Treatment, and why there is still no vaccine
The drugs against this parasite are old, effective against the active stage, and useless against the dormant one. A vaccine exists, but only for sheep. Here is why, and what would have to change.
The short answer
Healthy people with a mild infection usually need no treatment. Pregnant women with a first infection, newborns, people with eye disease and people with weakened immunity are treated, mainly with pyrimethamine and sulfadiazine plus folinic acid. These drugs kill the multiplying parasite but not the dormant cysts, so they cannot cure a latent infection. No human vaccine exists; the only licensed vaccine protects sheep against abortion.
Who is treated
Most people with healthy immune systems recover from toxoplasmosis without treatment1. Treatment is reserved for the situations in which the parasite does harm: a first infection during pregnancy, congenitally infected newborns and infants, active disease in the eye, and infection in people with weakened immunity1.
The drugs
The standard combination is pyrimethamine with sulfadiazine, plus folinic acid to protect the bone marrow from pyrimethamine’s side effects. It has been the treatment of choice for decades, and a 2018 review of the whole field concluded that none of the alternatives, including pyrimethamine with clindamycin, atovaquone or the newer macrolides, or trimethoprim-sulfamethoxazole on its own, has been shown to be better2. Trimethoprim-sulfamethoxazole is cheaper, easier and widely used, and it is the drug that prevented recurrences of eye disease in the one trial that tested it3. In pregnancy, spiramycin has traditionally been used to reduce transmission to the baby, with pyrimethamine and sulfadiazine reserved for confirmed fetal infection; the only randomized comparison found fewer infected babies with the stronger combination, though not significantly so4. People with advanced HIV who carry the parasite take trimethoprim-sulfamethoxazole to prevent reactivation.
All of these drugs share one limitation, and it is the central fact of toxoplasmosis treatment: they act on the rapidly multiplying stage of the parasite and not on the dormant cysts2. A treated infection is a controlled infection, not a cured one. The cysts remain, and so does the possibility of reactivation if immunity fails, and of recurrence in the eye. Finding a drug that kills cysts is one of the field’s real unsolved problems, and several candidates are in early research.
Why there is no vaccine for people
There is a vaccine, but you would have to be a sheep. Toxovax is a live, weakened strain of the parasite that cannot form cysts, and it has been used since the early 1990s to prevent the abortions that toxoplasmosis causes in flocks5. It works because a live infection is what generates lasting immunity to this parasite.
That is also the problem. A review my lab published in 2022 summarized decades of attempts: many candidate vaccines partially protect animals, but full and lasting protection has only ever come from live-attenuated parasites, and a live vaccine cannot be given to people or to the animals we eat, because the parasite is zoonotic and a weakened strain could in principle revert6. Vaccines built from single proteins, DNA or nanoparticles have so far fallen short. The most promising directions, in my view, aim at different targets: stopping cats from shedding oocysts, and blocking the environmental stages of the parasite, which would cut off the infection at its source rather than trying to protect each person separately6.
What this means in practice
- If you are healthy and have just been told you have antibodies, you almost certainly need no treatment. Ask what the result means for you, and read the blood tests page.
- If you are pregnant with a confirmed first infection, or immunocompromised, treatment decisions belong with a specialist, and the drugs work best when started early.
- If you have had eye disease, ask about long-term prophylaxis; it is the one setting where prevention of recurrences has trial evidence.
- There is no vaccine to ask for. Prevention is food hygiene, and it works.
Sources
- Centers for Disease Control and Prevention. Treatment of Toxoplasmosis. Accessed 17 September 2026.
- Dunay IR, Gajurel K, Dhakal R et al. Treatment of Toxoplasmosis: Historical Perspective, Animal Models, and Current Clinical Practice. Clin Microbiol Rev 2018;31. PubMed · DOI
- Silveira C, Belfort R, Muccioli C et al. The effect of long-term intermittent trimethoprim/sulfamethoxazole treatment on recurrences of toxoplasmic retinochoroiditis. Am J Ophthalmol 2002;134:41-6. PubMed · DOI
- Mandelbrot L, Kieffer F, Sitta R et al. Prenatal therapy with pyrimethamine + sulfadiazine vs spiramycin to reduce placental transmission of toxoplasmosis: a multicenter, randomized trial. Am J Obstet Gynecol 2018;219:386.e1-386.e9. PubMed · DOI
- Buxton D, Innes EA. A commercial vaccine for ovine toxoplasmosis. Parasitology 1995;110 Suppl:S11-6. PubMed · DOI
- Arranz-Solís D, Saeij JPJ. New Avenues to Design Toxoplasma Vaccines Based on Oocysts and Cysts. Front Immunol 2022;13:910961. PubMed · DOI