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Backgrounder: Beware Gateway Bugs Bringing Insect-Borne Disease
By Joseph A. Davis
Insects are a lethal environmental threat that make news only rarely. And while the human body count from insect-borne disease may be higher in other parts of the world, Americans need to know more about the bugs in the backyard, especially as the problem is worsening with climate change, porous borders and other factors.
The fickle, fitful media does occasionally make major stories out of Zika and West Nile viruses, Lyme disease and the like. But after injecting a few initial jolts of fear, it often slacks off and pays little attention to the background of how and why these threats come to be.
Environmental journalists can help. But first, a few basics, and then a rundown of some of the most significant insect-borne diseases, in this two-part Issue Backgrounder on insect-borne disease.
Tick-borne and other diseases
Mosquito-borne and other diseases
|A sandfly taking a blood meal through human skin. The insect is a vector for Leishmaniasis, which causes skin lesions and is sometimes fatal. It is mainly found in tropical and subtropical regions, but there have been cases in the Southwest United States. Photo: World Health Organization/S. Stammers|
For starters, ticks and mosquitoes are probably the worst culprits — and favorite scapegoats. But various flies, chiggers, roaches, lice, fleas, midges and assassin bugs are also involved in spreading disease. (By the way, many of us misuse the term “insect” when we mean to include arthropods of many kinds, whether six- or eight-legged … or even more-legged. This article uses the term in a broad non-taxonomical way.)
Bottom line: All of these can be “vectors” of disease — carrying and spreading illnesses that affect humans and other animals.
You get the idea. Fix your screens. Wear your DEET.
But also be aware that each species of insect thrives in particular environmental conditions and fits into particular ecosystems. Many go through complex life cycles that intersect with humans and other animals in particular ways. Analyzing these connections is important for understanding and controlling the transmission of disease.
And it is important to know that as environmental conditions constantly change, so do the disease impacts on humans. Climate change brings warmer temperatures, changes in drought and flood conditions, fire and plant ecosystems.
Man-made climate change is not the only way we humans are influencing insect-borne disease. For example, we perform major deforestation and destruction of grasslands. In some places, such as Chicago, we literally “drain the swamp” in hopes of ending insect-borne disease. And we use chemical insecticides that sometimes promote resistance in insects.
It’s not just how we handle nature — it’s also how we handle ourselves. For instance, we concentrate our populations in crowded cities, making us easier targets and helping spread illness. And we hop on airplanes and cross the world with communicable diseases and insect hitchhikers. This also means that the problem is far from confined to the developing world.
So to help you better understand and cover this story, here are some key resources, along with a rundown of some of the most significant diseases that arthropods help transmit to humans. Part one focuses on tick- and other insect-borne diseases (also see our separate TipSheet on ticks). Part two focuses on mosquito-borne disease.
There are some obvious go-to sources that can anchor any story on insect-borne disease, especially the Centers for Disease Control and the World Health Organization. One interesting bit of reading for disease geeks is the journal Emerging Infectious Diseases, published by the CDC, where you can find discussion of some mysterious and cutting-edge plagues of tomorrow. You might follow STAT and Kaiser Health News, though they focus more on healthcare policy than things that itch.
If you look at the standard medical journals, you will find stories. Better yet: the Journal of Vector Borne Diseases, which is open access (free). Or the International Journal of Mosquito Research, the Journal of Tropical Diseases & Public Health, or the PLOS Neglected Tropical Diseases blog. The Twitter feed of author/expert @Laurie_Garrett is also worth following.
Tick-borne and other diseases
Lyme Disease: This one scares Americans, and rightly so. Untreated, it can result in disabling joint pain, memory loss and extreme tiredness. Humans catch it from the bite of a tick.
It is no exotic tropical disease. It was named after Lyme, Conn., where investigations of a cluster of cases in 1975 led to a fuller understanding of the disease. But the disease has been around for thousands of years in North America and Europe. It is found on other continents as well.
