Topography as a Contextual Variable in Infectious Disease Transmission

Clinical Laboratory Science, Spring 2004 by Montoya, Isaac D

OBJECTIVE: This paper examines whether or not: topography is a contextual variable that indirectly influences the transmission of infectious diseases. Age, gender, race/ ethnicity, education level, economic status, injection drug use, and high-risk sexual behavior are known to influence infectious diseases transmission, but the effects of topography are often overlooked.

DESIGN: A sample of 395 drug users were chosen from census tracts based upon a target profile of drug use behavior and demographics for the city of Houston. HIV was chosen as the infectious disease used to test this hypothesis.

SETTING: Residents of 16 census tracts in Houston, Texas participated in this study.

RESULTS: The findings revealed that census tracts that were 'isolated' by topographic barriers, such as bayous, parks, railroad tracks, railway yards, major thoroughfares, freeways, and unique street grids had fewer cases of HIV than census tracks that were more accessible to thru-traffic.

CONCLUSION: The research findings suggest that future research studies should consider topography as being contextually related to infectious disease transmission.

ABBREVIATIONS USED: HIV = human immunodeficiency virus

INDEX TERMS: Census tracts; drug user; HIV; infectious diseases; topography.

Clin Lab Sci 2004;17(2):95

The purpose of this paper is to present a qualitative case study that examines whether or not topography is a contextual (or control) variable that is related to the transmission of infectious diseases. Control variables such as age, gender, race/ethnicity, education level, and economic status as well as causal variables such as injection drug use and high-risk sexual behavior are known to influence infectious disease transmission, but the effects of topographic barriers, either natural or man-made, are often overlooked.1-3 The link between topography and infectious disease transmission is indirect in nature, if the link exists at all. Topographic features can indirectly affect infectious disease transmission by serving as barriers to the direct causes of the spread of infectious diseases. For instance, it is plausible to consider that topographic barriers, such as rivers or railway yards, could act to reduce drug trafficking by impeding the ease of access to and exit from areas where drug dealing occurs. The logic follows that with reduced drug trafficking, there would be fewer needles shared among drug users and thus lower incidences of infectious disease. HIV will be used as a proxy for infectious disease transmission. This paper will examine the prevalence of HIV in 16 census tracts in Houston TX to determine if future research studies should include topography as one of the several context-related, control variables that affect HIV transmission.

A careful review of the literature suggests that there is evidence that topographic features play a role in the spread of HIV. One study affirms that HIV transmission is constrained equally by geographic location as well as by the interpersonal ties of social networks.4 In her study of Uganda, Obbo finds that one region of Uganda had a low prevalence of HIV because it was geographically isolated from other regions with higher incidences of HIV.4 Potterat demonstrated that the constraints of geographic area may influence, or even determine, behavior patterns associated with the risk of acquiring gonorrhea, and can determine the magnitude of that risk for any given population.5 And another research study reports that socioeconomic, cultural, land use, terrain, behavioral, and risk exposure are each factors in the spread of HIV and that they influence each other in complex ways.6

Wallace discovered that research on factors that influence the transmission of HIV generally has ignored questions of context, even though any individual who contracts the virus is defined by social, economic, geographic, historical, and other context-related factors.7 An important consequence of ignoring context-related factors is the finding that HIV transmission has been exacerbated by public policies that allow the degradation and the subsequent desertification of low-income, minority areas.8-10 These policies have disrupted the socio-geographic-centered networks that confine HIV transmission to tightly clustered 'socio-geographic' spaces.7 The consequences of such policies have led to more rapid propagation of the disease and to the spread of HIV into the more suburban areas surrounding those that have been 'hollowed out' by poor fire and garbage services.7,11

In a study that served as a precursor to this one, Hu and collaborators identified geographic areas, separated by ZIP code boundaries, that were associated with higher incidences of HIV in the Newark, New Jersey metropolitan area.6 Hu found that in ZIP codes which had the highest prevalence of HIV, there also tended to be the highest percentages of African-Americans who lived in multifamily units. But more interesting to this study, Hu also found that high percentages of HIV coincided with natural topographic features. The Watchung Mountains divide Newark and the Oranges (the cities of Orange, East Orange, South Orange, West Orange, and Maplewood) from other cities in Essex County, and this mountain range may have been a contributing factor to the disparate HIV rates among ZIP codes. Newark and the Oranges have higher incidences of HIV-prevalent ZIP codes than those in the more suburban areas of Essex. Hu and his associates did not consider topography as a context-related variable, and in this case it may have been outweighed by other factors, for there were racial and housing disparities on each side of the mountain range. However, topography would have been clearly influential had the racial composition and percentage of multi-member housing been the same on both sides of the divide.