Peanut allergy

Signs and symptoms

Symptoms of peanut allergy are related to the action of Immunoglobulin E (IgE) and other anaphylatoxins which act to release histamine and other mediator substances from mast cells (degranulation). In addition to other effects, histamine induces vasodilation of arterioles and constriction of bronchioles in the lungs, also known as bronchospasm. At least 11 peanut allergens have been described.

Symptoms can include mild itchiness, urticaria, angioedema, facial swelling, rhinitis, vomiting, diarrhea, acute abdominal pain, exacerbation of atopic eczema, asthma, and cardiac arrest. Anaphylaxis may occur.

Cause

The exact cause of someone developing a peanut allergy is unknown. One study indicated that exposure to soy milk or soy products was positively correlated with peanut allergies. However, an analysis of a larger group in Australia found no linkage to consumption of soy milk, and said that the appearance of any linkage is likely due to preference to using soy milk among families with known milk allergies. The same NEJ study also indicated that infants who are truly peanut allergic are significantly more likely to have been exposed to one or multiple peanut-oil containing skin creams when they had rashes as infants.

Food allergies seem less common in developing countries. The hygiene hypothesis is an attempt to understand why this is the case. Proponents of the hypothesis say that the relatively low incidence of childhood infections in developed countries contribute to an increased incidence of allergic diseases. The hypothesis may also explain why first-born children are more likely to have an allergic disease. The method used in cooking peanuts may result in the different incidence of allergies in developed nations in the west and undeveloped areas, as peanuts are much more frequently roasted in the west, whereas in other parts of the world peanuts are most frequently boiled, fried, or eaten raw.

A number of specific proteins have been linked to the allergy including Ara h 1, Ara h 2, and Ara h 3.

Timing of exposure

There is evidence that eating peanuts early in life may decrease later allergies. The American Academy of Pediatrics, in response, rescinded their recommendation to delay exposure to peanuts along with other foods. The academy also found no reason to avoid peanuts during pregnancy or while breastfeeding.

Diet during pregnancy

A 2014 study found that peanut consumption by pregnant women without peanut allergies was associated with a decreased likelihood that their children would develop peanut allergies. A 2003 study found no link to maternal exposure to peanuts during pregnancy or during breast-feeding.

Routes of exposure

While the most obvious route for an allergic exposure is unintentional ingestion, some reactions are possible through external exposure. The NEJ study noted that peanut allergies are much more common (about 7 times) in infants who had oozing, crusted skin rashes as infants. Airborne particles in a farm- or factory-scale shelling or crushing environment, or from cooking, can produce respiratory effects in exposed allergic individuals. Empirical testing has discredited some reports of this type and shown some to be exaggerated. Residue on surfaces has been known to cause minor skin rashes, though not anaphylaxis. In The Peanut Allergy Answer Book, Harvard pediatrician Michael Young characterizes this secondary contact risk to allergic individuals as rare and limited to minor symptoms. Some reactions have been noted to be psychogenic in nature, the result of conditioning and belief rather than a true chemical reaction. Blinded, placebo-controlled studies by Sicherer et al. were unable to produce any reactions using the odor of peanut butter or its mere proximity.

Diagnosis

Diagnosis of food allergies, including peanut allergy, begins with a medical history and physical examination. National Institute of Allergy and Infectious Diseases guidelines recommend that parent and patient reports of food allergy be confirmed by a doctor because "multiple studies demonstrate 50% to 90% of presumed food allergies are not allergies."

Skin prick testing

Skin prick tests can be used to confirm specific food allergies. Skin prick tests are designed to identify specific IgE bound to cutaneous mast cells. During the test, a glycerinated allergen extract drop is placed on the patient's skin. The patient's skin is then pricked through the drop. This procedure is repeated with two controls: a histamine drop designed to elicit an allergic response, and a saline drop designed to elicit no allergic response. The wheal that develops from the glycerinated extract drop is compared against the saline control. A positive allergic test is one in which the extract wheal is 3mm larger than the saline wheal. A positive skin prick test is about 50% accurate, so a positive skin prick test alone is not diagnostic of food allergies.

Oral food challenge

The "gold standard" of diagnostic tests is a double-blind placebo-controlled oral food challenge. At least two weeks prior to an oral food challenge, the person is placed on an elimination diet where the suspected allergen is avoided. During the oral food challenge, they are administered a full age-appropriate serving of a suspected allergen in escalating size increments. They are continuously monitored for allergic reaction during the test, and the challenge is stopped and treatment administered at the first objective sign of allergic reaction.

