VECTOR-BORNE DISEASES CAUSED BY MOSQUITOES, TICKS AND FLIES

VECTOR-BORNE DISEASES

anopheles pupa Vector-borne diseases are human illnesses caused by parasites, viruses and bacteria that are transmitted by vectors (usually mosquitos, ticks, lice or blood-sucking insects). Every year there are more than 700,000 deaths from diseases such as malaria, dengue, schistosomiasis, human African trypanosomiasis, leishmaniasis, Chagas disease, yellow fever, Japanese encephalitis and onchocerciasis.

The burden of these diseases is highest in tropical and subtropical areas, and they disproportionately affect the poorest populations. Since 2014, major outbreaks of dengue, malaria, chikungunya, yellow fever and Zika have afflicted populations, claimed lives, and overwhelmed health systems in many countries. Other diseases such as Chikungunya, leishmaniasis and lymphatic filariasis cause chronic suffering, life-long morbidity, disability and occasional stigmatisation.

Distribution of vector-borne diseases is determined by a complex set of demographic, environmental and social factors. Global travel and trade, unplanned urbanization, and en The following table is a non-exhaustive list of vector-borne disease, ordered according to the vector by which it is transmitted. The list also illustrates the type of pathogen that causes the disease in humans.

See Separate Articles: MOSQUITOES factsanddetails.com

Vectors (Usually Mosquitoes, Ticks or Blood-Sucking Insects)

Vectors are living organisms that can transmit infectious pathogens between humans, or from animals to humans. Many of these vectors are bloodsucking insects, which ingest disease-producing microorganisms during a blood meal from an infected host (human or animal) and later transmit it into a new host, after the pathogen has replicated. Often, once a vector becomes infectious, they are capable of transmitting the pathogen for the rest of their life during each subsequent bite/blood meal. [Source: World Health Organization (WHO), March 2, 2020]

Vector-borne diseases account for more than 17 percent of all infectious diseases, causing more than 700 000 deaths annually. They can be caused by either parasites, bacteria or viruses. Malaria is a parasitic infection transmitted by Anopheline mosquitoes. It causes an estimated 219 million cases globally, and results in more than 400,000 deaths every year. Most of the deaths occur in children under the age of 5 years.

Dengue is the most prevalent viral infection transmitted by Aedes mosquitoes. More than 3.9 billion people in over 129 countries are at risk of contracting dengue, with an estimated 96 million symptomatic cases and an estimated 40,000 deaths every year. Other viral diseases transmitted by vectors include chikungunya fever, Zika virus fever, yellow fever, West Nile fever, Japanese encephalitis (all transmitted by mosquitoes), tick-borne encephalitis (transmitted by ticks). Many of vector-borne diseases are preventable, through protective measures, and community mobilisation.

Vector-Borne Diseases and Their Vectors

Vector — Mosquitos: 1) The Aedes genus transmits chikungunya (virus), dengue fever (virus), lymphatic filariasis (parasite), Rift Valley fever (virus), yellow fever and Zika (virus). 2) anopheles genus transmits lymphatic filariasis (parasite) and malaria (parasite). The culex genus transmits Japanese encephalitis (virus), lymphatic filariasis (parasite) and West Nile fever (virus).

anopheles egg 2) Vector — Flies: 1) Blackflies transmit Onchocerciasis (river blindness, parasite); 2) Sandflies transmit Leishmaniasis (parasite) and sandfly fever (phlebotomus fever, virus); 3) Tsetse flies transmit sleeping sickness (African trypanosomiasis, parasite)

3) Vector — Ticks transmit: 1) Crimean-Congo haemorrhagic fever (virus); 2) Lyme disease (bacteria); 3) Relapsing fever (borreliosis, bacteria); 4) Rickettsial diseases (eg: spotted fever and Q fever, bacteria); 5) Tick-borne encephalitis (virus); and 6) Tularaemia (bacteria).

