Latin name: Sitophilus granarius
Source material: Whole insect bodies
An insect, which may result in allergy symptoms in sensitised individuals.
Sitophilus granarius is a common pest in stored grain, and a common kitchen pest.
The grain weevil is a member of the family of beetles known as Curculionidae, or true weevils. It is tiny, and chestnut brown or black. It has an elongated snout on the front of the head, at the tip of which are the mouth parts. Grain weevils are associated with all kinds of whole grain. Besides the grain weevil, there are two other almost indistinguishable species of weevil that are important pests of grain: the rice weevil (S. oryzae) and the maize weevil (S. zeamais). (1)
Females produce numerous eggs six times a year, and deposit them inside grain kernels.
The allergy-causing components persist after processing (cleaning and milling) of the grain. The principal exposure occurs while bakers are exposed to the grain weevil allergens, together with other dust and flour allergen, but also during milling procedures.
At least 23 different proteins have been identified as potential allergens of grain weevil, of which no less than 11 proteins were capable of binding IgE. (2, 3, 4) In an evaluation of Danish bakers skin-prick test-positive to extracts of grain weevil, 54% (n = 66) had elevated levels of IgE against whole-body extracts of grain weevil. At least 11 proteins, denoted Ag 1-11, were capable of binding IgE. Ag 4 seemed to be the most powerful allergen, and was recognised by IgE from all 34 patients. IgE directed against Ag 1 was found in sera from 30 patients (88 %), and IgE directed against Ag 2, Ag 6, Ag 8, and Ag 9 in sera of 22 (64%), 19 (56%), 18 (53%), and 16 (47%) patients, respectively. The remaining allergens bound less than 25% of the sera. All individual sera reacted with at least four different proteins. (2)
No allergens have been characterised to date.
A German study has shown that another member of the genus, S. oryzae, the rice weevil, caused IgE-mediated allergy among people living in a former grain store. This study found several binding components in rice weevil, ranging from 30 to 70 kDa, and also observed cross-reactivity to the grain weevil. (2, 5, 6)
a. IgE-mediated reactions
Although grain weevil can be found in homes, it has been reported primarily as an occupational allergen, (2, 4, 7 )and one of the causes of ‘Millworker’s asthma’, (1, 8) ‘grain-worker's asthma’, (9) and ‘Farmer’s lung’. (7) Hypersensitivity pneumonitis has also been described. (10)
Two laboratory workers, who spent a considerable time each day handling grain infested by grain weevil, developed allergic rhinitis and asthma; one after exposure to weevil dust for only six months, and the other after three years. The former worker also noticed sneezing when she was cooking using flour. One of the workers was so sensitive to weevil dust that she had to leave the laboratory; the other worker, by using an extractor fan and wearing a mask, could remain free of symptoms. Skin-prick testing with extracts of grain weevil faecal dust was positive. Consequently, 100 patients attending an allergy clinic for seasonal hay fever symptoms were also skin-prick tested with extracts of S. granaries, of which 51 were positive. In 18, the wheal size was at least 6 mm in diameter. The authors concluded that the significance of the positive responses in these 18 patients was unexplained. (1, 8)
In view of these two patients, a pilot study was undertaken on 75 volunteer millworkers and controls to determine the extent of sensitivity to grain weevil. A millworker was defined as anyone who worked in a flour mill or mill producing animal feed from mixed cereals, or who worked in grain-storing silos. Skin-testing with weevil and mixed flour extracts found that 57% had a positive response to the weevil extract, and 68% a positive response to the mixed flour extract. In the control group, 34% were positive to the weevil extract and 17% to mixed flour. Of the initial 75 millworkers, 15 were selected for further study, based on a positive skin response to the weevil, and a history of a productive cough and chest tightness and wheezing when exposed to mill dust. The forced expiratory volume in one second (FEV1) showed significant reductions of 20% and 15% in two subjects following inhalation of weevil extract. One worker developed wheezing and cough. Twenty-five of the control subjects with positive skin responses to the grain weevil demonstrated no significant FEV1 changes to a similar provocation inhalation test. (8)
In a more recent South African study that examined mill employees in the maize wet-milling industry to determine sensitisation to common allergens and to maize products, of 75 patients tested for specific IgE, 18 were found to have specific IgE to storage mite and 13 (20%) to storage mite and grain weevil. (11)
A study of 66 Danish bakers whose skin-prick tests were positive to extracts of S. granarius reported that 54% had elevated levels of IgE against whole-body extracts of S. granarius. A range of allergens were detected, and a heterogenous response found with a range of sensitisation to the various allergens. (2) Whether the bakers were symptomatic following exposure to this organism was not reported and the authors cautioned that “as for baker's allergy, it is evident that many other factors in the baker's environment besides grain weevil may cause allergy, including flour components (e.g. wheat or other grain proteins), baking-improving additives (e.g. fungal enzymes), and bakery-infesting pests (mites and other insects).
An earlier study of 440 Swedish farmers evaluating the Phadiatop® reported that 29 were RAST-positive to G. intestinalis, and six to S. granarius. The calculated overall prevalence of a positive RAST to G. inteslinalis and S. granarius in the whole farming population (n= 2578) of Gotland was therefore 5.2% and 1.1% respectively. (12)
b. Other reactions
Compiled by Dr Harris Steinman, firstname.lastname@example.org
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- Bartie C, Calverley AE, Rees D. Sensitisation to maize in the wetmilling industry. Curr Allergy Clin Immunol 2006;19(3):138-41.
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