Joint Pain That Starts After an Infection

A few weeks after a bout of food poisoning or a urinary infection, a knee swells. Then an ankle. The original illness is long over, nothing grows on any test, and antibiotics make no difference at all.

This is reactive arthritis: joint inflammation triggered by an infection elsewhere, rather than infection of the joint itself. The distinction sounds academic and it governs the entire treatment.

Triggered, not infected

In septic arthritis, bacteria are inside the joint and it is an emergency. In reactive arthritis, the joint is sterile — the immune system, having responded to an infection somewhere else, goes on reacting in the joints afterwards.

That is why joint fluid grows nothing, why antibiotics do not help the arthritis, and why the treatment is anti-inflammatory rather than antimicrobial.

Two very different things that both cause a swollen joint

	Septic arthritis	Reactive arthritis
Onset	Hours to a couple of days	One to four weeks after an infection
Joints involved	Usually one	Often several, asymmetric, lower limb
Fever	Usually, often high	Mild or absent
Joint fluid	Bacteria present	Inflammatory but sterile
Urgency	Emergency — same day	Prompt assessment, not an emergency
Treatment	Drainage and antibiotics	Anti-inflammatories; not antibiotics

Which infections trigger it

• Gut infections — campylobacter, salmonella, shigella, yersinia; see norovirus and food poisoning for the acute illness
• Chlamydia, which is the commonest genitourinary trigger; see what a full STI panel covers
• Occasionally other urinary or respiratory infections
• Rarely, following certain vaccinations — far less often than claimed
• In many cases no trigger is ever identified, which does not change the diagnosis

The original infection is often mild and sometimes passes unnoticed, which is why the connection is missed. A gut upset three weeks before a swollen knee rarely feels related to the person experiencing it.

What it looks like beyond the joints

Reactive arthritis is not only a joint condition, and the associated features are often what makes the diagnosis.

• Red, painful or light-sensitive eyes, which need assessment; our article on pink eye covers the ordinary causes that this is not
• Pain where tendons attach to bone — the heel is classic
• A whole finger or toe swollen rather than one joint
• Lower back and buttock pain from inflammation of the sacroiliac joints
• Skin changes on the palms or soles, and nail changes
• Mouth ulcers, which are usually painless
• Urinary symptoms without infection

Who gets it

Reactive arthritis is uncommon after any single infection — a small percentage of people who have the triggering illness — and a few things shift the odds.

• Age twenty to forty is the usual range, though it occurs outside it
• Men and women are affected in similar numbers after gut infections; genitourinary triggers are reported more often in men
• Carrying the HLA-B27 gene makes it more likely and more likely to be severe
• A previous episode makes another more likely
• The severity of the original infection does not predict it well
• Family history of related conditions, including ankylosing spondylitis and psoriasis

None of this is worth testing for in advance. It matters because it explains why one person in a household affected by the same food poisoning develops joint pain and nobody else does.

How it is diagnosed, and why the tests are mostly negative

There is no test that confirms reactive arthritis. It is a clinical diagnosis made by recognizing the pattern and excluding the things it resembles.

1. Joint aspiration where a joint is hot and swollen, principally to exclude infection
2. Inflammatory markers, usually raised; see what CRP and ESR mean
3. Tests for the triggering infection — stool, urine or chlamydia testing
4. Tests to exclude other causes, including gout and rheumatoid arthritis
5. HLA-B27, which is associated but neither confirms nor excludes
6. Imaging, which is often normal early on

Patients frequently find the run of negative results unsettling, as though nothing has been found. In this condition the negatives are the finding.

One question comes up repeatedly and deserves a direct answer: whether this is the start of a lifelong arthritis. For most people it is not. The majority of cases resolve and do not return, and even among those who have a second episode, most do not go on to a chronic condition. The minority who do are identified over months rather than at the first appointment, which is frustrating and is the honest position.

Living with it while it settles

The months before it resolves are the hard part, and a few practical things make a measurable difference.

• Keep moving within the limits of the pain; immobility stiffens joints faster than the arthritis does
• Use anti-inflammatories regularly rather than occasionally, if they are suitable for you
• Ask about physiotherapy early rather than after months of guarding the joint
• Report new eye symptoms the same day rather than waiting for the next appointment
• Expect fluctuation; a bad week does not mean the condition is progressing
• Tell any clinician treating you that this followed an infection, because the connection is easily lost between departments

Treatment and what to expect

Anti-inflammatory drugs are the mainstay, sometimes with steroid injection into a badly affected joint. Where the trigger was chlamydia, that infection is treated — for its own sake and for partners, not because it will resolve the arthritis.

• Most cases settle within three to six months
• A minority run longer, and a small proportion become chronic
• Recurrence happens, sometimes after another infection
• Gentle continued movement preserves function better than rest
• Persistent cases are managed by rheumatology, sometimes with a biologic
• Treating the trigger promptly may reduce the chance of it happening at all

If joint pain follows an infection and nobody has connected the two, saying so can shorten the process considerably. Our article on what happens at an infectious disease consultation covers how that history gets taken, and you can reach us through the contact page.

Frequently asked questions