Vectors — 1) Aquatic snails transmit Schistosomiasis (bilharziasis, parasite); 2) Fleas transmit plague (transmitted from rats to humans, bacteria) and Tungiasis (Ectoparasite); 3) Lice transmit Typhus (bacteria) and Louse-borne relapsing fever (bacteria); and 4) Triatome bugs transmit Chagas disease (American trypanosomiasis, parasite)

Lyme disease is a tick-borne infection passed on by deer ticks. No vaccine is available, Symptoms include fever, headache, extreme fatigue, aching joints and muscles, mild neck stiffness and bulls eye rashes. Treatment is usually with antibiotics. If untreated, lyme disease can lead to heart, nervous system and joint disorders.

St Louis encephalitis virus can cause severe neuroinvasive illness including encephalitis, but more often causes symptoms including fever, nausea, headache, vomiting and tiredness. It is spread by the southern house mosquito

Dengue Fever

aegypti mosquito Dengue fever is a nasty, viral disease transmitted by the “Aedes” mosquito, usually the “ Aedes aegypti” , the same mosquito species responsible for transmitting human viruses such as Zika, chikungunya and yellow fever.

Dengue is the most prevalent viral infection transmitted by Aedes mosquitoes, which also transmits yellow fever. More than 3.9 billion people in over 129 countries are at risk of contracting dengue. The diseases sickens up to 100 million people every year (there are also asymptomatic cases) and causes an estimated 40,000 deaths every year. There are several strains of dengue fever (the four main one are immunologically related). [Source: World Health Organization (WHO), March 2, 2020]

Research in 2013 suggests some 390 million people are infected with the virus each year, most of them in Asia. That's about one in every 18 people on Earth, and more than three times higher than the World Health Organization's previous estimates. According to Associated Press: “Known as "breakbone fever" because of the excruciating joint pain and hammer-pounding headaches it causes, the disease has no vaccine, cure or specific treatment. Most patients must simply suffer through days of raging fever, sweats and a bubbling rash. For those who develop a more serious form of illness, known as dengue hemorrhagic fever, internal bleeding, shock, organ failure and death can occur. And it's all caused by one bite from a female mosquito that's transmitting the virus from another infected person. [Source: Associated Press, November 15, 2013]

See Separate Article DENGUE FEVER factsanddetails.com

Yellow Fever

aedes Yellow fever is the second most dangerous mosquito-borne disease after malaria. It is a viral infection transmitted by “Aedes aegypti” mosquitos and is endemic to zones in tropical Africa and South America. Urban yellow fever is carried the “ Aedes aegypti” mosquito. Jungle yellow fever is an enzootic disease transmitted among nonhuman primate hosts by a variety of species of mosquitos.

Yellow fever famously halted progress on the Panama Canal in the 1900s and shaped the history of Atlantic coast cities from Philadelphia to Rio de Janeiro. Although a yellow fever vaccine has been available since the 1930s, the disease continues to afflict 200,000 people a year, a third of whom die, mostly in West Africa.

Yellow fever victims often suffer from jaundice (a liver disease that yellows the skin), hence the name yellow fever. Other symptoms include headache, chills, nausea, and vomiting. Severe cases can infect the blood, liver and kidneys.

Dengue fever and yellow fever are so closely related they are regarded as sister diseases. Yellow Fever sometimes picked by mosquitos from monkeys that carry the disease with no ill effects. Tissue and blood samples are taken from hunted monkeys to find source of disease and define infected area.

