Dangerous processionary caterpillars: These symptoms in dogs are an emergency

When you walk your dog in woodland or a park in spring or early summer, you may pass them without noticing: processionary caterpillars. What looks like a harmless “caterpillar parade” can quickly turn into a genuine emergency for dogs, cats and children. The most dangerous part is that many owners do not spot the early warning signs in time.

Why processionary caterpillars are so dangerous for dogs (pine processionary moth and oak processionary moth)

The pine processionary moth (Thaumetopoea pityocampa) and its close relative, the oak processionary moth, are moth species. The problem is not the adult moths but the larvae: caterpillars that often move in long nose-to-tail lines across the ground.

Their bodies are covered in tiny, highly irritating hairs. These hairs:

• snap off with even the slightest touch or vibration
  
• are carried by the wind and end up on grass, soil, tree trunks, clothing and shoes
  
• release a potent toxin that attacks skin, mucous membranes and airways
  

The dangerous hairs do not even need to touch the dog directly - sniffing near a nest or a line of caterpillars is often enough.

Dogs are particularly at risk because they explore with their nose close to the ground, sniff, lick and may even pick caterpillars up.

Typical symptoms in dogs after contact with processionary caterpillars

The first signs usually appear within minutes to an hour after contact. The mouth, tongue and nose are most commonly affected.

Early warning signs around the head

• sudden, heavy drooling
  
• frantic rubbing of the muzzle with paws or along the ground
  
• licking, smacking, gagging or “chewing at the air”
  
• redness and swelling of the lips, tongue or lining of the mouth
  
• redness and swelling of the nose or around the eyes
  

Many dogs clearly show that something is “burning”: they become restless, yelp, or pull away when you try to touch their muzzle.

Severe reactions in the mouth and tongue

The toxins carried on the caterpillar hairs can seriously damage tongue tissue. This may lead to:

• rapidly increasing, doughy swelling of the tongue
  
• a tongue that turns purple or dark red
  
• strongly foul-smelling saliva
  
• parts of the tongue dying off in the days that follow
  

If the tongue swells severely, the dog can barely breathe - this is an immediate, life-threatening emergency.

General symptoms and serious emergencies

Depending on how many hairs were taken in and how sensitive the dog is, other symptoms can occur - sometimes dramatic ones:

• vomiting and diarrhoea, occasionally with blood
  
• sudden lethargy, weakness, unsteady gait
  
• rapid, shallow breathing and whistling breathing sounds
  
• pronounced swelling of the head, throat or chest area
  
• circulatory collapse, up to and including unconsciousness
  

If breathing difficulty, fast swelling and a general collapse occur together, this indicates an allergic shock - an absolute emergency where every minute matters.

First aid: what owners should do immediately

If you suspect your dog has come into contact with processionary caterpillars or their hairs, act quickly but calmly.

Immediate steps right after exposure

• move your dog away from the danger area so it cannot touch more caterpillars or nests
  
• keep your own distance and avoid touching your dog’s head if possible
  
• put on disposable gloves if you have them
  
• gently rinse your dog’s mouth, tongue and lips with lukewarm water
  
• do not rub or scrub; rinse carefully or dab lightly
  

Do not try to remove caterpillars with your hand - it spreads the hairs even further.

If you are at home and have access to a mild bicarbonate solution (water with a little bicarbonate of soda or baking powder), you can use it to rinse carefully. Try to prevent your dog from swallowing the liquid.

What owners must avoid

• do not apply creams, gels or sprays to the mouth or tongue lining
  
• do not give “home remedies” such as corticosteroid tablets without veterinary instruction
  
• do not allow your dog to eat or drink until a vet has assessed the situation
  
• do not crush or burn caterpillars - this releases even more hairs into the air
  

Veterinary treatment: what happens at the clinic

After initial rinsing, the rule is: get to a vet or an emergency veterinary service immediately. When you phone ahead, mention your suspicion of processionary caterpillars.

At the practice, depending on severity, the vet may:

• inject a strong antihistamine and anti-inflammatory medication
  
• provide pain relief to reduce burning sensations and mouth pain
  
• give oxygen and stabilise circulation if there is breathing difficulty
  
• closely monitor the tongue and throat area
  

Timing is crucial: the sooner the dog is treated, the lower the risk that parts of the tongue will die off or that shock will develop.

In severe cases, the dog may need to stay in for observation so breathing, heart and circulation, and the progression of swelling can be monitored continuously. Occasionally, surgery is later required to remove dead tissue from the tongue.

How to spot processionary caterpillars in woodland and parks

Knowing what the nests and caterpillars look like makes it far easier to protect your dog.

• Nests: whitish, cotton-wool-like webbing on pine or oak branches, often in clusters
  
• Caterpillar lines: multiple caterpillars travelling in a long line across paths, forest floor or grassland
  
• Season: depending on the area, mainly spring to early summer
  

Many local authorities now put up warning signs at woodland edges or near playgrounds. If you see a warning, keep your dog on a lead and away from pine and oak stands.

Prevention: how to protect your dog in the long term

A few behaviour rules can significantly reduce the risk:

• avoid known affected areas in spring
  
• in those zones, keep your dog on a lead and steer it away from ground webbing and caterpillar lines
  
• do not let your dog forage or roll in heavily affected grassland
  
• after each walk in risk areas, visually check coat, paws and muzzle
  

Owners of very curious dogs or puppies should be especially vigilant. Young animals put everything in their mouths and often react more strongly.

If people come into contact with processionary caterpillars

Processionary caterpillars are not only a problem for animals. People can also react strongly to the hairs. Typical symptoms include itchy raised welts on the skin, eye irritation, hoarseness and coughing.

After contact, these steps can help:

• rinse affected skin with plenty of water and a mild wash gel; do not scratch
  
• change worn clothing immediately and wash it separately
  
• rinse eyes thoroughly with saline solution if they are affected
  
• if you develop breathing difficulty, facial swelling or a feeling of weakness, go straight to A&E
  

Why even dried hairs remain dangerous

A particularly deceptive point: the stinging hairs remain active for months, even if the caterpillar has long since died. Old nests, leaf litter and stored timber can still trigger reactions for a long time.

For example, if you store firewood from affected areas, you may unknowingly bring the hairs into your home or garden. For this reason, affected trees should only be treated or removed by trained professionals.

If your dog repeatedly walks in a risk area

If you live in a region where processionary caterpillars occur regularly, contact your local forestry service or parks department for guidance. There are often maps of affected zones or notices about closures.

You can also discuss with your vet how to recognise a reaction as early as possible and what to keep ready for emergencies - for instance, a list of the nearest out-of-hours veterinary services. Conscious, forward planning can save a dog’s life in a crisis, long before panic sets in and you can no longer act effectively.