Tropical diseases are undermining intellectual development in countries with poor health care—and they’re coming here next
But last year, Yolken found a microbe that seems to sap intelligence, not sanity. In the October issue of Proceedings of the National Academy of Sciences, he described a study that included mental tests followed by physical screenings. Investigators from Johns Hopkins, Baltimore’s Sheppard Pratt Health System, and the University of Nebraska–Lincoln were startled to find a previously unsuspected virus,Acanthocystis turfacea chlorella virus 1, or ATCV-1, lurking in the throats of two of every five of their Baltimore research subjects.
Despite the decades Yolken has spent analyzing microbes and mental health, this finding astonished him. “It’s something that we wouldn’t have suspected would actually have any effect on humans or animals,” he told me. For one thing, ATCV-1 is a chlorovirus, which infects algae, and viruses that transcend species usually do so in modest increments within their kingdom. Yet this pathogen scaled the evolutionary ladder in a single bound as it jumped from algae to humans.
But the study’s baseline cognitive tests unveiled the true shocker. When compared with those who did not harbor the virus, those infected were about 10 percent slower to make calculations and had a reduced attention span, suggesting that the virus compromised their ability to calculate, to focus, and to process visual information—disadvantages in the classroom, on the job, and in other familiar learning situations. “American Researchers Discover ‘Stupidity Virus,’ ”Newsweek trumpeted when the news got out; a more measured Wired UK headline read, “Human cognition-affecting virus discovered.”
The lowered mental functioning was independent of potentially confounding factors, including age, socioeconomic status, education, place of birth, or smoking status. Gender and race made no difference. Repeating this experiment in a larger population yielded the same results, and when the research team tested mice before and after exposing them to the virus, they found 1,000 gene changes in brain regions known to be important to memory and learning. These infected mice also took 10 percent longer to navigate a maze and showed reduced attention spans, compared with the uninfected controls. Although critics suggested that they had found not an IQ-lowering microbe but rather sample contamination, Yolken refuted this suggestion in an article in the February Proceedings of the National Academy of Sciences.
Infection and Intelligence
Pseudo-nitzschia and ATCV-1 join a panoply of microorganisms that have long been suspected, and sometimes known, to change how well we think. These newcomers are getting attention because they struck in the West, where a functional public health infrastructure allows for disease surveillance and the investigation of mysterious sources of disability. But pathogens that affect cognition have traditionally infected poor people of color in the developing world, so their ravages and long-term effects have been more likely to go unanalyzed and untreated. In developing countries, points out Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, infection by such pathogens is the norm, not a headline-worthy exception.
The mental effects of pathogens, including worms and other organisms that cause schistosomiasis (or snail fever), leishmaniasis (caused by sand flies), and dengue fever, have cast a long shadow over the heavily political and seemingly endless debates over race and intelligence. Studies like a 2010 report from Randy Thornhill, an evolutionary biologist at the University of New Mexico, and a 2011 research study from Carleton University in Ottawa, have strongly correlated low average IQs from various nations with high rates of infectious disease. A 2011 article Thornhill published in the journal Intelligence calculated that, in the United States, allowing for education and wealth, “Infectious disease was the best predictor of average IQ.”
Thornhill’s “parasite-stress” theory maintains that parasites sap the brain’s energy in several ways. The newborn human brain uses fully 87 percent of the body’s “metabolic budget,” an amount that diminishes with age and maturity, and if a young brain cannot meet these initial high-energy requirements, its growth and development suffer. Microbes and larger parasites drain this energy by feeding on tissues and lodging in the digestive tract, where they siphon off nutrients and iron. Additionally, viruses divert energy from their hosts to crank out copies of themselves. This stolen energy normally would fuel brain building and other metabolic needs of the child.
The result, said Thornhill’s 2011 article, is that across the United States, “there is a negative association between infectious-disease stress and average intelligence” as measured by IQ score. To calculate the effects of disease on intelligence outside the United States, Thornhill used data from Richard Lynn and Tatu Vanhanen’s 2006 book IQ and Global Inequality, which listed average IQ scores for more than 113 countries. These ranged from a high average of 107 in Hong Kong to a low of 63 in Ethiopia. The lowest IQs were found in the poorest countries of Africa, and the authors suggest that the differences were heritable but partly due to variations in the wealth of nations.
Scientists who, like Lynn, believe in heritable racial difference in cognitive ability talk of intelligence, but they measure IQ. Moreover, they tend to conflate the two, even though many cultural and socioeconomic factors separate them. No consensus on the definition of intelligence exists: valuable intelligence varies with culture. IQ tests assess only some components of mental ability and achievement, those that are highly valued and frequently practiced in affluent Western culture, and they do so inconsistently.
Brink Lindsey, the author of Human Capitalism: How Economic Growth Has Made Us Smarter—and More Unequal (2013), cogently argues in a May 2013 web article from The Atlantic, that whatever aspects of intelligence IQ tests measure, these factors are not universal. They are assessed more accurately for affluent Westerners than for others. However, Thornhill’s analysis revealed that IQ is more closely correlated to the health of nations than their wealth.
A Panoply of Disease
One dramatic health difference between rich and poor countries is the prevalence of neglected tropical diseases, or NTDs, which afflict a billion people worldwide, most of whom live in the kind of extreme poverty that characterizes the Global South.
Apocalyptic images of dramatic medical crises such as AIDS and Ebola captivate the West and spur altruism, but when it comes to the NTDs that chronically compromise health and challenge mental abilities, our myopia has been profound. Ebola-racked Guinea, Liberia, and Sierra Leone also harbor the highest-known concentrations of hookworm victims. Since 2013, 11,000 people have died from Ebola in sub-Saharan Africa, but 10 million people—nearly half the population of these countries—suffer from at least one NTD or malaria or both. And NTDs plague extremely poor denizens of the subtropics not only of sub-Saharan Africa, but also of Saudi Arabia, Brazil, India, China, Indonesia, and Mexico. They are, in the words of Peter Hotez, the dean at Baylor, “great disablers rather than killers.”
Yet the HIV disease prevalent on the African continent and throughout the developing world deranges thinking, too. Children who acquire HIV prenatally from their mothers risk central nervous system disease that can cause a spectrum of brain dysfunction from encephalopathy to subtle cognitive impairment. Language disorders and developmental delays can result because HIV crosses the blood-brain barrier to contribute to various types of neuronal injury. A 2010 longitudinal study published in the journal AIDS examined more than 300 HIV-positive children with an average age of 12 and found that they fell into the low-average scale for memory, speed of processing, and verbal comprehension. The researchers suggested that neurodevelopmental problems in children and adolescents with HIV might be linked to “changes in proinflammatory monocytes”—immune-system changes that the virus provokes.
Cognitive delays plague many people who are infested with parasitic worms, including 807 million–1.2 billion people with ascariasis, 604–795 million suffering from trichuriasis, and 576–740 million with hookworm infection, concentrated in sub-Saharan Africa, Asia, and Latin America. Each of these diseases hinders economic prosperity by decimating the workforce.
Such disastrous cases are only the most dramatic manifestation of an epidemiological sea change: tropical diseases—and their neglect—are not limited to the tropics any more. They’re now very much at home in the United States. The Big Five diseases—Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, and trichomoniasis—are quite common here among the poor, Hotez says. “While sub-Saharan Africa accounts for many of the world’s NTDs, somewhat paradoxically, most of the world’s NTDs can be found among the poor living in wealthier countries, including the ‘Group of 20’ nations. Houston and Texas … represent ‘ground zero’ for many of America’s neglected tropical diseases.”
In addition to life-threatening heart and respiratory devastation, this suite of communicable neuropsychiatric conditions saps the brainpower as well as the strength of poor people of color at home, just as it does abroad. Yet many doctors in the United States remain in denial. When the patient of a doctor at the Baylor College of Medicine received a letter saying that blood he had donated was being rejected because of a positive test for Chagas disease, the doctor exploded, “The test is wrong. That disease doesn’t exist in the United States!”
But it does, and this troubling scene is replayed across the country as American blood donors learn that they have a Third World disease. Tropical medicine experts agree that at least 330,000 U.S. citizens have Chagas disease, the most common parasitic disease in the Americas, and estimates range as high as one million. It infects six million to seven million more people in Latin America. This chronic, silent parasitic infection leads to fatal heart or intestinal damage in two of every five sufferers, and it also causes intellectual slowing. It can be treated, but the lack of awareness by doctors in the United States means that it often isn’t.
The insect vector for Chagas is an unwelcome immigrant—the triatomine or “kissing” bug, which lives in the cracks of substandard housing and passes on the parasite to people by defecating while sucking their blood. Chagas causes as many as one in 10 neurological problems in Mesoamerica. In the United States, it mainly affects poor Hispanic communities, but taken altogether, the infections that ravage the developing world now imperil the bodies and minds of at least 14 million U.S. residents.
Blame the Climate
Why are tropical diseases flourishing here? Xenophobes may indict immigrants who bring microbes north with them, but the real culprit is the U.S. climate. Many microbes function only within a narrow temperature range, and the life cycles of the parasites in question often require heat. This country obliges with temperatures that are considerably warmer than those in much of the affluent West, particularly those in northern Europe. “The U.S. is somewhat unusual in being a wealthy nation much of whose population lives in very warm, humid regions,” Stan Cox, a senior scientist at the Land Institute, told The Washington Post in July.
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