See Separate Article YELLOW FEVER factsanddetails.com

Malaria

Malaria is a disease caused by single-cell parasitic protozoans that are transferred to humans by particular kinds of mosquitos. It a can damage the nervous system, kidneys and liver. Severe cases can quickly lead to death. The word “malaria” is derived from the Italian words “mala aria” , which means "bad air" and is based on the mistaken belief in the past that the disease was caused by bad swamp air. [Source: Michael Finkel, National Geographic, July 2007]

Leishmaniasis bone marrow biopsy Malaria is a parasitic infection transmitted by Anopheline mosquitoes. It causes an estimated 219 million cases globally, and results in more than 400,000 deaths every year. Most of the deaths occur in children under the age of five. This better than before but still bad. Malaria affected up to 300 million people and killed nearly 800,000 every year in the early 2000s, twice as many people as in the 1980s. [Source: World Health Organization (WHO), March 2, 2020]

Malaria is one of the world's deadliest diseases and the world's number one parasitic killer. Endemic in 106 nations and threatening half the world’s population, its threat is greatest in Africa, where the WHO says a child dies of malaria about every 45 seconds. About 90 percent of the victims are children under 5 in poor countries in Africa. A child dies of malaria every 12 to 30 seconds and about 25 percent of all childhood deaths in the Third World are the result of malaria.

Jerry Adler wrote in Smithsonian magazine: “A. gambiae has been called the world’s most dangerous animal, although strictly speaking that applies only to the female of the species, which does the bloodsucking and harms only indirectly. Its bite is a minor nuisance, unless it happens to convey the malaria parasite, Plasmodium falciparum, for which it is a primary human vector. [Source: Jerry Adler, Smithsonian magazine, June 2016]

See Separate Article MALARIA: ITS HISTORY, PARASITE AND ITS LIFE-CYCLE factsanddetails.com

Leishmaniasis

Leishmaniasis is a parasitic disease found in tropical and subtropical areas of the world. Transmitted by the bite of some species of sand fly, it comes in cutaneous (skin) and visceral (internal organ) forms. Cutaneous leishmaniasis is characterized by open or closed skin sores. Visceral leishmaniasis typically develops over several months and is characterized by fever, enlargement of the liver and spleen and anemia.

Leishmaniasis affects about 15 million people a year, mostly in tropical areas. It begins with a bite that doesn't heal and a wound that gets bigger. Many victims have nasty looking lesions. If untreated the lesions can take months and even years to heal and leave behind nasty looking scars. Leishmaniasis can resembles leprosy in its external form. It slowly kills victim in visceral form by eating the internal organs.

Leishmaniasis is transmitted to humans by sand flies that thrive in animal dung used for cooking and thatched used for making roofs in poor villages. The flies that transmit the disease are smaller than mosquitos and easily pass through mosquito netting. They congregate in large number in banana groves and bamboo stands often found near poor villages.

Visceral Leishmaniasis (Black Fever Kala-Azar)

Visceral leishmaniasis, also known as black fever and kala-azar, is the world’s second deadliest parasitic killer after malaria. Spread by sand flies, it causes itchy skin lesions, fever, swelling of the spleen and liver and serious weight loss. A serious form eats away at a victim’s skin and mucosa tissues and can cause death.

Black fever kills about a half million people worldwide annually, nearly all of them poor. In its developed form the disease causes the liver and spleen to expand so much they to bulge from victims rib cage. Describing a 15-year-old boy in Bihar state in India with black fever, Stephanie Strom wrote in the New York Times, “Dilip Manjhi...lay listless on a string bed....Dilip’s spleen extended eight centimeters below his rib cage and was rock hard. His liver protruded three centimeters below his ribs.” About 90 percent of the cases are in Bihar, Bangladesh, Nepal, Sudan and northeastern Brazil. In India the disease is known as kala azar.

Preventing and Treating Visceral Leishmaniasis

Leishmaniasis is cured by grueling course of intravenous medication. The most widely prescribed medicine was discovered more than a century ago and has severe side effects. No vaccines or preventative drugs are available for the disease. Like tsetse flies, the sand flies occupy certain regions, often in the rain forest. Forced by poverty and desperation, more and more people are moving to these areas. The current treatment can be toxic and costs $500 in India.

Leishmaniasis can be cured and prevented with a series of cheap injections from a drug called paromomycin — an inexpensive antibiotic used in some places to treat diarrhea that was found promising in the 1960s but never made it to the market place because it was determined that there was not enough profit in it to justify carrying out expensive trials.

