Toxoplasma infection, testosterone, and sex ratios
The temporal dynamics of testosterone levels in infect- ed hosts remain unclear, though indirect evidence suggests a systematic change over time – potentially involving an initial increase followed by an irreversible decline. High testosterone in females is often associated with an increased secondary sex ratio (proportion of male offspring) (James 2010). Studies in both humans and mice indicate a higher secondary sex ratio shortly after infection, with some human cases reaching a 2.6: 1 ratio (260 sons per 100 daughters) (Kaňková et al. 2007a,b). However, over time, infected females tend to have more daughters (Kaňk- ová et al. 2007a,b). An ecological study identified Toxoplasma prevalence as one of the stronger predictors of low secondary sex ratios – ranking third among 16 examined factors, after son preference and fertility rates (Dama et al. 2016).
A decline in testosterone levels over time may be explained by the deteriorating health status associated with chronic infection (Flegr et al. 2014b, Flegr and Escudero 2016), as poor health typically reduces testosterone pro- duction. Alternatively, host compensatory mechanisms may act later in infection to lower testosterone levels and counteract its immunosuppressive effects.
Despite clear associations between Toxoplasma infection and changes in testosterone levels, fundamental questions remain. Does Toxoplasma actively manipulate host hormone levels, or are these changes incidental effects of infection? If manipulation occurs, is the goal to enhance transmission – either alimentary or sexual – or to suppress the host’s immune response for prolonged persistence? Alternatively, could individuals with naturally high testosterone and weaker immunity simply be more susceptible to infection? Given testosterone’s immunosuppressive properties (Roberts et al. 2004, Foo et al. 2017), increased sexual transmission may be a secondary consequence rather than a primary adaptive strategy of the parasite.
Further research is needed to clarify the complex interplay between Toxoplasma, testosterone, immunity and host behaviour, shedding light on whether observed hormonal changes serve a functional role in parasite transmission or are merely byproducts of infection.
Publication Date: 2025-05-20