When you’re out and about, whether that’s for a dog walk or a picnic in the park, you should be careful who you might be sharing the woodland with.

Usually, oak processionary moth (OPM) caterpillars are seen from May to July and this toxic caterpillar can be bad news for both animals and humans.

It's important to note that these caterpillars may be spotted outside of this time frame and you should keep away from them at all times.

What are OPM caterpillars and how do they affect pets and humans? Let’s take a look.

News and Star: OPM caterpillars are harmful to animalsOPM caterpillars are harmful to animals (Image: Canva)

Blue Cross has explored the insect and set up a guide to help people better understand the small but dangerous critters via its website.

What is an oak processionary moth caterpillar and what makes it toxic?

OPM caterpillars are a species of moth and their caterpillars are found during the spring in nests on oak trees.

It’s the hairs on these creatures that cause irritation as they contain a substance called thaumetopoein which can be harmful to animals and humans.

You might not have to touch a caterpillar to come into contact with its hairs though as they can become detached and blow about on a windy day.

The name of these caterpillars comes from the way they move around together in a procession.

They’ll sit together on a branch or inside a tree trunk and create a white, silk-like nest.

News and Star: The hairs from OPM caterpillars can cause harm to pets and humansThe hairs from OPM caterpillars can cause harm to pets and humans (Image: Getty Images)

When travelling from oak tree to oak tree, they walk along the ground in a procession.

Cats are at risk of coming across these caterpillars because they often run up into trees while dogs may meet the caterpillars during one of their processions along the ground.

The hairs can easily stick to a dog’s face or paws when they’re out and about.  

What do OPM caterpillars look like?

These caterpillars have a grey body, black head and long white hairs which are more noticeable than the shorter ones.

If you see an OPM caterpillar, you should report it to the Forestry Commission via its Tree Alert online form.

What can happen if a dog or cat touches an OPM caterpillar?

Your dog or cat may have the following symptoms if they have sniffed, touched, licked, picked up or tried to eat a caterpillar or its nest:

  • Excessive drooling
  • Swollen tongue
  • Conjunctivitis
  • Gagging
  • Vomiting
  • Difficulty breathing
  • Inflamed or swollen mouth

You should call your vet for advice if your pet has come into contact with an OPM caterpillar and is suffering any of the above symptoms or is uncomfortable.

Keep an eye on your pet for a couple of days even though serious allergic reactions are rare.

No vaccine or spot-on treatment exists for these toxic caterpillars but you should try to keep your dog or cat away from them and any nests if you see them before it’s too late.

Try calling them so they come back to you and hopefully avoid running into trouble.

News and Star: OPM caterpillars are often spotted on or in oak trees but they also move between trees by walking along the groundOPM caterpillars are often spotted on or in oak trees but they also move between trees by walking along the ground (Image: Canva)

Where might I find an OPM caterpillar?

These caterpillars have either been seen in or are likely to be seen in the following counties:

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

OPM caterpillars could travel to other counties so it’s important dog owners keep an eye out for them when on walks.

You can keep up to date with sightings of the caterpillars via the Forestry Commission’s map.

What happens if humans touch an OPM caterpillar?

Humans that come into contact with an OPM caterpillar could get a rash or experience breathing difficulties.

They might also get sore, itchy or irritated eyes and throat.

For humans, the symptoms are usually not medically serious and are likely to pass in a few days but you should speak to a pharmacist who can help you with relief of the symptoms.

While serious allergic reactions are rare, you should speak to a doctor if you experience them.

If your dog has an OPM caterpillar attached to its coat and you need to remove it, don’t touch it with bare hands.

Blue Cross suggests wearing rubber gloves or using a thick pile of tissues to make sure you can keep away from hairs of the caterpillar.

Find out more about OPM caterpillars via the Blue Cross website.