A single hot, puffy joint in a cat is a different problem from the general stiffness of old age. One joint that swells over 12 to 24 hours, feels warmer than the matching joint on the other leg, and makes the cat carry the limb is usually infection, trauma or an immune reaction, and only one of those three waits comfortably. Measure the joint in millimeters, take the temperature, and let 24 hours of records and a 39.5 C reading decide the visit.
Heat, fluid wave and pain in 2 minutes
Use the back of your hand, not your fingers: it reads temperature better. Hold it on the joint for 5 seconds, then on the same joint of the other leg for 5 seconds. A difference you can feel after 5 seconds on each side is real heat. For fluid, press one side of the swelling with 1 finger and watch the opposite side: a bulge that rises 3 to 5 mm is a fluid wave, meaning the capsule is distended rather than the bone being thick. Pain shows as a 1 to 2 second hold of the limb or a bite when you flex past 30 degrees. If the puffiness sits at the toes and pads with a normal joint line, work from swollen paw instead.
The 24-hour observation window
Trauma and mild inflammation improve inside 24 hours. Write down 3 numbers at hour 0, hour 12 and hour 24: circumference in mm, the temperature to 0.1 C, and grade of weight bearing from 1 to 4. A joint 5 mm smaller at hour 24, with 20 percent more weight on the leg, can keep being watched. A joint that is unchanged or 2 mm larger at hour 24 moves to a visit, because a bite abscess declares itself between 24 and 72 hours and a septic joint gets worse, never better. Photograph the joint beside the tape at each check, from 30 cm.
Temperature above 39.5 C changes the plan
Normal rectal temperature in a cat is 38.1 to 39.2 C. Read 39.5 C and above as a systemic signal: a cat with a hot joint and 39.5 C is far more likely to have infection than a strain, and the visit moves from 48 hours to 6. At 40.0 C or higher, or with breathing over 40 breaths a minute at rest, go the same evening. Under 37.5 C is the other emergency, because a cold cat with a swollen joint may be septic. Take the temperature before touching the joint, since handling adds 0.2 to 0.4 C for 10 minutes.
Three groups of causes, and how they differ
Infection comes first in any single joint: a cat bite delivers bacteria 5 to 10 mm deep, the skin closes over it, and an abscess forms in 24 to 72 hours with heat, fluid and a cat that stops eating. Look for 2 small puncture marks 5 to 10 mm apart, a 1 cm scab, or a claw wound on the same leg. Trauma comes second: a fall from 1.5 m or a twist on landing distends a capsule, with swelling worst at 36 to 48 hours. Immune-mediated disease comes third and fools owners: it can start in 1 joint and reach 3 or 4 within 2 weeks; a cat under 5 years with a hot joint, no wound, no fever above 39.5 C and no response to 48 hours of rest belongs here. A cat over 8 years old with a firm, painless swelling growing over 4 to 8 weeks needs imaging, not observation. A soft, movable 1 to 3 cm lump in the groin or armpit is more likely a node, and swollen lymph nodes have a different workup.
Grade the joint on a 4-level scale
| Grade | Difference from the other leg | Heat after 5 s | Fluid wave | Act within |
|---|---|---|---|---|
| 1 | 2 to 4 mm | Barely warmer | None | 48 hours |
| 2 | 5 to 8 mm | Clearly warmer | Faint | 24 hours |
| 3 | 9 to 15 mm | Hot | Clear, 3 to 5 mm | 6 to 12 hours |
| 4 | Over 15 mm | Hot, skin tight | Strong, 5 mm or more | 1 to 2 hours |
Grade 1 with a normal temperature can be watched 48 hours. Grade 2 with 39.5 C is a 6-hour visit, because heat plus fever is the pattern infection makes. Grade 3 or 4, or any joint that leaks fluid or has a black or purple patch of skin, goes today.
Measure in millimeters, on both legs
Wrap a soft tape around the joint at its widest point, note the millimeter, then do the other leg. Read twice, keep the larger number. A 5 mm side-to-side difference is the line where a joint counts as swollen. On the hock, measure at the point of the heel and 1 cm above it; on the carpus, at the top of the paw and 2 cm up. Do not clip fur to see better: a 3 mm coat adds 6 mm to the circumference and turns a normal joint into a swollen one on paper.
When to see a vet
Go within 1 to 2 hours for a joint that is leaking fluid or pus, black or purple skin over the swelling, a temperature at or above 40.0 C, breathing over 40 breaths a minute, a cat that cannot stand, or a wound under 12 hours old that is still bleeding. Book within 6 hours for grade 2 or worse with a temperature at or above 39.5 C, a joint that grew 5 mm in 12 hours, refusal to eat for 24 hours, or a puncture wound anywhere on the leg. Book within 12 to 24 hours for grade 2 without fever, or lameness starting 24 to 48 hours after a fall. Book within 48 hours for grade 1 that is not improving, or a single firm swelling in a cat over 8 years old. Never give a human painkiller while you wait, and check the household poison list first: ibuprofen and acetaminophen are both toxic, and 1 tablet of 200 mg ibuprofen can injure the kidneys of a 4 kg cat. Check the household poison list before treating anything at home.
Record sheet: 6 lines a day
- Time, and which joint: carpus, elbow, hock or stifle, left or right.
- Circumference in mm at the marked spot, both legs.
- Heat: warmer, clearly warmer or hot, judged after 5 seconds.
- Fluid wave: none, faint or clear, with the bulge in mm.
- Temperature to 0.1 C, taken before handling the joint.
- Appetite as a percentage of the usual portion, and whether the cat walked, hopped or refused to move.
Five steps for today
- Compare both legs with the back of your hand for 5 seconds each, and write down warmer or hotter.
- Measure both joints with a soft tape in mm, twice, and mark the spot with a pencil dot.
- Take the temperature before touching the joint; 39.5 C or higher pulls the visit forward to 6 hours.
- Part the fur over the joint and look for 2 punctures 5 to 10 mm apart, then photograph the area from 30 cm.
- Do not massage, do not apply heat, and do not give any human painkiller; repeat the measurements at hour 12 and hour 24.
Terms mentioned in this article
Frequently asked
How do I tell a swollen joint from a swollen paw?
Feel above and below the joint. A swollen paw is puffy at the toes and pads with the joint itself normal, while a swollen joint is widest at the joint line and 5 mm or more larger than the same joint on the other leg. Press one side with 1 finger: a bulge that rises 3 to 5 mm on the opposite side means fluid inside the capsule.
Is a hot joint always an infection?
No, but infection is the one to rule out first in a single joint. Heat plus a temperature at or above 39.5 C, a puncture wound 5 to 10 mm deep, or a joint that grows 5 mm in 12 hours points that way, and a cat bite abscess usually shows itself within 24 to 72 hours. Trauma and immune-mediated disease also cause heat, so the temperature and the 24-hour trend separate them.
Can I give my cat a painkiller while I wait for the appointment?
No. Ibuprofen and acetaminophen are both toxic to cats, and 1 tablet of 200 mg ibuprofen can cause kidney injury in a 4 kg cat. Keep the cat quiet, do not massage or apply heat to the joint, and write down circumference, temperature and weight bearing at hour 0, 12 and 24 so the vet has a trend instead of a single reading.