Doctors and the public incorrectly diagnose countless suspicious red bumps and rashes as spider bites every year.

Of these, much of the blame falls on the brown recluse, a somewhat drab, venomous spider native to parts of the U.S. Yet, brown recluse bites are actually quite rare, says Rick Vetter, a retired professor of entomology at the University of California Riverside. 

These misdiagnoses not only unfairly demonize spiders minding their own business, but they can also distract from other serious, treatable medical conditions that are the real cause of symptoms.

The Misunderstood Brown Recluse Spider

One reason behind the frequent misdiagnoses is that doctors and the public know very little about brown recluses, making the spiders an easy scapegoat.

In many cases, however, having a basic understanding of brown recluses can rule out this spider bite diagnosis. 

For starters, the range for brown recluses is limited to about 15 states within the south-central U.S. So, if you live in the Western half of the country, the Upper Midwest or the Northeast, it’s likely not related to a recluse.

Brown recluses also tend to flourish in human-modified areas, just like rats and cockroaches. In most cases, when there’s one recluse there are dozens, if not more.

So, if you’re unable to find any spiders at the alleged attack site, a brown recluse likely isn’t responsible.

What Does a Brown Recluse Look Like?

Adding to the confusion, most people couldn’t identify a recluse from a lineup of harmless brown spiders.

“To most people, there are four spiders in America – tarantula, black widow, brown recluse and either a jumping spider or a garden spider,” says Vetter, adding a bit of humor: “It’s easy to identify a tarantula and a black widow — and maybe a jumping spider. All the other 4,000 species in North America [must be] brown recluses.”

A dorsal view of the brown recluse spider shows its violin-shaped trademark. Unfortunately, the misidentification of other spiders as brown recluses leads to a false perception of their widespread presence, creating an exaggerated sense of risk.

In reality, as the name suggests, recluses tend to keep to themselves. They never build their webs out in the open, but rather hide them in tight, dark spaces.

Read More: The Bewitching Dance of Australia’s Peacock Spiders

What Does a Brown Recluse Bite Look Like?

In the rare event that a brown recluse does bite a human, only about 10 percent of cases have serious consequences. Vetter outlines four tiers of recluse bites: 

• Tier 1 represents the majority of bites: “Virtually nothing happens,” he says, and the bite heals on its own.

• Similar to tier 1, tier 2 bites heal on their own. The only difference is that these bites also bring with them a symmetrical rash around the bite area. 

• Only about 1 in 10 recluse bites fall into tier 3. These bites turn necrotic, meaning blood flow gets shut down around the bite and cells that are deprived of oxygen start to die.

• Tier 4 holds less than 1 percent of recluse bites, which leads to a condition called “systemic loxoscelism.” In these rare-yet-potentially-deadly cases, the capillary bed is destroyed causing hemoglobin to float freely in the blood. As a result, bite victims have dark, almost cola-colored urine. Bites in this tier require immediate medical attention.

What Else Could This Bump Be?

While the medical literature is full of instances of conditions being misdiagnosed as brown recluse bites, that leaves the burning question: What are they really?

The culprits fall into several broad categories: 

• Bacterial, fungal or viral infections

• Reactions to medication

• Bites from insects or other arthropods

• Other medical conditions

Also, why are medical professionals so quick to overlook these other conditions in favor of a recluse diagnosis?

Part of the problem, in Vetter’s opinion, starts in medical school, where an incorrect diagnosis of a spider bite can be used to teach students – creating a misdiagnosis feedback loop.

Read More: Why Do Mosquitoes Love Biting Some People More Than Others?

Consequences of Misdiagnosis

Misdiagnosing a skin lesion as a recluse bite comes with a wide range of consequences.

On the most harmless end of the spectrum, the problem fixes itself, and you have a nice story to tell your friends. “ ‘I survived a spider bite’ just sounds cooler than ‘I survived a bacterial infection,’ ” jokes Vetter.

However, on the opposite end of the spectrum, a misdiagnosis could mean overlooking a serious condition that requires medical attention. 

For example, Lyme disease, which responds well to early antibiotic treatment, may be wrongly identified as a recluse bite because both can cause a rash around the bite site. Failure to properly treat Lyme can allow the disease to progress and lead to lasting health problems.

In the same vein, methicillin-resistant Staphylococcus aureus or MRSA, an antibiotic-resistant and deadly bacterial infection, can be confused with recluse bites. If left untreated, MRSA can enter the bloodstream and quickly turn deadly. 

Making Informed Diagnoses

Spider bite diagnoses seem to have eluded the principles of evidence-based medicine, some experts argue.

“It’s amazing,” Vetter says, “how [doctors] are so willing to blame spiders.”

To ensure accurate diagnoses for all conditions, it’s essential to give credit where credit is due and rely on evidence rather than mere hunches or hype.

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