The national fight to eradicate health disparities has often focused on structural concerns such as a lack of access to quality care and the dangers of toxic environments. Research has confirmed the relationship between the extreme stress of poverty and the risk of disease. But to a great degree, the exact pathway between extraordinary life challenges and disease has remained a mystery.
Two Robert Wood Johnson Foundation (RWJF) Health & Society Scholars, Allison Aiello, PhD, (2003-2005) and Jennifer Dowd, PhD, (2006-2008) joined Sheldon Cohen, PhD, a professor of psychology at Carnegie Mellon University, to explain one possible map for that pathway at their recent symposium, the Social Determinants of Susceptibility: At the Intersection of Social and Infectious Epidemiology, held at Columbia University. Their work asks: When people are exposed to the same infectious agents, who gets sick and why?
Hidden Challenges to Good Health
Cohen opened the session with a discussion of the ways that stress increases the risk of infection. A leading researcher in the field of stress and health, his groundbreaking 1991 study was the first to show that psychological stress could increase the likelihood of developing the common cold.
“Overall there is strong evidence that our resistance to infectious disease is not just attributable to our exposure to pathogens, but also is affected by our physical and social environments, our beliefs and coping skills. Particularly, the work points to the importance of socioeconomic environments, and major and chronic adverse events in disrupting our body’s ability to fight disease,” Cohen said.
Cohen also reported the results of his recent study “Chronic Stress, Glucocorticoid Receptor Resistance, Inflammation and Disease Risk,” published in March 2012 in the journal PNAS, showing that stress lowered the immune system’s ability to calm inflammation.
His ongoing investigation of the insidious impact of stress on health has produced evidence of links between anxiety-producing life events and poor respiratory health in children and adults, cardiovascular disease (CVD), and other chronic ailments. He also discovered that adults who grow up at the lower end of the socioeconomic ladder are generally more likely to have immune systems that are less resistant to disease.
Stress, Immunity and Health Risks
Dowd, Aiello and Cohen have focused on how infectious agents, such as viruses, might impact health disparities. They are looking at how viruses behave in the body when a person is subjected to many of the social determinants that we associate with disparities—poverty, difficult life circumstances and other factors that can weaken the immune system.
“Lifestyle, behavioral and environmental factors are certainly not the only influencers of chronic illnesses,” said Aiello, an associate professor of epidemiology at the University of Michigan School of Public health. “We know, for example, that approximately 15 percent of cancers have an infectious etiology.”
“And even though we are aware that stress is related to health disparities, the role of the immune system is not well understood,” said Dowd, a professor of epidemiology at the City University of New York School of Public Health.
Working together, Aiello and Dowd found that people with lower socioeconomic status had higher rates of many common latent infections such as cytomegalovirus (CMV), Epstein-Barr virus, and Herpes Simplex Virus 1. These infections were acquired earlier in life in low-income populations, among African American and Hispanic adults and children. The viruses were also more likely to reactivate in response to stress-related suppression of the immune system.
Aiello’s research investigated how these higher rates of infection and virus activation shape overall health.
The Silent Path to Illness
Aiello began by looking at the relationship between dementia and CMV, a common herpes virus. Rates of dementia are disproportionately high among African Americans and Hispanics. CMV is present in nearly 80 percent of the population. The virus is usually considered harmless, according to the Centers for Disease Control and Prevention.
Yet when Aiello analyzed data from the Sacramento Area Latino Study on Aging she found a link between higher CMV antibody levels and reduced cognitive function. “We looked at 1,789 people who enrolled in the study at age 60. Cognitive function was measured over 10 years. We found a seven-point difference on a test of cognitive abilities among the study participants with higher levels of CMV antibodies,” she said.
In more recent work with a multiracial elderly population, Aiello found a significantly higher rate of cognitive decline and dementia among individuals with higher CMV antibody levels.
CMV appears to silently alter the immune system if it is repeatedly activated. “Most people have the virus in their bodies by age 4, so perhaps if some people reactivate the virus more often than others that may negatively impact the immune system,” Aiello said.
“We also know that depression [which contributes to health disparities] is linked to dementia. Study participants with CMV were 1.5 times more likely to have depressive symptoms, indicating that CMV may reactivate in response to depression as well,” she added.
“Additional studies have confirmed our findings and linked CMV to higher mortality and frailty in elderly women. But the point, in reference to disparities, is that none of this happens in a vacuum. Clearly social patterns are important in shaping the link between infection and chronic disease. Since most people are exposed at an early age, why might CMV be more detrimental to one person than another?” Aiello asked.
A possible route to eradicating the health issues linked to CMV, Aiello suggested, may lie “in reducing exposure to CMV and other persistent infections. A vaccine might also be helpful, but improvements in housing infrastructure, infection control in day care, and reductions in social stressors may help to reduce exposure, virus reactivation rates and ultimately, related health disparities.”