Oak Processionary Moth Caterpillars and Dogs

Yes, oak processionary moth (OPM) caterpillars can be harmful to dogs. Their bodies carry hairs containing an irritating substance called thaumetopoein, which can cause health risks to pets and to humans¹. In dogs and cats, contact can cause excessive drooling, a swollen tongue, conjunctivitis, gagging, vomiting, difficulty breathing and an inflamed or swollen mouth¹.

The good news is that serious allergic reactions are rare, and most pets only experience slight irritation or discomfort¹. If your dog shows symptoms or is in discomfort after coming into contact with an OPM caterpillar, call your vet for advice, and keep an eye on your pet for a couple of days¹. There is no vaccine or spot-on treatment that protects against the hairs, so the practical protection is keeping your dog away from the caterpillars and their nests, especially from May to July when they are active¹.

The caterpillars are found almost exclusively in oak trees, and in the UK they are generally found in the south east of England, though they could be spreading¹. If you see caterpillars or a nest, do not touch or move them yourself: report the sighting to the Forestry Commission using the Tree Alert online form¹.

Symptoms to watch for after contact

The hairs on oak processionary moth caterpillars contain an irritating substance called thaumetopoein, which can cause health risks to pets and humans¹. For a dog, the most likely route of contact is the mouth and face: sniffing, licking or picking up a caterpillar with its nose, mouth or tongue. That is why the symptoms Blue Cross lists centre on the mouth, throat and eyes.

Watch for these signs in dogs and cats after any contact with an OPM caterpillar or its nest¹:

• Excessive drooling
• A swollen tongue
• Conjunctivitis
• Gagging
• Vomiting
• Difficulty breathing
• An inflamed or swollen mouth

Not every symptom will appear, and the severity varies from pet to pet. Serious allergic reactions are rare, and Blue Cross advises that pets should only experience slight irritation or discomfort, but it is best to keep an eye on your pet for a couple of days after contact¹. Symptoms that involve breathing, or vomiting that does not settle, are the ones to take most seriously.

If your dog or cat is suffering with any of these symptoms, or is in discomfort after coming into contact with an oak processionary moth caterpillar, call your vet for advice¹. Do not wait to see whether things worsen on their own if your pet is visibly struggling. For guidance on getting help outside normal opening hours, see our page on emergency vets and out-of-hours care, and for the kit that helps you handle a pet health problem at home, see our guide to pet first aid kits.

How serious reactions are, and when to call the vet

It helps to keep the risk in proportion. Blue Cross, whose guidance was reviewed by a veterinary surgeon in March 2023, states that serious allergic reactions are rare and that pets should only experience slight irritation or discomfort¹. Most dogs that sniff at a caterpillar will have an unpleasant hour or two, not an emergency. But "rare" is not "never", and the symptoms list includes difficulty breathing, which in any animal is a reason to get professional advice promptly.

The practical rule is straightforward. If your dog or cat is suffering symptoms or is in discomfort after coming into contact with an OPM caterpillar, call your vet for advice¹. Say what you saw: whether your pet touched a caterpillar, a nest, or the ground beneath an oak tree, and roughly when. That helps the vet judge how closely to monitor your pet.

After the initial call, watch your pet for a couple of days¹. Look for symptoms that persist or worsen rather than fade, particularly any continued difficulty with breathing, repeated vomiting, or swelling in the mouth or tongue that does not go down. A pet that stops eating or drinking because its mouth is sore also needs the vet's attention.

One pattern worth knowing about in any household illness is symptoms appearing in several members at once. The fire and rescue service's carbon monoxide guidance notes that when others in your household, including pets, experience symptoms at a similar time, that points to a shared cause in the home². OPM irritation is different: it comes from contact with caterpillar hairs outdoors. But if you and your dog both develop irritated skin, eyes or mouths after the same walk under oak trees, that shared timing is a useful clue to mention to both your pharmacist and your vet.

Recognising the caterpillars and their nests

Knowing what to look for lets you steer your dog away before contact happens. Blue Cross gives three identifying features¹.

The caterpillars. OPM caterpillars have grey bodies, black heads, and very long white hairs that are noticeable among the rest of their much shorter hairs¹. It is those hairs that carry the irritating substance, so the caterpillar's most distinctive feature is also its dangerous one. A curious dog sees a small, hairy, slow-moving thing on a trunk or path: exactly the kind of object many dogs want to sniff or mouth.

