People may use the term caterpillar rash to describe when the skin reacts to contact with certain species of butterfly or moth caterpillars. Most reactions are harmless and will resolve on their own, but some cases can be more severe.

Caterpillar rash, or lepidopterism, occurs when people develop a response to contact with either butterflies or moths in their larval form, or caterpillars.

This usually appears as a rash on the skin, but people can also develop a systemic response, which is when the reaction spreads into the body, causing indirect symptoms such as nausea.

Although there are nearly 165,000 different species of caterpillars in the world, there are only 150 types that can cause harm to humans, including 50 species that live in the United States.

The asp caterpillar, Megalopyge opercularis, is the most dangerous caterpillar in the U.S. It is the larval form of the flannel moth. People may also refer to it as the puss caterpillar.

Most of the time, caterpillar rash is not clinically significant and will resolve by itself. The exceptional case is lonomism. This develops when people come into contact with caterpillar species of the genus Lonomia, which are native to South America. Lonomism requires immediate medical attention.

This article discusses the symptoms, causes, diagnosis, and treatment of caterpillar rash. It also offers advice for preventing caterpillar rash.

An adult flannel moth resting on a leaf, its caterpillars may cause caterpillar rash 1Share on Pinterest
ePhotocorp/Getty Images

As the name suggests, the main sign of caterpillar rash is hives or a rash that appears on the skin. There are several different types of rash that may develop depending on the species causing the symptoms. The appearance of the rash may be:

  • red
  • raised
  • bumpy
  • bruised

Other symptoms of caterpillar rash can include:

With lonomism, a person may also develop the following internal symptoms:

Most of the time, symptoms resolve themselves within 24 hours of a sting (when a caterpillar hair penetrates the person’s skin).

However, Lonomia caterpillar species contain a more powerful venom, which can cause internal bleeding. Symptoms can last for 6 months after contact with this caterpillar.

It is rare, but in some situations, exposure to caterpillar venom can cause anaphylaxis. This causes the following symptoms of severe allergy:

A person should call 911 if they think someone is experiencing anaphylaxis.

The cause of caterpillar rash is exposure to caterpillars. Exposure varies according to the place and season, but key risk factors include working in jobs that involve climbing trees or interacting with plants. Children may also be more likely to touch or pick up caterpillars.

Additionally, the risk of contact between harmful caterpillars and bare skin is higher if people do not cover up their skin with protective clothing such as long sleeves and gloves.

Healthcare professionals use differential diagnosis to identify the cause of caterpillar rash, meaning they assess the different possible causes of symptoms before coming to a conclusion.

There are no tests for diagnosis, so healthcare professionals rely on medical history and situational awareness, such as if there are likely to be harmful caterpillars in the area.

Conditions to exclude during differential diagnosis for caterpillar rash include the following:

Caterpillar rash cases often go without diagnosis or people reporting them, but research indicates that climate change may be causing increasing numbers of some harmful caterpillar species.

Caterpillar rash typically goes away on its own. Because of this, the goal of treatment is generally to avoid additional stings by removing any caterpillar hairs remaining on the skin and managing symptoms. Treatment options for caterpillar rash include:

Typically, nonsteroidal anti-inflammatory drugs are not effective in reducing pain from caterpillar rash.

If any caterpillar hairs are visible, people can remove them using forceps or adhesive tape.

Learn more about how to treat caterpillar rash here.

Education is one of the main methods of prevention. This includes learning the different caterpillar species that can cause a reaction and when and where they are likely to exist.

People can avoid caterpillar rash by learning how to identify harmful types and avoiding touching them. A person may be safest to avoid contact with all caterpillars.

As well as learning about the species, people can find out where caterpillars are most common in the local area and when their peak season is. This makes it easier to avoid them.

Additionally, people can protect their skin through their choice of clothing. For example, wearing long-sleeved shirts and trousers can help to minimize exposure by reducing the amount of bare skin.

Using pesticides can reduce the prevalence of caterpillars, but the knock-on effects on the environment can have severe consequences. Populations of many birds, land animals, and aquatic species are experiencing a decline due to pesticide use, and insects such as butterflies are important for ecosystems’ survival.

Caterpillar rash develops after the skin comes into contact with a harmful species of caterpillar. The rash can appear in different ways depending on the species causing the reaction. Other, systemic, symptoms may include nausea, headache, and wheezing.

Most of the time, systemic symptoms resolve themselves within 24 hours. Exceptions to this are if people develop lonomism, which results from a particular group of toxic caterpillars, or if someone experiences anaphylaxis.

People can reduce their chances of caterpillar rash by improving their awareness of harmful species to avoid, which types occur in their area, and when peak caterpillar season is. Most caterpillars are harmless, but if someone is unsure, it is safest to avoid contact with all caterpillars.

A person should speak with a healthcare professional if they are concerned about any symptoms they experience after contact with caterpillars. People should call 911 immediately if they think someone is experiencing anaphylaxis.