How to survive America's most dangerous spider: They're spreading across US - and you often don't feel them until a wound gapes open and your urine turns black. Here's where they hide in homes... and what you must do immediately after a bite

First comes the bite – often so painless that victims don't even realize it has happened.

Then, within minutes, the venom gets to work, and the pain begins.

The skin begins to burn and throb, punctuated by electric shock-like zaps. As tiny blood vessels supplying the skin are destroyed, first there is florid bruising and then, in more severe cases, an open, festering wound.

Within a week or two, a hardened black ulcer can form over the dead tissue, eventually falling away as the wound begins to heal.

But in rare cases, the damage doesn't stop at the skin. The venom attacks red blood cells, turning urine Coca-Cola-colored and, in the most serious cases, causing kidney failure and even death.

This is what it can be like to be bitten by a brown recluse – one of America's most dangerous spiders.

And unlike America's other notoriously dangerous spider, the black widow – instantly recognizable by its glossy black body and distinctive red markings – the brown recluse is remarkably easy to miss.

About the size of a quarter, the drab brown spiders spend much of their lives hidden away in cracks, cardboard boxes and other dark, undisturbed corners of the home.

So what should you do if you spot one?

Rick Vetter, one of the world's leading authorities on brown recluse spiders, who has studied them for more than 30 years, has a simple answer: 'Smash it.'

And there may soon be more reason for Americans to heed that advice.

The true number of attacks is unknown, but reports to US poison centers suggest hundreds of Americans are bitten or suspected of being bitten every year. In 2024, 827 cases were recorded, including 29 that resulted in major medical complications.

Brown recluses have traditionally been concentrated in the warmer South and Midwest – but there are signs they are gaining a foothold farther north.

They’ve been spotted in at least 10 counties in Michigan, well beyond what experts have traditionally considered the spider's native range.

And scientists warn that rising temperatures could accelerate that northward creep.

A major study mapping how the spider's habitat could change as temperatures rise predicted that suitable territory would shift northwards, potentially allowing brown recluses to invade parts of Nebraska, Minnesota, Wisconsin, Michigan, South Dakota, Ohio and Pennsylvania where they have historically been rare or absent.

And climate may not be the only way they get around.

Brown recluses have a particular fondness for tight, undisturbed spaces – including the folded flaps of cardboard boxes – and can survive long periods without food.

This means they can inadvertently be transported hundreds of miles in shipments and household belongings, occasionally turning up far outside their natural range.

And when encounters do happen, the consequences can be terrifying.

Kansas mother-of-four Britagne Miller was left fighting for her life earlier this summer after being swarmed by around 15 spiders during a family camping trip.

'I felt like 15 spiders climbed up my legs and into my shirt, and I'm terrified of spiders,' she said.

At first, Miller appeared to be fine. But four days later, a bruise near one of the bites on her calf began rapidly swelling and became agonizingly painful.

'It was like shooting down my leg into my foot, and it felt like it was being stabbed,' she said.

Doctors discovered a huge blood-filled swelling in her calf and operated to remove nearly 300ml of blood clots. But Miller was already suffering from two rare blood-clotting disorders and a compromised liver, making surgery exceptionally dangerous.

'I'm terrified of surgery, that I'm going to go to sleep and not wake up or that I'm going to bleed out,' she said.

Her condition continued to deteriorate. She developed serious blood and staph infections and doctors believed she had entered end-stage liver failure.

'They gave me three days to up to a month to live,' Miller said. 'So that was scary.'

Miller survived and was eventually discharged from hospital, although she now needs a liver transplant.

Experts said the location and nature of her injuries raised suspicions that she had been bitten by brown recluse spiders, which are common in Kansas, although the species was never definitively identified.

Other victims have not been so lucky.

In one published case, a healthy 19-year-old man developed worsening pain, fever, chills, nausea and vomiting after a suspected brown recluse bite. He suffered kidney injury and was hospitalized, appeared to improve and was discharged – only to deteriorate again days later when the venom began destroying his red blood cells.

