There are thousands of diseases that exist in this world. Most diseases affect humans, as well as animals. The deadliest animal that vectors a majority of the world’s deadliest diseases is the mosquito. Although there are many different species of mosquitoes, as a whole, they vector an extreme amount of diseases that have affected the world and human populations and have caused an immense amount of deaths over many years. Mosquitoes are the most important arthropod affecting human and animal health around the world. It is important to know some of the most important and dangerous diseases that mosquitoes vector and some that have caused the most deaths by a mosquito borne/vectored disease. Those diseases that will be discussed include the following: Dengue fever, West Nile Virus, Malaria, and Yellow Fever. It is also important to know how to prevent and control mosquito-borne diseases to help stop the spread of diseases and protect yourself and others from these diseases.
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Dengue fever is mainly vectored by the mosquito Aedes aegypti and is known as “Breakbone fever” because of its many symptoms, which include muscle and joint pain during the febrile phase. According to Halstead, the important factors of Dengue are the mosquito interactions and human contributions to dengue epidemic behavior. Halstead includes the determinants of dengue epidemic behavior to be the following: mosquito density requirements for transmission, the basic reproduction number, the spatial heterogeneity of breeding specifically Aedes aegypti, the effect of temperature, and rainfall contribution. The minimum density of Aedes aegypti that allows for the transmission of viruses is not exactly known and does not show that transmission occurs in a particular region. Dengue fever is one of the most common diseases that is vectored by mosquitoes. According to Dr. Brundage from Texas A&M University’s Entomology Department, Dengue has about 4 serotypes, each of them being 65% related to one another. All four of the serotypes are distributed worldwide throughout what is called the “Dengue belt.” The “Dengue belt” is found to range from 30º North and 30º South latitude. This warm weather range is where the climate is most suitable for mosquitoes.
According to lecture notes provided from Dr. Brundage, Dengue fever’s distribution includes 2.5 billion people at risk of getting the disease. There are approximately 50-100 million infections of Dengue fever every year. About 500,000 of those infected have to be hospitalized due to the severity of the infection. In the areas in which Dengue occurs, the fatality rate is 5%. If the proper care is given immediately after being diagnosed with the infection, the fatality rate can be dropped down to less than 1%. Every 20-40 years, there are worldwide outbreaks of Dengue fever. The disease’s peak frequency all depends on the availability of the vector, in this case the mosquitoes. When discussing the availability, it is important to know that the mosquito is more prevalent in the summertime.
To know the prevalence of Dengue fever, it is important to know the mosquitoes that vector the disease as well as the life cycle of those vectors. The two main vectors of Dengue are the females of Aedes aegypti and Aedes albopictus species. Aedes aegypti will mostly feed on humans and tends to rest and feed indoors, and it’s prevalence is mostly associated with urban areas. Aedes albopictus will usually migrate further from the breeding site. It is responsible for moving along the Dengue belt and spreading disease. Unlike the other vector, Aedes albopictus is mostly associated with rural areas and habitats. Both vectors are very good at invading new areas because they have no predators. These mosquitoes usually lay their eggs in flood water and the eggs can resist desiccation for up to a year.
Specifically for the Dengue virus, there is a 10 day incubation period after a mosquito acquires the disease in which it remains in the mosquito. During the incubation period, the virus moves from the blood into the mosquito’s midgut, next the hemolymph, and finally the salivary glands and is then ready to go into the host. When the mosquito acquires the infection, it is able to infect humans with the disease and thus remains infectious for the rest of her life. These mosquitoes are able to transmit the virus with as little as 10 viral particles and can send Dengue into the human host. The virus continues to replicate in the mosquito and get peak virus particles 7-10 days post-infection in the midgut, and peak virus particles 12-18 days post-infection in the salivary glands. As for the infection in humans, after they are bitten by the mosquitoes, the viral particles infect immature dendritic cells (immune system cells). These skin dendritic cells migrate to lymph nodes and replicate here. They take over the macrophages (white blood cells) and use them to replicate the disease.
