OPM caterpillar's on a oak tree

Dogs, cats and toxic oak processionary moth caterpillars

The hairs on oak processionary moth caterpillars contain a substance that is dangerous to pets and people.

Oak processionary moth (OPM) caterpillars are usually seen from May to July and crawl around in processionary lines, nose-to-tail. They are generally found in the south east of England in the UK, but they could be spreading. 

If you see an OPM caterpillar, it's important to keep your dog or cat away.

OPM cluster in bright sunlight cropped
Oak processionary moth caterpillars clustered on a tree

What is an oak processionary moth caterpillar?

Oak processionary moth caterpillars (OPM) are a species of moth whose caterpillars nest on oak trees during the spring. The hairs on oak processionary moths caterpillars contain an irritating substance called thaumetopoein, which can cause health risks to pets and humans. Their thousands of hairs can easily come away from their bodies and blow around in the wind.

OPM caterpillars are found almost exclusively in oak trees and they get their name from the way they crawl around the branches in a procession. They often cluster together and build white, silk-like nests in the branches or trunks of oak trees. Sometimes they walk along the ground between oak trees in a procession.

Dogs' and cats' noses often keep close to the ground, so they are at risk of getting the caterpillars’ hairs in their nose, mouth or paws when exploring outdoors. Cats are also at risk when climbing trees.


OPM caterpillars are most likely to pose a risk to pets and people from May to July, but it’s best to keep away from them at all times.

What will happen if my pet touches an OPM caterpillar?

If your dog or cat has touched, licked, sniffed, picked up, or tried to eat an OPM caterpillar or their nest while exploring, you might notice symptoms including:

  • excessive drooling
  • swollen tongue
  • conjunctivitis
  • gagging
  • vomiting
  • difficulty breathing
  • an inflamed or swollen mouth

If your dog or cat has come close to or has touched a caterpillar, it is likely that the caterpillar’s hairs have come into contact with your pet. If your dog or cat is suffering with any of the above symptoms or is in discomfort after coming into contact with an oak processionary moth caterpillar, call your vet for advice. 

Serious allergic reactions are rare and they should only experience slight irritation or discomfort, but it's best to keep a close eye on your pet for a couple of days.

Can I protect my pet against OPM caterpillars?

There is no vaccine or spot-on treatment to prevent the caterpillars’ hairs from irritating your dog or cat. The best thing to do is to keep your pet away from caterpillars and their nests if you spot them in time.

Making sure your dog has good recall can help you to call them away if you notice them getting close to an OPM caterpillar.


Do not attempt to move a OPM caterpillar or nest yourself – oak processionary moth caterpillars can also cause health risks to people.

How to recognise an oak processionary moth caterpillar

OPM caterpillar's on a oak tree
Oak processionary moth caterpillar nest

There are some signs that can help you to identify OPM caterpillars.

  • OPM caterpillars have grey bodies, black heads and very long white hairs that are noticeable among the rest of their much shorter hairs
  • They are almost exclusively found on oak trees
  • They move nose-to-tail in a distinctive procession and are often clustered together
  • Their nests are white, silken and dome or teardrop-shaped, and are found on the trunks or branches of the tree. White, webbing, silk-like trails made by the caterpillars will be found near to the nest.

Where am I likely to find OPM caterpillars?

OPM caterpillars have either been found in, or are likely to be found in, the following counties: 

  • Bedfordshire 
  • Berkshire
  • Buckinghamshire
  • Essex
  • Hampshire
  • Hertfordshire
  • Kent 
  • London
  • Surrey 
  • Wokingham 
  • West Sussex

They may also have made their way to other parts of the country, so keep an eye out for them on dog walks.

For more in information on the locations of oak processionary moth caterpillars, take a look at the Forestry Commission map.

What happens if I touch an OPM caterpillar?

OPM caterpillars can also cause health risks to people because of their fine hairs, so it's important not to touch or get close to the caterpillars or their nests. If you need to remove a caterpillar from your pet's coat, we suggest wearing rubber gloves or using a thick wad of tissues to create a barrier between your skin and the caterpillar's hairs.

If you do come into contact with an OPM caterpillar, you may notice symptoms including: 

  • a rash
  • sore, itchy or irritated eyes and throat
  • breathing difficulties

Although unpleasant, symptoms are not usually medically serious and pass in a few days. If you have been affected, you should consult a pharmacist for relief from the symptoms, or a doctor for serious allergic reactions, although these are rare.

What to do if you have spotted an OPM caterpillar

Oak processionary caterpillars are not native to the UK and were accidentally imported from mainland Europe for tree planting schemes.

You can report any sightings of OPM caterpillars to the Forestry Commission by using the Tree Alert online form, to help them minimise the effects on oak trees caused by these caterpillars.

Images and photos: Supplied by Forestry Commission – Ralph Parks and H Kuppen

Page details


• 31 March 2023

Next review

• 31 March 2026

Approved by
Katy Alexander

Veterinary Surgeon MRCVS