An army of caterpillars is storming London and beyond, leaving a toxic trail in its wake.

The caterpillars, technically the larvae of oak processionary moths (OPM), are causing a rash of violent illnesses in London and the country’s southeastern region that include asthma attacks, vomiting and fevers.

The outbreak is so intense that health officials have issued a stern warning: Beware the white-haired caterpillar.

Indeed, several severe cases have already been reported, according to BBC News.

"During this time I had spells of feeling violently sick," one gardener told the news agency. "I thought I might have shingles. The rash got worse and the left side of my face became covered in this sore irritating rash."

The larve's most toxic substance is a protein called thaumetopoein, which is found mostly in the caterpillars' hairs. These insects typically have around 63,000 hairs, which are ejected as they trudge along. The hairs can easily become airborne.

"The caterpillars' thousands of tiny hairs contain an urticating, or irritating, substance called thaumetopoein," the Forestry Commission notes on its website. "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."

The protein stays active in each hair for as many as five years — exponentially increasing the risk of someone coming into contact with the protein.

Closeup of toxic caterpillars. The caterpillars are covered in thousands of hairs containing a protein that can cause severe illness. (Photo: Sarah2/Shutterstock)

To fight the problem, the Forestry Commission has launched an extensive pesticide campaign, in addition to laying traps in the trees where the moths spend most of their short lives. In all, some 600 sites are being treated for the caterpillars.

While the outbreak isn’t expected to last — treatment is scheduled to extend into early June at the latest — London isn’t likely to see the last of the caterpillar plague.

The species, the Telegraph reports, likely hitched a ride to the U.K. on Dutch trees used for construction projects. Once the moths reach a certain age, the pesticides are no longer effective — and then it’s back to anxiously bracing for the next springtime invasion.

Toxic caterpillars invade London
Swarms of white-haired insects are triggering a citywide health alert in London.