Vector-borne Diseases in the United States
Vector-borne diseases—those transmitted to people by intermediary organisms, primarily insects and ticks—have become significantly more common and more widespread in the United States in the past decade. Reported cases of diseases caused by infected ticks, mosquitoes, and fleas tripled from 2004 to 2016, with nearly 100,000 cases reported in 2016. This increase has been fueled by many factors, including changes in rainfall, temperature, and extreme weather events that have enhanced the abundance or distribution of insects and ticks—in many cases the result of climate change. Changes in land use, human settlement, and outdoor activity patterns have also led to more contact between people and disease-spreading organisms. The result is a growing U.S. public-health challenge.
What is a disease vector?
Vectors are organisms that serve as living “shuttles,” carrying disease-causing bacteria, viruses, and parasites from one person or animal to another.
Although large animals can transmit diseases to humans, the term “vector” almost always refers to blood-feeding insects and ticks.
In some cases, these vectors transmit pathogens from person to person. In other cases, they transmit pathogens to people from an animal host, such as a bird or rodent (in which case the disease is known as a vector-borne “zoonotic”).
Are vector-borne diseases a problem in the United States?
Yes. The most recent summary of U.S. vector-borne diseases by the Centers for Disease Control and Prevention (CDC) documents more than 640,000 cases of 16 vector-borne diseases from 2004 to 2016. Lyme disease, spread by ticks, is by far the most prevalent overall, although the relative importance of different diseases varies by region.
Many people who have never reported being ill—perhaps because symptoms were too mild for them to seek medical care—nonetheless show evidence in their blood of past vector-borne infections, indicating that the true number of such infections is considerably greater than the number documented in official statistics.
Some U.S. vector-borne diseases, like plague, are ancient global scourges and have been in the United States for much of its history. Others are newly emergent; the first known case of human Zika virus disease was in 1952 (in Africa), with the first documented U.S. case of locally acquired Zika not occurring until 2016.
Some U.S. vector-borne disease cases are the result of domestic transmission; for example, the vast majority of U.S. cases of West Nile disease today involve people infected by mosquitoes in the United States. Others are caught by travelers abroad; almost all U.S. Zika virus cases, for example, have involved people infected while traveling in Latin America and the Caribbean.
U.S. territories face particularly severe vector-borne-disease threats, in part because mosquitoes thrive in warm and wet climates. In Puerto Rico, the U.S. Virgin Islands, and American Samoa, mosquito-borne diseases such as Zika, dengue, and chikungunya are of particular concern. Outbreaks of dengue have also occurred in Hawaii.
Which disease vectors are most common in the United States?
Ticks—small, blood-feeding arachnids (a class of arthropods separate from insects)—are responsible for almost 95 percent of all vector-borne disease cases in the United States. The most prevalent U.S. tick-borne diseases are:
Lyme Disease, caused by the bacterium Borrelia burgdorferi, accounts for more than two-thirds of all U.S vector-borne disease cases.
The bacterium is transmitted to people through the bite of an infected blacklegged tick (commonly referred to as “deer ticks” in the eastern and upper midwestern United States).
About 30,000 cases are diagnosed and reported every year, but health surveys and blood tests suggest that this represents only 10 percent of actual annual infections.
Symptoms include fever, chills, fatigue, joint and muscle pain. Early stages can be treated with antibiotics, but in some patients—especially those not treated early—symptoms can persist for years.
Increasing incidence and prevalence have been linked to residential development in wooded areas, increases in deer populations (which carry ticks), and growing proximity of deer to humans in some urban settings.
Rocky Mountain Spotted Fever (RMSF), caused by the bacterium Rickettsia rickettsii, is transmitted by the bite of an infected American dog tick (in eastern and southern United States), wood tick (in the U.S. West), or brown dog tick (in the Southwest).
Symptoms include fever, headache, and a rash; RMSF is treatable with antibiotics, but can be deadly if not diagnosed quickly and treated properly.
Most RMSF cases occur in communities with large numbers of free-roaming dogs, especially in the southeastern United States.
Anaplasmosis and Ehrlichiosis, both caused by rickettsial bacteria, are transmitted by the lone-star tick, prevalent in the southern and southeastern United States. Treatable with antibiotics, they feature flu-like symptoms that resemble those of RMSF.