There are two main types of infections in Dengue fever: Dengue Shock Syndrome and Dengue Hemorrhagic Fever each with the same 3 stages/phases of symptoms. Dengue Hemorrhagic Fever consists of massive amounts of bleeding. There is more worry about the secondary infection, Dengue Shock Syndrome. If a person is infected initially with the disease’s serotype 1, then followed by infections by serotypes 2 or 3, the person will start bleeding and then eventually die. Also, if a person gets infected with serotype 3 then serotype 2, the person will also die. Dengue fever’s symptoms consist of 3 phases or stages. The first stage of the disease is known as the Febrile Phase. In the febrile phase, a person infected generally exhibits a 104-105ºF fever within 4-7 days after being bit. The person gets a high fever due to the fever trying to kill the virus using the person’s body heat. The febrile phase is associated with a headache behind the eyes, acute pain in muscles and joints, possible peticial hemorrhages (tiny spots of hemorrhages), and bleeding under skin causing red rash (2-5 days after fever). A secondary rash that occurs looks like measles, which comes with increased skin sensitivity which is very uncomfortable. The second stage of Dengue fever is known as the Critical Phase. In this phase, there is an increase in capillary permeability, which means that plasma is now leaking out of the capillaries. This is the primary indicator of the severe form of the disease where hemorrhaging starts. This phase consists of symptoms of acute abdominal pain, which can lead to metabolic acidosis and intravascular coagulation (blood clots). In this stage, a person is at highest risk of death. In the third stage, the Recovery Phase, the bodies of those that were able to survive through the Critical Phase reabsorb all the plasma that leaked. These people have frequent urination, which is the way to know a person is in recovery mode. The primary risk factor of this disease is if the person infected has had a previous infection. When a person has had a previous infection, the Dengue virus creates some of the same proteins as well as different ones. There are some antibodies that make Dengue fever worse when introduced to a prior infection. A way to find if a person has been infected with Dengue fever is by looking out for some of the major warning signs. These include hypovolemic shock (low blood volume), bleeding from gastrointestinal tract, liver failure, overall general weakness, multiorgan failure, and death.
This disease, although could be fatal and may be difficult to find, there are tests available to determine if someone has this disease. The tests that could be done to diagnose a person with Dengue are an antibody typer, a PCR test, or a liver function test. Once diagnosed, the only treatment for this disease is to give support to the person infected. Extra nutrients can be given to keep a person comfortable, but there is no 100% curable vaccine available for Dengue fever. This is because there are 4 different serotypes. Because there are antibodies that cause Dengue Shock Syndrome, a Yellow Fever virus was genetically engineered to produce the antibodies for all 4 of the serotypes all at once and a limited vaccine was produced and approved (Dec. 2015). This vaccine that was approved was for use on people infected that were over 6 years old and under 45 years old. The limited vaccine was proven to be 65% viral in Brazil, Argentina, and Mexico. Those that are under 6 years old and older than 45 years old are the ones most at risk due to the limited range of people and the age.
Another very important mosquito borne disease to be discussed is West Nile Virus. West Nile Virus, according to Dr. Brundage, was introduced into the United States in 1999 when there was an outbreak in NYC. It was first seen in Texas in 2002 in Houston, Tx. West Nile Virus, or WNV, is vectored primarily by Culex mosquitoes. According to Hayes and Gubler, the main mosquitoes responsible for the spread of West Nile Virus are Culex pipiens, C. restuans, C. quinquefasciatus, and C. tarsalis. This disease is maintained in a bird-mosquito-bird cycle in which it is then transferred by mosquitoes to both humans and horses which become incidental hosts. This virus can be lethal for birds and has affected about 320 avian species, primarily corvids (crows, magpies, and jays), house sparrows, house finches, and grackles. These birds are highly competent reservoirs for mosquito infection with WNV, according to Hayes and Gubler. In this cycle, birds are able to allow WNV to replicate and amplify the network population of the virus. Rabbits, rodents, squirrels, and alligators can reinfect Culex mosquitoes with WNV. Humans and other mammals, however, cannot reinfect mosquitoes and are determined as dead-end hosts for West Nile Virus. When observing incidental infection, approximately 20-30% actually develop severe symptoms of the disease. These symptoms include flu-like symptoms like fever, headache, malaise, anorexia, fatigue, and back and muscle pain. It then increases to other symptoms which include eye pain, sore throat, gastrointestinal distress, and sometimes rash. Of these people infected, it was found that 1 in every 140 patients will get the neuroinvasive form of the disease which consists of encephalitis, meningitis, flaccid paralysis, and sometimes tremors. For this disease, a person older in age is at higher risk to get the neuroinvasive form of the disease. Of those that get the neuroinvasive form of the disease, about 10% die because the disease moves to all other organs. According to Hayes and Gubler, 80% of infections result in no symptoms, 20% results in self-limited West Nile Fever, and less than 1% result in the neuroinvasive form of the disease.
