Fragile but Potentially Deadly: Mosquitoes Transmit Illnesses to Millions Every Year by H

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Infectious Diseases, ID Snapshot Fragile but potentially deadly: Mosquitoes transmit illnesses to millions every year by H. Cody Meissner M.D., FAAP Illnesses due to viruses, bacteria or parasites can be transmitted by mosquitoes, oftentimes without affecting the mosquito. Estimates suggest that as many as 700 million people annually are infected by mosquito-borne illnesses around the globe, and nearly 1 million people die from the illness. Symptoms range from mild influenza-like illness to encephalitis, coma and death.

Viruses that may be transmitted by the bite of a mosquito include the agents of dengue, Zika, yellow fever, chikungunya, Japanese encephalitis, West Nile, La Crosse, Rift Valley fever, Eastern and Western equine encephalitis and St. Louis encephalitis.

Which of the following statements is false?

a) Human infection with a mosquito-borne virus occurs when a male mosquito injects saliva while ingesting a blood meal.

b) Viruses carried by a mosquito vector are known as arboviruses.

c) The connection between insect vectors and disease was recognized in 1881 when Cuban physician Carlos Finlay proposed that yellow fever may be transmitted by mosquitoes rather than by human contact.

d) Person-to-person transmission of mosquito-borne viruses is rare but can occur.

e) The word flavivirus is derived from flavus (meaning yellow in Latin) based on the jaundice that occurs in patients with yellow fever. Flaviviruses include the agents of yellow fever, West Nile, dengue, Zika and Japanese encephalitis.

Answer: a is false

Despite a fragile appearance, mosquitoes have adapted well to the environment in order to ensure survival. Mosquitoes pass through four stages in a lifecycle: egg, larva, pupa and adult. After the mosquito lays her eggs in stagnant water, larvae derive nourishment from other organisms in the water. A snorkel-like breathing tube enables larvae to survive below the water surface and to survive in small volumes of water because they do not require oxygen dissolved in the water.

An Anopheles stephensi female is gorged with blood and beginning to pass unwanted liquid fractions of the blood to make room in her gut for more of the solid nutrients. Small, temporary collections of standing water (such as bird baths, tarpaulin folds, discarded cups or old tires) serve as effective mosquito nurseries. After emerging from the water, the adult mosquito is able to beat its wings 500 times a minute, making it extremely maneuverable.

A mosquito has a variety of ways to find prey, including chemical, visual and heat sensors. Both male and female mosquitoes feed on nectar and plant juices to provide a source of energy in the form of sugars as well as nutrients such as lipids. In many species, the mouthparts of the females are adapted for piercing the skin of an animal host and sucking blood. The female needs to obtain protein and other nutrients from blood before she can produce eggs.

Female mosquitoes find a suitable host by detecting organic substances such as carbon dioxide and octenol (a metabolic product of linoleic acid) that are contained in human breath and sweat. The proportion of carbon dioxide, octenol and other compounds make body odors from some people preferable to that of others. A large part of the mosquito's olfactory system is devoted to sniffing out blood sources. Of the dozens of types of odor receptors on its antennae, many detect chemicals found in perspiration.

The flexible hypopharynx penetrates the integument of the host using sharp teeth and is moved until a capillary is located. Once a capillary is penetrated, blood is pumped out and saliva is injected into the host prior to and during the meal. Mosquito saliva contains a number of factors, including anticoagulants and platelet inhibitors that prevent the female's proboscis from becoming occluded by blood clots. In addition, saliva may contain viruses or other pathogens that gain access to the blood of the host.

Most mosquito species feed at dawn or dusk. During the heat of the day, these mosquitoes rest. Some mosquitoes such as the Asian tiger mosquito feed during the daytime.

Transfusion of blood and other blood products as well as organ transplantation can transmit arboviruses if the virus is present in the donor's blood or organs. Blood and organs often are screened for viruses before being transfused or transplanted. At times, vertical transmission or mother-to-child transmission of arboviruses has occurred in infected pregnant and breastfeeding women.

Aedes aegypti (yellow fever mosquito) is more likely to spread Zika, dengue and chikungunya viruses than Aedes albopictus (Asian tiger mosquito) because A. aegypti feed on multiple people to complete a blood meal and therefore are more likely to feed on a person who is viremic with Zika virus at the time of the bite.

The Centers for Disease Control and Prevention surveillance system (ArboNET) tracks insect-borne diseases. Studies suggest A. albopictus and A. aegypti are concentrated in the Southeastern parts of the United States, but how far north or west these disease-bearing mosquitos may extend is uncertain. Epidemics of yellow fever have occurred as far north as Boston.

Dr. Meissner is professor of pediatrics at Floating Hospital for Children, Tufts Medical Center. He also is an ex officio member of the AAP Committee on Infectious Diseases and associate editor of the AAP Visual Red Book.