Processionary caterpillars and dogs: symptoms and what to do

You're walking through a pine forest in Spain or the south of France in early spring. Your dog noses a line of fuzzy caterpillars crossing the trail, and within minutes they're drooling heavily and rubbing their face in the dirt. That's the classic picture of processionary caterpillar exposure in dogs, one of the most common seasonal emergencies for vets across southern Europe.

If you live in Europe, are stationed there or are traveling with your dog, this is a hazard worth knowing about before you need to. The caterpillars don't bite or sting. The damage comes from thousands of microscopic irritating hairs that embed in the tongue, mouth and eyes and release a potent toxin. Acting fast can be the difference between swelling that settles and losing part of the tongue. Here's how to recognize it, what to do in the first minutes, what never to do and how to avoid it.

What processionary caterpillars are

Two closely related species cause most problems for dogs in Europe:

Both get their name from the way they travel in nose-to-tail lines, or "processions." From their third growth stage, the caterpillars carry barbed, harpoon-shaped hairs that break off easily when they feel threatened. The hairs contain a protein toxin that triggers intense inflammation. Two things catch dog owners off guard:

• Your dog doesn't have to touch a caterpillar. Hairs can drift in the air and stay on the ground where a procession has passed.
• The hairs stay irritating for a long time, including in empty nests, dead caterpillars and the soil under infested trees.

Where and when your dog is at risk

Spain and southern Europe. Pine processionary caterpillars live in mountain forests as well as city parks, gardens and housing developments with pine trees. Along the Mediterranean coast and in southern Spain, processions can start as early as December or January; at higher, colder elevations they may not come down until April or May. Mild winters tend to push the season earlier. If you see white silk tents high in the pines, expect caterpillars on the ground sooner or later.

The UK and northern Europe. Oak processionary moth has been established in London and parts of southeast England since the mid-2000s, and it's widespread in the Low Countries and Germany. The caterpillars mostly stay on the trees, but nests and hairs can fall onto paths and grass in parks, and dogs that sniff around oak trunks are at risk.

North America. Neither species is established in the US or Canada. The US does have its own stinging caterpillars, such as the puss caterpillar in the South, and the same first-aid principles apply.

Symptoms of processionary caterpillar exposure in dogs

Signs usually show up within minutes and are hard to miss:

• Sudden, heavy drooling, sometimes foamy.
• Pawing at the mouth or rubbing the face on the ground, restlessness and whining.
• Swelling of the tongue, lips and face, which can progress quickly.
• A bright red tongue at first, then pale, purple or black patches over the following hours: that's tissue dying (necrosis).
• Vomiting or retching, especially if your dog bit or swallowed a caterpillar.
• Red, watery eyes or swollen eyelids if hairs reached the eyes.
• Hives or red skin on thinly haired areas like the belly or between the toes.

Severe cases can involve trouble breathing, as the tongue and throat swell, or a body-wide allergic reaction (anaphylaxis).

What to do if your dog touches a processionary caterpillar

The first few minutes matter. Work through these steps while someone else calls the vet:

1. Move your dog away so they can't sniff the caterpillars or the ground again.
2. Protect your hands. Use gloves (dishwashing, latex or nitrile) or, in a pinch, a plastic bag. Don't touch the muzzle or tongue with bare hands, and don't rub your own eyes.
3. Rinse the mouth and muzzle with plenty of lukewarm water. Pour it from a bottle or use a gentle stream, with your dog's head tilted down so the water runs out of the mouth instead of being swallowed. Keep going for several minutes.
4. Don't wipe or scrub. Rubbing breaks the embedded hairs and releases more toxin. Let the water do the work.
5. Stop your dog from pawing at the face by holding them calmly.
6. Head to the vet without waiting, even if your dog seems to improve. If the face is swelling, loosen or remove the collar.

Keep these steps with the rest of your dog first aid plan, because there's no time to look them up in the moment.

What the vet will do and how dogs recover

At the clinic, the team will finish flushing the mouth, often under sedation so it can be done thoroughly, and give injectable medication to calm the inflammatory and allergic reaction. If there's significant swelling, breathing trouble or vomiting, your dog may need IV fluids, oxygen and a few hours of hospital monitoring.