Less prevalent tick-borne diseases found in the United States include babesiosis, tularemia, and illnesses caused by the Heartland, Bourbon, and Powassan viruses—the latter capable of causing life-threatening meningitis and encephalitis and showing a worrisome increase in prevalence in recent years.
Mosquitoes—a type of fly, with most species requiring a blood meal for the females to become fertile and reproduce—transmit the viruses that cause West Nile disease, Zika virus disease, dengue fever, and chikungunya, as well as the parasite that causes malaria.
West Nile disease is the most prevalent U.S. mosquito-borne illness.
People get infected when bitten by mosquitoes that have previously bitten birds infected with West Nile virus.
Since the first U.S. case was identified in New York in 1999, West Nile virus has spread to every state in the contiguous United States and some U.S. territories—largely via migratory birds.
Thousands of human cases are diagnosed every year (about 2,500 in 2018), but most infected individuals do not exhibit serious symptoms so are never diagnosed.
In a small fraction of individuals—especially people over 60 years old—the virus causes life-threatening neuroinvasive disease.
No vaccine has been approved for use in people.
Zika virus disease is spread from person to person by infected mosquitoes and can also be transmitted by sex.
A major health risk associated with Zika virus is transmission of the virus from pregnant women to their fetuses, which can cause birth defects including microcephaly (a neurological condition in which a newborn’s head and brain are significantly smaller than normal).
Zika virus can also cause Guillain-Barré syndrome, in which the immune system attacks nerve cells, progressively weakening muscles and potentially leading to temporary paralysis.
Dengue fever is caused by any of four related dengue viruses, all carried by mosquitoes.
The disease’s symptoms include a high fever, headache, and muscle, bone, or joint pain. Repeat infections can be life-threatening.
Though prevalent in the U.S. South in the 19th and first half of the 20th centuries, mosquito-control efforts largely relegated the disease’s Western Hemisphere presence to South and Central America and the Caribbean. In recent years, dengue has re-emerged to cause sporadic outbreaks in the southern continental United States.
Chikungunya was first recognized in Africa in the 1950s, but in recent decades has appeared in the Americas. It is caused by a virus and causes fever and joint pain.
Though transmission rates remain low in the United States, this potentially debilitating disease has in recent years been creeping northward from the Caribbean.
Malaria, caused by Plasmodium parasites, is transmitted by mosquitoes.
Symptoms include severe fever, chills, and flu-like symptoms, which can progress to include serious and even fatal complications.
Virtually all U.S. malaria cases today occur in travelers who contracted the disease abroad, primarily in sub-Saharan Africa and South Asia.
Malaria can be prevented or treated by drugs but some strains have developed resistance.
Less prevalent mosquito-borne diseases found in the United States include Saint Louis encephalitis, LaCrosse encephalitis, and eastern equine encephalitis.
Fleas are blood-feeding insects that can transmit pathogens from person to person or from other infected mammals, such as rodents, to humans.
Plague, caused by the bacterium Yersinia pestis, can cause serious illness, but is treatable with antibiotics and is exceedingly rare in the United States.
Typhus, a bacterial infection that in some forms can be transmitted to people from mice via fleas, is a scourge associated with poverty and unsanitary conditions that has recently reemerged in some U.S. suburban areas—including in Texas and Hawaii and among communities of homeless people in Southern California.
Is climate change affecting U.S. vector-borne diseases?
Human-caused climate change—including changes in temperature, rainfall, and seasonality—is affecting the survival, reproduction rates, and geographic ranges of disease-spreading ticks and insects, as well as the ranges and abundance of migratory birds, mice, and deer that serve as reservoirs of vector-borne pathogens.
The relationships among all these variables are complex, but scientists have been able to observe and predict some climate-related impacts with confidence:
Consistent with the fact that insects and ticks are cold-blooded and depend on environmental warmth for their survival and reproduction, there is evidence that climate-change-related increases in U.S temperatures will increase or are already increasing the length of the U.S. insect season. Climate change in recent years has also been associated with a well-documented northward expansion of the range of mosquitoes that carry dengue, chikungunya, and other traditionally subtropical diseases—though movements of people and cargo have also contributed to this spread.
An important consequence of warmer temperatures is that disease-causing microbes picked up by an insect during a blood meal can replicate and migrate to that insect’s salivary glands much faster, making it more likely they will get transmitted to another person before that insect dies.