He ultimately required three trips to the ER and two hospital stays.

And, in exceptionally rare cases, victims have died.

A healthy three-year-old girl in Tennessee developed dark urine after a suspected brown recluse bite and was rushed to hospital, where she suffered a cardiac arrest and died during a blood transfusion.

Another published report documented the death of an adult from severe destruction of red blood cells following brown recluse envenomation.

Despite their fearsome reputation, brown recluses are not aggressive – and will generally flee rather than attack a human.

But they thrive inside homes, where they wedge themselves into dark, tight and rarely disturbed spaces – behind picture frames, beneath windowsills and furniture, and inside closets, attics, basements and cardboard boxes.

And where there is one, there may be many.

In one extraordinary case, researchers collected 2,055 brown recluse spiders from a single family home in Kansas in just six months. They were found in every room, including bedrooms, bathrooms and the kitchen.

Yet the family of four had lived there for six years without suffering a single known bite.

That's because most bites occur only when the spider suddenly finds itself trapped against human skin.

'A lot of bites occur in bed,' said Vetter. 'A person may roll over on a spider, and the spider has a last-ditch defensive response.'

Others happen while getting dressed.

'A lot of women get bitten too when they put on skinny jeans,' he said. 'The spiders may crawl into the jeans at night and then, when someone puts them on, the spider gets pushed against the skin, causing it to bite.'

Laura Rosenwald, an entomologist specializing in spiders at the National Pest Management Association, said another danger is reaching into belongings that have been left untouched for months.

'Brown recluses love undisturbed areas, where they wedge themselves into flat, tight spaces, like inside a cardboard box, where you can't see them,' she said.

'And then bites occur when people stick their hands into boxes that they haven't looked in for a while and accidentally trap a spider.'

The good news is that the vast majority of brown recluse bites do not result in serious illness. Estimates suggest around 90 percent heal without major complications.

But there is no antivenom for brown recluse bites in the US, meaning doctors largely have to treat the symptoms while the body deals with the venom.

Immediately after a suspected bite, experts recommend washing the area thoroughly with soap and water, applying an ice pack and, if possible, raising the affected limb to reduce swelling.

Patients should then seek medical advice – particularly because the bite can initially appear relatively innocuous before becoming more serious over the following hours or days.

The most important warning sign is dark urine.

This can indicate that the venom is destroying red blood cells throughout the body, a potentially life-threatening complication known as systemic loxoscelism. Fever, chills, vomiting, dizziness and a widespread rash can also signal a serious reaction.

'It's a particular risk in children,' said Vetter. 'The most critical symptom is having dark urine, usually Coca-Cola colored. That's when immediate treatment is needed.'

And if possible, experts recommend taking a photograph of the spider – or safely bringing the dead spider with you – so that it can be properly identified.

That's important because there is no test that can prove someone has been bitten by a brown recluse, and wounds blamed on the spider are frequently found to have other causes.

There are also steps people can take to reduce the chances of ever being bitten.

Experts recommend regularly vacuuming and clearing clutter from attics, basements, closets and other areas that are rarely disturbed, making them less attractive hiding places.

Cardboard boxes can be swapped for sealed plastic storage containers, while clothing, shoes and bedding should be kept off the floor and shaken out before use – particularly if they have been left untouched for some time.

Sticky traps placed along walls can also help reveal whether brown recluses are living in a home, while gloves should be worn when handling old boxes or other stored belongings.

But despite the gruesome injuries they can cause, Rosenwald stressed that the spiders are not actively hunting humans.

'I always tell people these spiders really aren't heat-seeking missiles, they are not looking to bite you,' she said.

'Most of the time, when they bite, it's really only because the spider is trapped or pressed up against something.

'Taking steps to minimize places for the spider to live in your home, putting down sticky traps, and calling pest exterminators when you see them, can all reduce the risk.'