When Josh and Kristine Donovan noticed a bruise behind their daughter's knee, they thought it was an ordinary bug bite — until the mark grew and 5-year-old Kailyn came down with a fever.

Doctors first told the Mendon, Mass., parents that it appeared to be a spider bite and prescribed antibiotics.

But when the girl's condition did not improve, her parents sought answers from a pediatric infectious-disease doctor at the UMass Memorial Medical Center.

The surprising and terrifying diagnosis: It was a bite from a black widow, one of the most venomous spiders in North America.

“I didn’t know what to say,” Josh Donovan told ABC affiliate WCVB. “It’s not something you would expect, especially in Massachusetts.”

He said the UMass Memorial doctor explained that “the black mark on her leg was actually the venom from the spider.”

William Durbin, the specialist who is treating Kailyn, told the Boston Globe that the spider bite's deep color signaled necrosis, or cell injury.

“She had a very distinctive bite, which was very scary for her parents and of course the doctors, too,” Durbin told the newspaper. But, he said, the child is in good health and will make a full recovery.

Still, the Donovans said they want other parents to be aware.

“If you think it's something, just keep looking for an answer,” Kristine Donovan told WCVB. “I kind of had a feeling it was pretty bad, and I just kept kind of pushing to have it checked out.”

She told the Globe that she suspects the spider bit her daughter in their back yard in Mendon, in southeast Massachusetts. “We haven’t gone anywhere, so it had to have probably been in our back yard,” she added.

The Donovans told the newspaper that they plan to have an exterminator treat their home.

The couple could not immediately be reached for comment.

Black widows, which have small, black bodies adorned with red hourglass-like spots, are found in the United States, mostly in the South and West, hidden in outdoor areas such as barns, sheds and woodpiles, according to the University of Maryland Medical Center. Black widow bites are rarely seen in New England, but they are not unheard of, according to the Regional Center for Poison Control and Prevention, which serves Massachusetts.

The bites are described as a “pinprick,” though some people may not feel them at all. Symptoms include muscle cramps and muscle weakness; numbness, nausea and vomiting; trouble breathing; and seizures, according to the University of Maryland Medical Center.

People who suspect they have been bitten by a black widow spider are urged to seek immediate medical help, though death from such bites is rare.

Read more: