Crypto in cats is short for cryptosporidiosis, an intestinal infection caused by a microscopic parasite called Cryptosporidium. The parasite invades the lining of the small intestine, causing watery diarrhea and dehydration. Most infections in healthy adult cats are mild or produce no symptoms at all, but crypto can become serious in kittens, elderly cats, or cats with weakened immune systems.
How Cats Get Infected
Cryptosporidium spreads through the fecal-oral route. An infected cat sheds tiny egg-like structures called oocysts in its stool. These oocysts are already infectious the moment they leave the body, which makes them especially easy to spread in multi-cat households, shelters, and catteries. Another cat picks them up by grooming contaminated fur, sharing a litter box, or drinking contaminated water.
Once swallowed, the parasite hatches in the small intestine and attaches to the cells lining the gut wall using specialized surface proteins. It then gets engulfed by the host cell, setting up a tiny compartment where it feeds off nutrients from the cat’s own cells. This process damages the delicate finger-like projections (called microvilli) that absorb nutrients, which is why diarrhea and poor nutrient absorption follow.
The species most commonly found in cats is Cryptosporidium felis, though cats can occasionally carry other species as well.
Symptoms to Watch For
Most Cryptosporidium infections in cats are subclinical, meaning the cat carries and sheds the parasite without ever looking sick. When symptoms do appear, they typically include:
- Watery diarrhea: the hallmark sign, sometimes with significant fluid loss
- Dehydration: especially dangerous in small kittens
- Weight loss or poor growth in young cats with prolonged infections
- Decreased appetite
In a healthy adult cat with a functioning immune system, the infection is usually self-limiting. The diarrhea resolves on its own as the cat’s immune response clears the parasite. However, in immunocompromised cats, such as those with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), the infection can become severe, prolonged, and potentially life-threatening. A persistent crypto infection that won’t clear up often signals an underlying health problem that needs investigating.
How Crypto Is Diagnosed
Cryptosporidium oocysts are extremely small, far tinier than most other intestinal parasites. A standard fecal float test at your vet’s office can miss them entirely. More reliable options include specialized stool tests that use antibodies to detect the parasite or PCR testing, which identifies the parasite’s genetic material in a stool sample. Because shedding can be intermittent, your vet may recommend testing more than one stool sample if crypto is suspected but the first test comes back negative.
Treatment and Recovery
There is no single drug that reliably eliminates Cryptosporidium in cats. Because most infections in otherwise healthy cats resolve on their own, treatment focuses heavily on supportive care: keeping the cat hydrated, maintaining nutrition, and managing diarrhea while the immune system does its work.
For cats with persistent diarrhea, vets may try antiparasitic medications, though results vary. Adding dietary fiber or probiotics has helped some cats resolve lingering symptoms. Cats that are severely dehydrated, especially kittens, may need fluid therapy to replace what they’re losing through diarrhea.
If a cat’s crypto infection drags on despite treatment, your vet will likely look for an underlying immune problem driving the persistence.
Cleaning and Preventing Spread
Cryptosporidium oocysts are notoriously tough to kill. Most common household disinfectants, including bleach, don’t reliably inactivate them. Quaternary ammonium cleaners (the active ingredient in many pet-safe sprays) also fall short. The disinfectants proven to work against Cryptosporidium are limited: hydrogen peroxide at concentrations of 6% or higher, and steam sterilization are among the few effective options.
There’s a practical workaround, though. When oocysts dry out at room temperature, they lose infectivity fairly quickly. Exposure to dry conditions at normal room temperature (around 73 to 77°F) for 60 to 90 minutes reduced infectious oocysts by more than 99.9% in laboratory testing. So keeping surfaces clean and dry, combined with thorough mechanical removal of fecal material, is a realistic strategy for households.
In multi-cat environments, scoop litter boxes at least once daily to remove oocysts before they can spread. Wash litter boxes with hot water and allow them to dry completely. Isolate symptomatic cats if possible, and wash your hands thoroughly after handling litter or cleaning up diarrhea.
Can You Catch Crypto From Your Cat?
C. felis can infect humans, but the risk from pet cat ownership appears to be very low. A CDC review documented 14 confirmed human infections with C. felis across North America, South America, Africa, and Europe. Ten of those 14 cases involved people with compromised immune systems. A separate study examining the relationship between Cryptosporidium infection and animal exposure in people with HIV found no significant risk associated with cat ownership.
That said, the parasite doesn’t require direct contact with a cat. One documented human case involved a person in Rome who didn’t own a cat but likely encountered oocysts in the environment from the city’s large stray cat population. For people with healthy immune systems, basic hygiene (hand washing after litter box duty, keeping cats off kitchen surfaces) is sufficient. People who are immunocompromised should take extra precautions and have someone else handle litter box cleaning when possible.