New RVC research identifies changing epidemiology of harmful foetal disease

A new study from the Royal Veterinary College (RVC) highlights the changing epidemiology of congenital toxoplasmosis (CT), a foetal disease which affects approximately 190,000 pregnancies around the world each year, and the need for more extensive research to understand the underlying causes responsible for these changes.

Congenital toxoplasmosis occurs when pregnant women become infected with the parasite Toxoplasma gondii (T. gondii) for the first time, passing the infection to the foetus. Of the 190,000 cases of CT reported annually, three per cent of infected infants die before one month of age, sometimes before birth, and those who survive often experience problems with their vision and development.

Approximately one-third of the world’s human population has been exposed to T. gondii, which causes toxoplasmosis in humans and wild and domesticated warm-blooded animals. Transmission of the parasite can occur when people consume meat containing parasite cysts or raw unwashed foods, or water, containing parasites shed in the faeces of infected cats.

Analysing data from more than a quarter of a million people from 19 countries, the study confirmed reports that rates of human toxoplasmosis have been declining over the past six decades in high income countries. However, despite this overall decline, the researchers found that transient spikes in CT in high prevalence countries can occur due to an epidemiological ’peak shift’ where more women become infected for the first time whilst pregnant, rather than before motherhood.

The study also highlights conspicuous gaps in current understanding of the epidemiology of T. gondii in lowand middle-income countries that should be addressed in future research, as well as the need for new diagnostic methods to better understand the reasons for the changes in infection rates.

The study also suggests that countries which are predicted to experience an increase in cases of CT might benefit from the temporary introduction of prenatal screening programmes to diagnose and treat CT. The researchers also encourage re-evaluation of the cost-effectiveness of these screening programmes to consider additional conditions caused by CT, including possible neuropsychiatric effects.

Dr Gregory Milne, lead researcher and postdoctoral scientist at the Royal Veterinary College, said:

"Toxoplasma causes a large public health burden, from severe congenital disease among infants, to fatal infections among people with compromised immune systems, to other more subtle changes in host behaviour. It is therefore promising news to find consistent evidence of decreases in parasite exposure in many populations and countries. We show that more data are needed to assess the trajectories of exposure trends in lower-income countries.

"Our findings nonetheless caution against complacency: in high-prevalence countries, despite decreasing parasite exposure, cases of congenital disease may counterintuitively increase as more women acquire primary infections in pregnancy.

Milne, G.C., Webster, J.P., Walker, M. (2022). Is the incidence of congenital toxoplasmosis declining? Trends in Parasitology (in press).

The full paper will be available from Tuesday 15 November 2022 at:­asitology/­fulltext/S­1471-4922(­22)00256-2

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