Thank you for your inquiry.
Unfortunately I cannot give you an evidence-based answer to your questions. The best I can say is that to my knowledge, and according to a literature search, there has only been one reported case of anaphylaxis to fish oil and none to krill oil. I have copied for you below a link to the case report of anaphylaxis to fish oil, and as you can see from reading the article, there is no directly documented relationship between the ingestion of the fish oil capsule and the patient’s previously reported allergy to crab.
Thus, at this point in time, all one can say is that anaphylaxis to fish oil or krill oil is evidently extremely rare. On this basis alone, it would seem that there is very little risk regarding the ingestion of fish oil and krill oil in patients allergic to fish and shellfish respectively.
There are no large scale investigations into this issue, but there is one small study performed by Ray Slavin and associates which was published in Allergy and Asthma Proceedings (abstract copied below) that also gives some reassurance regarding the safety of the ingestion of these oils in patients allergic to fish or shellfish.
Finally, there is an interpretation of Dr. Slavin and associates data posted on the American College of Allergy, Asthma, and Immunology website. I have copied below the link to this interpretation as well.
In summary, there is no definitive answer to your questions, but the absence of any significant number of episodes of anaphylaxis to these oils and the small study done by Slavin, et al., indicate that there is very little risk regarding their ingestion in patients who are allergic to fish and/or shellfish.
Thank you again for your inquiry and we hope this response is helpful to you.
Fish-oil capsule ingestion A case of recurrent anaphylaxis: Canadian Family Physician July 2012 vol. 58 no. 7 e379-e381
Allergy Asthma Proc. 2008 Sep-Oct;29(5):528-9. doi: 10.2500/aap.2008.29.3159.
Are fish oil supplements safe in finned fish-allergic patients?
Mark BJ, Beaty AD, Slavin RG.
Section of Allergy and Clinical Immunology, Division of Immunobiology, Department of Internal Medicine, Saint Louis University School of Medicine, St. Louis, Missouri 63104, USA.
Fish oil supplements are popular alternative medicines. Many manufacturers label their products with the warning "avoid this product if you are allergic to fish." The objective of this study was to determine if finned fish (FF)-allergic patients could safely tolerate fish oil supplements. Six FF-sensitive subjects as determined by history and skin testing were selected. They were skin tested with two different fish oil supplements and given an oral challenge of each supplement 1 hour apart. Vital signs were measured at baseline and at 20-minute intervals after each challenge. Spirometry was measured at baseline and 1 hour after each challenge. Six of six patients with positive skin tests to at least one FF had negative skin tests to both fish oil supplements. All six subjects then had negative oral challenges to both supplements. In this pilot study, FF-sensitive patients tolerated fish oil supplements.
Fish Allergies and Omega-3 Supplements (ACAAI).
Phil Lieberman, M.D.