Increased rainfall and humidity in some areas have also been linked to a documented increase in the geographic range of U.S. ticks, with some research suggesting ongoing expansion of the northern range of Lyme-transmitting ticks and an earlier beginning of the annual tick season.
Early in their development, mosquitoes go through an aquatic larval and pupal stage dependent on the presence of standing water. Scientists expect that areas set to experience climate-related increases in rainfall are also likely to face higher mosquito populations, increasing the risk of West Nile, dengue, and other mosquito-borne diseases.
These relationships are complex, however; extreme rainfall in urban areas can flush mosquito larvae from stormwater systems, and some mosquitoes breed very well in the stagnant water that can linger during dry periods.
Increased drought has been associated with declines in ticks in some U.S. regions. By contrast, mosquito populations have been found to be relatively drought tolerant in urban areas, since even small amounts of water that collect on and around buildings and homes is enough to harbor mosquito larvae for the few days required for maturation.
In drought-affected rural areas, climate-related increased use of irrigation could increase standing water and, therefore, mosquito survival.
Human behaviors in response to climate change and related socioeconomic factors may also affect vector-borne disease rates.
As temperatures rise, for example, strategies to keep cool (such as staying indoors with windows closed and air-conditioning on) could reduce disease incidence by decreasing human exposure to vectors. Those who cannot afford air conditioning (or even screens)—already at higher risk—can be expected to bear much of the increased burden of disease.
While the above factors indicate that climate change is likely to exacerbate a number of vector-borne disease challenges in the United States, scientists note that changes in human behaviors unrelated to climate change—including increasing travel to parts of the world where vector-borne diseases are more common than in the United States—make it difficult to predict the true impact of climate change on vector-borne diseases.
How can vector-borne disease transmission be controlled?
Some of the most effective existing approaches to vector control require little or no technology. However, achieving effectiveness requires long-term commitment at the state, municipal, and community levels.
For mosquitoes, community-level campaigns have focused on reducing mosquito breeding environments by, for example, putting screens over rain barrels and eliminating standing water from trash, gutters and drains. But more than 80 percent of local U.S. vector-control programs are inadequate in various ways according to a 2017 survey by the National Association of County & City Health Officials.
Individuals can take actions to reduce their personal risk of vector-borne diseases by wearing insect repellent, wearing long-sleeved shirts and long pants treated with the insect repellent permethrin, and controlling ticks and fleas on pets.
There are also number of new biotechnologies under development that could help control vector-borne diseases. Some have been tested in laboratory populations or in limited outdoor tests, but their potential benefits and risks have yet to be fully assessed. Among them:
Researchers have engineered lines of male insects bearing killer genes that lead to death before sexual maturity is achieved—except when the insects are reared in the laboratory with special diets. These genetically altered male insects, grown to maturity in the laboratory, can be released to mate with wild females, whose offspring (lacking the lab diet) then die too young to reproduce or transmit disease.
Another approach involves infecting pest-insect populations with Wolbachia, a bacterium that reduces a host insect’s capacity to carry and transmit some disease organisms and can induce sterility in mosquitoes. Among the lingering uncertainties of this approach is whether Wolbachia increases the number of other pathogens in insects even as it reduces the target pathogen, and whether Wolbachia can cause problems by spreading to other insects.
Scientists have altered the DNA of some laboratory-reared insects in ways that greatly reduce their ability to spread disease—then have added another genetic change known as “gene drive” that enhances the inheritance of this engineered trait into virtually all of those insects’ offspring, quickly reducing the health risks posed by those insect populations. Some see the approach as one of the most promising for reducing the global toll of malaria and other scourges, but the technology also poses unresolved ecological risks and policy and ethical quandaries.
LAST UPDATED APRIL 15, 2019
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key references for those who want to dig deeper
Basics about vector-borne diseases:
Vital Signs: Trends in Reported Vectorborne Disease Cases — United States and Territories, 2004–2016, published in the CDC’s May 1, 2018 issue of Morbidity and Mortality Weekly Report, summarizes trends in the occurrence of nationally reportable vector-borne diseases.
The Division of Vector-Borne Diseases, a division of the CDC, is a science-based resource for staying up-to-date on vector-borne disease research, prevention, and control.
The National Association of County & City Health Officials published a profile of American mosquito control programs and their competency, titled Mosquito Control Capabilities in the U.S., 2017, and found that 84 percent of U.S. vector control programs need improvement.
