Kawasaki disease, and juvenile idiopathic arthritis (JIA) can each present as a febrile child with a rash, joint pain, or a cardiac finding, and the three are frequently confused. They are separated by mechanism, by age, by the pattern of fever and rash, by which joint is involved, and above all by what happens to the heart.
The reason to separate them carefully is that the important decisions are time-limited. Kawasaki disease needs treatment with (IVIG) within 10 days of fever onset, and rheumatic fever prophylaxis depends on a streptococcal infection being recognised. A child who is treated late for Kawasaki disease may carry a coronary aneurysm for life.
Three diseases, three mechanisms
The three diseases differ first in what the immune system attacks. Kawasaki disease is a vasculitis — inflammation of the vessel wall — of medium-sized arteries; acute rheumatic fever is an immune cross-reaction in which the response to a group A streptococcal throat infection also attacks the child’s own tissues; JIA is a chronic synovitis, inflammation of the joint lining, except in , which is autoinflammatory (driven by the innate, non-specific arm of the immune system). In the table, RF is rheumatoid factor, an autoantibody that defines one category of JIA.
| Kawasaki disease | Acute rheumatic fever | Juvenile idiopathic arthritis | |
|---|---|---|---|
| Mechanism | Medium-vessel vasculitis | Immune-mediated cross-reaction after streptococcal pharyngitis | Chronic immune-mediated synovitis; systemic JIA is autoinflammatory |
| Trigger | Unknown; probably infectious in a susceptible child | Rheumatogenic group A Streptococcus | Unknown |
| Typical age | 9-11 months; about 80% under 5 years | 5-15 years | Oligoarthritis 1-2 years; polyarthritis school age or adolescence; systemic JIA any age |
| Sex | Slight male predominance | Equal; chorea more common in girls | Female predominance in oligoarticular and RF-positive polyarticular disease |
| Fever | Sustained, high, resistant to antipyretics and antibiotics, at least 5 days | A minor criterion; not the presenting focus | Systemic JIA: daily quotidian spikes. Other subtypes: usually afebrile |
| Rash | Polymorphous maculopapular exanthem | Erythema marginatum (uncommon, under 10%) | Systemic JIA: evanescent pink rash appearing with the fever spike |
| Joints | Sometimes involved, especially in atypical presentations | Migratory, transient, asymmetric, large joints | Persistent: oligoarthritis affects up to 4 joints; polyarthritis at least 5, symmetric small joints when RF-positive |
| Heart | Coronary artery dilation and aneurysm | Valvulitis, mitral valve first; pancarditis; subclinical carditis on Doppler | Pericarditis in severe systemic JIA is the exception |
| Time-critical step | IVIG within 10 days of fever onset | Treat streptococcal pharyngitis within 9 days to prevent a first attack; long-term prophylaxis after an attack | Early, effective disease-modifying treatment before erosive damage |
| Prevention | None specific | secondary prophylaxis | None specific |
The fever pattern
Of the features that can be checked at the bedside, the pattern of the fever is the most useful discriminator, and it costs nothing to establish.
- Kawasaki disease — fever that does not respond to antipyretics or to antibiotics, lasting 5 days or more without treatment, and continuous rather than spiking.
- Systemic JIA — one or two high spikes every day with the child returning towards normal in between, the pattern, with the rash appearing at the height of each spike. Parents often give a better account of the rhythm than a chart does.
- Acute rheumatic fever — fever is present in most cases but is a minor criterion (a supporting rather than defining finding in the diagnostic rules), and the child’s illness is usually dominated by joint pain.
Persistent fever in a child with no clear focus is also the presentation of serious bacterial infection, urinary tract infection, and leukaemia, so the fever pattern is a starting point rather than a conclusion.
The rash
A rash occurs in all three diseases, but each looks different and appears at a different moment, so where and when it is looked for matters.
- Kawasaki disease — a pink maculopapular rash on trunk and limbs, indistinguishable from a viral exanthem on its own. Its role is to count towards the diagnostic criteria, not to identify the disease.
- Acute rheumatic fever — erythema marginatum: non-pruritic reddish rings with a pale centre, on trunk and proximal limbs, appearing in under 10% of cases. The distinction from the migratory erythema of Lyme disease rests on exposure history.
- Systemic JIA — an evanescent salmon-pink rash that comes and goes with the fever spikes; it may be visible only when the child’s temperature is high, so it should be looked for during a spike rather than between them.
- Scarlet fever, a differential for all three because of the shared , produces a fine sandpaper-like rash that is accentuated in skin creases () with circumoral pallor, together with pharyngitis and a positive streptococcal test.
Strawberry tongue — a red tongue with prominent papillae — occurs in Kawasaki disease, scarlet fever and toxic shock syndrome, and is therefore not a discriminator by itself. It is the company it keeps that decides: in Kawasaki disease the other principal features or a coronary abnormality are present; in scarlet fever there is pharyngitis, a sandpaper rash and a positive test; in toxic shock syndrome there is shock and multi-organ dysfunction.