By most accounts, Lyme disease is the most prevalent tick-borne disease in the United States, and its incidence has grown significantly in recent decades.
As environmental conditions constantly change,
so do the disease impacts on humans.
The vector that spreads Lyme disease to humans is usually the deer tick, a blood feeder so small that it often goes undetected. The disease is caused mainly by a spirochete bacterium called Borrelia burgdorferi. If detected in time, it can usually be successfully treated with antibiotics. But too often it is not.
Deer ticks infest, among other species, white-tailed deer and the white-footed mouse, two of the most common reservoirs of the bacteria in the United States. Some environmental approaches to controlling Lyme involve control of these species. Reducing deer populations seems to have reduced Lyme transmission in some cases. There are even mouse bait boxes that treat mice with insecticide rather than killing them.
Lyme is certainly an environmental disease — and is disturbing people because they can catch it when they are outdoors enjoying nature. Wearing repellents and long pants sealed at the ankle helps stop ticks. But clearing unnecessary brush and tall grass, a way of reducing tick habitat, can also help.
Some studies have suggested that reforestation of abandoned colonial farmland in New England led to a rise in Lyme disease. Other studies suggest that the fragmentation of woodland for suburban development, which favors white-footed mice, is a factor promoting Lyme.
Other Tick-Borne Encephalitis: The bad news is that there are many other serious tick- (and mosquito-) borne viral diseases, which, though less common than Lyme, can often attack the human nervous system. The catalog includes, but is not limited to: La Crosse virus, Jamestown Canyon virus, St. Louis encephalitis virus, Powassan virus and Eastern equine encephalitis virus.
Tularemia: A disease of animals and humans caused by the bacteria Francisella tularensis. It can be spread in several ways, but an important vector is the bite of ticks, deer flies and other arthropods. Although it can be life-threatening, it can be successfully treated with antibiotics. The disease is endemic in the United States, mainly because of reservoir species like rabbits and hares. Reported U.S. cases in recent years have rarely been over 200 per year.
|Lymphatic filariasis patient in India with a visible enlargement of the left foot and leg. The disease is transmitted by blood-feeding black flies and mosquitoes. Photo: World Health Organization|
Rocky Mountain Spotted Fever: A serious disease caused by Rickettsia bacteria transmitted by the Dermacentor tick. Despite the name, it is widely distributed throughout the United States, Canada and Central America. Most people who get it experience a fever, headache and rash. It can be lethal if not treated promptly with the proper antibiotic. The incidence of Rocky Mountain Spotted Fever has been increasing in recent years, reaching about 4,000 cases annually in the U.S.
Bubonic Plague: The good news: bubonic plague is down. The disease, caused by the bacterium Yersinia pestis, is transmitted to humans by the bite of a flea. While the disease devastated Europe as the “Black Death” in the late Middle Ages, it is today rare in the developed world. It can be easily treated with antibiotics. Typically, there are only about seven U.S. cases per year, often in the arid West. The fleas that cause plague usually come from reservoir species of rodents. In the middle ages, it was rats. In the United States today, it is likely wild rodents such as prairies dogs, chipmunks and ground squirrels. Outbreaks can happen when the ecological barriers isolating these wild species from humans are disturbed.
Leishmaniasis: A disease caused by a protozoan parasite transmitted by the bite of the sandfly. It causes skin lesions and is sometimes fatal. It is found in tropical and subtropical regions and is rarely found in the United States, although cases have been found in Texas and Oklahoma.
Filariasis: A family of roundworm parasitic diseases transmitted by blood-feeding black flies and mosquitoes. It takes various forms, some which have quite ugly manifestations like elephantiasis and river blindness. Filariasis affects some 120 million people in 73 countries, but it is not found in the United States.
