Fact check: Image doesn't show a poisonous caterpillar, but touching it can cause allergic reactions

·5 min read

The claim: Image shows a poisonous caterpillar that can cause anaphylactic shock in people

Out of the tens of millions of insects roaming the planet, caterpillars are perhaps the most beloved. These soft-bodied babies, or larvae, of butterflies and moths have inspired children's books, and their cocooned metamorphosis is often taken as a symbol of change and transformation.

But one social media post is urging caution against some members of the very caterpillar family.

"It's that time of the year again that these caterpillars are back," reads a graphic shared on Facebook on Sept. 29, 2020, that depicts a fuzzy yet barbed black-and-white insect. "Do not touch them, they are poisonous!"

The post warns that touching these caterpillars will cause "your throat to swell shut" because of an allergic condition called anaphylactic shock.

The post also includes an image of a child whose neck and chest are covered in a red rash. The graphic makes no mention of whether contact with the caterpillar can cause skin reactions.

The post has been shared more than 9,000 times, most of those in the last week as it found new life online. USA TODAY reached out to the Facebook user for comment.

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While the caterpillar isn't named anywhere in the post, experts told USA TODAY the fuzzy black-and-white insect is a hickory tussock moth caterpillar, a species native to southern Canada and the northeastern U.S.

USA TODAY could not verify the image of the child with rashes.

The post is wrong. The hickory tussock moth isn't actually poisonous. And alarm over it stretches the truth, experts say.

"(This caterpillar) is not anything spectacular in the sense of something to be alarmed about," entomologist Akito Kawahara, associate professor and curator at the Florida Museum of Natural History at the University of Florida, told USA TODAY.

In some people, touching the caterpillar can cause allergic reactions, but anaphylactic shock – the most severe allergic reaction – is not commonly one of them.

How do allergic reactions occur?

The post describes the caterpillar as being poisonous and leading to a severe allergic reaction. But experts say those are two different things, and one doesn't apply here. The caterpillar isn't poisonous.

Allergens, and the allergic reactions they cause, are typically specific from one person to the next, unlike poisonous substances, which broadly affect many people through a toxic rather than an immune response, dermatopathologist Dr. Eric Hossler of Geisinger Medical Laboratories in Pennsylvania explained.

Allergic, or hypersensitivity, reactions are the result of the immune system's overexaggerated response to what it perceives as a foreign invader. These invaders are typically environmental substances, like pollen or mold, but they are generally harmless to most people.

"There are many different types of allergies, and most of them are very mild," Hossler told USA TODAY.

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Hypersensitivity can progress into something more severe, like anaphylactic shock (commonly called anaphylaxis by the medical community), when an allergen gets directly into the body, Hossler explained.

"(These reactions) can go on to be more severe. ... For instance, a kid gets stung by a bee and the venom causes swelling, trouble breathing and a drop in blood pressure, that would be anaphylaxis," he said.

Caterpillar shown isn't poisonous, contact doesn't result in anaphylaxis

Hickory tussock moth caterpillars aren't themselves poisonous, experts say. Handling them can cause skin irritation, but nothing commonly on par with anaphylaxis.

"(The caterpillar's) hairs cause severe dermatological reactions in some individuals. ... It's not really a toxin or a venom in the normal sense," David Wagner, an entomologist and professor of ecology and evolutionary biology at the University of Connecticut, told USA TODAY. "It's more like a fairly serious allergen for some people."

Hossler agreed, saying the insect's hairs contain an irritating substance that can cause a reaction in some individuals, typically a very mild skin irritation in the form of an itchy rash that fades after a day or two.

What makes the hairs particularly irksome is that they are barbed, a defensive mechanism against would-be predators, Kawahara and Wagner explained. Once the little barbs lodge themselves into the skin – even the eyes in some cases – it typically takes a tweezer or similar instrument to get them out.

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However, to even develop a rash after contact is quite uncommon, said Hossler, who has seen cases associated with the caterpillar.

"Many people handle these caterpillars and other related caterpillars and have no reaction whatsoever," he said. "You almost have to squish it or rub it on your arm to get a reaction... I've picked up many of these and never had any problems."

Developing anaphylaxis is also exceedingly rare, Hossler added.

He said he is aware of one case report where a hickory tussock moth caterpillar spurred anaphylaxis, but typically even if children put the caterpillar in their mouths, the worst reaction they got was irritation of the tongue or the mouth, not anaphylaxis.

"It's theoretically possible (to get anaphylaxis), but if it's happened, it would be exceptionally rare," Hossler said.

Our rating: Partly false

Based on our research, we rate PARTLY FALSE the claim an image shows a poisonous caterpillar that can cause anaphylactic shock in people. Experts say the insect shown, the hickory tussock moth caterpillar, is not poisonous, but its hairs can cause allergic reactions in certain individuals, particularly with excessive touching. Anaphylactic shock, a rare but severe allergic reaction, is exceptionally rare in relation to this caterpillar.

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This article originally appeared on USA TODAY: Fact check: Image doesn't show a poisonous caterpillar

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