The disease burden of ocular toxoplasmosis in Denmark in 2019: Estimates based on laboratory testing of ocular samples and on publicly available register data

Abstract

Background: Toxoplasma gondii is an important zoonotic protozoan parasite with worldwide distribution. Information on the contribution of ocular toxoplasmosis to the disease burden caused by this parasite is limited or lacking from many countries.

Methods: We estimated the minimum occurrence of ocular toxoplasmosis in Denmark using results from direct detection of T. gondii DNA with qPCR and determination of the Goldmann-Witmer coefficient on ocular samples submitted by ophthalmological clinics and departments to the national reference laboratory in 2003-2019. In addition, we inferred incidence estimates using retrospective data that are publicly available in the National Patient Register, and we used unstructured expert elicitation as the basis for sensitivity analyses. We estimated the disease burden of ocular toxoplasmosis in 2019 in disability-adjusted life years (DALYs).

Findings: Ocular samples from 263 individuals (median age 57 years, range 2-88) had been tested with at least one of the methods during 2003-2019, and 42 (16%) tested positive (median age 65 years, range 14-85). In 2019, five (16%) of 31 tested individuals were positive, giving a minimum annual incidence estimate of 0.09 per 100.000 population. From this, we calculated a disease burden of at least 4 DALYs (95% confidence interval, 3-5). The age range suggested that this figure represented postnatally acquired ocular toxoplasmosis. The disease burden of ocular toxoplasmosis due to congenital toxoplasmosis has been previously estimated to be at least 12 DALYs, resulting in an estimated minimum total disease burden due to ocular toxoplasmosis of 16 DALYs. In 2005-2018, the mean annual number of diagnoses of ocular toxoplasmosis reported to the National Patient Register was 186, and the corresponding disease burden estimate was 134 DALYs (95% confidence interval, 113-158). Sensitivity analyses focusing on incidence and severity resulted in disease burden estimates in the range of 9-523 DALYs.

Interpretation: Because most diagnoses of ocular toxoplasmosis are based on clinical observations, ophthalmoscopy, and serology without confirmatory testing, the disease burden caused by ocular toxoplasmosis is likely substantially higher than our minimum estimates. Our results indicate that ocular toxoplasmosis contributes to the disease burden caused by T. gondii in Denmark, but uncertainty about the incidence and severity precludes reliable estimation of its importance.

Keywords: DALY; Disease burden; Ocular toxoplasmosis; Toxoplasma gondii; Toxoplasmosis.

Fig. 1

Number of positive test results (black) and negative test results (gray) from qPCR and/or calculation of the Goldmann-Witmer coefficient for Toxoplasma gondii on ocular samples at the national reference laboratory for parasites, Denmark, 2012–2019, by year.