Mosquito-borne and other diseases
Malaria: A mosquito-borne disease caused by a microscopic parasitic organism (Plasmodium) that lives in the bodies of both Anopheles mosquitoes and humans in various stages of a complex life cycle. People get it from the bite of a mosquito. An estimated 212 million cases of malaria occurred worldwide in 2015, according to the CDC. Of those affected, some 429,000 died, mostly children in Africa.
In the United States, only about 1,700 cases are found each year (most acquired in other countries by returning travelers), although there are concerns that could change for the worse. The reasons are many why the incidence of malaria is lower in the United States, and include much more than environmental conditions. In fact, malaria was once endemic in the United States — the original mission of the CDC was malaria control — but it was declared eliminated in 1951.
|Worker fogging with pesticides in Bali, Indonesia, to prevent dengue and other diseases transmitted by mosquitoes. Photo: World Health Organization/B. Chandra|
Probably the most important reason why malaria is not endemic in the United States is that the country has a strong public health system that has mounted an aggressive and comprehensive campaign against malaria. Public health programs watch for malaria cases, prescribe preventive medication, promote preventive practices (e.g., mosquito nets), prescribe anti-malarial medications, coordinate with mosquito-control programs (including some serious insecticides), remove mosquito habitat and take other measures. Where virtually no humans have malaria, mosquitoes do not transmit it from human to human.
Yet Anopheles mosquitoes capable of transmitting malaria do exist in much of the United States. Parts of this country already have a climate favorable to mosquitoes, and if you live in a wet or marshy area, you know that mosquitoes can be plentiful there. On top of that, malaria can be introduced, or reintroduced, by a human who has acquired it elsewhere. This happens. According to the CDC, 63 outbreaks of locally transmitted malaria took place in the United States between 1957 and 2015. Fortunately, they were knocked down by our health system.
Climate change has often been mentioned as threatening human health via malaria. But this will still be far more worrisome in other parts of the world than in the United States. Increases in temperature (and decreases in mosquito-killing frosts), as well as rainfall can increase mosquito populations. Worldwide, 3.2 billion people in 106 countries and territories live in malaria-prone areas.
Increased incidence of malaria in parts of the Americas from which people often travel to the United States could further raise risks. Several other reporting angles to pursue include the development of malaria strains resistant to antimalarial drugs and the emergence of insecticide resistance in some mosquitoes.
Zika Virus: A viral disease transmitted by mosquitoes, Zika has been in the news a lot in the last two years. The disease is especially worrisome because when it infects a pregnant woman, it may cross the placental barrier to harm the fetus by causing microcephaly and other birth defects. Zika is spread by the bite of the Aedes aegypti and Aedes albopictus mosquitoes. Most cases have no symptoms or mild ones, such as fever, red eyes, joint pain, headache and rash. In adults, Zika infection can sometimes cause Guillain-Barré syndrome, a serious condition. It can be passed from human to human through sexual contact.
While Zika was first noticed in Africa in the 1940s, it spread eventually to the Americas, where a 2015-16 epidemic in Brazil and other countries brought it to world attention. The World Health Organization declared the Zika outbreak a public health emergency in February 2016, although it lifted that declaration (after robust world response) in November 2016. The emergence of Zika in South American, Central American and Caribbean countries eventually caused the CDC to issue travel advisories, especially for pregnant women (or those planning to become pregnant), whom it urges to avoid Zika areas.
Many of the Zika infections diagnosed in the United States are acquired abroad by travelers. But the CDC has also found some locally transmitted (endemic) Zika in the country, including areas of Miami and Brownsville, Texas.
Yellow Fever: A viral disease transmitted by mosquitoes (mainly Aedes aegypti), that can be serious enough to be fatal. Today it is mostly of historical significance to residents of the United States, but it is still prevalent in parts of Africa and South America. Yellow fever occurs mostly in the tropics and subtropics, because that is where Aedes aegypti mosquitoes mostly live. An effective vaccine exists. About 45,000 people died of the disease in 2013, according to the WHO, most of them in Africa.
