What are insect-borne diseases?

Causes and Symptoms

Insects such as mosquitoes, blackflies, tsetse flies, eye flies, houseflies, fleas, and lice are responsible for transmitting pathogens that inflict disease and illness on hundreds of millions of people each year. Mosquitoes and blackflies are the most medically and economically destructive of the bloodsucking insects. Mosquito-borne diseases kill more than one million people every year and infect hundreds of millions more. These diseases are generally one of three types. The first usually causes fever, rashes, joint pain, and occasional fatal infections. The second type is hemorrhagic, which causes bleeding from the mucous membranes and occasionally leads to fatal shock. The third type causes symptoms of encephalitis, an inflammation of the brain, and is identified with several epidemics.

The better-known mosquito-transmitted diseases include malaria, yellow fever, dengue fever, and West Nile virus. Malaria, responsible for the deaths of at least one million people in 2002, produces a high fever and chills and does not provide lasting immunity to a relapse. The disease is treated with quinine, but patients can still develop the deadly blackwater fever. Yellow fever is more deadly than malaria and is endemic in Central and South America, the West Indies, and Africa. There is no treatment for yellow fever, although at-risk individuals can be vaccinated to prevent infection. Patients with yellow fever may contract hepatitis and suffer renal (kidney) failure, hemorrhage (blood loss), shock, and death.

Dengue fever, also called breakbone fever, is rarely fatal, but it causes a high fever accompanied by severe pain and stiffness in the joints. The World Health Organization (WHO) estimates that between 50 and 100 million people are infected with Dengue fever annually; in 2010 there were a reported 1.6 million cases in the Americas. The disease is endemic in the tropics, where about 40 percent of the world’s population lives. There is no vaccine or cure. Another form, hemorrhagic dengue, produces shock syndrome and follows a previous infection with dengue. The disease may infect people in as many as sixty-one countries.

West Nile virus was first reported in the United States in Queens, New York City, in 1999. In 2012, 5,674 human cases were reported, with 286 deaths. The virus also causes high mortality in many species of birds and results in numerous horse deaths. Most infected people are symptom-free, with mild cases causing flulike symptoms such as fever, coughing, and weakness. Serious infections can progress to more severe headaches, high fevers, tremors, and partial paralysis, as well as encephalitis.

Biting blackflies are a nuisance that can affect tourism, agriculture, forestry, and recreation. For animals, these bites can cause weight loss, stress-related illnesses, and reduced egg, milk, and meat production. One of the most widespread of the blackfly diseases is onchocerciasis, or river blindness, a disease that can be transmitted to humans, cattle, and other large mammals. The serious effects can occur long after the initial fly bite and are caused by a worm that is transmitted to the host. The worm curls up in lumps on the body, causing coarsening of the skin, depigmentation (loss of color), and intense itching that can drive a person to suicide in severe cases. If the worm enters the eyes, then it can cause reduced peripheral vision, night blindness, and complete loss of vision. Blackfly fever, found in the New England area of the United States, is a severe reaction to bites that causes fever, nausea, headaches, and swollen lymph
glands.

Tsetse flies, houseflies, and eye flies are responsible for numerous infections and diseases. The tsetse fly’s bite can transmit parasites and cause sleeping sickness, a West African disease in humans. The parasite causes enlarged glands, loss of appetite, and extreme lethargy. The patient eventually lapses into a coma, and the disease is fatal if not treated. A similar fatal disease called nagana occurs in animals, especially imported cattle and horses, cattle, goats, camels, and pigs.

Houseflies are nonbiting insects that can transfer pathogens from infected septic matter (feces) to human foodstuffs, sores, or mucous membranes. These pathogens can transmit cholera germs or cause diarrhea and eye infections. Eye flies drink fluid from eyes or blood from sores, ulcers, and small wounds on humans and animals. These very tiny black flies transmit bacteria, especially to children, and spread eye sores such as conjunctivitis (pinkeye) and other infections.

Biting midges are small flies that can transmit several diseases. Sandfly fever causes a short illness with fever in humans. Bartonellosis, or Carrion’s disease, occurs in South America in Peru, Ecuador, and Colombia in higher altitudes. Mild cases usually cause a slight fever, but severe cases can have an 80 percent mortality rate. After five to eight weeks, fever survivors develop large wartlike tumors all over the body, which cause pain and irritation. Leishmaniasis
is a parasitic disease and ranges from a mild form with fevers and anemia to a disfiguring type with skin wounds to another form that can cause internal organ damage and death. Most forms infect animals and accidentally transfer to humans. The disease affects approximately twelve million people worldwide, with over one million new cases every year. Other biting midges transmit diseases such as the Oropouche virus, which is found in tropical America. Severe flulike symptoms, with fever and vomiting, can last for up to two weeks but are usually not fatal.

Flea-borne murine (mice) typhus produces a typhuslike fever, eye rash, headache, chills, and general achiness. The most infamous disease transmitted by fleas, however, is the plague, which killed a quarter of the population of Europe in the fourteenth century. People become infected by receiving a flea bite, handling infected animals, or breathing infected respiratory droplets. The plague causes fever, chills, seizures, and severe headaches, followed by swollen lymph glands (or buboes) in the armpit, groin, or neck. Untreated septicemic plague is initially hard to diagnose because it invades the bloodstream directly, eventually spreading to the liver, kidney, spleen, lungs, eyes, and/or lining of the brain. This type of plague has a 40 percent mortality rate with treatment and a 100 percent fatality rate if untreated. Pneumonic plague, which is contracted by inhaling infected drops from another person or animal, causes severe, overwhelming pneumonia, shortness of breath, high fever, and blood in the phlegm. Life-threatening complications include shock, high fever, problems with blood clotting, and convulsions. Untreated pneumonic plague is
almost always fatal, but a specific antibiotic treatment must be started fifteen to eighteen hours after symptoms appear in order to be effective.

