Neglected Tropical Diseases in Texas

Overview of the NTDs in Texas and the American South

The major NTDs in Texas and other areas of the American South are listed in Box 1. Among their common features is the observation that most of these conditions cause chronic disabilities, which disproportionately affect people living in extreme poverty [3], [5], [13][15]. Another key feature is that NTDs are important examples of health disparities mostly affecting people of color, particularly African American and Hispanic minorities, largely because of the poverty link [3], [5], [13][15].

 Neglected Tropical Diseases in Texas

Parasitic Infections

  • Chagas disease
  • Cutaneous leishmaniasis
  • Cysticercosis
  • Toxocariasis
  • Trichomoniasis

Bacterial and Viral Infections

  • Murine typhus
  • Tuberculosis in diabetes mellitus
  • Dengue
  • West Nile virus

Neglected Parasitic Infections

Among the parasitic infections, Chagas disease (American trypanosomiasis caused by Trypanosoma cruzi infection) received renewed attention in 2012 based on recently published estimates of large numbers of people infected in the Western Hemisphere, including the high prevalence rates among pregnant women and subsequent maternal-to-child transmission [16], [17]. Of note, the first reported case of mother-to-child transmission in the U.S. was announced on July 6, 2012 by the Centers for Disease Control and Prevention (CDC) [18]. The CDC estimates that 300,000 cases of Chagas disease are found in the U.S. [19], whereas other investigators have suggested that almost as many cases occur in Texas alone [20]. Several kissing bug vector species are widespread in Texas and capable of transmitting T. cruzi; a significant percentage of these vectors are polymerase-chain-reaction (PCR) positive for T. cruzi [19], [21]. In South Texas, a high percentage of dogs, which are natural hosts, are also infected with T. cruzi [22], and a risk map for humans acquiring Chagas disease in Texas has been developed [21]. However, the extent to which T. cruzi transmission to humans actually occurs in the state is unknown [14]. There is an urgent need to increase surveillance for human T. cruzi infection in the region, possibly through seroprevalence studies, as well as for studies that attempt to document the extent of autochthonous transmission and mother-to-child transmission. In this sense, we are still at the “tip of the iceberg” in terms of our understanding of the epidemiology of Chagas disease in Texas and elsewhere in the American South. An alternative metaphor is that we have only seen the “ears of the armadillo” (similar to the ears of the hippopotamus metaphor sometimes used for malaria in Africa), referring to the nine-banded armadillo (Dasypus novemcinctus), which is native to Texas (Figure 1).

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Information is also scant for several other key NTDs in Texas. As with Chagas disease, these NTDs appear to be widespread in different areas of the state, but supporting surveillance and transmission studies are either sporadic or missing. Cutaneous leishmaniasis (CL) is another vector-borne parasitic protozoan infection, caused by Leishmania spp. and transmitted by sand flies of the genus Lutzomyia. Human cases of autochthonous CL caused by Leishmania mexicana infection have been recognized in Texas, primarily in the south-central region, since 1903 [23]. In 2008, nine cases were reported in northern Texas, not far from the Dallas-Fort Worth area [24]. There are several important animal reservoirs of Leishmania spp. in the Americas. In Latin America, rodents serve as an important animal reservoir for L. mexicana, and the Southern Plains woodrat has been implicated in Texas [23] and elsewhere in the southern U.S. Widespread infection of foxhounds in the U.S. with visceralizing L. infantum is also of concern, but the true extent of veterinary and human transmission in Texas and the rest of the U.S. is largely unknown [25]. A recent modeling study suggests that the range of reservoirs and sand fly vectors for CL is likely to expand deeper into the U.S., possibly in association with climate change [26], and thus, a northward expansion of CL infection in humans is conceivable.

Among the helminthic infections, neurocysticercosis (NCC) is now a major cause of epilepsy in Texas [27]. Most of the recent cases of NCC are believed to have been imported through immigration from Latin America [27], but autochthonous transmission still remains a possibility. Toxocariasis (Toxocara canis and Toxocara cati infection) is widespread in the American South, particularly among African American and Hispanic minority populations [28]. A covert form of this NTD has been linked to asthma and developmental delays [29], but the prevalence of toxocariasis in Texas and its potential contribution to chronic sequelae in the state have not been accurately determined.

Neglected Viral and Bacterial Infections

West Nile virus (WNV) infection, a mosquito-transmitted arbovirus infection, emerged in Houston, Texas in 2002 [30], where it occurs more commonly among people living in proximity to bayous lined with vegetation and other bodies of stagnant or slow-moving water [31]. Texas experienced a historic peak in WNV cases in 2012 affecting several areas of the state [32]. Like other vector-borne NTDs, WNV infection has been linked to poverty and its associated conditions [13], [33], [34]. A study in Houston in 2004 found that 7% of homeless people were positive for WNV infection, and that seroprevalence rose to 17% for those who did not seek shelter at night [34]. Risk factors for severe disease from WNV infection include hypertension, diabetes, and alcohol and substance abuse [35][39], which are all chronic morbidities that often go untreated in marginalized populations. In Texas and elsewhere, WNV infection was recently identified as an emerging etiologic agent of chronic renal disease and kidney failure [40].

Dengue emerged in South Texas in 1980, with additional outbreaks recognized in 1999 and 2005, and where conditions related to poverty also represent major risk factors for infection [41], [42]. Studies to determine the prevalence of dengue virus infection among residents of city-pairs on the U.S.-Mexico border have shown much higher rates of acute and past infection in Texas than would have been anticipated based on how infrequently the disease is recognized and reported [41][43]. In 2004 and 2005, recently contracted dengue virus infections were found among an estimated 2–4% of the residents of Brownsville, Texas, compared to 7–32% of residents of Matamoros, Mexico, with part of the difference having been ascribed to socioeconomic factors [41], [42]. The under-recognition of an ongoing dengue outbreak in the U.S. was recently highlighted in Key West, Florida [44], [45] and emphasizes the need for better surveillance and education of clinicians about NTDs in the U.S. Severe dengue has occurred in the continental U.S. and is always a concern where frequent dengue virus infections occur. Preliminary studies indicate that dengue may have already emerged in Houston (unpublished data). International air travel increases the risk for importation of dengue virus and possible outbreaks [46], an especially salient factor given Houston’s role as a major international air travel hub and the presence of the mosquito vector.

Among the major bacterial infections, murine typhus (Rickettsia typhi infection, transmitted by cat fleas) is emerging in South Texas [47]. An important evolving scenario lies at the interface of infectious diseases and the pandemic of chronic disease. Substantial evidence documents type 2 diabetes as the most important major risk factor for tuberculosis (TB) along the Texas border with Mexico [48], [49], increasing the risk of active TB three-fold. Similar observations have now been made in TB high burden countries across the globe [50]. Altered gene expression in the host and altered immune responses to several other pathogens in diabetes have been identified [51][54]. Studies conducted in South Texas were instrumental in uncovering the relationship between TB and diabetes such that additional studies in Texas might help in determining if similar relationships exist for other neglected diseases. The interaction between TB and type 2 diabetes illustrates how a neglected disease may interface with a chronic noncommunicable disease (CNCD). Some data suggest that the NTDs themselves manifest much like the CNCDs with respect to their chronic morbidities [55], and may certainly account for a hidden burden of CNCD-related morbidity [56].

 

Andrus J, Bottazzi ME, Chow J, Goraleski KA, Fisher-Hoch SP, et al. (2013) Ears of the Armadillo: Global Health Research and Neglected Diseases in Texas. PLoS Negl Trop Dis 7(6): e2021. doi:10.1371/journal.pntd.0002021