Itch mites found a feast in cicada eggs

The mysterious case of itch mites that sent scores of suburbanites to doctors with unexplained welts has been cracked.

Guess what? Periodical cicadas were to blame.

"We know the culprit. We know why they were there," said Ed Zaborski, a research scientist with the Illinois Natural History Survey.

Health officials were puzzled in early August when hundreds of people began showing up in emergency rooms or calling doctors and public health officials complaining of quarter-sized red welts and rashes that appeared seemingly out of nowhere.

It quickly was determined the bites were not dangerous, just incredibly itchy. Microscopic mites were thought to be the cause, though why it happened in the Chicago suburbs remained an elusive question.

Zaborski, a mite expert, was called by the Illinois Department of Public Health to investigate. Using mite outbreaks in Kansas and Nebraska in 2004 and 2005 as a base to proceed, he and other scientists assumed pin oak trees were the source.

The mite in question has been identified as Pyemotes herfsi, the same species that appeared in those states. It was the nature of its sheltered home here that initially stymied investigators, however.

These mites are parasites that need access to an insect host in a sheltered location. In the other states, the mites had been feeding on midge larvae inside raised lumps, known as galls, on oak leaves.

There, the mite was able to reproduce huge populations, Zaborski said.

So his first instinct was to check oak trees in areas where the calls were coming from, mainly DuPage and Cook counties.

"There were very few oak trees at all, let alone pin oaks, in these locations," he said. "It was very clear something was going on, but nobody could find anything."

After another fruitless Friday in the lab, Zaborski left his bag of leaves and twigs and went home, still stumped.

"By Monday, I had welts on me. I'd been bitten," he said. "I started poking around some more. Most of the twigs, if not all of the twigs, had cicada egg nests."

Female cicadas lay eggs on tree branches in "nests" of 10 to 20 eggs each. Since hundreds of eggs can be laid on a single branch, they become an abundant source of hosts in a protected environment. In a sense, it was mite heaven, and their numbers were able to swell to outbreak proportions.

These mites aren't particular to a specific insect host but do need a protected environment. They're here all the time but don't usually have the opportunity to thrive.

"The only reason we saw the numbers (of mites) we did was because there were so many cicada egg nests," Zaborski said. Female mites gave birth to fully developed adults, which became abundant enough to be noticed -- because of the bites -- in early August.

"We did get hundreds of calls about it because people were so itchy and uncomfortable and couldn't figure out what was going on. They knew it wasn't mosquito bites," said Kitty Loewy, spokeswoman for the Cook County Department of Public Health.

The periodical cicada emergence varied by location, as did the appearance of mites.

"We can make a reasonable guess that this won't be an annual event," Zaborski said.

Seventeen years from now may be another matter.