Shrimp allergy is seen in more than 1% of the Canadian population . Our results demonstrate that almost all of our patients with shrimp allergy were also sensitized to house dust mite. This is similar to small studies on an Asian population which have found a majority of patients with shrimp allergy have positive skin tests to HDM. One demonstrated that patients who identify as shrimp allergic, both those that react on oral food challenge and those who can tolerate shrimp, are almost all positive on skin testing to HDM . Another showed that 72% of shellfish sensitized individuals have skin test positivity to HDM .
Alternatively when looking at shrimp sensitization in those with HDM positivity, the minority show positivity to shrimp . This may be explained by the role of tropomyosin in the shrimp and HDM allergies. Tropomyosin is the major allergen in shrimp [5, 6] and therefore, sensitization to the protein would be likely found in a majority of shrimp allergic individuals. These patients would have a greater potential to cross react with Der p 10 of HDM. This would explain why we, along with other groups, have found a high frequency of HDM sensitization in shrimp allergic patients. When looking at patients with HDM allergy for shrimp sensitization, Der p 10 is not the major allergen in HDM allergy. Therefore, it may be expected that the number of patients who are HDM allergic with shrimp sensitization would be lower, as a minority of patients with HDM allergy are sensitized to the tropomyosin. A previous study found that HDM allergic patients were more likely to be Der p 10 negative with a HDM allergy alone compared to combined HDM and shrimp allergy, which did have higher levels of Der p 10 IgE .
We found there to be an increased odds of having a sizeable HDM skin test wheal size (≥5 mm) if patients also had a shrimp skin test wheal size (≥5 mm) compared to patients with a smaller shrimp skin test wheal size (<5 mm). Interestingly, a previous study showed there was a non-significant smaller wheal size to D. pteronyssinus in patients with seafood allergy than those without seafood allergy . While we did not compare the wheal size for HDM in shrimp allergic patients to those not shrimp allergic, a larger HDM skin test (≥5 mm) was more likely if the patient had a large shrimp skin test (≥5 mm).
Based on the protein homology theory, HDM allergic individuals would be expected to be sensitized to the tropomyosin component of HDM for cross reactivity to occur. We did not have the ability to look into the specific components (Der p 10 and Pen a 1) of our patients’ HDM and shrimp allergy to find sensitization to tropomyosin. Interestingly, there are studies of patients who do not have elevated levels of specific IgE to tropomyosin that are positive to dust mite who develop symptoms after consuming shellfish . This demonstrates that there may be alternative allergenic proteins involved with the cross-reactivity between these two allergens, and some alternative allergens have been identified .
An alternative explanation for the high frequency of HDM sensitization in our patients may be high HDM exposure and coincidental HDM sensitization. Increased HDM exposure may result in increased sensitization to HDM . There is variable evidence for different factors such as humidity or damp house environment as a cause of increased household HDM. A Canadian study assessed this idea, and while there was higher amounts of HDM in Vancouver (which is more humid) compared to Winnipeg, the relative indoor humidity level was not a significant factor for the higher levels of HDM in Vancouver . Our patients, from Southern Ontario, also live in an area of increased humidity. But based on the aforementioned evidence, increased outdoor humidity may not lead to increased HDM sensitization. To further characterize this, we would need to compare a control group (perhaps patients with food allergies other than shellfish) to compare the levels of HDM sensitization in our area.
Our data also allowed us to look for a correlation between size of skin test and severity of reaction to shrimp. There was not a significant relationship, which was confirmed when looking at the shrimp skin test quartiles by ordinal logistic regression. These results are similar to previous studies which found no association between skin test size and clinical reaction severity in food allergy (shrimp was not included in this study) .
One limitation of our study was that it was completed in only one clinic in Southern Ontario with a limited sample size. The findings may differ in a multicenter study. The location of this study may be a factor as high humidity in Southern Ontario was likely a factor in the high proportion of HDM sensitization. Also, we did not perform oral challenges to shrimp in our patients to confirm shrimp allergy. It is likely that some patients would have passed an oral challenge so they would have been excluded from the analysis. And as noted previously, we did not perform serum shrimp specific IgE or HDM specific IgE levels on all of our patients. Further objective data would have been interesting to analyze.
Future research should address a number of questions that this study has generated. We would like to see if our patients would have a resolution of shrimp allergy if they were treated with HDM immunotherapy. There have been previous cases showing that food allergy symptoms to shrimp have resolved after HDM immunotherapy. There is conflicting evidence for development of shrimp allergy with dust mite immunotherapy. One study, where subcutaneous immunotherapy was given to HDM allergic patients who were not sensitized to shrimp, showed that patients did not develop positive skin test or symptoms with shellfish consumption . Another study, assessing specific IgE to Pen a 1 before and after sublingual immunotherapy to HDM, did not find any patients who formed antibodies to shrimp tropomyosin . Alternatively, there is a study that showed an increase in shrimp IgE in some patients after receiving dust mite immunotherapy . Other studies could assess whether different areas of the country have different levels of HDM sensitization in shrimp allergic patients. One may postulate that the coastal areas or more humid areas would have a higher proportion of these patients and dry areas fewer. Also, as previously mentioned, it would be fascinating to look at the HDM sensitization in all food allergic patients and compare this to shrimp allergic patients.