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People who are treated in hospital for infections may have a higher risk of developing early-onset Alzheimer’s disease years later.

at the Karolinska Institute in Sweden and his colleagues analysed the health records of around 290,000 people in Sweden with Alzheimer’s disease and 1.4 million people matched by age and sex who hadn’t been diagnosed with the disease.

Among those diagnosed with Alzheimer’s before the age of 60, 17.4 per cent had been treated in hospital for an infection at least five years earlier, compared with 9.8 per cent of people matched by age without an Alzheimer’s diagnosis.

Bacterial infections and those of the urinary and genital organs were most strongly linked to a higher risk of Alzheimer’s disease, but it was also seen with viral infections and those of the gastrointestinal and central nervous systems.

The researchers accounted for differences in people’s education and their family history of neurodegenerative disease and other conditions, including cardiovascular disease and diabetes.

Many previous studies have hinted at potential links between Alzheimer’s disease and specific pathogens, such as herpesvirus. The findings build on this by adding evidence that there is a general link between infection and the condition.

“Despite an abundance of preclinical data… there are not many well-conducted observational studies in this area. Therefore, the findings of the current study should be seen as an important addition,” says at McGill University in Montreal, Canada.

“The results are broadly consistent with the concept – supported by some 500 papers using a wide variety of approaches – that microbes play a major role in Alzheimer’s disease,” says at the University of Manchester, UK.

However, the results don’t necessarily show that infections raise the risk of Alzheimer’s. Another possibility is that early undetected changes in the body due to Alzheimer’s disease could make people more prone to infections. Alternatively, other factors may have increased the risk of both Alzheimer’s disease and infections, such as immune dysfunction or alcohol or drug abuse, says at the University of Helsinki in Finland.

“To translate this work into clinical practice, we need to investigate whether strategies aimed at reducing infections lower the risk of subsequent dementia,” says at the London School of Hygiene & Tropical Medicine.