During the Spanish-American War in 1898, more American soldiers died of diseases like yellow fever than from combat. Major Walter Reed became an Army hero largely by demonstrating that mosquitoes transmitted yellow fever. This knowledge allowed control of yellow fever and completion of the Panama Canal.
West Nile Virus: A viral disease transmitted by the bite of mosquitoes, it has caused much concern (along with big news coverage) in the United States. Although it was first discovered in other parts of the world (Africa and the Middle East) as far back as the 1930s, West Nile showed up in the United States around New York City in roughly 1999. It is transmitted by several species of the Culex mosquito. Since peaking around 2002, deaths in the U.S. from West Nile have not increased. In recent years, they have hovered between 100 and 200 per year.
West Nile is usually not a serious health threat. Only one in five people who are infected with the virus show any symptoms, typically similar to those of the flu. But about 1 percent of those infected develop more serious neurological illness — meningitis or encephalitis. These can be serious and sometimes fatal, especially in people with weak immune systems: the very young, the very old and people taking immunosuppressant drugs. Hence the wisdom of avoiding mosquito bites, through repellents, long clothing, netting, etc., as well as mosquito-reduction measures around the home and neighborhood.
West Nile also infects numerous bird species, which often suffer much higher mortality from it than humans do. Birds serve not only as a reservoir of infection, but can help spread the virus when they migrate.
Dengue is the fastest-growing mosquito-borne
viral disease in the world. There have been outbreaks in
Key West, South Texas and Hawaii.
Dengue: A mosquito-borne disease caused by one of four related viruses that occurs in tropical areas. Symptoms include high fever, headache, vomiting, muscle pain, joint pain and a characteristic rash. It has been nicknamed “breakbone fever” because of the pain.
Dengue has spread globally and become much more of a problem in the decades since World War II. It is the fastest-growing mosquito-borne viral disease in the world, and its incidence has increased 30-fold in the last 50 years. The reasons for the increase in dengue are several. Increased growth and crowding in tropical cities may be a factor, along with increased global travel. But dengue mosquitoes thrive in warm and wet places, and global climate change is also considered a likely factor.
Worldwide, about 2.5 billion people live in areas where dengue can be transmitted, and 390 million are infected yearly. About 96 million of those actually become ill. Some of those cases turn into a more severe form of the disease. An estimated 10,000 to 20,000 of them die. Most cases of dengue diagnosed in the United States involve people who have traveled abroad. But there is greater risk for people living near the U.S.-Mexican border. In 2009, there was an outbreak in Key West, Fla. There was also an outbreak in South Texas in 2005, as well as a small one in Hawaii in 2001.
Chikungunya: A viral disease transmitted by mosquitoes that is usually excruciatingly painful. Typical symptoms include fever, joint pain, headache, muscle pain, joint swelling or rash. While it had existed primarily in Africa, Asia and Europe for a long time, it was found in the Americas for the first time in 2013. First reported in Caribbean islands, it is now being found more widely, including in many U.S. states. In 2016, there were an estimated 171 cases in the United States, almost all of them travel-related.
The above list hardly exhausts the discussion of insect-borne disease. There is sleeping sickness, caused by Tsetse flies. There are a host of viral hemorrhagic fevers, some of which involve insects. We didn’t even mention typhus. And there are emerging diseases we hardly understand yet, like the Bourbon virus (that comes from ticks rather than whiskey), which showed up very recently in the Midwest. Or the Heartland virus, transmitted by the Lone Star Tick.
Joseph A. Davis is director of SEJ’s WatchDog Project, and writes SEJournal Online’s Backgrounders and TipSheet columns.
* From the weekly news magazine SEJournal Online, Vol. 2, Nos. 32 & 33. Content from each new issue of SEJournal Online is available to the public via the SEJournal Online main page. Subscribe to the e-newsletter here. And see past issues of the SEJournal archived here.