Four parasites cause most of the diarrhea that gets called worms: Giardia, coccidia, Tritrichomonas foetus, and roundworm. A single fecal float catches some of them and misses others, which is why a PCR panel earns its place in a cat with 3 or more loose stools a day. Treatment is the easy part. The hard part is the recheck: 3 samples, taken on alternate days, because 1 clean sample after treatment proves very little.
The 4 parasites and how each one is found
Size and test matter more than the name on the report. A Giardia cyst is 10 to 20 micrometers, a coccidia oocyst is 20 to 30, and a roundworm egg is 70 to 90, large enough to spot at 100x magnification by someone who looks at feces all day. Tritrichomonas is the exception: it has no cyst stage and dies on the slide, so PCR is the practical route.
| Parasite | Size | Best test | Signs appear | Recheck at |
|---|---|---|---|---|
| Giardia | 10 to 20 micrometers | Antigen test or PCR | 5 to 10 days after exposure | Day 3 |
| Coccidia (Cystoisospora) | 20 to 30 micrometers | Fecal float with centrifugation | 5 to 7 days | Day 5 |
| Tritrichomonas foetus | 8 to 12 micrometers | PCR, since culture takes 7 to 10 days | 5 to 14 days | Day 14 |
| Roundworm (Toxocara) | 70 to 90 micrometers | Fecal float, 3 g sample | 21 to 35 days in adults | Day 14 |
Two of the four are found by looking, and two need a chemical test. That split explains most of the arguments about results: a float can be clean and the cat can still have Giardia, because cysts are shed in waves and a 3 g sample from a good day may hold none.
Why 1 fecal exam is not enough
Shedding is intermittent. A cat can pass cysts for 2 days, test negative on day 3, and be positive again on day 5. Centrifugation at 1,200 to 1,500 rpm for 5 minutes beats a simple smear, and 3 g of fresh feces beats 1 g scraped off the top of the box. Collect with a clean spoon into a sealed pot, keep it at 4 C if the sample waits more than 2 hours, and bring the whole thing rather than a smear on a swab.
Diarrhea that lasts 3 days or more deserves a panel, not a repeat float. In a kitten under 6 months, 12 hours of watery diarrhea can cost 50 g per kg of body weight, which in a 1 kg kitten is 50 g out of 1,000.
PCR, float, and antigen: what each one answers
- Float with centrifugation: 15 to 30 minutes, 20 to 60 dollars, sees eggs and oocysts, misses Giardia about half the time.
- Antigen test for Giardia: 10 minutes in house, 25 to 50 dollars, good sensitivity, no species information.
- PCR panel: 1 to 3 days, 90 to 220 dollars, detects DNA from 4 or more organisms at once, does not tell you whether the organism is alive.
- Culture for Tritrichomonas: 7 to 10 days, 60 to 120 dollars, rarely worth the wait when PCR exists.
Because PCR cannot separate live from dead DNA, a positive result 2 days after treatment may mean nothing. That is the reason the recheck schedule starts on day 3 and spreads over 11 days, and the reason 3 negative samples carry far more weight than 1.
The 3 rechecks, on alternate days
Sample on day 3, day 5, and day 14 after the last treatment. The first 2 sit 48 hours apart, which is the interval that catches a wave of shedding instead of a single lucky sample. The day-14 test is the one that decides whether the cat is clear, because it sits past the prepatent period of most of the 4 parasites. A cat that is negative on all 3 is treated as clear; a cat positive on any 1 goes back for another round and another 3 samples.
Write the dates on the pots. Three unlabelled samples in 1 bag is the most common reason a recheck has to be repeated, and a repeat costs another 90 to 220 dollars.
Kittens lose water faster than adults
A kitten with diarrhea can lose 50 to 100 g per kg of body weight in 24 hours. Weigh daily on a kitchen scale to the nearest 5 g, and treat a loss of 50 g per kg as the line where fluid support is discussed. Water needs run 60 to 80 ml per kg per day on top of normal intake, and a 1 kg kitten that stops nursing for 12 hours is behind by 30 to 40 ml before anyone notices.
Offer small meals 4 to 6 times a day rather than 2 large ones. A 5 kg adult with 3 days of diarrhea and no vomiting can usually be managed at home with monitoring; a kitten cannot, because it has no reserve.
Cleaning the house, in minutes and degrees
Giardia cysts survive 7 days or more in damp litter and on a wet bowl. Steam clean litter trays and bowls at 60 C for 5 minutes, or run them through a dishwasher on the hot cycle, and let them dry fully before refilling. Standard bleach at 1 part in 32 does not reliably kill cysts, and no amount of scrubbing fixes a tray that stays damp. Scoop twice a day, change litter completely every 7 days, and give each cat its own bowl and its own towel.
Wash hands for 20 seconds after litter duty, and do not let a child share a bed with a cat that is shedding. Giardia and roundworm both have a human risk, and the household measures are the same ones that stop the cat from being reinfected.
When to go the same day
- Blood in the stool, or stool that is black and tarry.
- Vomiting plus diarrhea, which doubles the fluid loss.
- A kitten under 8 weeks with 3 or more loose stools in 12 hours.
- No eating for 24 hours, or a kitten that stops nursing for 12 hours.
- Gums that feel tacky, or skin that stays tented when lifted.
Older cats get a different workup first. Chronic diarrhea in a cat over 10 is as likely to be thyroid, kidney, or bowel disease as a parasite, so the panel should travel with a blood profile. The senior panel lines show what to include, and the pre-anesthetic panel covers the baseline to have on file before any procedure. For test prices in one place, see the cost tool.
The 5-step version
- Bring 3 g of fresh stool in a sealed pot, not a swab.
- Ask for a PCR panel when diarrhea lasts 3 days or more.
- Sample on day 3, day 5, and day 14 after treatment ends.
- Weigh a kitten daily and treat a 50 g per kg loss as the fluid line.
- Steam trays at 60 C for 5 minutes, scoop twice a day, wash hands 20 seconds.
Frequently asked
My cat had a clean fecal test. Can it still have Giardia?
Yes. Cysts are shed in waves, so a 3 g sample from a quiet day can hold none, and a single float misses Giardia often enough that 3 samples over 5 to 7 days are the standard. If diarrhea lasts 3 days or more, ask for an antigen test or a PCR panel rather than a second float.
Why 3 rechecks instead of 1?
Because 1 negative sample after treatment can sit between 2 waves of shedding and tell you nothing. Sampling on day 3, day 5, and day 14 covers the prepatent period of all 4 parasites and gives 2 samples 48 hours apart, which is the interval most likely to catch a cat that is still shedding.
Do I need to treat the house as well as the cat?
Yes, or reinfection is likely. Cysts survive a week or more in damp litter and on wet bowls, and bleach at 1 part in 32 does not reliably kill them. Steam trays and bowls at 60 C for 5 minutes, dry them fully, scoop twice a day, change litter every 7 days, and wash hands for 20 seconds.