The Culex mosquitoes that vector West Nile Virus tend to infect people more during early summer, with peaks around August and September. Most reported incidents have been found to occur in rural areas. The United States has more than 60 mosquito species that have been infected with WNV. However, this does not mean that all of these species are competent vectors. The major vectors of West Nile Virus are the following mosquitoes: Culex pipens, Culex restaurans, Culex tarsialis (Western US), Culex migipalpans (California), Ochlerotatus canodensis, and Ochlerotatus cantator. The spread of disease in these mosquitoes begins with the feeding on blood from a human with the disease, the virus then heads to the posterior midgut of the mosquito, then attaching to the cells of the midgut quickly, and then move to the salivary glands where it replicates. The virus requires the salivary proteins in order to infect the host with the disease.
Malaria disease is another mosquito vectored disease that is extremely important in human health. Malaria is a disease that begins with the protozoan parasite in the Plasmodium family. There are 4 different plasmodium species that invade the mosquitoes and allow for the infection of malaria: Plasmodium vivax, Plasmodium falciparum, Plasmodium malariae, and Plasmodium ovale. The most dangerous of these is Plasmodium falciparum, which accounts for 90% of those infected with the disease. If infected with the other species of plasmodium (10%), a person will exhibit the symptoms, get severely ill and go into organ failure, but the disease will not be fatal. These plasmodium species are able to infect mainly Anopheles mosquitoes. Anopheles quadrimaculatus is found primarily in the East, Anopheles pseudopunctipennis is found in the United States and South America, and Anopheles freeborni is found in the West. Malaria disease is found to have approximately 300-500 million cases per year which can result to 1.5 to 2.7 million deaths per year. 90% of these deaths are on the continent of Africa. It is found that currently 40% of the world population (3.4 billion people) live in malaria endemic places. Malaria expands to over 106 countries and territories all trying to stop the spread of malaria. In the United States, there are approximately 1,500-2,000 reported cases per year. These are usually recent travelers who don’t get the malarial medications before travelling abroad. There are other ways of acquiring malaria without being bit by a mosquito, one way is via blood transfusion from someone who has been infected with malaria. Another way of getting malaria is at an airport in which an infected mosquito gets on a plane with a person and occurs in instances where planes arrive from quarantined countries and the doors were not allowed to be open until they were sprayed with insecticides and cleared.
The life cycle of malaria begins with two host parasites, a mammalian host and a mosquito host. After the Anopheles mosquito is infected, it gets to the host through the salivary glands and is then passed onto the human. Then, the sporozoites from the plasmodium cause the virus to be motile but cannot cause infection due to its incubation period. Once the virus gets into the liver cells, the sporozoites change form and turn into schizonts, which then undergo division into cells to form a new version called merozoites. These merozoites are really flexible and can reproduce asexually or sexually. They go into the bloodstream and float around until they find red blood cells (erythrocytes) and replicate asexually. Some of the merozoites sexually reproduce and differentiate into sexual stages turning into gametocytes. These gametocytes then re-infect the mosquito.
Malaria has an interval of about 7-30 days before the first clinical signs appear. The plasmodium that has the shortest incubation period is Plasmodium falciparum and the one that has the longest incubation period is Plasmodium malariae. The malaria virus presents its symptoms either mildly or extremely acute/severe. The mild version of the disease, known as the uncomplicated version, gives a short attack. A person infected that gets the mild version of the disease will exhibit generic flu-like symptoms that can be attributed to a common cold. When a person infected exhibits the complicated, or severe, symptoms can result in serious organ failure, abnormalities in the blood and metabolism, abnormal behavior, seizures, may fall into a coma, has severe anemia, urinating blood, has respiratory distress, kidney failure, and low blood sugar. As for treatment for malaria, it depends on the severity of the disease. If the virus or disease is least virulent, a person can get medical care and drugs and be able to survive. For this, the form of the disease is needed to be identified as well as the correct type of blood that is needed. According to Thera, the vaccines available for malaria that target the sexual stages would prevent neither infection nor disease in the person who is vaccinated.
