Is there a vaccine against toxoplasmosis?

If you search the internet for a toxoplasmosis vaccine, you will simply get the answer: "There is no vaccine against toxoplasmosis". This could change in the future.

The Immunology Blog
By Dr. Annabelle Eckert

With regard to its clinical symptoms, a distinction is made between 3 forms of the disease:

One third of the world's population is already infected with Toxoplasma gondii

The European Commission's website CORDIS - which publishes scientific research results from the EU - states that toxoplasmosis is "one of the neglected parasitic infectious agents" and "will now be targeted more effectively through public health measures". About one third of the world's population is infected with this intracellular parasite.2

Everything is a matter of opinion

Ocular toxoplasmosis is both one of the rarest forms of toxoplasmosis and the most common posterior uveitis of the eye. About a quarter of patients with a history of retinochoroiditis have a visual acuity of less than 0.1 in the affected eye. In most cases, the initial manifestation in the eye is a focal necrotising retinochoroiditis. The inflammatory reaction begins in the inner retinal layers and spreads to the other layers. An important difference between congenital and acquired ocular toxoplasmosis is that in the acquired form usually only one eye is affected and the inflammatory reaction is limited to a single lesion.

The congenital form is different. Here, clusters of inflammatory foci are recognisable as active lesions and post-inflammatory scar areas. Other possible signs of inflammation include vitritis, vasculitis and anterior uveitis with decompensation of intraocular pressure. Ocular complications such as epiretinal membranes, cystoid macular oedema, vitreoretinal traction, choroidal neovascularization and serous retinal detachment can be identified and visualised using optical coherence tomography.

An active TG lesion can be detected by fluorescein and indocyanine green angiography. In cases of pronounced vitritis, ultrasound examination of the eye is used. Point-like echoes in the vitreous, thickening of the posterior hyaloid membrane, and focal retinochoroidal thickening are frequently observed. In addition to these non-invasive diagnostic techniques, invasive collection of vitreous or aqueous humour samples and serum samples provides clear evidence of active infection. For example, an 8:1 ratio of anti-toxoplasma antibodies in the eye to serum suggests active ocular toxoplasmosis.3-5

What is the TOXPOX project all about?

The aim of the EU-funded TOXPOX project is to develop an effective and safe recombinant vaccine against toxoplasmosis. Recombinant viral vectors are designed to stimulate a specific protective immune response through the presentation of specific TG antigens. In the TOXPOX project, recombinant viral constructs based on poxviruses and also on lentiviruses were constructed and expressed the desired TG antigens. The various recombinant vaccines were tested in vivo in the mouse model. Specific antibody responses could be induced here. TOXPOX has thus developed a safer vaccine strategy that needs to be investigated in further research studies.6

Is vaccination against toxoplasmosis possible?

The toxoplasmosis vaccine Toxovax® is already used in the animal world. In sheep flocks, this vaccine is used to prevent toxoplasmosis-related abortions. Toxovax® is a live attenuated TG vaccine strain. However, there is a risk of reversion to the wild type.6,7 Immunisation of humans against toxoplasmosis is the aim of various scientific studies. One of them deals with a live attenuated toxoplasmosis vaccine (strain RH:ΔNPT1).8 In the mouse model, this vaccine elicits a strong immune response. More on this in a future blog post.

References
  1. https://www.rki.de/DE/Content/Infekt/EpidBull/Merkblaetter/Ratgeber_Toxoplasmose.html;jsessionid=601FAFFA8B0362C01061DAC4A99A60A4.internet102#doc2390224bodyText7
  2. https://cordis.europa.eu/article/id/151498-vaccine-against-toxoplasmosis/de
  3. Stokkermans T. J. et al. (2021). Toxoplasma Retinochoroiditis. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493182/
  4. Agarwal A et al. (2016). Toxoplasma Retinochoroiditis. In: Gupta V., Nguyen Q., LeHoang P., Herbort Jr. C. (eds) The Uveitis Atlas. Springer, New Delhi. https://doi.org/10.1007/978-81-322-2506-5_44-1
  5. Mushtaq F. et al. (2019). Primary Acquired Toxoplasma Retinochoroiditis: Choroidal Neovascular Membrane as an Early Complication. Cureus 11(2): e4001. doi:10.7759/cureus.4001
  6. https://cordis.europa.eu/article/id/151498-vaccine-against-toxoplasmosis/de
  7. Yang W.B. et al. (2019). Immunization With a Live-Attenuated RH:ΔNPT1 Strain of Toxoplasma gondii Induces Strong Protective Immunity Against Toxoplasmosis in Mice. Frontiers Microbiology, 13 August 2019.