Hospital staffers diagnosed Dante with multisystem inflammatory syndrome in children, or MIS-C, a rare but life-threatening complication of Covid-19 in which a hyperactive immune system attacks a child’s body.

children diagnosed with Covid have developed MIS-C.

About 80 percent of MIS-C patients are treated in intensive care units, 20 percent require mechanical ventilation, and 46 have died.

Throughout the pandemic, MIS-C has followed a predictable pattern, sending waves of children to the hospital about a month after a Covid surge.

At Medical University of South Carolina Shawn Jenkins Children’s Hospital, for example, doctors last month treated 37 children with Covid and nine with MIS-C — the highest monthly totals since the pandemic began.

Although most children who develop MIS-C had been healthy, 80 percent develop heart complications.

Some patients develop heart rhythm abnormalities or aneurysms, in which artery walls balloon out and threaten to burst.

Adults also can develop a post-Covid inflammatory syndrome called MIS-A; it’s even rarer than MIS-C, with a mortality rate seven times as high as that seen in children.

Although MIS-C is new, doctors can treat it with decades-old therapies used for Kawasaki disease, a pediatric syndrome that also causes systemic inflammation?

Chou acknowledges that her study — which found genetic variants in just 17 percent of patients — doesn’t solve the puzzle.

Most children with MIS-C test negative for Covid, suggesting that the body has already cleared the novel coronavirus from the nose and upper airways.

That led doctors to assume that MIS-C was a “postinfectious” disease, developing after “the virus has completely gone away,” said Dr.

Lael Yonker noticed that children with MIS-C are far more likely to develop gastrointestinal symptoms — such as stomach pain, diarrhea and vomiting — than the breathing problems often seen in acute Covid.

In some children with MIS-C, abdominal pain has been so severe that doctors misdiagnosed them with appendicitis; some actually underwent surgery before their doctors realized the true source of their pain.

Yonker, a pediatric pulmonologist at Boston’s MassGeneral Hospital for Children, recently found evidence that the source of those symptoms could be the coronavirus, which can survive in the gut for weeks after it disappears from the nasal passages.

The body breaks down viral RNA very quickly, Chou said, so it’s unlikely that genetic material from a Covid infection would still be found in a child’s stool a month later.

Although the first doctors to treat MIS-C compared it to Kawasaki disease — which also causes red eyes, rashes and high fevers — Arditi said MIS-C more closely resembles toxic shock syndrome, a life-threatening condition caused when particular types of strep or staph bacteria release toxins into the blood.

Toxins released by the bacteria can trigger a massive overreaction from key immune system fighters called T cells, which coordinate the immune system’s response, said Arditi, the director of the pediatric infectious diseases division at Cedars-Sinai Medical Center.

In a typical response to a foreign substance — known as an antigen — the immune system activates only about 0.01 percent of all T cells, Arditi said.

That allows superantigens to activate 20 percent to 30 percent of T cells at once, generating a dangerous swarm of white blood cells and inflammatory proteins called cytokines, Arditi said.

Although multiple studies have found that children with MIS-C have fewer total T cells than normal, Arditi’s team has found an explosive increase in a subtype of T cells capable of interacting with a superantigen.

“He’s back to all these activities,” said DeMaino, noting that Dante’s doctors rechecked his heart six months after his illness and will check again after a year.

Although children who survive Kawasaki disease have a higher risk of long-term heart problems, doctors don’t know how MIS-C survivors will fare