New research links persistent allergies to Parkinson’s risk

Chronic allergic diseases, including asthma and allergic rhinitis (hay fever), may increase the risk of developing Parkinson’s disease by contributing to persistent inflammation in the body, perhaps by maintaining elevated levels of immune cells in circulation, according to a large study from South Korea.

While the mechanisms remain unclear, future studies should explore whether treating allergic inflammation could lower the risk of Parkinson’s or slow its progression, the researchers wrote. The study, “Allergic disease as a risk factor for Parkinson’s disease: a possible role of eosinophil,” was published in npj Parkinson’s Disease.

Neuroinflammation (inflammation in the brain or spinal cord) is a hallmark of Parkinson’s. Allergic diseases can trigger chronic inflammation throughout the body, and evidence suggests this persistent immune activity may also affect the brain. Although inflammation is a normal immune response to threats, uncontrolled inflammation can gradually damage healthy cells. In Parkinson’s, the resulting damage to dopamine-producing nerve cells disrupts motor control and contributes to the characteristic Parkinson’s symptoms.

To explore whether allergic diseases may increase the risk of developing Parkinson’s, the researchers used data from the Korean National Health Insurance Service. They looked at more than 5 million adults aged 40 or older who underwent health examinations in 2009 and were followed through 2019. Those who had been diagnosed with Parkinson’s were excluded.

The researchers focused on three allergic diseases: asthma, allergic rhinitis, and atopic dermatitis. Asthma causes the airways in the lungs to become inflamed and narrowed, making breathing difficult. Allergic rhinitis is an inflammation of the inside of the nose triggered by airborne allergens such as pollen or pet dander, and usually causes sneezing and a runny nose. Atopic dermatitis causes dry and itchy skin.

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Asthma, hay fever raise Parkinson’s risk

Asthma, allergic rhinitis, and atopic dermatitis occurred in 1.1%, 12%, and 0.1% of the more than 5 million study participants, respectively. During an average follow-up of 10.1 years, 44,621 participants developed Parkinson’s. This means that on average, fewer than one adult out of every 1,000 developed Parkinson’s every year during the study. Nearly two-thirds (63.7%) were diagnosed when they were age 65 or older.

Patients with asthma had a 16% higher risk of developing Parkinson’s than those without asthma. Allergic rhinitis increased the risk by 18%. The risk was higher in those with two or more allergic diseases; for example, having both asthma and allergic rhinitis increased the risk by 28%. However, atopic dermatitis was not linked to a higher risk of developing Parkinson’s.

Three or more hospital visits a year also increased the risk of developing Parkinson’s. Other factors contributing to a stronger link between allergic diseases and Parkinson’s included being younger than 65 and using corticosteroids, medications used to suppress the immune response. For other medications, results were mixed.

To better understand the link between inflammation and Parkinson’s, the researchers compared blood samples from 234 adults with Parkinson’s and 468 healthy adults. They measured eosinophils and neutrophils, two types of immune cells that respond by moving into the threatened area of the body. When too many of these immune cells remain in circulation, they can contribute to lasting inflammation.

Patients had higher numbers of both eosinophils (105.9 vs. 57.7 cells per microliter) and neutrophils (3,633 vs. 2,846 cells per microliter) in the blood. They were also more likely to have unusually high numbers of eosinophils (200 cells per microliter or more) compared with healthy adults (11.5% vs. 3.6%), “suggesting a potential association between eosinophils and [Parkinson’s],” the researchers wrote.

“In this large, nationwide population-based cohort study, we found that individuals with allergic airway diseases, including asthma and allergic rhinitis, had an increased risk of developing [Parkinson’s],” the researchers concluded. “The risk was further elevated in patients with more frequent outpatient visits and multiple coexisting allergic conditions, indicating a dose–response relationship.”