Lice are responsible for several diseases, including louse-borne typhus, which comes from louse feces. Infection occurs through a scratch, contact with mucous membranes, or inhalation. Symptoms include fatigue, muscle aches, headache, coughing, rapid onset of fever, and a blotchy rash on the chest or abdomen. If untreated, the disease can cause delirium, low blood pressure, coma, and possibly death. Treatment with antibiotics usually provides a prompt cure. Brill-Zinsser disease is similar to a mild form of typhus and is a reoccurrence of louse-borne typhus, sometimes waiting as much as thirty years between outbreaks.

Symptoms of louse-borne relapsing fever include head and muscle aches, nausea, appetite loss, dizziness, coughing, vomiting, and the abrupt onset of fever. Severe infections cause the liver and spleen to swell and make breathing painful. Trench fever, or Wolhynian fever, occurred in Central Europe during World War I and World War II and is generally rare and seldom fatal. There may be no symptom or headaches, muscle aches, fever, and nausea.

Chagas’ disease is found in South and Central America. Transmission of bacteria occurs through infected feces, and symptoms range from mild and flulike to severe chronic cardiac or digestive system disease. Estimations indicate that eight to eleven million people living in in Mexico, Central America, and South America are infected with Chagas' disease . Between 10 and 30 percent of infected people will develop chronic, life-threatening symptoms or heart failure, and about will fifty thousand die each year from infection. There is no vaccine or cure for Chagas’ disease.

Treatment and Therapy

For most insect-borne diseases, treatment includes medical intervention such as antibiotics and vaccines to prevent transmission. As of 2013, vaccines for most of the diseases were still under development, and many of the diseases have no known cure. Educating people on how to avoid infection and treat the symptoms is essential.

Biological control involves reducing a target population by introducing a predator, pathogen, parasite, competitor, or toxin produced by a microorganism. This method of control was used as early as 1889 in California and became popular in the early twentieth century. Insecticides (or pesticides) replaced biological control and proved to be very successful in eliminating the insect vectors. Early pesticides were composed of natural botanicals (plant products) such as nicotine, rotenone, and pyrethrum mixed with chemicals such as lime sulfur, arsenic, mercuric chloride, and soaps. The scientific development of insecticides began as early as 1867, and the structures of botanical insecticides were known in the 1920s. In 1939, the insecticide properties of the first synthetic insecticide, DDT, were discovered. DDT has provided a major benefit in the control of typhus, trench fever, and malaria, and it is still used in indoor insecticides for malaria control.

Pesticides may lose their effectiveness as the parasites become resistant to a specific toxin or insecticide. The focus in the early twenty-first century is on developing pesticides known as third-generation insecticides, mainly aimed at affecting mosquito development. Since these insecticides are developed for mosquitoes, they are less disruptive to the environment and less toxic to humans and other organisms.

Other preventive measures include insect growth regulators (IGRs), which are species-specific and prevent larvae from developing. Genetic control, which involves introducing sterile males to the population or the release of insects with only male-producing properties, results in so few females that the population declines. Genetic control methods also may involve population replacement in which the genetic structure is modified in a particular species of insect to prevent disease transmission. A gene can be inserted that would cause the insect to die after breeding and would be passed to offspring. A different gene insertion would kill the females, which are the bloodsuckers in most species, but not the males.

In addition to pesticides and insecticides, which may be applied over a large area, the elimination of insect breeding and gathering areas is very effective. General measures to prevent infection include using personal repellents on people and livestock, wearing light-colored clothing and long sleeves and pants during feeding times, stabling livestock during peak biting periods, managing sewage and fecal waste, using netting or fine mesh screening, reducing the number of strays and wild animals that can act as hosts, and keeping the air moving with fans.

Perspective and Prospects

Insect-borne diseases have been infecting people throughout history. Malaria was noted in ancient times and written about prior to the first century CE. Over time, some people speculated about a connection between insects and illness, but the scientific equipment and knowledge were not yet available to prove the existence of the pathogens. Initial observations led to improved hygiene and better housing, which greatly reduced plague and typhus. Malaria and yellow fever, however, did not respond to these developments.

In 1877, Sir Patrick Manson discovered that some mosquitoes can carry a parasite that infects people. This discovery and subsequent work by many others stimulated the research into insect-borne diseases. The insect transmission of plague and typhus took ten to twelve years to understand, and others took decades. There were no specific drugs or vaccines for the diseases until 1925, except for quinine for malaria, which did not always work.

Research continues in an effort to find successful preventive and control measures, including the development of more effective pesticides, insecticides, and genetic controls. Attention must be paid to illnesses in humans and animals and the potential insect connection in order to fight existing diseases and recognize new or mutating ones. For example, some researchers speculate that bloodsucking stable flies may have caused the initial spread of human immunodeficiency virus (HIV) from chimpanzees to humans.

Although many insect-borne diseases were greatly reduced by 1970, they have since made a dramatic recovery in warm areas of the world, perhaps because of the evolution and adaptation of the pathogens. In addition, social problems such as poverty, famine, and war, which often result in overcrowded and unsanitary conditions, offer ideal breeding areas and easy transmission for numerous insect-borne illnesses and diseases.

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