Tyrophagus putrescentiae (38%), Blomia kulagini (34%), and Acarus siro and Chortoglyphus arcuatus (24%). Fifty percent of the 50 patients were also sensitised to Tenebrio molitor and 36% to Blatta orientalis (11).
A 47-year-old African-American female elementary schoolteacher presented with itchy, watery eyes, rhinorrhoea, postnasal drainage, and nasal congestion complicated by recurrent epistaxis for 2 months. She had had similar symptoms the previous year from September to May, but was symptom-free during the summer. Her symptoms began within 1 hour after entering the classroom and improved in the evening at home, on weekends, and vacation. A detailed history of her classroom environment revealed the presence of Mealworms that were used to teach the children about life cycles. A skin-specific IgE test was positive and associated with a delayed reaction, starting 8 hours after exposure and persisting for 24 hours. Removal of the Mealworms from the classroom resulted in complete relief of her symptoms (5).
Compiled by Dr Harris Steinman, firstname.lastname@example.org
- Street, R. 1999. "Tenebrio molitor" (On-line), Animal Diversity Web. Accessed November 14, 2005 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Tenebrio_molitor.html.
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