Reactive arthritis

Short answer

Reactive arthritis is joint inflammation triggered by an infection somewhere else in the body — usually a gut infection (food poisoning) or a sexually transmitted infection like chlamydia — that begins one to a few weeks after the infection itself. It typically swells the large joints of the lower limbs, the knees, ankles and feet, usually on one side, and can bring heel pain, a "sausage" toe, eye redness and urinary symptoms. It most often affects adults roughly 20 to 40, and the good news is that it usually settles on its own within a few months to a year.

Note: this article explains what reactive arthritis is and is not medical advice. A hot, very painful joint with a fever can be a joint infection and needs urgent care — see the red flags below. Only a clinician can diagnose reactive arthritis.

The delay is why almost nobody connects the dots

If a joint has swollen up weeks after an infection, you are not imagining the link and it is not a coincidence you invented — the delayed connection is exactly how this condition works, and it is real and recognised. It is also not your fault that no one joined the dots earlier: the gap between infection and joint is precisely what makes reactive arthritis easy to miss, even for doctors, unless the recent infection is mentioned.

The strange and defining thing about reactive arthritis is the gap. You have a bout of food poisoning or a urinary or genital infection, it passes, and then a week or several later a knee or ankle blows up — and the two feel completely unrelated. By the time the joint hurts, the original infection may be gone, so people rarely mention "I had a stomach bug three weeks ago" to the doctor looking at their swollen knee. That missing link is exactly what makes the diagnosis, so it is worth knowing about.

This is not something to sort out at home, and this article will not try to diagnose you. What it will do is give you the pattern — a joint flaring up in the weeks after an infection, in a young-ish adult — so you can recognise it, mention the recent infection, and get it assessed. The reassuring part, which most people do not know, is that reactive arthritis usually improves and often resolves entirely.

What it feels like

The symptoms follow a recognisable shape, drawn from how doctors identify it.

Joints, lower limbs, usually one side. Pain, swelling and stiffness most often in the knees, ankles and the joints of the feet — the large joints of the lower body — and characteristically asymmetric, hitting one side rather than mirroring across the body. That lopsidedness is one thing that separates it from rheumatoid arthritis.

Heel and foot pain (enthesitis). Like its relatives in the spondyloarthritis family, reactive arthritis inflames the spots where tendons attach to bone, so pain at the back of the heel or in the sole of the foot is common and often the most stubborn symptom.

A "sausage" toe or finger (dactylitis). A whole toe or finger can swell along its length rather than at one joint — the same sausage-digit sign seen in psoriatic arthritis.

The three-part pattern: joints, eyes, urinary. Classically reactive arthritis can combine arthritis with eye inflammation (red, irritated eyes) and urinary or genital symptoms such as discomfort passing urine. The important caveat: most people do not get all three together, so their absence does not rule it out. But eye redness or urinary symptoms alongside a swollen joint after an infection is a strong clue.

Low back or buttock stiffness. Because it belongs to the spondyloarthritis group, some people get inflammatory back or sacroiliac-joint stiffness too.

The infections that set it off

Reactive arthritis is a reaction to an infection, not an infection in the joint itself — the joint is inflamed but not infected. Two routes account for most cases.

Gut infections. A bout of bacterial food poisoning — the kind that causes diarrhoea — can trigger it a week or more later. Here it affects men and women roughly equally.

Genital/urinary infections. Sexually transmitted infections, chlamydia in particular, are the other common trigger, and this route is more common in men.

The joint symptoms typically start one to a few weeks after the triggering infection — long enough that the infection may already have cleared, which is why the timeline is so easy to miss. There is also a genetic thread: people who carry the HLA-B27 gene are more prone to reactive arthritis and to the wider spondyloarthritis family it belongs to.

Reactive arthritis vs the arthritis it gets confused with

Because it is inflammatory and swells joints, reactive arthritis can be mistaken for rheumatoid or psoriatic arthritis. A few features point specifically at it:

• A recent infection — a gut bug or an STI in the previous few weeks — is the biggest tell, and the one only you can supply.

• Lower-limb, asymmetric joints (one knee, one ankle) rather than the symmetric small-hand-joint pattern of rheumatoid arthritis.

• Eye or urinary symptoms alongside the joints.

• It usually resolves over months, unlike the chronic course of rheumatoid arthritis.

• Blood antibodies (rheumatoid factor) are typically negative, which helps a doctor separate it from rheumatoid arthritis.

The overlap with psoriatic arthritis and ankylosing spondylitis is real — they are all in the spondyloarthritis family and share heel pain, sausage digits and HLA-B27 — so the recent-infection history is often what tips the diagnosis toward reactive arthritis specifically.

How it is diagnosed

There is no single test that confirms reactive arthritis. A doctor puts it together from the story — the recent infection is central — plus the joint exam and tests that point at the trigger and rule out other causes. That can include stool or urine tests or swabs to identify the original infection (even if it has settled), and a sample of fluid from the swollen joint, importantly to exclude a joint infection and gout, which need different treatment. Blood tests for rheumatoid factor and inflammation help place it against other arthritis. HLA-B27 may be checked as supporting evidence but is not required.

The single most useful thing you can bring is the timeline: the infection, when it happened, and when the joint started.

What your own data can add

A wearable cannot diagnose reactive arthritis — that is clinical, and the recent-infection history is the key. But reactive arthritis sits in the spondyloarthritis family, and inflammatory arthritis in that family shows a measurable flare signature: an activity-tracker study that included axial spondyloarthritis found that flares showed up as a real drop in daily movement, and related work in inflammatory arthritis saw resting and night-time heart rate and HRV rhythm shift around flares. When inflammation is active, the body works harder and moves less, often before you have fully registered it.

