Mosquitoes and infectious disease

Definition

Mosquito-borne infectious diseases are diseases transmitted through the bites of infected mosquitoes. Mosquitoes act as biological vectors by transmitting pathogens or parasites from one host to another. The most prevalent infectious diseases transmitted by mosquito bites are malaria, yellow fever, dengue fever, Chikungunya, West Nile fever, various types of encephalitis, and Oroya fever, a bacterial disease. The role of mosquitoes in transmitting malaria was discovered by British doctor Ronald Ross on August 20, 1897; the day was subsequently declared World Mosquito Day, which has been used by various governments and health organizations to promote awareness of mosquito-borne diseases.

Causes

Mosquito-borne infectious diseases are caused by the presence of the transmitted virus or Plasmodium parasites in the blood of human and nonhuman animals. More than one hundred types of viruses and parasites are transmitted to humans and animals through mosquito bites. Malaria is caused by infection with Plasmodium falciparum or P. vivax parasites. These parasites develop in the mosquito’s body and are then passed on when the mosquito injects saliva during feeding (biting).

West Nile fever results from the transfer of the West Nile virus (WNV) from infected birds to humans by the Culex mosquito. After multiplication in the human blood, WNV is transported to the brain, where it causes inflammation of the brain tissue.

Dengue fever is caused by the dengue virus, one of the four viruses common to tropical and subtropical climates. It is spread from one infected person to another by the Aedes mosquito. As the dengue virus multiplies and damages cells, an infected person begins to show symptoms similar to those of other mosquito-borne infections.

Chikungunya fever is caused by an arbovirus infection transmitted primarily through A. aegypti and A. albopictus mosquitoes. The mosquitoes feed on an infected person during the viremic period of that person’s infection (that is, within five days from the onset of the mosquito bite and symptoms) and then transmit the virus to other humans.

Though Zika virus, which is also spread by the A. aegypti and A. albopictus mosquitoes, had long been around in Asia and Africa, it was first detected in the Western Hemisphere in Brazil in 2015. By 2016, the virus had spread to such an extent that the World Health Organization (WHO) declared it an international health emergency. Additionally, after conclusive studies and testing proved a direct causality between the presence of the Zika virus in a pregnant woman and birth defects, mainly microcephaly (an unusually small head often accompanied by some level of brain damage), the WHO strongly advised pregnant women to avoid traveling to a number of countries into which the Zika virus had spread. Experts also warned that the virus could be spread through sexual intercourse. By July 2016, the first case of locally transmitted Zika virus was reported in the United States in Florida. The main outbreak in the Americas subsided by the summer of 2017. As of December 2022, the WHO reported eighty-nine countries and territories were impacted over the course of the outbreak, and though cases remained low, transmission still occurred in several countries in the Americas.

In 2018, the Centers for Disease Control and Prevention (CDC) announced that the incidence of mosquito-, tick-, and flea-borne diseases in the United States tripled between 2004 and 2016. Tick-borne diseases accounted for about 75 percent of the cases reported; mosquito-borne diseases accounted for most of the remaining 25 percent. While tick-borne diseases increased steadily, mosquito-borne diseases fluctuated as epidemics of such diseases as WNV, dengue, chikungunya, and Zika broke out and subsided. Many of these outbreaks occurred in Puerto Rico; cases in US states were most often the result of the infected person traveling to an area where an epidemic was ongoing, although some locally transmitted cases occurred in Hawaii, Florida, and Texas.

The WHO has called mosquitoes the world's deadliest animal. Mosquito-borne diseases, according to the WHO, are responsible for more than one million deaths annually; malaria alone caused 619,000 deaths in 2021.

According to scientists, the earth’s warming climate has also led to a dramatic increase in the world’s mosquito population. This increase has led to the presence of mosquito-borne diseases in places where they had been more rare or had never been detected before. For example, in 2023, nine cases of malaria were detected in Florida, Maryland, and Texas, the first such cases in two decades. Cases of dengue and chikungunya had also been detected in the southern United States and France.

Risk Factors

Causal risk factors for mosquito-borne infectious diseases include mosquito bites; living in habitats with stagnant water, which are ideal environments for mosquito breeding; international travel to mosquito-endemic areas; habitation in areas with large mosquito populations; occupations involving exposure to woodlands and forests; having a suppressed immune system; and receiving a blood transfusion with infected blood product.

Symptoms

The common symptoms associated with mosquito-borne infectious diseases are high fever, back and joint pain, rash, eye pain, chills, headache, malaise, muscle weakness, flu-like symptoms, hypotension, and fatigue. There are additional physiological and clinical symptoms specific to each disease type. The additional symptoms of the various forms of encephalitis include brain inflammation, brain damage, and coma; death can follow.

Yellow fever is associated with additional symptoms, including “furry” tongue, irritability, slowed pulse, decreased urine volume, bloodshot eyes, constipation, facial flushing and proteinuria, and hepatic coagulopathy that produces hemorrhagic symptoms such as black vomit, nose bleeds, gum bleeds, and bruising. In the late stages of yellow fever, infected persons develop hypotension, shock, metabolic acidosis, acute tubular necrosis, myocardial dysfunction, cardiac arrhythmia, seizure, and coma. Recovery from yellow fever generally confers long-lasting immunity against subsequent infection.

