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.

• 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”).

• 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.

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.

• 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.

• 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.