By Jaroslav Flegr
Background Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii, affects about a third of human population worldwide. In the past ten years, numerous studies had shown that infected subjects have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting the adverse effects of various diseases.
Methods A cross-sectional internet study was performed on a population of 4,499 Toxoplasma-free and 786 Toxoplasma-infected subjects and looked for factors which positively or negatively affect the risk of SARS-CoV-2 infection and likelihood of a severe course of Covid-19.
Findings Logistic regression and partial Kendall correlation controlled for sex, age, and size of the place of residence showed that latent toxoplasmosis had the strongest effect on the risk of infection (OR = 1·50) before sport (OR = 1·30), and borreliosis (1·27). It also had the strongest effect on the risk of severe course of infection (Tau = 0·146), before autoimmunity, immunodeficiency, male sex, keeping a cat, being overweight, borreliosis, higher age, or chronic obstructive pulmonary disease. Toxoplasmosis augmented the adverse effects of other risk factors but was not the proximal cause of the effect of cat keeping (in the form of higher likelihood of Covid infection and higher severity of the course of infection), which was observed especially in a subset of Toxoplasma-infected subjects (Tau = 0·153). Effects of keeping a cat were detected only in subjects from multi-member families, suggesting that a cat could a vector for the transmission of SARS-CoV-2 within a family.
Interpretations Toxoplasmosis is currently not considered a risk factor for Covid-19 and Toxoplasma– infected subjects are neither informed about their higher risks nor prioritised in vaccination programs. Because toxoplasmosis affects a large segment of the human population, its impact on Covid-19-associated effects on public health could be considerable.