Older Americans already dread shingles, and they may now have one more reason to do so: A painful episode could raise their odds for cognitive decline.

A new study found a 20% higher risk that an older person would experience "subjective cognitive decline" if they'd gone through a bout of shingles.

That was especially true for men who already carried the APOE4 gene, known to raise the odds for cognitive decline and dementia.

"Our findings show long-term implications of shingles and highlight the importance of public health efforts to prevent and promote uptake of the shingles vaccine," said corresponding study author Dr. Sharon Curhan, of the Channing Division for Network Medicine at Brigham and Women's Hospital in Boston.

"Given the growing number of Americans at risk for this painful and often disabling disease and the availability of a very effective vaccine, shingles vaccination could provide a valuable opportunity to reduce the burden of shingles and possibly reduce the burden of subsequent cognitive decline," she added in a hospital news release.

The study was funded by GlaxoSmithKline, which makes the Shingrix shingles vaccine. It was published Aug. 13 in the journal Alzheimer's Research & Therapy.

The new study comes on the heels of related research published in July. That six-year study found that people who got so-called newer "recombinant" shingles vaccines, such as Shingrix, spent 17% more time living free of any diagnosis of dementia, compared to folks who had gotten the older "live" form of the vaccine, Zostavax.

Shingles is caused by the same virus that triggers chickenpox in youth, the varicella zoster virus. That virus then goes dormant, but can remerge to cause shingles as immune system defenses wane in the senior years.

According to Curhan's team, the data on whether or not shingles could raise dementia risks have been mixed.

They decided to look at subjective cognitive decline, defined in a hospital news release as "an individual’s self-perceived experience of worsening or more frequent confusion or memory loss."

Cognitive decline can be a precursor to Alzheimer's disease or dementia, although that's not always the case.

The new study looked at data on over 149,000 people enrolled in ghe Nurses’ Health Study, the Nurses’ Health Study 2 and the Health Professionals Follow-Up Study. They filled out questionnaires every two years, which included items on shingles and cognitive decline.

Any history of shingles was tied to a 20% higher odds for subjective signs of cognitive decline, the researchers said. Carrying the APOE4 gene, linked to dementia, pushed the risk even higher for men who had gotten shingles, but that wasn't true for women.

How might the shingles virus harm the brain?

According to Curhan's group, it might damage blood vessels in the body and/or brain, harming cognition over time.

“We’re evaluating to see if we can identify risk factors that could be modified to help reduce people’s risk of developing shingles,” Curhan said. “We also want to study whether the shingles vaccine can help reduce the risk of adverse health outcomes from shingles, such as cardiovascular disease and cognitive decline.”