Arthropod-borne diseases are transmitted by arthropods, members of the invertebrate phylum Arthropoda, which includes insects, spiders, and crustaceans. Mosquitoes, fleas, ticks, lice, and flies are the arthropods that usually act as vectors for various pathogens (disease-causing microorganisms), including bacteria, viruses, helminths (parasitic worms), and protozoa. Transmission of these pathogens to humans by the arthropod vector can cause a variety of human diseases, including malaria, yellow fever, Chagas disease, and dengue fever. These and other arthropod-borne diseases can result in a wide range of effects, from mild flulike symptoms to death. Some survivors of arthropod-borne diseases can suffer chronic, crippling aftereffects.
While arthropod-borne diseases are a major concern worldwide, developing countries are the most affected. These diseases tend to occur primarily in tropical countries—the endemic zones of the pathogens and the arthropods that harbor them. However, these diseases can also spread when people travel between infected and noninfected areas, or when infected arthropods are inadvertently transported. Natural disasters, wars, poverty, and overpopulation can facilitate outbreaks of disease, since they may create conditions that are ideal for transmission or may cause a breakdown in the health care and public health systems.
Humans contract arthropod-borne diseases when a pathogen, such as a bacteria or virus, is transmitted from its reservoir (natural host) to a human via the arthropod vector. The most common arthropod vectors are flies, fleas, ticks, mosquitoes, and lice. Transmission from arthropod to human occurs either mechanically or biologically. In mechanical transmission, the arthropod deposits pathogens onto a surface from which a host either absorbs or ingests them. For example, a housefly may deposit bacteria onto food that is then eaten by a human. In biological transmission, the arthropod injects the pathogens directly into the body of the host; for example, when a mosquito bites a human.
WORDS TO KNOW
ARTHROPOD: A member of the largest single animal phylum, consisting of organisms with segmented bodies, jointed legs or wings, and exoskeletons.
RESERVOIR: The animal or organism in which the virus or parasite normally resides.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
The effects of arthropod-borne disease range from mild to severe. Arthropod-borne diseases, such as encephalitis and malaria, are characterized by symptoms such as headaches, fevers, weakness, and anemia. Some diseases can be fatal, and others, while not causing death, may have chronic effects that decrease quality of life.
Arthropod-borne diseases have shaped the course of history. From 1343–1351, several forms of plague caused by the bacterium Yersinia pestis were likely carried to humans by fleas on black rats. The event became known as the black plague or Black Death, which killed over two-thirds of the population of urban areas in Asia, one-third of the population of the Middle East, and between one-third and two-thirds of the population of Europe. Plague continued to ravage European cities sporadically, but never as it did during the Black Death. Isolated outbreaks of plague still occur, affecting under 5,000 people annually, but epidemic plague largely disappeared in Europe just before the turn of the nineteenth century—well before the advent of antibiotics. Scientists debate the reasons for its disappearance, but many point to increased sanitation and the possibility that Yersinia pestis-carrying fleas diminished as brown rat populations replaced black rats in Europe.
Until the mid-twentieth century, arthropod-borne diseases were an endemic health problem. American cities battled outbreaks of mosquito-borne yellow fever. Yellow fever, along with malaria, a disease involving a Plasmodium protozoan also transmitted by mosquitoes, stopped French construction of a canal through Panama during the 1880s when it claimed the lives of over 20,000 workers. The same diseases claimed an additional 5,000 lives when the United States completed the Panama Canal project two decades later.
Arthropod-borne diseases remain a threat, especially in less-developed countries. Malaria remains the most widespread arthropod-borne disease in the world, killing one to two million people and affecting between 250 and 500 million people per year (weather conditions can cause large changes in numbers of cases), almost exclusively in the developing world. Dengue fever, a viral disease transmitted by mosquitoes, increased in prevalence during the late 1990s and early 2000s. In 2005, it was endemic to over 100 countries, with about 50 millions cases of dengue fever occurring each year. Dengue hemorrhagic fever is a complication of dengue fever that is fatal in about 5% of cases.
Arthropod-borne diseases occur worldwide, although they are more common in tropical areas such as are found in the Caribbean, Central and South America, Asia, the South Pacific, and Africa. Many regions in North America, Europe, and Australia are less affected by these diseases. Some arthropod-borne diseases are endemic to a particular country or locality, while others, such as malaria, are widely spread throughout the world.
Arthropod-borne diseases can be dispersed when infected individuals travel from a locality where they contracted the disease to an area where the disease is absent or less common. In addition, infected arthropods may be introduced to regions where the disease was previously absent, and, if conditions are favorable, the disease may gain a foothold in the new region. A variety of causes—from accidental transportation in food products to deliberate introduction of a species as a pest control agent—may be responsible for the transfer of an infected arthropod to an uninfected area.
The recommended treatment of an arthropod-borne disease depends upon the specific disease. Treatment often involves a course of antibiotics and, in some cases, a vaccine may be available for the specific disease. However, prevention measures are similar for all arthropodborne diseases.
