A new study suggests that infections and dementia could be linked, whereas autoimmune diseases are unlikely to increase the risk of this neurodegenerative disorder.
There has been an extensive debate over the possible connection, especially its most common form, Alzheimer's disease, but the evidence remains inconclusive.
Some recent studies suggest the risk of dementia increases following different hospital-diagnosed infections. However, research also indicates that systemic inflammation, rather than specific infections, might explain the elevated risk of the condition.
The new study published in JAMA Network by the University of Copenhagen researchers looked at data from 1,493,896 Danish residents born from 1928 to 1953 who were alive and in Denmark on January 1, 1978, and on the date of their 65th birthday.
From age 50 years onward, 45% of the participants were diagnosed with infections, most often respiratory, followed by gastrointestinal and urinary infections. Additionally, 9% of the participants were diagnosed with autoimmune diseases, primarily rheumatoid arthritis and polymyalgia rheumatica.
The study found that infections were associated with a 1.49-fold increased rate of dementia, especially within five years after contracting them. A higher dementia risk was linked to most infections, except for ear and cardiovascular.
Among those with autoimmune diseases, most of which cause inflammation, the increase of dementia risk was only 1.04-fold, especially after adjustment for infections.
The authors say they cannot make firm conclusions about the role of inflammation in dementia. They also note that the slight increase in the risk may be masked by the effect of autoimmune disease medications that either suppress severe inflammation or reduce dementia risk, as some studies suggest.
The researchers conclude that because the increased dementia risk is associated only with infections and not autoimmune diseases, it "may point toward a role for infection-specific processes rather than general systemic inflammation."
Dementia risk factors
Dementia is a general term for loss of memory, language, problem-solving, and other thinking abilities that interfere with daily functioning. Alzheimer's disease is the most common form of dementia, accounting for 60% to 80% of cases. An estimated six million Americans are currently living with the condition.
Increasing age is the strongest known risk factor for dementia. Women are more likely to develop Alzheimer's disease than men, but this may be because they live longer on average than men.
Although Alzheimer's disease does not run in families, scientists have identified over 70 genes that may increase the risk of developing the condition.
Other dementia risk factors include:
- High blood pressure in mid-life (ages 45 to 65), particularly for vascular dementia.
- Type 2 diabetes in mid-life, particularly for Alzheimer's disease and vascular dementia.
- Obesity in mid-life. It also increases the risk of developing other risk factors, such as type 2 diabetes.
- Lack of physical activity later in life.
- Unhealthy diet, high in saturated fat, sugar, and salt.
- High alcohol consumption (more than 12 drinks a week).
- Low levels of cognitive engagement.
- Depression in mid- or later life.
- Severe or repeated head injuries.
- Hearing loss that leads to social isolation, loss of independence, and problems with everyday activities.
- Social isolation.
- Air pollution.
- JAMA Network. Hospital-Diagnosed Infections, Autoimmune Diseases, and Subsequent Dementia Incidence.
- Alzheimer’s Association. Alzheimer's Disease Facts and Figures.
- Alzheimer Society. Risk factors for dementia.