Historically, soil-transmitted helminth infections were endemic throughout Appalachia and the southern United States. The Rockefeller Sanitary Commission invested upwards of $1 million in a campaign designed to eradicate these infections, decreasing hookworm infection from 41.7% among persons in the South in 1914 to 19.3% by 1930.23
Despite these interventions, soil-transmitted helminth infection remained significant in areas of the United States, with some estimating that one in five Kentucky residents or 600,000 persons harbored these parasites in 1945.24
Although transmission of these infections within the United States has likely declined significantly in the intervening years because of improvements in sanitation practices, the current status of these infections is unknown.
Published studies of soil-transmitted helminth infection in the United States show that, as recently as 1982, some regions of Appalachia continued to sustain a significant burden of both T. trichiura and A. lumbricoides, likely from endemic sources. The high-quality studies summarized in this review illustrate that helminth infection was highly prevalent throughout the southern United States as well as Appalachia as recently as the early 1980s. Even less is known about soil-transmitted helminth infections in other parts of the United States outside the southern United States and Appalachia; however, those areas are generally thought to carry lower burdens of infection.
Despite the historically high burden of these infections seen in the United States, the published literature does not provide enough information to determine if soil-transmitted infections remain endemic in the United States, with the most recent high-quality study examining endemic transmission of infection occurring in 1982. Recent publications are case-based, study exclusively immigrant populations, or use a laboratory-based approach that makes determination of endemic transmission impossible.
Soil-transmitted helminth infections result in significant disease in those people infected; they are a cause of cognitive and physical growth impairment and anemia in children. Additionally, S. stercoralis
infection may result in potentially severe and fatal hyperinfection in immunocompromised hosts.25
All soil-transmitted helminth infections may lead to subtle long-term consequences such as poor school attendance and academic achievement as well as delays in growth and development from chronic malnutrition.1
Although the literature does not provide enough evidence to determine the status of soil-transmitted helminths in the United States, socioeconomic and environmental conditions may remain conducive to ongoing infection. The 1990 poverty rate for Appalachia was 15.2%, higher than the national rate of 13.1%, and in nonmetropolitan areas of central Appalachia, over 27% of the population lives below the poverty level, a rate over two times the national average.26
Substandard housing units are common, facilitating conditions that may encourage transmission of these infections. In Central Appalachia, an area encompassing 179 counties and cities in Kentucky, West Virginia, Tennessee, and Virginia, 57% of counties had substandard housing rates higher than the US rate (5.3%) in 1990.27
Historically, access to indoor plumbing and running water in Appalachia has lagged behind the rest of the United States, with one-half of homes in the region lacking access in 1964. Although sanitation has improved since the last high-quality studies were performed, an estimated 169,000 housing units remained without indoor plumbing as recently as 2000. Poor sanitation continues to persist in some areas of Appalachia, with 3% of the region lacking plumbing, and in some areas, plumbing is not complete in over 25% of housing units.28
This literature review had several limitations. First, search strategies relied on appropriate use of MeSH terminology and may have lead to incomplete identification of existing studies. Additionally, by relying solely on the published literature, this review was subject to publication bias, because studies with minimal or negative results may not have been included. Second, because only a point estimate of prevalence was reported in each study, it was not possible to ascertain the certainty of these estimates of prevalence. Third, most of the studies relied on a single stool screening examination and suboptimal diagnostic methodology. The sensitivity of a single stool examination to detect infection with soil-transmitted helminths may be very low because of variation in egg output and the small amount of stool examined, with reported diagnostic sensitivity of a single stool examination for S. stercoralis
ranging from 30% to 50%.29,30
These limitations each contribute to possible underreporting of the current distribution and endemic transmission of soil-transmitted helminths in the United States.
Soil-transmitted helminths were historically considered endemic in several areas of the United States. Although the prevalence of these infections has decreased from levels observed in the early part of the 1900s, it is unclear if these infections continue to be transmitted among those people living in Appalachia or throughout the southern United States. Changes to the US population as a result of immigration have led an increase in attention focused on imported soil-transmitted helminth infections, which are unlikely to contribute to local transmission given the medical care and standards of sanitation in most areas of the United States. However, little attention has been given to possible continued transmission of soil-transmitted helminths in historically endemic areas, where they may cause an underappreciated burden of disease. Because conditions in areas of the United States may still support endemic transmission and these diseases cause significant morbidity among those people infected, determining the status of disease is essential. Additional research, particularly assessment of possible transmission through community level investigations similar to those searches included in this review, should be performed to determine if soil-transmitted helminth infections continue to be endemic in the certain areas of the United States.