An Unexpected Stowaway:

Patterns of Tuberculosis Transmission and Human Travel

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Stacey L. Prow

sprow1@swarthmore.edu

At some point in undetermined history, the parent strain of human Tuberculosis - Mycobactrium bovis - underwent adaptation to jump from animals to humans and became the causative agent of tuberculosis in humans (Iseman, 1994).  Skeletal artifacts indicate that Tuberculosis (the causative agent of Tuberculosis in humans is caused by an infection of M. tuberculosis) has afflicted human kind since at least 3000-5000 BC, but, for the past 550 years, Tuberculosis has been pandemic in North America and Europe (Iseman, 1994).  The de facto breakdown of national and regional borders caused by technological changes in the transportation sector has vastly increased the transmissibility of Tuberculosis and other organisms.  This trend is not only seen with modern-day transportation but also in historical times during European expansion and colonization.  

Since the 1973 discovery of Tuberculosis-like bacilli in preserved Peruvian mummy tissue, there has been much debate whether Tuberculosis was endemic in prehistoric communities of Amerindians.  Most paleopathologists now agree that the infection was endemic in larger communities in the Americas, similar to the population-dependant nature of many of today’s infections (Clark et al.,1987).  There has been much debate as to what this revelation means for the high incidence of Tuberculosis in these populations in light of Spanish conquest of the region.  Whether the Amerindians were exposed to a more virulent strain of the disease or whether the disease epidemic resulted from enforced changes in ecological and environmental due to Spanish conquest is undetermined (Clark et al.,1987).  However, both cases tie the cause of the Tuberculosis epidemic to changes ultimately caused by human travel and, specifically, European colonization and conquest.

A more straightforward and recent example of Tuberculosis transmission due to modern travel is the case of the Yanomami Indians of Brazil.  The Yanomami Indians of the Amazon rainforest are the subject of recent studies to categorize and treat an emerging tuberculosis epidemic in a previously “naive” population.  This population was not exposed to TB until contact with individuals of European descent in the mid-1960s (Sousa et al., 1997).  By the 1980’s the infection, characterized by a high mortality rate, has spread throughout the population and become epidemic.  Observational studies show significant differences in the immune response to M. tuberculosis infection in Yanomami Indians and contemporary residents of the area, suggesting that TB, and other prevalent diseases in history, may represent a powerful selective pressure on the evolution of the human immune response (Sousa et al., 1997).

Technological advances and societal changes have enhanced the evolution of drug-resistant strains of M. tuberculosis in the United States and around the world (Iseman, 1994).  The preferential survival of drug-resistant bacteria through Darwinian selection involves either a global or localized survival advantage within a particular niche.  Natural selection for drug-resistant strains is gradually increasing the world’s Tuberculosis cases that involve drug-resistant organisms.  For instance, a recent report on the state of TB cases in the United States reported that TB cases are at an all-time low, but of those cases, a much larger proportion involve drug-resistant strains (CDC, 2005).  From 2003 to 2004 in the United States, the incidence of multi-drug resistant TB (TB that is resistant to at least two first-line therapies) increased 13.3 percent (CDC, 2005).  Historically, most drug-resistant cases have arisen due to failed treatment of an individual; however, these strains are able to transmit to new patients, creating a pre-formed drug resistant infection that is increasingly mobile due to modern travel (Iseman, 1994).

Differential microbial characteristics, selective pressures from the use of broad-spectrum antibiotics (as well as the misuse to narrow-spectrum antibiotics), and social and technological changes that enhance the transmission of Tuberculosis and other infections are all key factors contributing to the rise in drug-resistant infections.  However, the human-mediated spread of active infections has increased in tandem with the technological progress of travel.  The increasing mobility of humans will exponentially increases the likelihood that horizontal gene transfer will cause the exchange of drug-resistant suites of genes between noxious and commonplace infections.  Public health officials are increasingly worried about the possibility that Enterococci (bacteria responsible for many common infections that has gained resistance to many conventional therapies) will spread vancomycin resistance to other genera of bacteria (Kingman et al., 1994).  Even more alarming to public health officials, is the possibility of gene transfer between organisms that already evade treatment by the first-line-of-defense drugs.

Drug-resistant infections are becoming a major public health concern in two major arenas: hospitals (and other such institutional settings) in developed countries and the larger communal population in developing countries.  Both of these settings create a genetic merry-go-round that increases the likelihood of gene transfer.  These ecosystems are not isolated from each other and have a high potential to impact populations not included in these two categories.  Hospitals play host to a large reservoir of microbial species, increasing the probability of the horizontal transfer of a suite of drug-resistance genes from one bacterium to another.  Even worse is the probability that an already difficult-to-treat pathogenic bacteria gains resistance to the antibiotic currently used as the last line of treatment.  In developing countries, the public health concern in the misuse of antibiotics due to their high cost.  The use of sub-therapeutic levels of antibiotics significantly increases the likelihood of developing resistant bacteria.

The transmissibility of infectious diseases, like Tuberculosis, has increased in step with the advent of transportation and travel technology since long before the European decimation of the “naive” American Indian population.  Technological changes have also facilitated the transportation of foods, thereby, increasing the importation (or exportation) of disease-causing bacteria, both drug-resistant strains as well as less virulent strains.  The seemingly exponential increase in travel in the last century has greatly increased not only the number of people traveling but also the number of insidious diseases.  Endemic infections confined to certain regions in history now have the capability to become worldwide epidemics and the rising tide of drug-resistant infections will only increase the severity of the public health problem in the future.  To sustain these infectious diseases, microbes need a human population that is sufficiently large so that the disease can shift from one local area to another, until antibody count is able to recede in some areas of the population (Diamond, 1999).  Modern travel has effectively increased the population size of even the remotest places.  It seems like only a matter of time until such epidemics occur, especially since the air travel time to your destination of choice is much less than the incubation time of most diseases. 

 

References

Center for Disease Control Press Release. U.S. Tuberculosis cases at an all-time low in 2005, but drug resistance increasing. 2005.

Clark, G.A.; Kelley, M.A.; Grange, J.M.; Hill, M.C.; et al. Mycobacterial Disease in Human Populations: A Reevaluation [and Comments and Reply]. Current Anthropology, Vol. 28, No. 1.  1987: 45-62.

Diamond, J. Guns, Germs, and Steel. 1999.

Iseman, M. Evolution of Drug-Resistant Tuberculosis: A Tale of Two Species. PNAS, Vol. 91, No. 7. 1994: 2428-2429.

Kingman, S. Resistance and European Problem, Too. Science, New Series, Vol. 264, No. 5157. 1994: 363-65.

Sousa, A.O.; Salem, J.I.; Lee, F.K.; Vercosa, M.C. Cruaud, P.; Bloom, B.R., et al. An Epidemic of Tuberculosis with a High Rate of Tuberculin Anergy among a Population of Previously Unexposed to Tuberculosis, the Yanomami Indians of the Brazilian Amazon. PNAS, Vol. 94, No. 24. 1997: 13227-13232.

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