I had an interesting question today about the cat-associated parasite Toxoplasma gondii. It can cause serious infection in people that ingest it, particularly in immunocompromised individuals and pregnant women, but disease is rare. Since cats can pass one form of the parasite in their feces, the question was whether using flushable kitty litter is a bad idea, since it would result in Toxoplasma being discharged into the sewage system.
On one hand…
- Water is a source of Toxoplasma exposure.
- Food contaminated by Toxoplasma-contaminated water is a also a source of exposure.
- Municipal water was determined to be a possible source of exposure in at least one Toxoplasma outbreak (Bowie et al. 1997).
- The form of Toxoplasma in cat feces is hard to kill, so it could survive routine water treatment measures.
On the other hand….
- Cats rarely shed Toxoplasma. They typically do so only for a short period of time after their first exposure (usually when they're quite young), so the vast, vast majority of household cats are not shedding the parasite.
- There’s a massive dilutional effect when something goes down the drain. To constitute a risk, the parasite would have to come out of the cat, survive waste water treatment and be discharged into the environment, then either make it into a drinking water source (with more dilution and more treatment) or reach someone’s mouth through other routes such as on food or from contaminating the general environment (e.g. soil, recreational water bodies). Even if some Toxoplasma were present in cat feces in flushable litter, the odds that someone susceptible would encounter enough Toxoplasma from this source to cause disease is exceedingly remote.
I don’t think using flushable litter constitutes a public health risk.