Researchers probe Kawasaki disease causes as infections plunge amid COVID outbreak

TOKYO — Since the coronavirus pandemic began, the number of young children affected by Kawasaki disease in Japan has sharply declined, according to a nonprofit agency that studies the mysterious illness.

The disease, discovered by Tomisaku Kawasaki in 1967, affects mostly children aged under 5, causing inflammation of the blood vessels. Additional symptoms can include redness of the eyes, a deeply red-colored tongue and a full-body rash of the same color. Viewing the sudden change in patient numbers as an opportunity for scientific insight, experts are moving forward with research into its causes.

In serious cases, the illness can cause lumps in the coronary arteries, the blood vessels that transport blood to the heart, damaging the heart muscles. Such lumps form in about 3% of patients and can leave aftereffects.

The number of children affected by Kawasaki disease was on the rise prior to the coronavirus pandemic, reaching around 17,000 in 2019. This figure represented a rate of 370.8 per 100,000 children under the age of 5, the highest rate of any country in the world.

According to a survey by the nonprofit Japan Kawasaki Disease Research Center, the number of patients declined to 11,173 in 2020 and remained under 12,000 in 2021 and 2022. The rate of occurrence also fell to 250.6 per 100,000 people in 2020, dipping below 300 for the first time in eight years, and remained in the 200 range for the following two years.

While mechanisms behind the onset of the disease remain murky, researchers have some theories. For the portion of the population made susceptible to the illness due to some yet-unknown factor, exposure to viruses or bacteria or other microorganisms can act as a trigger for an excessive immune system response. Based on patient surveys, there is thought to be no single particular virus or microorganism to blame.

Research center director Yoshikazu Nakamura, a public health expert and professor emeritus at Jichi Medical University, said that before the pandemic, the month of January saw the highest number of patients, followed by August. In Taiwan, South Korea and other areas, meanwhile, many patients are seen in the early summer. The seasonal variation makes sense if multiple microbes are behind the symptoms, Nakamura explained.

Nakamura pointed to two potential causes for the decline in cases of Kawasaki disease. First, heightened hygienic measures during the spread of COVID-19 that greatly reduced seasonal influenza cases may have halted the spread of the microbes or viruses that trigger the illness. The other is the more home-oriented lifestyle with fewer outings. In some cases, infants’ diseases are caused by viruses or bacteria transmitted from their caregivers. There is the possibility that fewer outings by parents or other closely residing relatives has reduced the number of opportunities for this type of infection.

Despite some patients having made fewer trips to medical institutions during the pandemic, few with Kawasaki disease’s distinctive symptoms such as continuous high fever and red, swollen limbs are thought to have failed to seek medical attention.

Nakamura said, “I’d like to find out and clarify which of the two possible factors was in play by comparing the changes in coronavirus patient numbers over the three years since 2020 with region-by-region breakdowns of the pandemic. In doing so, we may come closer to uncovering the causes of Kawasaki disease. I also want to see the number of patients for this year, as societal activity undergoes massive change.”

(Japanese original by Sooryeon Kim, Lifestyle, Science & Environment News Department)