How things go depends on how many hairs were involved and how quickly treatment started:

• Mild or quickly treated cases: swelling settles within a day or two and dogs recover completely.
• Cases with necrosis: over the next several days, the damaged part of the tongue darkens, becomes clearly outlined and may slough off. Losing the tip or an edge is most common. Most dogs adapt well and eat and drink normally again, though they may need soft food and pain control while healing.

Keep an eye on the eyes, too. If hairs got into them, conjunctivitis or corneal injuries can follow. If you notice tearing, redness or squinting in the following days, see your vet; our guide to dog eye discharge explains what different discharge can mean.

How to protect your dog on walks

Prevention works far better than any treatment:

• Skip pine forests in season, especially January through May in Spain. If you can't, keep your dog on a short leash and stick to wide trails.
• Watch the ground. A procession is a line of brownish, hairy caterpillars about 1.5 in (4 cm) long each, sometimes stretching several feet. Give it a wide berth.
• Avoid walking under infested trees on windy days, when loose hairs spread farther. Near oaks in the UK, steer your dog away from trunks with silk nests.
• Don't let your dog dig under pines in spring, where caterpillars bury themselves to pupate.
• Train a rock-solid "leave it" so your dog backs off whatever they're sniffing; see teach your dog leave it and drop it. A reliable recall, covered in how to teach your dog to come when called, lets you call them away in time.
• Plan trail routes with the season in mind and carry extra water for rinsing; more tips in our guide to hiking with your dog.

Caterpillar nests near your home

If you're living in Europe with pines or oaks nearby, plan ahead:

• Don't remove or burn nests yourself. Disturbing them scatters hairs that can affect you, your pets and your neighbors. Hire a licensed pest control company.
• Control is seasonal: nest removal in fall and winter for pine processionary, collar traps around trunks to catch caterpillars as they climb down, and pheromone traps for the moths in summer.
• Report nests in public spaces. In Spain, contact the local town hall (ayuntamiento); many treat parks every year. In the UK, oak processionary sightings can be reported to the Forestry Commission through its TreeAlert service.
• Encourage natural predators such as great tits and other insect-eating birds with nest boxes.

Frequently asked questions

Can my dog get hurt just by sniffing a processionary caterpillar?

Yes. When a dog's nose gets close, the caterpillar releases its hairs defensively, and they lodge in the nose, tongue or eyes. Hairs can also linger on the ground where a procession passed or drift through the air near infested trees. If you saw your dog sniffing caterpillars, rinse with lukewarm water and call your vet even before symptoms appear.

Can a processionary caterpillar kill a dog?

It's uncommon, but it can happen. Life-threatening cases involve tongue and throat swelling that blocks breathing, anaphylaxis, or a dog that swallowed several caterpillars. That's why vets always treat exposure as an emergency. With fast veterinary care, the vast majority of dogs pull through, although some lose part of the tongue.

How quickly do symptoms start?

Irritation starts within minutes, with drooling, restlessness and face rubbing as the first signs. Swelling builds over the first hour, and tongue color changes from necrosis may only become obvious over the following hours or days. A dog who seems fine shortly after contact isn't necessarily out of danger.

Are empty nests and the soil under the trees still dangerous?

Yes. The irritating hairs remain active for a long time, so old nests, dead caterpillars and the soil where caterpillars buried themselves all remain a risk. Keep your dog from sniffing or digging around infested trees even when you no longer see live caterpillars.

Do processionary caterpillars affect people and cats too?

Yes. In people they cause itchy rashes and eye and airway irritation, and children are especially vulnerable because they like to touch caterpillars. Outdoor cats can develop mouth and eye injuries similar to those in dogs, so the same precautions apply to them.

Sources

• Forestry Commission (UK): guidance on oak processionary moth.
• Royal Horticultural Society (RHS): oak processionary moth advice.
• European and Mediterranean Plant Protection Organization (EPPO): datasheets on Thaumetopoea pityocampa and Thaumetopoea processionea.
• Spanish Ministry for the Ecological Transition (MITECO): forest health information on the pine processionary moth.
• Merck Veterinary Manual: hypersensitivity reactions and anaphylaxis in small animals.