A Florida mother is speaking out about the dangers of a seemingly harmless backyard pest after her son landed in the emergency room with alarming symptoms. Andrea Pergola, of Land o’ Lakes, said she was in the backyard with her son, Logan, who was picking up tree branches when he brushed up against a caterpillar, Fox 13 reported.

In a lengthy post on Facebook, Pergola said that within seconds, her son, whose age was not revealed, started experiencing painful symptoms on his arm.

“He instantly felt a sharp, stinging pain and his arm went numb. Within 5 minutes he was dizzy, had lost color, was complaining of the worst pain he had ever felt & his eyes weren’t super focused,” Pergola wrote, in the June 16 post.

Pergola said her son had brushed up against a southern flannel moth, one of the most venomous caterpillars found in the U.S. She immediately tried to put garlic on his arm to pull out venom, but it kept getting worse. She drove him to a local emergency room where he grew nauseated and lethargic.

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“He didn’t fight the IV (which would be the norm) because he said whatever would help at that point was fine,” Pergola wrote. “Before they could get the IV in he began to shake as someone who was cold would do, partially from fright and partially from the venom moving through his body.”

Pergola said the red rash spread from his body to his chest, and that he was administered medication within an hour of getting stung.

“Logan did not pick it up; it was on a branch and he brushed his arm against it,” she wrote, as a warning to parents. “Each dot on his arm represents a place where he was stung – well over 20 injection sites. He is a healthy, strong, young man & it knocked him out. I can’t even imagine a small child or elderly person.”

The southern flannel moth is one of the most venomous caterpillars in the U.S., according to the University of Florida entomology and nematology department. It’s found in states from as far north as New Jersey to as south as Florida, and as far west as Arkansas and Texas.

According to researchers, some individuals react to stings more severely than others, with symptoms ranging from headache, fever and nausea to low blood pressure, seizures and convulsions.

“Please if you are stung or your kid is stung get to the ER and let them treat you,” Pergola said. “I can assure you, we didn’t realize he had been stung so many times (the entire site was on fire and then went numb) and normal over-the-counter Benadryl would not have treated this at home.”

Pergola’s post has been shared nearly 400,000 times.