By Hugh Roddy, MD
Introduction
Human immigration, poverty, and climate change are associated with emerging neglected tropical diseases (NTD) in New Texas. “New Texas” is a term coined by Peter Hotiz, a prominent public health official, to describe evolving human and environmental conditions that give rise to vector-related illnesses. A vector is an organism, typically a biting insect or tick, that transmits a pathogen (bacteria, parasite, virus) from one animal or plant to another.
The growing number of world travelers returning home, passing through, and relocating to Texas increases the risk for NTD. Notably, invasive non-native plants that promote vector survival hitchhike along with the people movers.
Texas poverty is a social determinant of NTD. Within our wealthy economy are pockets of poverty. In 2016, over 4 million Texans lived below the poverty line. The state ranks 41st for food insecurity and educational attainment.
Tropical weather is expanding northward from equatorial regions to the southern U.S.. The warming effects amplify the transmission of vectors. Texas temperatures rose 0.5 to 1 degree F over the past 100 years, and the trend is surging. By 2050, Texas is projected to have 80 to 100 days over 95 degrees compared to 40 days over the previous average of the last 40 years. As I write this on February 26, 2024, the temperature is 93 degrees.
Chagas, Leishmaniasis, and West Nile viruses are each vector-related illnesses. Changing environmental conditions and human activity increases the probability that more Texans will encounter these serious diseases in the future.
Chagas
Chagas disease an infectious disease in humans caused by a infectious pathogen, the T. cruzi parasite. The scenario is this: A animal reservoir, such as an infected rodent or dog, is bitten by the vector insect, triatomine reduviid. The vector then bites a human and transfers T.cruzi parasite to his bloodstream. Reduviid vector, is commonly called the “kissing bug” because it seeks a blood meal in moist areas on the human face at night, mainly when the victim sleeps.
Chagas has an acute phase that occurs shortly after the initial infection, during which the parasite rapidly reproduces in the human bloodstream. The victim is essentially without symptoms but may experience fever or conjunctivitis. A chronic phase may occur around 20 years later with the manifestations of damaging lesions in the heart (myocarditis) and GI system (enlargement of the esophagus and colon).
Impoverished areas in the U.S. southern states are increasingly vulnerable to Chagas due to climate factors. As temperatures soar above 30 Celsius (86 Fahrenheit), the feeding rate of kissing bug increases, and their life cycle shortens.
Chagas was reported in Texas as early as the 1930s among people who had not traveled (locally acquired). South Texas is especially vulnerable to the infection. The CDC estimates about 37,000 Texans are unknowingly infected. Texas cities with the highest prevalence are Laredo, McAllen, El Paso, and Brownsville. The prevalence is higher among those who immigrated from endemic areas (where the disease is regularly prominent) in Latin America.
Leishmaniasis
Leishmaniasis is a parasitic illness transmitted by many subspecies in the Leishmania genus. Leishmania sandfly vector(s) feed on mammalian reservoirs, including rodents and feral and domesticated dogs. Humans are infected when the sandfly takes a blood meal from primarily the face, neck, and arms.
Human Leishmaniasis is more prevalent in the U.S. than many people realize. Two common types are Cutaneous (CL) and Visceral (VL).
CL is generally not life-threatening. However, the skin and mucosal (a membrane rich in mucous glands) lesions may develop into disfiguring scars and destruction of mucous membranes in the mouth, eyes, and nose. The impoverished are disproportionately affected by both the illness and complications.
CL is endemic in the U.S. Locally-acquired (non-travel outside U.S.) cases have recently been reported in Texas and Oklahoma. The median number of cases reported yearly in Texas was six from 2008 to 2017; in 2020, eighty cases were identified.
VL , also known as kala-azar, is a life-threatening illness that causes chronic inflammation and devastating effects on multiple organs. VL heightens the risks for diabetes and cardiovascular, kidney, blood, and neurologic diseases.
VL may be asymptomatic, and the transmissible Leishmaniasis parasite persists in the blood for a long time, unbeknownst to the victim. There is growing concern that the parasite could be secondarily spread via blood transfusion, childbirth, or organ transplant. Given the mutability of the vector species, there is a risk that new and undiscovered sandfly subspecies could find a preference for human reservoirs that would enable human-to-vector-to-human transmission.
VL is endemic in underdeveloped countries plagued with conflict, war, and displacement. VL is rare in the U.S. but has been reported among deployed military soldiers, international travelers, and immigrants.
Leishmaniasis is sensitive to climate warming. The sandfly thrives, and the increase in temperature facilitates its migration north. Like Chagas, people living in substandard conditions are at high risk of exposure to the infected sandfly and subsequent disease.
West Nile Fever
A single-stranded RNA virus causes West Nile fever called the West Nine virus (WNV). It is a member of the Flavivirus genus, including viruses that cause dengue, yellow fever, and Zika.
Birds are the primary reservoir for the WNV. They develop sufficiently high viral loads to transmit the infection to the primary vector, Culex mosquitoes. Culex transmits the virus to humans through a blood meal of a mosquito bite. Humans are ‘dead-end hosts” because they don’t transmit the virus to other mosquitos that bite them.
Most people who contract West Nile fever experience no ailment or relatively mild flu-like symptoms. However, for some unlucky people, the virus invades the neurologic system, leading to encephalitis, convulsions, paralysis, and possibly death. For immunocompromised and aging populations, the risk of neuro-invasive disease increases.
Locally-transmitted cases of West Nile fever in Texas were first reported in 2002. A severe outbreak emerged in Texas in 2012. That year, 89 deaths occurred among the approximately 1,900 cases. Almost 50% of the deaths were reported in the DFW metroplex. In 2022, 42 cases of West Nile fever and seven deaths were reported.
Climate change spawns West Nile disease transmission; it affects the virus itself as well as the activity of the bird vectors. Climate warming also impacts human behavior as more time spent outdoors raises the likelihood of mosquito exposure.
Conclusion
Neglected tropical diseases pose a serious threat to human health as they are associated with severe morbidity and mortality. Ample documentation demonstrates that emergent risk for NTDs is related to ecological and climate factors.
Citizens of Texas would be well-served if they better appreciated the many adverse effects of Texas warming. Climate proposals withered in the 2023 Texas state legislature. The Republican-dominated legislators ignored efforts to reduce energy costs, decrease greenhouse emissions, and lessen the climate burden on poor and disenfranchised communities. Perhaps climate change should rank a high priority among the many political issues facing Texans today.
Selected Bibliography
Climate Change Proposals Withered at the Texas Capitol this Year, Texas Tribune June 2, 2023.
Curtin JM; Sronson NE. (2021) Leishmaniasis in the United States: Emerging Issues in a Region of Low Endemicity. Microarganisms 2021: 9 (3).
El Sayad A; Kamal M. (2020) Climatic changes and their role in the emergence and re-emergence of disease. Environ Sci Pollut Res Int (18).
Garza M. et al. (2014) Projected future distributions of vectors Trypanosoma cruz in North American under climate change scenarios. PLoS Negl Trop Dis: 8 (5).
Hotez PJ. (2018) The rise of neglected tropical diseases in the “new Texas”. PLoS Negl Trop Dis: 12 (1) .
Irish A. et.al. (2022) Updated estimates and mapping for prevalence and Chagas disease among Adults, United States. Emerging Infectious Diseases: 28(7).
West Nile virus. CDC.gov >westnile.
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