Infections among individuals with multiple sclerosis, Alzheimer's disease and Parkinson's disease.

A link between neurodegenerative diseases and infections has been previously reported. However, it is not clear to what extent such link is caused by confounding factors or to what extent it is intimately connected with the underlying conditions. Further, studies on the impact of infections on mortality risk following neurodegenerative diseases are rare. We analysed two data sets with different characteristics: (i) a community-based cohort from the UK Biobank with 2023 patients with multiple sclerosis, 2200 patients with Alzheimer's disease, 3050 patients with Parkinson's disease diagnosed before 1 March 2020 and 5 controls per case who were randomly selected and individually matched to the case; (ii) a Swedish Twin Registry cohort with 230 patients with multiple sclerosis, 885 patients with Alzheimer's disease and 626 patients with Parkinson's disease diagnosed before 31 December 2016 and their disease-free co-twins. The relative risk of infections after a diagnosis of neurodegenerative disease was estimated using stratified Cox models, with adjustment for differences in baseline characteristics. Causal mediation analyses of survival outcomes based on Cox models were performed to assess the impact of infections on mortality. Compared with matched controls or unaffected co-twins, we observed an elevated infection risk after diagnosis of neurodegenerative diseases, with a fully adjusted hazard ratio (95% confidence interval) of 2.45 (2.24-2.69) for multiple sclerosis, 5.06 (4.58-5.59) for Alzheimer's disease and 3.72 (3.44-4.01) for Parkinson's disease in the UK Biobank cohort, and 1.78 (1.21-2.62) for multiple sclerosis, 1.50 (1.19-1.88) for Alzheimer's disease and 2.30 (1.79-2.95) for Parkinson's disease in the twin cohort. Similar risk increases were observed when we analysed infections during the 5 years before diagnosis of the respective disease. Occurrence of infections after diagnosis had, however, relatively little impact on mortality, as mediation of infections on mortality (95% confidence interval) was estimated as 31.89% (26.83-37.11%) for multiple sclerosis, 13.38% (11.49-15.29%) for Alzheimer's disease and 18.85% (16.95-20.97%) for Parkinson's disease in the UK Biobank cohort, whereas it was 6.56% (-3.59 to 16.88%) for multiple sclerosis, -2.21% (-0.21 to 4.65%) for Parkinson's disease and -3.89% (-7.27 to -0.51%) for Alzheimer's disease in the twin cohort. Individuals with studied neurodegenerative diseases display an increased risk of infections independently of genetic and familial environment factors. A similar magnitude of risk increase is present prior to confirmed diagnosis, which may indicate a modulating effect of the studied neurological conditions on immune defences.