A new longitudinal study tracking 730 adults in two Indigenous Bolivian communities reveals that dementia incidence is similar to rates in the U.S. and Europe, challenging earlier assumptions that the condition was remarkably uncommon in preindustrial lifestyles. The findings appear in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association.
Observations suggested that dementia was rare among Indigenous communities living preindustrial lifestyles in the Bolivian Amazon. But the rate at which new cases develop tells a different story.
Tracking Rates Across Tsimané and Mosetén Communities
The study, published in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, followed 730 adults who did not have dementia during their initial assessment and checked back nearly five years later. Researchers identified 22 new dementia cases, resulting in an overall incidence rate of 7.40 cases per 1,000 person-years. A separate nationally representative study published in JAMA Network Open by Yale researchers analyzed thousands of observations from about 3,300 people aged 65 and older and found that many individuals with dementia go undiagnosed, and two-thirds of patients with a documented diagnosis fail to report it themselves.
For individuals under 80 years old, the rate of new diagnoses did not dramatically differ from rates seen in populations in the United States and northern Europe. At follow-up assessments, overall dementia prevalence stayed below 2%. Margaret Gatz, lead study author and professor of psychology and gerontology at the University of Southern California, pointed out that the outcomes contradict previous beliefs.
The surprising finding was that there weren’t big differences in dementia incidence. That challenges us to identify the risk factors for developing dementia in the Tsimané population.
Margaret Gatz, professor of psychology and gerontology at the University of Southern California
Co-author Ben Trumble, a professor in Arizona State University’s School of Human Evolution and Social Change, clarified that the small overall percentage of cases relates to how long individuals survive.
(This study) shows that not only is the number of people with dementia very low in this population, which could be explained by mortality, but also the number of people who ever develop dementia is low.
Ben Trumble, professor at Arizona State University
Mortality Rates Shape Remote Community Survival Times
Participants diagnosed with dementia faced more than six times the mortality risk compared to those with normal cognition.
Bolivian Cognitive Impairment Differs from Classic Alzheimer's Disease
The physiological presentation of cognitive impairment in the Bolivian communities appeared different from classical Alzheimer’s disease. Many affected individuals struggled more with attention, reasoning, and managing tasks than with memory. Decades of historical research, beginning with findings from Arnold Pick at the University of Prague examining a 71-year-old man named August H., and later microscopic investigations by Alois Alzheimer on a 51-year-old woman named Auguste D. published in 1907, established the foundational presence of cellular tangles and plaques that define classic neuropathology.
Preliminary blood analyses showed connections between dementia cases and neurofilament light chain proteins—a broad marker of neurodegeneration—but did not find correlations with the amyloid or phosphorylated tau protein profiles typically associated with Alzheimer’s.
In most Tsimané with cognitive impairment or dementia, the pattern of atrophy in the brain is not typical of Alzheimer’s disease. Rather, there are changes indicative of vascular contributions to cognitive impairment and dementia.
Margaret Gatz, professor of psychology and gerontology at the University of Southern California
