OPM caterpillar's on a oak tree

Cats and toxic oak processionary moth caterpillars

Key points

  • Oak processionary moth (OPM) caterpillars have hairs containing a substance that irritates a cat’s eyes and throat
  • They are usually seen from May to July and crawl around in processionary lines, nose-to-tail
  • Generally found in the south east of England in the UK, but could be spreading

What is an oak processionary moth caterpillar?

OPM cluster in bright sunlight cropped

Caterpillars are oak processionary moth (OPM) at the larval stage of the insect’s life cycle.

They have thousands of hairs which can come off their bodies and are blown about by the wind. The hairs contain an irritating substance, called thaumetopoein, which makes pets and people itchy and can cause other problems (see below).

OPM caterpillars are found almost exclusively in oak trees and you can usually see them crawling around the branches or trunk in lines, nose-to-tail. They often cluster together and build white, silk-type nests in the branches or trunks (not in the leaves) of trees. Sometimes they walk along the ground between trees in this procession, which makes them intriguing to cats. Unlike other caterpillars, they do not live on fences or walls

They have some very long white hairs which are really noticeable among the rest of their much shorter hairs. 

They 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 cat touches an OPM caterpillar?

Cats are inquisitive creatures and love to hunt so they may well come across these critters while in the garden or out exploring. 

As the hairs of the caterpillar contain a hazardous substance, it’s a good idea to keep your pet (and yourself) away from them.

If your cat has touched, licked, sniffed, picked up, or tried to eat caterpillars or their nests while exploring, you might notice symptoms including excessive drooling, swollen tongue, conjunctivitis, gagging, vomiting, difficulty breathing or an inflamed or swollen mouth. Generally symptoms are no more serious than irritating.

Because cats’ noses are just above ground level and they like to climb trees, they tend to get the caterpillars’ hairs in their noses and mouths when they meet one of these creepy crawlies.

If your cat is suffering after coming into contact with an OPM caterpillar, call your vet for advice.

What should I do if my cat sniffs out an OPM caterpillar?

If you spot your cat going close to an oak processionary moth caterpillar or their nest, pick them up quickly and move them away from danger.

If your cat has come close to or has touched a caterpillar, it is likely that the caterpillar’s hairs have some into contact with your pet. If your cat experiences any of the symptoms listed above or is in discomfort, call your vet for advice. Serious allergic reactions are rare, but keep a close eye on your pet for a couple of days.

Can I protect my cat against OPM caterpillars?

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

OPM caterpillar's on a oak tree

Where am I likely to find OPM caterpillars?

OPM caterpillars have either been found in, or are likely to be found in, the following areas. They may also have made their way to other parts of the country however, so do keep an eye out for them on dog walks.

London:  Brent, Bromley, Camden, Croydon, Ealing, Greenwich, Hackney, Hammersmith & Fulham, Haringey, Harrow, Hillingdon, Islington, Kensington and Chelsea, Kingston Upon Thames, Lambeth, Lewisham, City of London, Merton, Newham, Richmond Upon Thames, Slough, Southwark, Sutton, Tower Hamlets, Wandsworth, Waltham Forest, and City of Westminster

Berkshire: Bracknell Forest, Pangbourne, West Berkshire, Windsor and Maidenhead

Buckinghamshire: South Buckinghamshire

Essex: Thurrock

Hertfordshire: Watford

Surrey: Elmbridge, Epsom and Ewell, Guildford, Mole Valley, Runnymede, Spelthorne and Woking

What happens if I touch an OPM caterpillar?

People can develop rashes and sore, itchy or irritated eyes and throat, and breathing difficulties from exposure to OPM caterpillars. 

Experts strongly advise people not to go near or touch caterpillars or their nest because of the health risk from their fine hairs.

If you need to remove a caterpillar from your cat’s coat, we suggest wearing rubber gloves or using a thick wad of tissues to create a barrier between your skin and the caterpillars’ hairs. 

The effects of OPM contact on people, although unpleasant, are not usually medically serious and pass in a few days. People who have been affected should consult a pharmacist for relief from the symptoms, or a doctor for serious allergic reactions, although these are rare.

Spotted an oak processionary moth caterpillar?

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

You can report any sightings of OPM caterpillars to the Forestry Commission to help them minimise the effects on oak trees caused by these caterpillars. http://www.forestry.gov.uk/opm

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

— Page last updated 07/02/2022