Clinical Experience
IgE-mediated reactions
Allergic rhinitis and asthma following exposure to B. mori has been described. (8, 10, 17, 18) However, occupational exposure is the main source of atopy and asthma from silkworms. (19) Occupational hypersensitivity pneumonitis has been reported. (20, 21)
A range of studies, mostly from Japan, have suggested B. mori to be an important allergen in asthma and allergic rhinitis. In a study, patients with asthma and severe allergic rhinitis showed a prevalence of sensitisation to moth (asthma 69%, rhinitis 46.8%) comparable to that of cedar pollen and mite, recognised as the most common sensitisers in Japan. (8, 10, 18) The clinical relevance of IgE antibodies to moth was confirmed with broncho-provocation tests.
Another Japanese study found that a third of 267 Japanese patients with allergic rhinitis have IgE against silkworm. (22) A Japanese study of 80 nasal-allergic medical students showed that 18 (22.5%) were positive to silkworm moth allergen, a positive rate as high as those for Candida or ragweed. Of the symptomatic group of 26 patients, 5 (19.2%) were shown to have positive reactions to silkworm moth; while of the 48 asymptomatic subjects, 6 (12.5%) were positive to silkworm moth. (17)
A Japanese study investigating the prevalence of IgE antibodies to chironomid midges in bronchial asthmatic patients around the Lake Suwa area, in comparison with those of the Matsumoto area (control area), found that of 123 Japanese adult patients with bronchial asthma, 33 (50.8%) were positive to mite and 28 (43.1%) to silkworm. The prevalence to the chironomids was 11 (16.9%) to Chironomus yoshimatsui, 8 (12.3%) to Chironomus plumosus and 3 (4.6%) to Tokunagayusurika akamusi. The number of positive tests to silkworm in the Lake Suwa area was higher than in other areas. (23)
However, other studies suggest that cross-reactivity between silkworm and Chironomid may play a role. In a Japanese study evaluating specific IgE to B. mori and Chironomus yoshimatsui in 51 house-dust-mite-sensitive asthma patients, none of whom had definite histories of exposure to these insects or evidence of insect-induced asthma, 30 (59%) had raised serum-specific IgE to moth and 25 (49%) showed positive IgE antibodies to midge (similar frequency to Japanese cedar pollen, a well-known cause of allergy in Japan). RAST-inhibition studies indicated cross-allergenicity between these two insects, and also the existence of species-specific allergens. (10)
Of 56 randomly-selected Japanese patients with asthma, but without any occupational exposure to insects, on intracutaneous tests 69.6% and 53.6% were positive to silkworm wing and caddisfly wing extract respectively. In addition, 57.1% had positive intracutaneous reactions to a chironomid whole-body extract. Of those with positive skin tests to silkworm, serum IgE to silkworm wings, caddisfly wings, and chironomid whole bodies was detected in 82% (32 of 39), 83% (25 of 30), and 81% (26 of 32) of patients respectively. (18)
Two studies showed occupational asthma and sensitisation to silkworm in 29% and 34% of silk filature workers, with the higher percentage in those who twisted the silk threads. (24, 25) Sensitisation to silkworm needs to be differentiated from sensitisation to silk per se. This is further illustrated by a clinical survey of two silk filatures that showed that 36.2% of the persons engaged in the processing of natural silk were suffering from bronchial asthma, while 16.9% of the total subjects had asthma of occupational origin. Skin-prick tests using crude silkworm cocoon and pupal allergen extract demonstrated that 28.8% of the subjects were sensitive to the silkworm-derived allergens. IgE antibodies specific to both cocoon and pupal allergens were demonstrable in the sera of patients with positive skin reactions and occupational asthma. (24)
In a study from mid-western USA, 43% of a group of asthmatic allergic children were RAST-positive to moth. (2)
Other reactions
B. mori is not only a source of aeroallergens; silk, excrement, dander of silkworm, silk products and even silkworm pupae can cause allergic reactions. (7, 12, 21, 26, 27) Sensitisation to silkworms frequently develops in those who work in sericulture. (26, 12) It is estimated that each year in China there are over 1 000 patients who suffer anaphylactic reactions after consuming silkworm pupae, 50 of whom received emergency room treatment for severe anaphylactic reaction. (28)
Immunoblot analysis has reported a protein of approximately 30 kDa to be the silkworm pupa's major allergen. (7)
A number of case reports are illustrative.
A 37-year-old French national visiting China developed anaphylactic shock after consuming oil-fried silkworm chrysalis. Thirty minutes later, he felt an itchy sensation in his mouth and on his face, accompanied by mild nausea and by flushing and swelling of his face. He began to experience difficulty breathing. The authors also list a summary of 13 prior cases of severe anaphylactic reaction caused by silkworm pupa consumption. (28)
A 21-year-old male was reported to have developed pruritus, urticaria, flushed appearance, hypotension, fainting and loss of consciousness 30 minutes after ingestion of silkworm pupae for the first time. (29) Similarly, a 20-year-old male experienced pruritus, urticaria, a flushed appearance, headache, hypotension, abdominal pain, vomiting and dyspnoea after ingestion of silkworm pupae for the first time. Onset of symptoms occurred within 30 minutes. (30)
Many reports describe adverse reactions occurring following the first ingestion of pupae, as demonstrated in a report of a 19-year old male and two females aged 37 and 54 who developed anaphylaxis after ingesting silkworm pupae for the first time. Onset of symptoms varied between 2 and 3 hours. Symptoms included urticaria, flushed appearance, headache, hypotension, abdominal pain, vomiting and dyspnoea. (31)
Five males and 3 females all aged 9 to 46 years of age were diagnosed with anaphylaxis following the ingestion of silkworm pupae for the first time. Symptoms included pruritus, urticaria, a flushed appearance, angioedema, abdominal pain, vomiting, nausea and dyspnoea. Onset of symptoms occurred within 30 minutes to 4 hours. (32, cited in 28)
Vegetable worm (Cordyceps sinensis), a medicinal fungus (not a worm) used in traditional Chinese medicine, has been shown to be cross-reactive with a protein present in silkworm pupae, with a study showing that patients reacting to the ingestion of B. mori pupae all experience allergic symptoms similar to those produced by the vegetable worm. Four of the five patients showed high levels of a serum-specific IgE to vegetable worm and silkworm pupae extracts. (33)