Still’s disease is a rare inflammatory condition that causes high spiking fevers, joint pain, and a distinctive salmon-colored rash. It belongs to a category of diseases driven by an overactive innate immune system rather than the antibody-driven processes behind more familiar autoimmune conditions like lupus or rheumatoid arthritis. The disease has two forms: one that strikes children (called systemic juvenile idiopathic arthritis, or sJIA, with an average diagnosis age around 5) and one that appears in adulthood, typically around age 40. Both share the same core features, though they differ in some important ways.
How Common Is Still’s Disease?
Still’s disease is genuinely rare. Studies from France, Japan, and Turkey estimate the adult-onset form affects between 1.6 and 6.2 new people per million each year, regardless of ethnicity. The overall prevalence sits between 1 and 34 cases per million people. Women are affected somewhat more often than men, making up roughly 67% of adult cases. The childhood form shows a more even split between boys and girls.
The Three Hallmark Symptoms
The combination of a distinctive fever pattern, a fleeting rash, and joint pain is what makes doctors suspect Still’s disease in the first place.
Fever
The fever in Still’s disease follows a “quotidian” pattern, meaning it spikes once daily and then returns to normal between episodes. These spikes typically happen in the late afternoon or evening, and body temperature can swing by as much as 4°C (about 7°F) within just four hours. In roughly 20% of cases, the temperature never fully returns to normal between spikes, or a second spike occurs in the morning, creating what’s called a double quotidian fever.
Rash
The rash is salmon-pink, flat or slightly raised, and tends to appear on the trunk and limbs. It can also show up on the palms, soles, and face. Unlike many inflammatory rashes, it usually doesn’t itch. It comes and goes quickly, often appearing alongside the fever spike and fading as the temperature drops. A hot shower or rubbing the skin can trigger it to appear, a response known as the Koebner phenomenon. Because it’s so fleeting, patients sometimes don’t notice it, and doctors may need to look for it during a fever spike.
Joint Pain and Arthritis
Joint pain is nearly universal. In one large multicenter study, 99% of adults and 91% of children reported it. Morning stiffness affects roughly half to two-thirds of patients. Over time, some people develop true joint inflammation with swelling and damage, while others experience pain without lasting structural changes. The adult form tends to cause less long-term joint damage on average than the childhood form, though chronic cases can still lead to significant erosion.
Other Symptoms to Watch For
Beyond the main trio, Still’s disease can produce a range of other symptoms. Sore throat is especially common in adults, affecting about 78% compared to only 26% of children. Swollen lymph nodes occur in 30 to 43% of patients. Some people develop serositis, inflammation of the lining around the heart or lungs, which is more common in adults (about 32%) than children (19%). An enlarged spleen or liver can also occur.
What Drives the Inflammation
Still’s disease is classified as an autoinflammatory disorder, meaning the immune system’s first line of defense (the innate immune system) becomes overactive without the usual trigger of infection. The exact cause remains unclear, but the inflammation is fueled by a loop of signaling proteins that ramp each other up.
Three of these proteins play central roles. One drives joint inflammation specifically. Another drives both fever and systemic symptoms, and when it’s found at extremely high levels in the blood, it correlates with active fevers. A third protein sits at the intersection, amplifying the entire inflammatory cascade. This understanding has directly shaped treatment: medications that block these specific proteins have transformed outcomes for people who don’t respond to conventional therapy.
How Still’s Disease Is Diagnosed
There is no single test that confirms Still’s disease. Instead, diagnosis relies on a combination of clinical features and blood work, along with ruling out other conditions that can look similar, including infections, cancers (particularly lymphoma), and other autoimmune diseases like lupus or vasculitis.
The most widely used diagnostic framework is the Yamaguchi criteria, which groups features into major criteria (fever, joint pain, the typical rash, and a high white blood cell count) and minor criteria (sore throat, swollen lymph nodes or enlarged spleen, abnormal liver function tests, and negative results for rheumatoid factor and antinuclear antibody tests). Meeting at least five of these criteria, with at least two being major, yields 96% sensitivity and 92% specificity.
