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Dear Dr. Roach: I always thought that I had a cast iron stomach; however, after a bout of suspected food poisoning about six months ago, I've had two incidences of violent vomiting for about a half hour. Both of the preceding two meals included shrimp, which I have eaten often in the past without a problem. The episodes were just a month apart. In trying to find an answer, I was surprised to read that shellfish allergy can develop suddenly in adults. I'm afraid to eat it again, and I'll follow up with my doctor for an allergy test.

-- M.C.W.

 Dear M.C.W.: Allergies to fish and shellfish are common, seen in about 3% in U.S. adults. The symptoms vary among different people affected. Of all the shellfish, shrimp is likeliest to cause an allergy.

       The usual allergic reaction to shrimp, called the IgE-mediated response, begins within minutes. The skin may show redness or warmth, itching or hives; people may notice tingling or swelling in the mouth or lips; and severe allergic reactions can cause dizziness, fainting, low blood pressure and confusion. It can sometimes lead to cardiac arrest. Severe allergic reactions can be extremely serious.

       Gastrointestinal symptoms can also occur as an immediate symptom, including severe vomiting, diarrhea and abdominal pain. However, stomach symptoms may also occur as a delayed type of allergic reaction. People often consider this to be "food poisoning" and may not recognize it as a possible allergy. These symptoms begin a few hours after eating, and include protracted vomiting or diarrhea.

       Finally, it's possible that you have had bad luck and have had several episodes of diarrheic shellfish poisoning from contaminated shellfish.

       An allergist is the ideal consultant to help sort out whether this really is an allergy. Both blood tests and skin tests may be helpful. I agree with you avoiding shellfish until you get evaluated.

       Incidentally, the ability to vomit and have diarrhea is a remarkable strength of the stomach and rest of the digestive system: You want to be able to get rid quickly of contaminated food.

Dear Dr. Roach: My 53-year-old son carries the cystic fibrosis gene. Will his sons and his nephews carry the gene?

-- C.S.

Dear C.S.: Cystic fibrosis is caused by genetic abnormalities in the CFTR gene on chromosome 7. There are many different mutations that can cause varying degree of severity, and some abnormal genes do not cause disease at all. However, the classic cause is a mutation called F508del, and when people have two copies of this gene, they will have the usual form of cystic fibrosis. 

       People with only one copy of an abnormal gene are carriers -- both copies need to be abnormal to have clinically apparent cystic fibrosis. I'm going to assume your son has one copy of the F508del and one normal gene. A full genetic analysis would be needed to prove that.

       According to classical genetics, half of your son's offspring, both sons and daughters, will carry the abnormal gene and the other half will carry the normal gene. If one of his children has a child with another carrier, that child will have a 1 in 4 chance of having cystic fibrosis, a 1 in 4 chance of having two normal copies of the gene, and a 1 in 2 chance of being a carrier. 

       His nephews have a chance of having the gene ONLY if your son's siblings are also a carrier for CF or has CF themselves. Any siblings should consider consultation with a genetic counselor. 

            Readers may email questions to ToYourGoodHealth@med.cornell.edu.

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