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Hospital-treated infections are associated with an increased subsequent risk of developing Alzheimer’s and Parkinson’s disease at a relatively early age, research shows.

The Swedish study found that infections treated with either in- or out-patient specialized care were linked with an increased risk of Alzheimer’s and Parkinson’s, primarily before the age of 60 years.

However, there was no association between hospital-treated infections and the other common neurodegenerative disease of amyotrophic lateral sclerosis (ALS).

The positive associations with Alzheimer’s or Parkinson’s remained after excluding infections in the 10 years prior to these two diseases being diagnosed, with repeated infections in early- and mid-life conferring the greatest increase in risk.

Elevated risk was seen with bacterial, viral and other infections and across different sites of infection, including those in gastrointestinal and genitourinary locations.

The researchers therefore suggest that the underlying mechanisms linking the two neurodegenerative diseases to infections may occur at a systemic level rather than be specific to particular pathogens or organs.

“We hypothesize that infectious events may be a trigger or amplifier of a preexisting disease process, leading to clinical onset of neurodegenerative disease at a relatively early age among individuals with disease predisposition,” they explain in the journal PLoS Medicine.

“Active monitoring and prevention of severe infections may therefore help to prevent or delay disease onset among high-risk individuals.”

Using several Swedish national healthcare registers, the team conducted a nationwide nested case-control study to examine the associations of hospital-treated infections requiring inpatient or outpatient care and the risk of three common neurodegenerative diseases.

Patients newly diagnosed with Alzheimer’s, Parkinson’s, or ALS disease in Sweden between 1970 and 1996 were each matched with five control individuals matched for sex and year who were randomly selected from the general population.

Overall, the study included 291,941 people with Alzheimer’s, 103,919 with Parkinson’s, and 10,161 with ALS, together with their matched control participants. After excluding infections diagnosed during 5 years before diagnosis, hospital-treated infection was associated with a 16% increased risk of Alzheimer’s and a 4% increased risk of Parkinson’s disease.

People with at least two hospital-treated infections before the age of 40 years had a 2.62-fold increased risk of Alzheimer’s, and Parkinson’s risk increased by 41%.

All these results were primarily attributable to AD and PD diagnosed before 60 years, with no association found for those diagnosed above this age.

For example, genitourinary infection was associated with the highest risk of Alzheimer’s before the age of 60 years, with risk increased 2.75-fold, while infection of the central nervous system was associated the highest risk of Parkinson’s disease, with a 39% increased risk.

Gastrointestinal infection was also associated with a higher risk of Alzheimer’s or Parkinson’s disease before the age of 60 years, with a 35% and 18% increased risk, respectively.

Jiangwei Sun, from the Karolinska Institute in Stockholm, and co-workers note the observational nature of their study means a causal link cannot be formally proven.

Nonetheless, they conclude: “Further studies are warranted to validate these findings, to elucidate underlying mechanisms, and to determine whether better control of hospital-treated infections could prevent or delay onset of neurodegenerative diseases, especially the ones with an onset relatively early in life.”

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