Malaria is characterized by high fever, shaking, and chills in its early stages. If untreated, these symptoms are followed by nausea and vomiting, high fever, dizziness, delirium, headache, and pain, and symptoms such as splenomegaly, decreased body temperature, hepatomegaly, sweating, fever and chills, fatigue, shortness of breath, anemia, pale skin, and extreme exhaustion in its final stages.

Symptoms of dengue fever also include swollen lymph nodes, rash, pain behind the eyes, decreased heart rate, severe muscle pain, severe weakness, headaches, and enlarged lymph nodes (in children).

Symptoms of Chikungunya fever usually manifest after an incubation period of three to seven days after a mosquito bite. Symptoms include sudden fever, joint pain with or without swelling, chills, headache, nausea, vomiting, lower back pain, and rash.

Though symptoms of Zika virus are typically relatively mild, including fever, rash, and joint pain, and generally last up to a week at most, the virus has also been linked to Guillain-Barré syndrome, a nervous system disorder that causes temporary paralysis.

Screening and Diagnosis

The presence of a mosquito-transmitted disease is often diagnosed after observing an associated fever that is common with all infectious disease types. Biological fluids such as saliva and urine are screened for the presence of a virus or parasites.

Symptomatic diagnosis is based on the history of subjective fever and on a rectal temperature and a splenomegaly. These methods are often used in parts of the world with limited laboratory facilities. Microscopic examination of blood films provide the most reliable diagnosis of the presence of the virus. Other tests include antigen tests (using venous blood) and molecular methods-based tests.

Treatment and Therapy

Treatment for a mosquito-borne infectious disease is designed to eliminate the viral and plasmodium parasite loads that cause the illness. For malaria, treatment and therapy involves one or more antimalarial prophylactic drugs, such as mefloquine, doxycycline, and the combination atovaquone and proguanil hydrochloride. In malaria-endemic areas, the drug treatment regime of choice is artemisinin combination therapy.

Yellow fever is treated using vaccines. The vaccine is an attenuated live-virus vaccine that has been used for several decades. A single dose confers immunity for ten years or more.

Antibiotics and antiviral medications are effective upon diagnosis of viral encephalitis. Corticosteroids and medications for fever are administered to reduce brain swelling and inflammation and to decrease the fever or treat headaches. Additional supportive treatment includes drinking extra fluids for hydration and bed rest.

Prevention and Outcomes

Mosquito control and eradication with chemical and biological agents is the key, first-line defense against mosquitoes and mosquito-borne infectious diseases. Insecticides (such as Malathion and permethrin) and repellants (such as NN-diethyl metatoluamide, or DEET) are common chemical control agents. Some garden plants, including lemon thyme and rosemary, are known mosquito repellants. Biological agents, such as predatory fish (mosquitofish) and dragonflies, feed on mosquito larva and adult mosquitoes. Other preventive measures include behavioral and physical modifications, such as staying indoors at night and using insecticide-treated sleeping nets and permethrin-treated clothing to cover as much of the body as possible.

Yellow fever vaccine, a live attenuated vaccine, is commonly used as a preventive agent. A single dose of the vaccine confers immunity for up to ten years in adults.

Bibliography

Goddard, Jerome. Infectious Diseases and Arthropods. Totowa: Humana, 2008. Print.

Hackett, Don Ward. "Celebrating the World's Deadliest Animal." Precision Vaccinations, 20 Aug. 2018, www.precisionvaccinations.com/zika-dengue-chikungunya-and-yellow-fever-viruses-are-all-transmitted-humans-mosquitoes. Accessed 2 May 2024.

Marquardt, William C., ed. Biology of Disease Vectors. 2nd ed. New York: Academic, 2005. Print.

McNeil, Donald, Jr., Catherine Saint Louis, and Nicholas St. Fleur. "Short Answers to Hard Questions about Zika Virus." New York Times, 29 July 2016. www.nytimes.com/interactive/2016/health/what-is-zika-virus.html. Accessed 2 May 2024.

Nolen, Stephanie. “Mosquitoes Are a Growing Public Health Threat, Reversing Years of Progress.” The New York Times, 29 Sept. 2023, www.nytimes.com/2023/09/29/health/mosquitoes-malaria-disease-climate-change.html. Accessed 2 May 2024.

Rosenberg, Ronald, et al. "Vital Signs: Trends in Reported Vectorborne Disease Cases—United States and Territories, 2004–2016." Morbidity and Mortality Weekly Report, vol. 6, no. 17, 2018, pp. 496–501. Centers for Disease Control and Prevention, www.cdc.gov/mmwr/volumes/67/wr/mm6717e1.htm?s‗cid=mm6717e1. Accessed 5 Mar. 2023.

Tolle, Michael A. “Mosquito-Borne Diseases.” Current Problems in Pediatric and Adolescent Health Care, vol. 39, no. 4, 2009, pp. 97–140. doi.org/10.1016/j.cppeds.2009.01.001. Accessed 5 Mar. 2023.

"Zika Virus." World Health Organization. 8 Dec. 2022, www.who.int/news-room/fact-sheets/detail/zika-virus. 5 Mar. 2023.