Oak processionary caterpillar: How dangerous is it?

Carsten Bindslev-Jensen, Professor at the Department of Clinical Research and Consultant at Department of Dermatology I and the Odense Research Centre for Anaphylaxis (ORCA), Odense University Hospital, answers the five most important questions.

Although the caterpillar has earned its dramatic nickname, it is not the insect itself that poses the greatest risk. The real problem is its microscopic irritating hairs, which can become airborne and be carried by the wind, causing intensely itchy skin rashes even in people who have never touched a caterpillar.

The species has attracted considerable attention in Odense this summer after spreading to several locations across the city. It has also been detected elsewhere on the island of Funen, including Kerteminde Municipality.

Below, Carsten Bindslev-Jensen explains what everyone should know about the oak processionary caterpillar.

1. Why does the oak processionary caterpillar cause skin rashes?

– Carsten Bindslev-Jensen

It is not actually the large, visible hairs on the caterpillar that cause the problem. Instead, the caterpillar is covered in thousands of microscopic irritating hairs measuring just 0.1–0.3 millimetres in length. When disturbed, it releases these hairs into the air.

The hairs are covered with tiny barbs and contain a protein called thaumetopoein. This activates the body's mast cells, which also play a central role in allergic reactions. However, it is important to stress that this is not an allergy.

Instead, it is a toxic reaction that can affect anyone, much like the reaction caused by stinging nettles, jellyfish stings or adder bites.

The first symptoms resemble hives and often disappear after a few hours. However, the inflammatory process in the skin continues, and later the same day many people develop an intensely itchy rash with small bumps.

Without treatment, the itching can persist for several days and may be extremely uncomfortable.

2. Can you develop a skin rash without touching the caterpillar?

– Carsten Bindslev-Jensen

Yes. The irritating hairs are so small that they can be carried several hundred metres by the wind. Naturally, the concentration of hairs decreases with distance from the caterpillars, but direct contact is not required for symptoms to develop.

The severity of the reaction depends largely on the number of irritating hairs a person is exposed to.

3. Is the oak processionary caterpillar dangerous?

– Carsten Bindslev-Jensen

For most people, exposure to the irritating hairs results only in skin reactions.

In rare cases, however, the hairs may be inhaled and trigger an asthma attack. This is seen primarily in people whose asthma is not well controlled, where the hairs can act as the trigger.

If the hairs enter the eye, they can become lodged in the cornea and require urgent assessment by an ophthalmologist.

Cases of anaphylactic shock, in which blood pressure drops suddenly, have also been reported, but these are extremely uncommon. My colleagues in cities such as Berlin and Munich, where the oak processionary caterpillar has been established for many years, encounter such cases only very rarely.

4. What should you do if you develop a skin rash or other symptoms?

– Carsten Bindslev-Jensen

The most effective treatment for the skin reaction is a prescription-only potent topical corticosteroid (group 3). Over-the-counter creams are generally not strong enough, so you should contact your GP to obtain a prescription.

The cream usually begins to work within 24 hours and only needs to be used for a few days, so there is little reason to worry about side effects.

Antihistamines have only a limited effect and mainly help relieve the initial acute reaction.

You can try rinsing the affected skin or using adhesive tape to remove the irritating hairs. However, because the hairs are covered with tiny barbs, it is difficult to remove them completely.

If you develop symptoms affecting your eyes or airways, seek urgent medical attention immediately. If you show signs of anaphylactic shock, call 112 without delay.

5. How can you protect yourself against the oak processionary caterpillar?

– Carsten Bindslev-Jensen

The best way to protect yourself is simply to avoid contact with the caterpillars and with areas where large numbers of irritating hairs may be present.