The Gates Foundation has donated $30 million to combat leishmaniasis. The Institute of One-World Health, a small San-Francisco-based charity supported with a $47.2 million grant from the Gates foundation, is currently trying to get paromomycin to the desperately poor who need by conducting trials to prove the drug is safe and effective. If the drug is approved a course of treatment that cost less than $10 per person could virtually eliminate the disease. In India, One-World is working the Indian drug company Gland Pharma to produce and sell the drug at cost. A big hurdle that still has to be overcome is getting the drug to remote villages where it is needed most.

Preventative measures focus on reducing man-fly contact. The flies are most active at dawn and dusk and bite mostly at night (or during the day when disturbed). They are about a third smaller than mosquitos so netting should be a fine mesh variety with at least 18 holes to the linear inch. This netting is sometimes intolerably hot in tropical areas. Regular mosquito netting sprayed with an insecticide is relatively effective.

Lymphatic Filariasis and Elephantiasis

Lymphatic filariasis is a disease that infects 120 million people, of which 43 million are sick. Caused by microscopic, thread-like, parasitic filarial worms (“wuchereria bancrofti” ) and transmitted by mosquitoes, it can cause fevers and asthma-like breathing difficulties. The worms reside in the lymph nodes and muscle tissues. In its most extreme form for is elephantiasis (See Below). Dog heartworm is also caused by filarial worms.

Elephantiasis (often misspelled and mispronounced as “elephantitis”) is a lymphatic disorder caused by filarial worms carried by mosquitoes. People with the disease, which is a leading cause of disability worldwide, can have severely swollen limbs and, in men, swelling of the scrotum. The legs, arms and male genitals swell to a huge size as a result of the accumulation of lymphatic

Lymphatic filariasis can be treated with a drug called diethylcarbamazine (DEC). DEC, used in conjunction with invermectin, a powerful drug donated by the drug giant Merck, reduces 99 percent of the filariasis parasite concentrations in the victim's blood. The disease can be prevented by taking the drug Albendazole

Encephalitis, West Nile Disease and Lyme Disease

Encephalitis is a tick-bourne disease that often produces fatal swelling of the brain. Symptoms include fever, headache vomiting, neck stiffness, pain in the eyes when looking at light, alterations in consciousness, seizures, paralysis or muscle weakness. Correct diagnosis requires hospitalization. There are vaccines for encephalitis and specific tick-borne encephalitis immune globulin. Encephalitis prevention includes avoiding places with ticks such as high grass, edges of forests, clearings.

The West Nile virus is named after the West Nile district of Uganda, where it was first identified. It is a virus that is spread by mosquitos, birds and humans. It is a close relative of yellow fever. The southern house mosquito (Culex pipiens mosquito) is the main vector for West Nile virus, which usually causes mild, flu-like symptoms but in severe cases can cause a form of encephalitis, an inflammation of the brain, or meningitis.

West Nile virus was first detected in North America in 1999. Since then, there have been around one million human infections with West Nile, nearly all of them caused by a mosquito bite. The disease was first identified in a woman in the West Nile district of Uganda in 1937. The disease appeared in Israel in the 1950s and Romania in 1996. Each outbreak followed an usually dry, hot spell. In 1999 the disease arrived in New York, probably at LaGuardia airport. It is thought to have arrived on a stowaway mosquito or in the blood of an infected person. The summer it arrived in the U.S. was very hot and dry.

Image Source: Centers for Disease Control and Prevention cdc.gov/DiseasesConditions

Text Sources: Centers for Disease Control and Prevention (CDC), U.S. Department of Health and Human Services; World Health Organization (WHO) fact sheets; National Geographic, New York Times, Washington Post, Los Angeles Times, The New Yorker, Time, Reuters, Associated Press, AFP, Lonely Planet Guides and various websites books and other publications.

Last updated May 2022

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