The procession. The caterpillars move nose-to-tail in a distinctive procession, and they are often clustered together¹. A line of caterpillars travelling head to tail along a branch or trunk is the behaviour that gives the species its name, and a cluster on the ground under an oak is a particular hazard for a dog that sniffs around the base of trees.

The nests. The caterpillars often cluster together and build white, silk-like nests in the branches or trunks of oak trees¹. On the trunk, a nest looks like a pale, webbed pad or pouch stuck to the bark; in the branches it can be a silken mass among the leaves. Nests can remain on the tree after the caterpillars have gone, and the hairs stay irritating, which is one reason the advice is to keep away at all times, not just in summer¹.

Because OPM caterpillars are found almost exclusively in oak trees¹, you do not need to inspect every tree on a walk. Focus your attention on oaks: the trunk, the main branches, and the ground beneath the canopy where caterpillars and shed hairs can fall.

The risk period: May to July, but keep away all year

OPM caterpillars are usually seen from May to July, and they are most likely to pose a risk to pets and people during those months¹. That is when the caterpillars are active, feeding and moving, and when a dog nosing around the base of an oak is most likely to meet one.

But the advice from Blue Cross is not to relax completely once August arrives. The caterpillars are most likely to pose a risk from May to July, but it is best to keep away from them at all times¹. Old nests, shed skins and fallen hairs can persist on and around a tree after the caterpillars themselves have gone, and those hairs do not lose their irritant simply because summer has ended. A nest you notice on an oak in February is still something to keep your dog away from and to report.

The May to July window overlaps with other seasonal hazards that dog walkers in England are advised to plan around. Derbyshire County Council notes that ticks can be active all year round but are most active from April to July, and sometimes later in the autumn³, so a summer walk under oaks in grassy country can bring both risks at once. Checking your dog over after a walk in that period covers more than one hazard. For the wider picture of seasonal pet risks, see our pets and animals section.

Where oak processionary moth is found in the UK

Oak processionary caterpillars are not native to the UK. They were accidentally imported from mainland Europe, arriving on trees brought in for tree planting schemes¹. Once here, they established themselves in oak trees, and they are now generally found in the south east of England, though they could be spreading¹.

Blue Cross lists the counties where OPM caterpillars have either been found, or are likely to be found¹:

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

If you walk your dog anywhere in those counties, especially where there are mature oaks in parks, commons, woodland edges and street trees, it is worth knowing what the caterpillars and nests look like. Surrey County Council's summer health and wellbeing advice includes information and advice on oak processionary moths among its seasonal hazards for residents⁴, which reflects how established the moth has become in that part of the country.

If you live outside the listed counties, the risk is lower today, but the guidance that the moth could be spreading means it is sensible anywhere in the country to keep your dog from mouthing hairy caterpillars or touching silk-like nests on oaks¹. A sighting well outside the south east is also exactly the kind of thing the Forestry Commission wants to know about through Tree Alert¹.

Keeping your dog away: leads, recall and oak trees

Because there is no vaccine or spot-on treatment to prevent the caterpillars' hairs from irritating your dog or cat¹, avoidance is the whole of the protection. That comes down to two things you can train and one decision you make on each walk.

Use a lead under and around oaks in the risk period. The habit of keeping dogs on leads in hazardous spots is well established in official safety advice for other countryside risks. Derbyshire County Council advises dog walkers to keep their dogs on leads close to water, because dogs running free risk sliding on slippery banks and ending up in deep or fast-flowing water⁵. Devon and Somerset Fire and Rescue Service tells dog owners to keep dogs on leads near fast flowing or flooding rivers, where banks are steep or unstable, and near cliffs⁶. The same principle applies to an oak tree with caterpillars on it: a dog on a lead cannot range under the canopy and sniff out a nest at the trunk's base.

Work on recall. Blue Cross advises that making sure your dog has good recall can help you call them away if you notice them getting close to an OPM caterpillar¹. Recall is worth practising before you need it, in safe places, so that it holds when the distraction is a cluster of hairy caterpillars on a path.

