Toxoplasmosis

Cats: what they do and what they do not do

Every warning about toxoplasmosis mentions cats. Few mention that the studies looking for a link between owning one and getting infected keep coming up empty. Both facts are true, and it helps to understand why.

By Jeroen Saeij · Updated 17 September 2026

What happens inside a cat

Cats and their wild relatives are the only animals in which Toxoplasma reproduces sexually¹. A cat is usually infected by eating an infected mouse or bird, or raw meat. For one to three weeks after that first infection it sheds oocysts, the parasite’s egg-like environmental stage, in its droppings. In careful experiments, cats shed more than 20 million oocysts each between the fourth and thirteenth day after infection, and afterwards most were immune to shedding again for years². So at any given moment, very few cats are shedding, and an adult cat that has lived with you for years is almost certainly not one of them.

Two more details shape the risk. Freshly passed oocysts are not infectious. They need one to five days in the open air to mature³. And cats are fastidious animals: in the same experiments, no oocysts were found on the fur of cats that had just shed millions of them². That is why touching a cat is not how people become infected.

What the studies of infected people show

If cats were an important direct source, people who own or touch them would be infected more often. That is not what studies find.

• In a study of pregnant women across six European cities, contact with cats was not a risk factor for a recent infection, while undercooked meat and contact with soil were⁴.
• In an American study of recently infected adults, the one cat-related exposure that raised the risk was having three or more kittens, which points to the real mechanism: kittens are the cats most likely to be shedding⁵.
• Public-health reviews reach the same conclusion: direct contact with cats is not thought to be a primary risk for human infection¹.

Where cats do matter: the environment

The oocysts a cat sheds in a garden, a sandbox, a field or a watershed stay infectious for many months. My lab found part of the reason: a family of proteins that lets the parasite survive drying, freezing and salt⁶. Those oocysts end up on vegetables, in untreated water, in the mouths of grazing animals whose meat we later eat, and in coastal waters where shellfish and sea otters concentrate them⁷⁸. Outbreaks traced to contaminated drinking water in Canada and Brazil⁹¹⁰, and a study showing that most congenital infections in North America begin with oocysts rather than meat¹¹, are cats’ real contribution to human disease. It is a contribution made through the environment, one cat at a time, and it is why keeping cats indoors and out of sandboxes is good for everyone, not only for cat owners.

What to do

If you are pregnant or immunocompromised³:

• Keep the cat. Nothing in the evidence justifies rehoming a cat.
• Change the litter daily, ideally by someone else, or with gloves and hand washing. Daily works because oocysts need at least a day to become infectious.
• Keep cats indoors and feed them commercial or well-cooked food, so they do not become infected in the first place.
• Avoid stray cats and new kittens during pregnancy, and do not adopt a new cat then.
• Wear gloves in the garden, cover sandboxes, and wash vegetables, because that is where the oocysts are.

And there is no point in testing the cat. A cat that has antibodies has already been infected and has almost certainly finished shedding. A cat without antibodies could become infected tomorrow. Neither result changes what you should do.

Sources

1. Elmore SA, Jones JL, Conrad PA et al. Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention. Trends Parasitol 2010;26:190-6. PubMed · DOI
2. Dubey JP. Duration of immunity to shedding of Toxoplasma gondii oocysts by cats. J Parasitol 1995;81:410-5. PubMed
3. Centers for Disease Control and Prevention. Preventing Toxoplasmosis. Accessed 17 September 2026.
4. Cook AJ, Gilbert RE, Buffolano W et al. Sources of toxoplasma infection in pregnant women: European multicentre case-control study. European Research Network on Congenital Toxoplasmosis. BMJ 2000;321:142-7. PubMed · DOI
5. Jones JL, Dargelas V, Roberts J et al. Risk factors for Toxoplasma gondii infection in the United States. Clin Infect Dis 2009;49:878-84. PubMed · DOI
6. Arranz-Solís D, Warschkau D, Fabian BT et al. Late Embryogenesis Abundant Proteins Contribute to the Resistance of Toxoplasma gondii Oocysts against Environmental Stresses. mBio 2023;14:e0286822. PubMed · DOI
7. Dabritz HA, Conrad PA. Cats and Toxoplasma: implications for public health. Zoonoses Public Health 2010;57:34-52. PubMed · DOI
8. Shapiro K, VanWormer E, Packham A et al. Type X strains of Toxoplasma gondii are virulent for southern sea otters (Enhydra lutris nereis) and present in felids from nearby watersheds. Proc Biol Sci 2019;286:20191334. PubMed · DOI
9. Bowie WR, King AS, Werker DH et al. Outbreak of toxoplasmosis associated with municipal drinking water. The BC Toxoplasma Investigation Team. Lancet 1997;350:173-7. PubMed · DOI
10. de Moura L, Bahia-Oliveira LM, Wada MY et al. Waterborne toxoplasmosis, Brazil, from field to gene. Emerg Infect Dis 2006;12:326-9. PubMed · DOI
11. Boyer K, Hill D, Mui E et al. Unrecognized ingestion of Toxoplasma gondii oocysts leads to congenital toxoplasmosis and causes epidemics in North America. Clin Infect Dis 2011;53:1081-9. PubMed · DOI