An invasive, toxic species of caterpillar has officials in the UK on edge. Authorities are now warning residents to avoid the caterpillars and their prickly, poisonous hairs that can irritate and kill.

“It's time to be vigilant! Oak Processionary Moth spotted in parks across Bexley,” the Royal Forestry Society tweeted last week. (Bexley is a south-east London borough.)

So far, officials have only seen nests and emerging larvae of the toxic oak processionary moth, or OPM (Thaumetopoea processionea). The larvae, aka caterpillars, aren’t expected to fully hatch and begin moving around and eating oak foliage until around mid-May, at which point they pose a danger to humans as well as the trees.

The caterpillars can be easily spotted by their name-sake nose-to-tail processions, but their most dangerous feature is their coat of toxic hairs. Each caterpillar has approximately 62,000 of the white bristles. The hairs are full of a toxic protein called thaumetopoein, and they can be ejected if the caterpillar feels threatened. Thus, humans can be exposed by touching the caterpillars or being brushed by shed hairs swept up in the wind.

The hairs’ thaumetopoein can then cause skin irritation, breathing troubles, fevers, and eye and throat irritation. In people who are allergic, the toxin can cause life-threatening reactions.

“At best, you can get contact dermatitis. At worst, you can die,” Jason Dombroskie, an entomologist with Cornell University, told the New York Times. “You can go into anaphylactic shock and have your airways close up. The airborne hairs set up a whole different ballgame.”

British officials are already taking actions to survey, monitor, and eliminate nests, which present as white, silken webbing on the trunks and branches of oak trees. In the meantime, they’re warning residents to look out for and stay away from the caterpillars.

One gardener who unknowingly had a run-in with the pests described her symptoms to the BBC, saying:

My first symptom was a rash on my tummy. I was unaware of what it was and thought at first it was a heat rash... I had spells of feeling violently sick. I thought I might have shingles. The rash got worse and the left side of my face became covered in this sore irritating rash. My left eye became very sore and weepy. I contacted my doctor and it was confirmed I had been severely affected by OPM and must keep away from the source as over time I had developed a severe allergic reaction.

OPM is a relatively new problem for the country. The species is native to southern Europe, where there are other native predators and environmental factors that keep them in check. The species was likely introduced to Britain by accident in 2005 as eggs on live oak imports. OPM has since spread and likely been reintroduced several times. The Forestry Commission notes that if they are left unchecked, they would likely “survive and breed in much of England and Wales.”