Emerging Vector-Borne Diseases in the United States: What is Next, and Are We Prepared?, 2016, is a report published by the National Academies of Sciences, Engineering, and Medicine that appears in the Global Health Impacts of Vector-Borne Diseases: Workshop Summary.
Ticks currently pose the biggest vector-borne disease threat in the United States. Studies that have analyzed this threat and possible ways to mediate it include Using citizen science to describe the prevalence and distribution of tick bite and exposure to tick-borne disease in the United States, 2018, published in PLOS One; Tick-Borne Zoonoses in the United States: Persistent and Emerging Threats to Human Health, 2017, published in the ILAR journal; Range Expansion of Tick Disease Vectors in North America: Implications for Spread of Tick-Borne Disease, 2018, published in International Journal of Environmental Research and Public Health, and Vectors as Epidemiological Sentinels: Patterns of Within-Tick Borrelia burgdoferi Diversity, 2016, published in PLOS Pathogens.
Vector-borne diseases and climate change:
In 2016, the U.S. Global Change Research Program released an in-depth scientific assessment of climate change and its impacts on human health in the United States. The Climate and Health Assessment discusses vector-borne diseases, as well as other threats to human health like air quality, extreme weather, and mental health and well-being.
The CDC has produced an easy-to-read Insects and Ticks fact sheet about the association between climate change and vector-borne diseases.
The 2001 article, Climate variability and change in the United States: potential impacts on vector- and rodent-borne diseases, published in Environmental Health Perspectives, describes the numerous factors that affect vector-borne disease transmission, including human and environmental factors.
In the 2014 article, Recent Weather Extremes and Impacts on Agricultural Production and Vector-Borne Disease Outbreak Patterns, published in PLOS One, the authors describe their use of satellite imaging to map extreme weather- in this case, the El Niño Southern Oscillation- and its relation to mosquito-borne disease outbreaks. A related study, Global risk model for vector-borne transmission of Zika virus reveals the role of El Niño 2015, 2016, published in the Proceedings of the National Academy of Sciences, used climate data to model weather conditions created by El Niño that likely exacerbated the 2015 Zika outbreak in South America. Impact of El Niño Southern Oscillation on infectious disease hospitalization risk in the United States, 2016, also published in the Proceedings of the National Academy of Sciences, addresses the effects of El Niño on infectious diseases, including vector-borne diseases, in the United States. More information on the climatic forces behind El Niño is made available by the National Weather Service.
Identifying climate drivers of infectious disease dynamics: recent advances and challenges ahead, 2017, published in Proceedings of Royal Society, describes the challenges of attributing health impacts to climate change, and identifies where climate science and epidemiology can work together to make progress for human health.
West Nile virus, climate change, and circumpolar vulnerability, 2016, published in WIREs Climate Change, presents a multidisciplinary synthesis on West Nile virus and climate change, describing the potential for the virus’s expansion and the vulnerability of the circumpolar north.
Other studies that have explored the impact of climate change on vector-borne diseases are Climate change and vector-borne diseases: a regional analysis, 2000, published by the World Health Organization; Climate Change and Spatiotemporal Distributions of Vector-Borne Diseases in Nepal- A Systematic Synthesis of Literature, 2015, published in PLOS One; Effect of climate change on vector-borne disease risk in the UK, 2017, published in The Lancet Infectious Diseases; and Impact of climate change on human infectious diseases: Empirical evidence and human adaptation, 2016, published in Environment International.
The importance of developing and strengthening public health policy in light the increasing prevalence of vector-borne diseases has been highlighted in Climate change and vector-borne diseases of public health significance, 2017, published in FEMS Microbiology Letters; Climate change and vector-borne diseases: what are the implications for public health research and policy?, 2015, published in Philosophical Transactions of the Royal Society B: Biological Sciences; and Opportunities and challenges in modeling emerging infectious diseases, 2017, published in Science.
Drought and immunity determine the intensity of West Nile virus epidemics and climate change impacts, 2017, published in Proceedings of the Royal Society of London B: Biological Sciences, describes the impact of climate change on the West Nile virus, particularly the role of drought as a primary driver of increased West Nile virus epidemics.
The Effects of Climate Change on GDP by Country and the Global Economic Gains from Complying with the Paris Climate Accord, 2018, published in Earth’s Journal, provides evidence that countries that comply with the 2016 Paris Climate Agreement will appreciate considerable economic gains, in part by reducing morbidity and mortality from vector-borne diseases.