Processionary caterpillars: a danger to dogs and cats

Every year, from May to July, dogs arrive at the vet with a swollen tongue after sniffing or licking a processionary caterpillar. Long confined to the north-east of the country, the insect is now settling in Wallonia as well. We are publishing this article in October because the next season is being prepared right now: empty nests remain urticating for years and are far easier to spot in bare trees.

Two species, two seasons

The oak processionary

Thaumetopoea processionea is a native species, present in Flanders since the 19th century. Outbreaks are concentrated in north-east Limburg, the province of Antwerp and the Kempen. The eggs overwinter in the crowns of oak trees and hatch in April or May. The caterpillars develop urticating hairs from the third larval stage, around mid-May. The night-time processions, from June to mid-July, mark the peak of the danger.

In Wallonia, the species had never been observed before 2018. The first outbreaks were spotted in 2019 in the Arlon and Eupen areas, and its range keeps expanding.

The pine processionary

Thaumetopoea pityocampa lives on pines and cedars. Its calendar is reversed: the caterpillars spend the winter in large white, cotton-like nests at the tips of branches, then descend to the ground in procession between January and May, with a peak of exposure in March. Historically Mediterranean, it is moving north with the warming climate and has already been observed in southern Belgium.

Why these hairs are so dangerous

A single caterpillar carries up to 700,000 urticating hairs measuring 0.2 to 0.3 mm, invisible to the naked eye. They release a toxic protein on contact with skin or mucous membranes, detach at the slightest disturbance, are carried by the wind over tens of metres and cling to shed skins and nests. You therefore do not need to touch a caterpillar to be affected.

Dogs are hit first because they sniff the ground and sometimes try to bite a caterpillar. Cats mostly contaminate themselves by licking their paws after walking across a procession.

Symptoms in dogs and cats

Signs appear within minutes to a few hours:

• heavy drooling, stringy saliva, frantic pawing at the mouth;
• swollen tongue, purplish red, then hard and painful;
• swelling of the lips, muzzle and sometimes the throat;
• vomiting, refusal to drink or eat;
• red, watery eyes if the hairs have touched the eye;
• breathing distress if the throat swells: noisy panting, bluish gums, sudden collapse.

Without prompt treatment, the affected part of the tongue can become necrotic and fall off within days, leaving the animal unable to drink and eat normally. The massive drooling and localised swelling of the mouth set this emergency apart from heatstroke.

What to do immediately

1. Protect yourself: put on gloves, the hairs are just as urticating for you.
2. Rinse thoroughly the mouth, tongue and eyes with lukewarm water, using a needleless syringe or a bottle, letting the water run forwards so the animal does not swallow it.
3. Never rub: rubbing breaks the hairs and releases more toxin.
4. Do not pull the hairs out by hand and do not give any human medication.
5. Call the vet immediately, even if the signs seem mild. In the evening or at the weekend, contact the on-duty vet (see how the on-duty vet service works).

The vet will give anti-inflammatories, antihistamines and painkillers. The faster the treatment, the better the chances of saving the tongue. In Zoolinko, the free health record, the 72-hour emergency access lets the on-duty vet consult your pet's file, linked to its microchip.

Prevention during the season

• From May to July, keep your dog on a lead near oak trees and stop it sniffing at the foot of isolated trees in full sun, the ones most often colonised.
• Learn to spot the nests: silky white clumps turning yellow or brown, pressed against the trunk or under the large branches; in heavy outbreaks they can exceed a metre in length.
• Near an outbreak, do not dry laundry outdoors from May to September, as the Walloon public service recommends.

These precautions come on top of the ticks and fleas: the risk seasons in Belgium calendar and our list of toxic plants.

Why worry about it in October

The caterpillars are gone, but not the danger. Abandoned nests, full of shed skins and hairs, remain urticating for years; Ecopedia speaks of up to eight years. A dog digging at the foot of an oak or a gust of autumn wind is enough to put the hairs back into circulation. Once the leaves have fallen, the nests are much easier to see: use the opportunity to inspect your garden and your usual walks.

Autumn and winter are also when the next generation's eggs wait in the crowns and when municipalities plan nest removals, ideally carried out from April to May, before the caterpillars become urticating. Finally, the pine processionary spins its winter nests precisely at this time.

Reporting a nest: what municipalities do

Never touch a nest yourself: no water jet, no garden-centre insecticide, no burning, no bin bag. Control is a job for professionals equipped with fine-filter vacuums.

• In Flanders, report the nest to your municipality or the fire brigade, who organise removal on public land and advise you for private property.
• In Wallonia, each affected municipality appoints a "référent chenille processionnaire". This contact advises owners and passes reports on to the Walloon Forest Health Observatory, which keeps the nest map up to date.
• In Brussels, the species remains rare; notify your municipality or Brussels Environment if in doubt.

For serious human cases, 112 and the Poison Centre (070 245 245) are the right numbers.