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Toxic caterpillars that can cause asthma attacks, vomiting and skin rashes have descended on parts of the UK, officials have warned.

Oak processionary moths (OPM), which are in their larval stage, have been spotted across London and the south east.

Hairs on the caterpillars can cause fevers and eye and throat irritations, the Forestry Commission said.

The organisation has issued a caution not to touch the species.

The biggest infestations of OPM were recorded in Greater London last year, stemming from Kingston upon Thames to Brent, according to the BBC .

Some infestations were also spotted in Bracknell Forest, Slough and Guildford.

OPM caterpillars were spotted emerging from egg plaques in mid-April, and trees were treated on April 23, the Forestry Commission added.

"The treatment programme is expected to continue until late May or early June," a spokesman said.

"After that the caterpillars will be too large to be affected by our preferred treatment product."

(Image: Forestry Commission)

The caterpillars

As a caterpillar, each OPM has about 62,000 hairs, which they can eject.

Hairs that fall to the ground can be active for up to five years. The moths only live for two to three days in July or August.

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It is thought that the moths were brought into the UK on trees imported from Europe for a landscape project.

OPM caterpillars are most easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions and the fact that they live and feed almost exclusively on oak trees.

They can sometimes be seen processing across the ground between oak trees, and clustering together as they feed on oak leaves.

The threat

(Image: Forestry Commission)

The caterpillars' thousands of tiny hairs which contain an irritating, substance called thaumetopoein. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems.

This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown into contact by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why nests should not be touched without protective clothing.

OPM caterpillars can threaten the health of several species of oak trees because they feed on the leaves. Large populations can defoliate, or strip bare, large parts of oak trees, leaving them vulnerable to attack by other pests and diseases, and less able to withstand stresses such as drought and flood.

They will only feed on other trees if they run short of oak leaves to eat, and have been seen on hornbeam, hazel, beech, sweet chestnut and birch trees.

Source: Forestry Commission

Precautions

(Image: Forestry Commission)

People in the affected areas can take these simple precautions to minimise the health risks to themselves and their pets and livestock:

DO NOT :

touch or approach nests or caterpillars;

let children touch or approach nests or caterpillars

let animals touch or approach nests or caterpillars; or

try removing nests or caterpillars yourself.

DO :

teach children not to touch or approach the nests or caterpillars;

train or restrain pets from touching or approaching them;

keep horses and livestock a safe distance from infested oak trees. Covering or stabling livestock can help;

see a pharmacist for relief from skin or eye irritations after suspected OPM contact;

call NHS111 or see a doctor if you think you or someone in your care has had a serious allergic reaction;

consult a vet if you think your pet or livestock has been seriously affected;

call in a pest control expert to remove infestations in your own trees;

Report any sightings to the Forestry Commission .

Tree surgeons, forestry and ground-care workers, and others working on or close to oak trees in the affected areas, should wear full protective clothing, and familiarise themselves with the signs of OPM presence and the regulations applying to handling and moving oak material.

Source: Forestry Commission