Control on and off the lead. London Fire Brigade advises dog walkers to make sure they can control their dog on and off the lead near water⁷, and the same standard of control is what protects your dog around oaks. Derbyshire's flooding guidance repeats the advice to keep dogs on leads close to water to keep them safe⁸, and Devon and Somerset Fire and Rescue Service lists keeping dogs on leads near quarries, clifftops and fast-flowing water among its top causes of rescues and how to avoid them⁹. The Met Office's strong wind advice adds keeping dogs on a lead at the coast¹⁰. These are all different hazards, but the message is consistent across the fire services, councils and the Met Office: in places where something can go wrong quickly, the lead is the control that works.

A simple checklist for walks in OPM country between May and July:

• Call your dog away if you notice them getting close to a caterpillar¹
• Do not let them sniff, lick or mouth caterpillars, nests, or debris under oaks¹
• Check the tree for white, silk-like nests before letting your dog explore beneath it¹
• Keep your dog on a lead near water, cliffs and fast-flowing or flooding rivers, where banks can be slippery, steep or unstable¹

For the broader job of planning walks and pet care around emergencies, see our guide to including your pets in a household emergency plan.

Never move a caterpillar or nest yourself

If you find caterpillars or a nest, leave them exactly where they are. Blue Cross is direct on this point: do not attempt to move an OPM caterpillar or nest yourself, because oak processionary moth caterpillars can also cause health risks to people¹. The hairs that irritate your dog's mouth will irritate your skin, eyes and airways too, and a nest can contain large numbers of them. Disturbing a nest, bagging one up, or brushing caterpillars off a branch all put you in contact with the hairs.

This fits a wider principle that runs through all the emergency services' advice on animal rescues: do not put yourself at risk to help an animal. London Fire Brigade tells dog walkers never to enter the water to rescue a dog, and not to get into danger themselves while trying to rescue a dog from water, because dogs often manage to scramble out on their own⁷. The same discipline applies here. Keep people, children and pets back from the tree, and let the professionals handle what is on it.

The reason professional handling matters is that the hairs contain thaumetopoein, an irritating substance that can cause health risks to pets and humans¹. Trained crews deal with OPM nests with the right protective equipment. Your job as a dog owner is containment and reporting, not removal.

Reporting a sighting to the Forestry Commission

You can report any sightings of OPM caterpillars to the Forestry Commission by using the Tree Alert online form¹.

When you report, the useful details are the ones you have already learned to notice as a dog owner: where the oak tree is, as precisely as you can describe it; what you saw, whether caterpillars with grey bodies, black heads and long white hairs moving in a procession, or a white, silk-like nest on the branches or trunk¹; and roughly how many caterpillars or nests there were. A photograph taken from a safe distance, without touching or disturbing anything, helps identification.

A report is worth making even if you are not completely sure what you have seen. The caterpillars are found almost exclusively in oak trees¹, so a hairy caterpillar cluster on an oak in one of the affected counties between May and July is a reasonable report. Outside the listed counties, a report is more valuable still, because it is the mechanism by which a spreading moth gets noticed.

If your own dog has already had contact before you spotted the hazard, deal with the pet first: call your vet for advice if your dog or cat is suffering symptoms or is in discomfort¹. Then make the report, so the next dog walker past that tree faces less risk than yours did.

What the caterpillars do to people, and where to get help

The hairs do not distinguish between species. They contain thaumetopoein, an irritating substance that can cause health risks to pets and humans¹, so an owner who picks a caterpillar off their dog with a bare hand, or brushes against a nest while pulling a dog away, can be affected too.

For people, the outlook is reassuring. Although unpleasant, symptoms are not usually medically serious and pass in a few days¹. Blue Cross advises consulting a pharmacist for relief, or a doctor for a serious allergic reaction, which is rare¹. The pharmacist is the right first stop for the common case of irritated skin, eyes or throat; the doctor is for the rare reaction that goes beyond that.

The practical sequence for a household where dog and owner have both had contact is therefore: call the vet for the dog¹, speak to a pharmacist for yourself¹, and report the sighting to the Forestry Commission¹. Nobody in that sequence should be touching the caterpillars or the nest to "clear them up", because that simply creates more contact with the hairs¹.

For related seasonal hazards to pets, from hot weather to cold, see our pets and animals section. If your dog gets into trouble in water or at the coast on the same kind of countryside walk, our page on what to do if your dog gets into trouble in water covers that hazard.