One Swollen Joint in a Cat: 48 Hours, 2 Tests

One swollen joint on one leg is a local problem until proven otherwise, and the 3 groups are trauma, infection, and immune disease. Infection is the one that ruins a joint fastest: cartilage can be destroyed in 24 to 48 hours, and a cat that walked in on Monday may never bend that knee normally again. Measure both sides, put the 48-hour clock on it, and ask for 2 tests rather than a course of painkillers.

First, prove it is 1 joint and not the whole leg

Run a cloth tape around the limb at 3 points: 2 cm above the joint, at the joint itself, and 2 cm below. Do the same on the opposite leg and write all 6 numbers down. A difference of 5 mm or more at the joint line is real swelling, while a difference that is largest above or below points at the bone or the soft tissue instead. Then feel with 2 fingers for heat, a soft pocket of fluid, and the spot that makes the cat pull away.

A paw swollen from the toes to the wrist, with no single hot joint, is closer to foreign bodies and cellulitis, and reads better against the swollen paw checklist. A cat with several joints swollen at once, on both sides, belongs with the general swollen joint workup instead.

The 3 cause groups

Trauma: a fall from 1.5 m, a bad landing, a bite from another cat, or a road scrape. The joint is hot for 24 to 72 hours, the cat is sore but still eats, and swelling peaks at day 2 or 3. A bite puncture often hides under a 2 mm scab and turns into an abscess in 3 to 5 days.
Septic arthritis: bacteria inside the joint, usually from a bite, a surgery, or a spread from a wound. Signs are a joint that is hot, tight, and painful enough that the cat refuses to put the paw down, a fever of 39.5 C or higher, and worsening over 24 hours rather than 3 days. This is the group that must be tapped and started on treatment within 48 hours.
Immune-mediated or neoplastic: a joint that has been thick for 2 to 6 weeks, mild lameness that comes and goes, and no fever. Immune polyarthritis can start in 1 joint and spread to 3 or 4 within 4 weeks. Bone tumors and synovial tumors are rare in cats but produce a firm, non-painful swelling that does not respond to anything for 4 to 8 weeks.

Why 48 hours is the line

Bacterial enzymes and neutrophil enzymes strip cartilage proteoglycan within 24 to 48 hours of a joint infection, and the loss does not come back. That is why the rule is not "wait and see for a week": a joint that is swollen, hot, and non-weight-bearing for 24 hours gets an exam the same day, and one that is painful but still used gets a firm 48-hour limit. After 48 hours without improvement, or at any point with a fever of 39.5 C, the next step is a tap, not another anti-inflammatory.

Time window	What to check	Numbers that matter	Action
0 to 6 hours	Weight-bearing, heat, 3 tape readings	5 mm difference at the joint line	Photograph, restrict to a 1.5 square meter room
6 to 24 hours	Fever, appetite, paw use	Below 39.2 C, eating half a meal	Call the clinic if no weight-bearing
24 to 48 hours	Size, heat, pain on flexion	Swelling smaller by 20 percent	Exam and tap if unchanged
48 to 72 hours	Fever, wound, new swelling above joint	39.5 C or higher, 1 cm spread	Same-day exam, imaging, drainage
3 to 7 days	Lameness score, muscle loss	Walks 5 steps without a limp	Recheck at 7 days
2 to 6 weeks	Second joint, weight, appetite	1 joint only, weight stable	X-ray and immune workup if it persists

The 2 tests worth asking for

Joint tap: a needle into the joint under brief sedation, pulling 0.2 to 1 ml of fluid. Normal synovial fluid is pale and clear, under 3,000 cells per microliter, and mostly mononuclear. A septic joint usually shows over 20,000 cells per microliter with more than 80 percent neutrophils, plus bacteria on a stained slide. The sample also goes to culture, which takes 2 to 5 days. Cytology costs 60 to 150 dollars and answers the question the same afternoon.

Radiographs: 2 views of the joint, at 90 degrees to each other, because 1 view hides fractures and bone lysis behind the other bones. X-rays show bone, joint space, and soft tissue, not cartilage, so early infection can look normal; a widened joint space or a 1 to 2 mm pocket of gas is a useful hint. Expect 120 to 300 dollars for 2 views, and know that a normal film at hour 24 does not cancel the tap.

Home care: 4 numbers that help

Restrict the cat to 1 room of 1.5 square meters with a litter box that has a 5 cm entry lip, and no jumping surfaces higher than 30 cm for 10 days. A warm compress at 38 C for 10 minutes, 3 times a day, eases stiffness in a joint that is already being treated, but never put heat on a joint that is hot, spreading, or has a bite wound, since heat helps bacteria as much as it helps tissue. Weigh the cat weekly: 10 percent weight loss in 4 weeks means the lameness is costing muscle, not just comfort.

Do not reach for human painkillers. A single 200 mg ibuprofen tablet is a dangerous dose for a 4 kg cat and can cause kidney injury and stomach ulcers within 24 to 72 hours; paracetamol destroys red blood cells and the liver at 1 tablet. If a topical product, a plant, or a human medicine is part of the story, run it through the toxin list before the cat grooms the joint again.

Red flags that skip the wait

The cat will not put the paw down at all for 12 hours, or cries when the joint is touched.
A temperature of 39.5 C or higher, or shivering with cool ears.
Swelling that spreads 3 cm above or below the joint, or a red line running up the leg.
A visible bite puncture, a 2 mm scab, or any wound near the joint.
Lameness that jumps to a second joint within 4 weeks, which points away from trauma.

Five steps for the next 48 hours

Take 6 tape measurements, 3 per leg, and write the numbers with the time.
Photograph the joint from 2 angles at a fixed distance.
Record temperature twice a day to 0.1 C.
Confine to 1.5 square meters, no stairs, no sofa, and no heat on a hot joint.
Ask for a tap and 2 radiographic views at the first exam instead of a 5-day painkiller trial.

One joint, 2 sides measured, 48 hours, 2 tests. That is enough to separate a bruise from an infection before the cartilage pays for the delay.

Terms mentioned in this article

Frequently asked

How do I know if the joint is infected?

Infection moves fast and comes with heat, tight swelling, refusal to bear weight, and often a temperature of 39.5 C or higher within 24 hours, while trauma peaks at day 2 or 3 and slowly improves. Only a joint tap settles it: over 20,000 cells per microliter with more than 80 percent neutrophils points to sepsis, and culture takes 2 to 5 days. Do not wait a week on painkillers.

Can I use a warm compress at home?

Only on a joint that is already being treated and is not hot or spreading: 38 C for 10 minutes, 3 times a day. Never apply heat to a joint with a bite wound, a fever, or swelling that is moving up the leg, because warmth feeds the bacteria as well as the tissue. Cold packs for 5 minutes are safer in the first 24 hours after a known knock.

Is one swollen joint ever arthritis?

Yes, immune polyarthritis can begin in 1 joint and reach 3 or 4 within 4 weeks, and it usually comes without fever at first. A joint that has been thick for 2 to 6 weeks with mild, shifting lameness needs radiographs and immune testing rather than antibiotics. Bone and synovial tumors are rare in cats and produce a firm swelling that ignores treatment for 4 to 8 weeks.