The Role of Ferritin
One of the most striking lab findings in Still’s disease is extremely elevated ferritin, a protein that stores iron. Normal ferritin levels range from about 40 to 200 ng/mL. In Still’s disease, levels are typically at least five times higher (above 1,000 ng/mL), and values between 4,000 and 30,000 ng/mL are not unusual. In extreme cases, levels as high as 250,000 ng/mL have been recorded. A specialized test that measures what percentage of that ferritin is glycosylated (a normal modification the body makes) can add further evidence: in Still’s disease, this percentage drops below 20%, compared to the 50 to 80% seen in healthy people. A glycosylated ferritin below 20% has a sensitivity of 78% and specificity of 64% for the disease.
Three Possible Disease Courses
Still’s disease doesn’t follow a single trajectory. Doctors recognize three broad patterns, though it can be hard to predict early on which one a person will follow.
- Monocyclic: A single episode of disease that resolves within months, sometimes up to a year, and doesn’t return. This accounts for roughly 19 to 44% of cases.
- Polycyclic: Flares that come and go over time, with symptom-free intervals between them. This pattern affects about 10 to 41% of patients.
- Chronic: Persistent, ongoing disease that doesn’t fully remit. This is the most common pattern, estimated at 35 to 67% of patients in studies from before biologic therapies became available. It’s more likely to lead to lasting joint damage.
Treatment Approaches
Treatment depends on how severe the disease is and whether the main problem is systemic inflammation (fevers, rash, organ involvement) or joint disease.
Corticosteroids are the first-line treatment and are used in the majority of cases, roughly 85% of children and 64% of adults in large studies. They’re effective at controlling symptoms quickly, but long-term use carries well-known side effects, so the goal is always to taper the dose as soon as possible.
For people whose disease doesn’t respond well to steroids, or who can’t reduce their steroid dose, biologic therapies that target specific inflammatory proteins have become the standard next step. When systemic symptoms like fever and rash dominate, medications that block the inflammatory protein most responsible for those symptoms tend to work best and are considered the first choice for refractory systemic disease. When joint inflammation and erosion are the bigger problem, a different class of biologic that targets the protein more closely tied to joint disease may be more effective and can also help with systemic symptoms. These biologics have a significant steroid-sparing effect, meaning they allow patients to reduce or stop steroids.
Macrophage Activation Syndrome
The most dangerous complication of Still’s disease is macrophage activation syndrome (MAS), a life-threatening condition in which the immune system spirals completely out of control. Immune cells begin destroying the body’s own blood cells and tissues, causing organ damage. It occurs in about 24% of children with the disease and 10% of adults.
Three factors independently raise the risk of developing MAS: an enlarged spleen, inflammation of the heart’s lining (pericarditis), and ferritin levels above 2,000. The mortality rate for MAS in Still’s disease patients is approximately 27%, and developing it significantly reduces overall survival. Signs include a rapid drop in platelet count, abnormal clotting, and paradoxically falling (though still elevated) ferritin. MAS requires immediate, aggressive treatment and is the primary reason Still’s disease demands close monitoring even when symptoms seem manageable.
Differences Between the Adult and Childhood Forms
While adult-onset Still’s disease and the childhood form share the same core features, they aren’t identical. Adults take slightly longer to get diagnosed (a median of about 7 weeks from symptom onset compared to 4 weeks for children). Adults more commonly have a sore throat, swollen lymph nodes, and inflammation around the heart or lungs. Children, on the other hand, are more likely to develop the characteristic rash and face a higher risk of MAS. Children also tend to need steroids more often and are more likely to accumulate long-term joint damage. Adults have a higher rate of anemia at diagnosis, with 37% showing low hemoglobin compared to 14% of children.