The joints
Joint involvement separates rheumatic fever from JIA by its behaviour over time — moving from joint to joint in one, persisting in the same joints in the other — and it also raises two alternatives that must not be missed: infection of the joint and leukaemia. In the table, ANA are antinuclear antibodies, autoantibodies against components of the cell nucleus.
| Pattern | Suggests |
|---|---|
| Migratory, transient, asymmetric, large joints (knee, elbow), after a sore throat | Acute rheumatic fever |
| Persistent involvement of up to 4 joints, most often the knee, in a toddler, with ANA positivity | Oligoarticular JIA |
| Persistent symmetric small-joint disease of hands and feet in an adolescent girl, with morning stiffness | RF-positive polyarticular JIA |
| Arthritis affecting 5 or more joints within the first 6 months | Polyarticular JIA |
| A single hot, exquisitely painful joint | Septic arthritis — aspiration is required, whatever else is being considered |
| Joint pain with bone pain and cytopenia, worst at night | Leukaemia — bone marrow examination is needed |
Morning stiffness and symmetric small-joint involvement belong to JIA; migration from one large joint to another over days belongs to rheumatic fever; a single inflamed joint belongs to septic arthritis until proven otherwise. Monoarticular joint swelling that is chronic and relatively painless, in a child, fits oligoarticular JIA, but tuberculous arthritis and chronic infection must also be considered.
The heart
Both Kawasaki disease and rheumatic fever can damage the heart, but they damage different parts of it, and the imaging finding tells you which disease you are dealing with:
- Kawasaki disease attacks the coronary arteries. The finding is dilation or aneurysm of the coronary arteries, graded by Z-score (the number of standard deviations from the expected diameter for the child’s body surface area). A normal echocardiogram early in the illness does not exclude the diagnosis, because aneurysms develop over the following weeks, so the study is repeated at 1-2 weeks and 4-6 weeks.
- Rheumatic fever attacks the valves. The findings are mitral regurgitation, then aortic regurgitation, with subclinical regurgitation detectable on Doppler echocardiography even when no murmur is audible. Pericarditis and myocarditis occur as part of pancarditis — inflammation of all three layers of the heart — and first-degree atrioventricular block is the common electrocardiographic finding.
- Systemic JIA causes pericarditis in its severe form, and rarely myocarditis. Coronary aneurysms are not part of its picture.
Echocardiography is therefore the single most useful discriminator in a febrile child with possible cardiac involvement, and it is indicated in both suspected Kawasaki disease and suspected rheumatic fever.
Laboratory clues
No single blood test separates the three diseases. The tests are most useful read alongside the clinical pattern: some point towards one disease, and two combinations flag a dangerous alternative. CRP is C-reactive protein and ESR the erythrocyte sedimentation rate, two general markers of inflammation. Pancytopenia means a fall in all three blood cell lines, and (MAS) is the uncontrolled immune activation that complicates systemic JIA.
| Test | What it suggests |
|---|---|
| CRP and ESR | Raised in Kawasaki disease and in active rheumatic fever; both may be normal in oligoarticular JIA |
| Antistreptolysin O (ASO) or anti-DNase B titre, rising on serial samples | Preceding group A streptococcal infection. A single raised value is common in healthy children of school age; a normal value does not exclude rheumatic fever, because titres are raised in roughly 85% of patients |
| Positive throat culture or rapid antigen test | Streptococcal pharyngitis; often negative by the time rheumatic fever appears |
| Platelets rising after day 7 of fever | Supports Kawasaki disease retrospectively |
| Sterile pyuria | Kawasaki disease, and useful for separating it from urinary tract infection |
| ANA | Positive in roughly three-quarters of oligoarticular JIA; marks a higher risk of |
| Rheumatoid factor | Positive on two occasions at least 3 months apart in the RF-positive polyarticular category |
| Pancytopenia with very high ferritin, high triglycerides and low fibrinogen | Macrophage activation syndrome until proven otherwise |
| Pancytopenia with blasts and bone pain | Leukaemia until proven otherwise |
Children who do not fit any of the three
The comparison works when a child looks like one of the three diseases. Three situations fall outside it, and in each the first priority is different.
The infant under 6 months with prolonged fever. This is the group in which Kawasaki disease is most often missed, because infants present with fewer features. An infant of 6 months or younger with fever for 7 days or more and no other explanation should have laboratory testing, and an echocardiogram if there is evidence of systemic inflammation, even with no principal features. Urinary tract infection and serious bacterial infection must be excluded in parallel.
The child with a single inflamed joint. Aspirate first. Septic arthritis, osteomyelitis and, less acutely, tuberculous arthritis all present this way, and JIA is a diagnosis of exclusion. Where the presentation is chronic, monoarticular and relatively indolent, oligoarticular JIA is likely but the exclusion list still has to be worked through.
The post-COVID hyperinflammatory presentation. Multisystem inflammatory syndrome in children is a distinct post-infectious illness following SARS-CoV-2 infection, with persistent fever, gastrointestinal symptoms, rash, conjunctivitis, myocardial dysfunction and, in some children, coronary involvement. It overlaps substantially with Kawasaki disease and is distinguished by age, by the epidemiological link to a preceding infection, and by a pattern of organ involvement with more frequent gastrointestinal and myocardial dysfunction and more marked inflammation. It is managed differently, and it is a reminder that a child who meets Kawasaki criteria today may not have the classic disease.
Order of operations
When a febrile child could have any of the three, the sequence that loses least time is:
- Document the fever pattern and duration, and examine the whole child — eyes, mouth, hands and feet, neck, skin, joints, and heart.
- Look for evidence of streptococcal infection with a throat swab or rapid test, and send serology ( and ) with a plan to repeat in 2-3 weeks.
- Do an echocardiogram if Kawasaki disease or rheumatic fever is possible. It is the test that separates the two.
- Send the inflammatory and supportive tests — full blood count, CRP and ESR, liver enzymes, albumin, urinalysis — which serve the Kawasaki algorithm and give a baseline for the others.
- Do not wait for a titre to come back before treating Kawasaki disease if the clinical criteria are met: IVIG within 10 days of fever onset is time-critical, and the serology results will not change the decision.
- Consider bone marrow examination and blood film if fever, cytopenia or bone pain suggest leukaemia, and consider MAS if the fever becomes continuous and the counts fall.