The next important vector-borne disease is Yellow Fever. Yellow Fever is a viral hemorrhagic fever that is vectored by the mosquitoes Aedes aegypti and Aedes albopictus and was the first virus that demonstrated transmission by mosquitoes. This disease has been isolated from blood feeding flies and ticks and was found to be able to be passed transovarially from mother mosquitoes to the eggs.. According to Alan D. T. Barrett and Stephen Higgs, the first documented outbreak of Yellow Fever was in 1648 in Yucatan, Mexico but originated in Africa and the spread of the disease is likely due to the shipping routes used that migrated the mosquitoes as well as those infected with the disease. This disease has now lead to 200,000 cases and 30,000 deaths, 90% of deaths occurring in Africa.
Yellow Fever consists of three types of transmission cycles: Sylvatic or Jungle Cycle, Intermediate Cycle, and Urban Cycle. The Jungle Cycle occurs mostly in South America and Africa and is enzootic in rainforests. This cycle is transmitted between monkeys or lower primates that infect the mosquitoes with the disease, who then infect the humans with the disease. For Jungle Cycle Yellow Fever, primates appear to be the only ones involved in the transmission cycle. The next cycle of YF is the Intermediate or Savannah Cycle which occurs only in Africa. This cycle takes place in savannah areas that are moist and where there is some human activity. The Intermediate Cycle is one that allowed YF to evolve and become an important human disease. This disease spreads from mosquitoes that feed on both primates and humans. The most common type of outbreaks for this disease have been reported to occur mostly in Africa. The next cycle is the Urban Cycle which is known to occur in South America and Africa, predominantly Nigeria, and vectored mostly by Aedes aegypti. This cycle mostly occurs in human travellers who move into dense urban centers. The outbreaks start in dense urban centers and spread outwards to where there is high rainfall, high humidity, and high temperatures. The following image (Figure 1) shows the three cycles of Yellow Fever and their vectors as well as how the disease spreads.
Yellow Fever has an incubation period of 3-6 days in which a person shows no symptoms of the disease. If infected, after 6 days mild cases of YF will show symptoms of fever, headache, muscle aches, sensitivity to light, loss of appetite, dizziness, redness of eyes, face or tongue, nausea, and vomiting.
Figure 1: The spread of Yellow Fever and its three cycles.
If these symptoms are acquired, they will usually improve and subside within several days. However, 15% of those infected who showed mild symptoms, 15% went on to acquire the toxic phase of the disease. If the toxic phase of YF is acquired, it can cause jaundice of the skin and whites of the eyes (hence the term Yellow Fever), abdominal pain and vomiting, decreased urination, bradycardia, bleeding from nose, mouth and eyes, heart, liver, and kidney problems/failure along with brain dysfunction which could all lead to death. There is a reported 10% of deaths for those who go into the toxic phase of Yellow Fever. For Yellow Fever, however, there is a vaccine available but is limited and difficult to acquire.
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With knowing these mosquito-borne diseases and the dangers of these diseases, it is important to know how to help prevent diseases and control the mosquito prevalence in certain areas. The goal is to minimize the harmful effects of mosquito bites and interrupt the pathogen transmission. Although eradication of mosquitoes and diseases is desirable, it is difficult to achieve. Some of the ways we can approach this goal is by preventing mosquito bites, suppressing mosquito populations, minimizing mosquito-vertebrate contact, reduce longevity of female mosquitoes, and reduce host susceptibility. To start the process of preventing mosquito-borne diseases, the mosquito distribution, abundance, and degree of pathogen activity need to be determined. For this, one of the tools that can be used are mosquito traps. These include light traps or aspirators that either trap the mosquitoes or kill them. Another way scientist are able to help is by testing mosquitoes for parasites by microscopy or using PCR to look at their DNA sequencing, or by testing dead birds to find the cause of death and further analyze the disease distributions.