The most effective prevention method is to avoid being bitten by the arthropod vector in the first place. This can be achieved by wearing clothing that covers bare skin, using repellants to deter insects, avoiding outdoor activities at times when the arthropods are most active, and sleeping under mosquito netting. Travelers may want to avoid visiting tropical countries where certain arthropod-borne diseases are common, and anyone traveling to countries where these diseases are endemic certainly should take precautions to prevent being bitten. Vaccinations exist for some of these diseases to prevent development of the disease if transmission occurs. However, vaccinations are not available for all arthropod-borne diseases, and everyone does not have access to those vaccines that do exist.
For mechanically transmitted infections, prevention measures include excluding insects from areas where food is prepared and served, washing or thoroughly cooking any food that may have come into contact with an arthropod, and avoiding water bodies inhabited by arthropods. If these precautions are taken, ingestion or absorption of possible pathogens is unlikely.
Arthropod-borne diseases spread rapidly when humans inhabit areas in high densities. This can occur during wars, where soldiers live in close quarters. It can also occur after natural disasters when homes are destroyed and people are forced to live close together in temporary shelters. It also occurs in poorer countries with large populations. An increase in the density of the human population leads to an increase in contact between humans and vectors, causing the rate of infection to rise.
Developing countries are most affected by arthropod-borne diseases. The World Health Organization estimates that up to 500 million cases of malaria occur each year, but fewer than 1,300 of these cases occur in the United States. An estimated one to two million people die every year due to malaria, and over 80% of the fatalities occur in Africa. This is primarily due to the poor living conditions—including lack of sanitation and the presence of stagnant water—that exist in many African regions. These conditions encourage the growth of arthropod populations. In addition, lack of access to high-quality health care in many areas limits prevention and treatment of disease, causing an increase in transmission, as well as, more serious outcomes when infection does occur.
Arthropod-borne diseases also have become a more significant risk in developed countries. For example, West Nile virus, a mosquito-borne disease, first emerged in the United States in 1999. This virus develops in birds and is transmitted to humans by mosquitoes. In 1999, 149 cases were reported in the United States. By 2003, over 9,000 cases were reported, including more than 250 fatalities. Methods to prevent the spread of West Nile virus focus on reducing the number of mosquitoes in an area and using personal protective measures (protective clothing, insect repellents, etc.) to prevent contact with mosquitoes.
One of the most common methods employed to combat arthropod-borne diseases is the use of insecticides to control the insect vectors. However, the sustainability of this method is questionable due to the emergence of insecticide-resistant arthropods. The widespread use of insecticides also can have unintended, negative environmental impacts. For example, the efficient insecticide DDT greatly reduced the number of malaria outbreaks in the 1950s and 1960s, but also caused extensive die-off of bird populations and other negative effects on the natural environment. Today, the World Health Organization recommends the reuse of DDT, but proposes that this use be limited to targeted areas only where it can efficiently kill large populations of disease-bearing mosquitoes with minimal negative environmental effects.
IN CONTEXT: SCIENTIFIC, POLITICAL, AND ETHICAL ISSUES
Based on data collected for a study on ectoparasitism and vector-borne diseases, Phillipe Brouqui, Didier Raoult, Andreas Stein, and other researchers at the Maladies Infectieuses et Tropicales argued that “[h]omeless people are particularly exposed to ectoparasites. The living conditions and the crowded shelters provide ideal conditions for the spread of lice, fleas, ticks, and mites.” The researchers also argued that “exposure to arthropod-borne diseases has not been evaluated systematically.”
A medical team visited shelters in Marseilles, France, for 4 consecutive years. Homeless volunteers were examined and received care during the study.
SOURCE: Brouqui, Phillipe, and Didier Raoult. “Arthropod-borne diseases in homeless.” Ann N Y Acad Sci. (October 2006, 1078: 223-35) and Brouqui Phillipe, Andreas Stein, et al. “Ectoparasitism and vector-borne diseases in 930 homeless people from Marseilles.” Medicine (Baltimore). (2005 Jan; 84(1): 61-8).
The development of vaccines is a growing area of interest. The World Health Organization's Initiative for Vaccine Research (IVR) was established to guide the development of vaccines for various diseases, and this program supports research on various arthropod-borne diseases, including dengue fever, Japanese encephalitis, malaria, and West Nile virus. The Bill & Melinda Gates Foundation also established the Malaria Vaccine Initiative (MVI) in 1999, with the goal of developing a vaccine for malaria and making it available in developing countries, including Africa.
See AlsoAnimal Importation; Bacterial Disease; Chagas Disease; Cholera; Contact Precautions; Demographics and Infectious Disease; Dengue and Dengue Hemorrhagic Fever; Emerging Infectious Diseases; Host and Vector; Japanese Encephalitis; Lice Infestation (Pediculosis); Rickettsial Disease; Rift Valley Fever; Malaria; Microorganisms; Mosquito-borne Diseases; Sanitation; Travel and Infectious Disease; Tropical Infectious Diseases; Vaccines and Vaccine Development; War and Infectious Disease; West Nile; Yellow Fever.
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