Oral food challenges pose risks. In a study of 584 oral food challenges administered to 382 patients, 48% (253) of challenges resulted in allergic reactions. 28% (72) of these challenges resulted in "severe" reactions, which were defined by the study as a patient having: lower respiratory symptoms; cardiovascular symptoms; or any four other, more minor, symptoms. Double-blind placebo-controlled oral food challenges are also time consuming and require close medical supervision. Because of these drawbacks to the double-blind placebo-controlled oral food challenge, open food challenges are the most commonly used form of food challenge. Open food challenges are those in which a patient is fed an age-appropriate serving of a suspected food allergen in its natural form. The observation of objective symptoms resulting from ingestion of the food, such as vomiting or wheezing, is considered diagnostic of food allergy if the symptoms correlate with findings from the patient’s medical history and laboratory testing such as the skin prick test.

Prevention

Peanut allergy may be preventable by feeding babies who are at high risk foods that contain peanuts when they are as young as four to six months of age.

Treatment

Currently there is no cure for allergic reactions to peanuts other than strict avoidance of peanuts. The principal treatment for anaphylaxis is epinephrine as an injection.

Epidemiology

The percentage of people with peanut allergies is 0.6% in the United States. In a 2008 study, self-reported incidence of peanut allergy was estimated to affect 1.4% of the population of the United States, triple the 0.4-0.6% rate found in a 1997 study. In England, an estimated 4,000 people are newly diagnosed with peanut allergy every year; 25,700 having been diagnosed with peanut allergy at some point in their lives. Peanut allergy is one of the most dangerous food allergies, and one of the least likely to be outgrown.

It is one of the most common causes of food-related death. However, there is an increasing body of medical opinion that the measures taken in response to the threat may be an over-reaction out of proportion to the level of danger: "About 3.3 million Americans are allergic to nuts, and even more—6.9 million—are allergic to seafood. However, all told, serious allergic reactions to foods cause just 2,000 hospitalisations a year (out of more than 30 million hospitalisations nationwide). And only 150 people (children and adults) die each year from all food allergies combined." Media sensationalism has also been blamed.

Frequency among adults and children is similar—around 1%—but at a study shows self-reports of peanut allergy are on the rise in children in the United States. The number of young children self-reporting the allergy doubled between 1997 and 2002. Studies have found that self-reported rates of food allergies is higher than clinically-observed rates of food allergies.

In 2013, Miranda Waggoner reported that the rates in self-reported incidence of the allergy, previously thought to be rare, could not be correlated with medical data confirming the self-reported incidence.

Hypoallergenic peanuts

North Carolina Agricultural and Technical State University received a patent in 2012 for a process to reduce allergens in peanuts 98 percent or more. The process treats roasted peanuts, removed from the shell and skin, with food-grade enzymes commonly used in food processing. The treatment consists of soaking the peanuts in an enzymatic solution. The treatment reduces two key allergens, Ara h 1 to undetectable levels and Ara h 2 by up to 98%. The resulting peanuts look and taste like roasted peanuts. The peanuts are not genetically modified.

The effectiveness of the process was demonstrated in human clinical trials at the University of North Carolina at Chapel Hill, using skin-prick tests.

N.C. A&T has signed an exclusive license for the process with Alrgn Bio. The company announced in October 2014 that batches of peanuts were available to food processing companies for evaluation. It said it would work with food processors and manufacturers to establish the process as "the industry standard for peanuts and peanut-derived ingredients."

Controversy

Sociologist Nicholas Christakis has said that measures taken (especially in schools) to ensure allergic children are not exposed to peanut allergens are disproportional to the actual risk of such exposure. Christakis has also said that popular responses to the danger of peanut allergies "bear many of the hallmarks of mass psychogenic illness."

Research

Immunotherapy, while being studied, is not ready for use in people outside of trials.

In those with mild peanut allergies, gradually eating more and more peanuts resulted in at least some short-term benefits. Due to the amount of evidence being small and the high rate of adverse effects, this is not currently recommended as treatment.

Sublingual immunotherapy involves putting gradually increasing doses of an allergy extract under a person's tongue. The extract is then either spat or swallowed. It is not currently recommended as treatment; however, it is being studied.

Epicutaneous immunotherapy involves giving the allergen through a patch. Trials look promising and are ongoing.

An early trial of injecting escalating doses of peanut allergen was conducted in 1996. However, one participant died seconds after injection from laryngospasm due to a pharmacy error in calculating the dose. The death abruptly ended one of the only studies on injected allergen desensitization to peanut allergies.

In 2011, a study was able to "turn off" peanut allergy in mice by attaching peanut protein to the mice's blood and injecting that into them. It has been successfully used in human studies of multiple sclerosis.