Pyelonephritis is a bacterial infection of the kidney itself, not the bladder, and in cats it is uncommon enough that it is often missed for weeks. The diagnosis is not made by a urine dipstick or by seeing white cells under the microscope; it is made by a quantitative urine culture with a known sampling method. Treatment runs 14 days at minimum, and the case is not closed until a repeat culture at 4 weeks after the last dose comes back clean.
Three ways bacteria reach the kidney
1. Ascending. This is the usual route. Bacteria climb from the bladder up the ureter, which is why any cat with repeated cystitis, crystals, or a bladder stone is a candidate. A ureter that is partly blocked gives bacteria a standing pool to grow in.
2. Bloodborne. Septicemia, endocarditis, a deep dental infection, or an infected wound can seed both kidneys at once. These cats look sicker: fever, no appetite, and often a source you can find on exam.
3. Direct spread. Rare, but a deep abscess or a penetrating injury next to the kidney can reach it. Imaging is what finds this one.
What the cat looks like
Fever of 39.5 to 40.5 C, pain when the kidney area is palpated, a hunched posture, and a cat that stops grooming. Older cats often show only poor appetite and weight loss over 2 to 3 weeks, and kittens can run a fever with no urinary signs at all. Some cases show no pus cells in the sediment at all, so a normal-looking urine sample does not rule it out.
The culture, and why the sample method changes the threshold
A colony count only means something if you know where the urine came from. The table gives the cutoffs most labs use to call a result positive rather than contamination.
| Sample method | Positive threshold | Contamination risk | Result time | Use when |
|---|---|---|---|---|
| Cystocentesis, ultrasound guided | over 1,000 CFU/mL | very low | 24 to 48 hours | default in every stable cat |
| Catheterized sample | over 10,000 CFU/mL | moderate | 24 to 48 hours | cat too small or too ill to hold still |
| Midstream, free catch | over 100,000 CFU/mL | high | 24 to 48 hours | last resort, needs a clean box |
| Bladder stone or tissue culture | any growth | none | 3 to 5 days | surgery or stone removal |
Three days of waiting, and what to do in them
Growth appears in 24 to 48 hours and the sensitivity panel needs another 24, so a full answer takes about 3 days. A cat with fever and kidney pain should not wait: most vets start a broad-spectrum antibiotic on day 1, then switch at day 3 once the panel names the organism and its resistances. Escherichia coli is the most common isolate, followed by Staphylococcus and Enterococcus, and the panel is what keeps a guess from becoming a 14-day mistake.
Collect the sample before the first dose. One dose of antibiotic can sterilize a culture for 5 to 7 days and leave you treating blind for 2 weeks.
Imaging: what the ultrasound is looking for
A renal pelvis wider than 2 mm, a dilated proximal ureter over 3 mm, a kidney that is enlarged with a thin cortex, or a stone casting a shadow. Ultrasound also tells you whether one kidney or both are involved, which changes the drug choice and the follow-up. CT is reserved for a cat that is not improving at day 5, or for planning surgery on a blocked ureter.
14 days, then 7 and 28 days after
An uncomplicated case gets 14 days. A cat with kidney involvement, a stone, or a history of relapse gets 21 to 28 days, because infected kidney tissue is harder to reach than bladder urine. The recheck schedule is the part owners skip: a culture at 7 days after the last dose catches a fast relapse, and the culture at 4 weeks is the one that defines cure. A cat that is culture-negative at 4 weeks is done; a cat that is positive needs another 21 days and a search for the reason, which is usually a stone, a partially blocked ureter, diabetes, or an immune problem.
Two relapses within 6 months justify a full workup: imaging, a urine culture every 8 weeks for 3 cycles, and a check for feline leukemia and immunodeficiency virus. Repeated courses without a culture are how resistant bacteria get made.
Five things to do at home
- Give the full course even if the cat looks normal on day 4, since stopping early is the main cause of relapse.
- Weigh weekly; a gain of 100 g over 7 days means appetite is back before the kidney has healed.
- Keep water intake up with 2 to 3 bowls in different rooms, and a fountain if the cat prefers moving water.
- Book the 4-week culture at the same visit you collect the first sample, so it is not forgotten.
- Watch for a return of the hunched posture or a fever, both of which mean the kidney is flaring again.
When it is an emergency
A cat that stops producing urine, cries when lifted, or has a temperature over 40.5 C needs to be seen the same day, because an obstructed or abscessed kidney can fail within 48 hours. So does any male cat that strains with nothing coming out; the obstruction side of this is covered in the six signs of a blocked cat. If the infection is found alongside a heart murmur or a seizure, the differential list widens, and vestibular disease in cats explains one common look-alike. Budget for the culture, the ultrasound, and 2 rechecks with the cost page; the culture is 1 of the few line items that pays for itself by preventing a second 3-week course.
Terms mentioned in this article
Frequently asked
Why is a 4-week recheck culture needed if the cat feels fine?
Because feeling fine and being cured are different states. Signs fade in 3 to 5 days while bacteria can persist in kidney tissue for weeks. A culture at 4 weeks after the last dose is the only proof the infection is gone, and it catches the relapse that would otherwise reappear 2 months later as a resistant strain.
Can I collect urine at home for the culture?
You can, but the threshold jumps to over 100,000 CFU/mL because of contamination, so a low count proves nothing. Aseptic cystocentesis at the clinic is the default and lets the lab call 1,000 CFU/mL positive. Collect before any antibiotic, since one dose can suppress growth for 5 to 7 days.
How long is treatment for feline pyelonephritis?
At least 14 days for an uncomplicated case, and 21 to 28 days when the kidney itself is involved, a stone is present, or the cat has relapsed before. Stopping at day 7 because the cat is eating again is the most common reason the infection returns within 6 weeks.