In order to prevent mosquito bites and infection, personal protection is recommended. This allows for the most direct and simple approach in preventing exposure to mosquito bites. People are advised to reduce their outdoor exposure during periods of peak mosquito activity which are dawn/dusk for Culex species or daytime for Aedes species. If outside, protective clothing like long sleeved shirts and long pants are recommended as well as head nets. Another way to prevent being bit by mosquitoes while asleep for those who live in areas with an abundance of mosquitoes, is to set up bed nets that are impregnated with pyrethoid to repel mosquitoes and kill those than contact the net. Chemical repellents like OFF or Cutter are recommended for the best way to repel mosquitoes while outdoors. There are also special chemicals and plants that help as spatial repellents for outdoor areas to prevent the abundance of mosquitoes.
To suppress mosquito populations, there are abatement or reduction tools that can be used. For example, for habitat modification, the attempt to reduce mosquito breeding habitats will most likely be effective. This extinguishes the areas in which mosquitoes breed and are then not able to reproduce in areas around you. There is also genetic control of a species in which scientists are able to suppress a mosquito population by introducing another population of organism that that specific species does not cooperate with or will not be a compatible host for the disease. There is also larval control in which the aerial or manual application of chemicals like oils or insect-growth-regulators that target immature mosquitoes can be used. For the control of adult mosquitoes, aerial or manual spraying of chemicals can also be used but the development of resistance to these can be a problem.
To reduce host susceptibility, vaccines can be introduced. Vaccines allow for the inoculation of a killed or weakened form of a disease agent that stimulates antibody production. This improves the immune response to vector-borne agents and reduces the disease by reducing host susceptibility. Chemoprophylaxis, which is the administration of medicine to prevent disease or infection, is also another way to prevent being disease. The types of chemoprophylaxis include antibiotics, anti-malaria compounds, anti-filariasis, and dog heartworm prevention. Some of the anti-malaria compounds available are chloroquine (widespread resistance for P. falciparum and P. vivax), mefloquine (mechanism of action is unknown; has neuropsychiatric side-effects), doxycycline (least expensive; requires daily medication), and atovaquone/proguanil (more convenient but more expensive). Some anti-filariasis medications available are Diethylcarbamazine (reduces microfilariae and causes less transmission), and Ivermectin (reduces microfilariae and causes less transmission but requires sustained treatment for 10-15 year life of adult worms; is a broad-spectrum antiparasitic). For dog heartworm prevention, Ivermectin is available as well as Moxidectin, which binds to parasite ion channels and disrupts neurotransmission which results in the death of the parasite.
Mosquito-borne diseases are extremely important and can lead to extreme symptoms and possibly fatality. It is important to know the way these diseases are spread and how to identify symptoms in order for health-care professionals to determine the disease and give a proper diagnosis and help either treat the disease or feel at ease with the circumstances. Dengue fever, West Nile Virus, Malaria, and Yellow Fever are only four amongst the many diseases that are vectored by mosquitoes. Although all present with differing symptoms, these are diseases that could change a person’s health indefinitely. The most prevalent mosquito that vectors many diseases is those of the Aedes species. However, since most people are not able to identify the mosquito with the naked eye, it is best to try to restrict contact with mosquitoes all together and to try to prevent and control the prevalence of mosquitoes.
Works Cited
- Barrett, Alan D. T., and Stephen Higgs. Yellow Fever: A Disease That Has Yet to Be Conquered. Annual Review of Entomology, 16 Aug. 2006, ento.annualreviews.org.
- Hayes, Edward B., and Duane J. Gubler. “West Nile Virus: Epidemiology and Clinical Features of an Emerging Epidemic in the United States.” Annual Review of Medicine, vol. 57, no. 1, 1 Sept. 2005, pp. 181–194., doi:10.1146/annurev.med.57.121304.131418.
- Thera, Mahamadou A., and Christopher V. Plowe. “Vaccines for Malaria: How Close Are We?”Annual Review of Medicine, vol. 63, no. 1, 10 Nov. 2011, pp. 345–357., doi:10.1146/annurev-med-022411-192402.
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