The reading only means something against your own baseline — "normal" is personal. In Welltory data from about 5,000 people who track with a wearable, resting heart rate alone spans the low 50s to the mid 70s, so no universal number flags inflammation; what matters is a shift away from your usual. Welltory measures heart rate, HRV, sleep and activity from your phone or watch, so if a joint flares in the weeks after an infection you have a record of how your body was behaving — useful context to bring to a doctor. (These figures describe a tracking cohort skewing older, around 40–72, not the whole population — they show how wide the spread is, not a target.)

What it cannot do: tell reactive arthritis from other inflammatory arthritis, or see whether a joint is inflamed or infected. Its job is to help you track how a flare rises and settles over the months this condition usually runs — and to notice if, unusually, it is not settling.

Will it go away? The reassuring part

For most people, yes. Reactive arthritis is usually self-limiting: it commonly clears on its own over roughly three to twelve months. That is a genuinely different outlook from rheumatoid or psoriatic arthritis, which are lifelong, and it is worth holding onto if you have just been diagnosed.

A minority are less lucky — the arthritis can become chronic or come back, especially in people who carry HLA-B27 — which is why follow-up matters even as things improve. But the default expectation is recovery, and treatment in the meantime is about controlling symptoms and clearing any ongoing infection rather than committing to years of medication.

It is worth setting expectations honestly on timing, because "a few months to a year" is a wide range and the middle of it can feel discouraging. Improvement is usually gradual rather than a clean switch-off: the swelling and pain ease in steps, with better and worse weeks along the way, and the heel or foot pain of enthesitis is often the slowest part to go. A wobble does not mean it is becoming chronic. What matters is the overall trend across weeks — and if that trend is genuinely flat or worsening past a couple of months, that is the cue to go back rather than wait the full year out.

Treatment, in brief

Management has two parts. First, treat any infection that is still active — antibiotics if there is a current bacterial infection, though antibiotics do not help the arthritis once the trigger has cleared. Second, control the joint inflammation and keep moving: anti-inflammatory painkillers (NSAIDs) are the mainstay, steroids may be used for stubborn joints, and physical activity or physiotherapy helps maintain movement and function. In the uncommon cases that turn chronic, disease-modifying drugs used in related arthritis may be added. Most people improve steadily.

Living with it while it runs its course

Because reactive arthritis usually resolves, most of what you do while it lasts is about staying comfortable and keeping function, not fighting a lifelong disease — and here the daily pattern genuinely helps.

The instinct with a swollen, painful joint is to rest it completely, but full immobility tends to leave joints stiffer and weaker. The more useful approach is gentle, steady movement within what the joint tolerates — the same "keep moving, adapt the load" principle that runs through the whole spondyloarthritis family. During the worst of a flare you dial the load down; as it settles you build back up. Enthesitis complicates this: because the disease inflames where tendons meet bone, pushing hard through heel or foot pain can aggravate the exact spot that is inflamed, so moderate beats either rest or grind.

This is where watching your own trend earns its place. Reactive arthritis runs over months, and it is genuinely useful to see whether it is on the mend or, unusually, not. If your activity is creeping back up and the flare signs are fading week over week, that is the expected recovery. If your movement stays suppressed and the joint is not settling after a couple of months, that is worth flagging, because it is the minority path — chronic or recurring disease — that benefits from earlier escalation. A tracker will not tell you which path you are on, but it makes the direction of travel visible instead of leaving you guessing between good and bad days.

And because reactive arthritis often follows a preventable infection, the other honest lesson is upstream: safe food handling and protection against sexually transmitted infections reduce the triggers in the first place. It is not a guarantee — plenty of infections are unavoidable — but it is the one genuinely preventive lever here.

When to see a doctor — and when it is urgent

• A swollen, painful joint in the weeks after a gut infection or an STI — mention the infection explicitly.

• Heel or foot pain, a sausage toe, or eye redness alongside the joint.

• Urgent: a single hot, very painful joint with a fever can be a joint infection (septic arthritis), a medical emergency that must be excluded fast — do not assume a hot joint is reactive arthritis.

• Eye pain or changes in vision, rather than just mild redness, need prompt eye assessment.

How to bring this up with your doctor

Lead with the timeline. This is the one condition where the sentence that unlocks the diagnosis is about something that already ended: "About three weeks ago I had food poisoning [or a genital infection], and now my knee is swollen and my heel hurts." The recent infection is the single most useful thing you can offer, and it is the thing a doctor cannot know unless you say it.

Bring the specifics. Which joints, one side or both, any heel or foot pain, any eye or urinary symptoms, and the date the infection started versus the date the joint did. If you track, note how your activity and resting heart rate behaved as the joint flared.

What to ask for.

• Whether this could be reactive arthritis, given the recent infection — say it plainly, because the infection link is easy to overlook.

• Tests to identify the triggering infection and to exclude a joint infection and gout (a joint-fluid sample), plus rheumatoid factor to help separate it from rheumatoid arthritis.

• What to expect for recovery, and what would prompt escalation if it does not settle.

If you are brushed off with "you just have a sore knee" when you had an infection weeks ago and now have a swollen lower-limb joint, name the infection again and ask whether reactive arthritis is being considered. That link is exactly what makes the diagnosis.

How we made it

Written from clinical sources — NIAMS, Mayo Clinic and Cleveland Clinic — for the causes, symptoms, course and diagnosis, plus a peer-reviewed activity-tracker study that included spondyloarthritis for the flare-and-movement angle. Where a popular detail is not in the primary sources (the old "can't see, can't pee, can't climb a tree" mnemonic), we left it out.

Written by the Welltory science team Data analysis by the Welltory data team Reviewed by Anna Elitzur, MD

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