Mosquito Diseases

Title: Mosquito Ecology
Date: 2012
Source: Connecticut Mosquito Management Program

Abstract: Mosquitoes are a diverse group of insects belonging to the fly order, Diptera. There are over 2500 species of mosquitoes which can be found on every continent except Antarctica. While North America is home to about 200 species of mosquitoes, in Connecticut there are currently 49 known species which can be found in a variety of habitats. Of this number, approximately one-half are considered pests to humans or livestock and can transmit organisms causing disease.  (Identification Guide to the Mosquitoes of Connecticut)

Only the female mosquito bites because she requires a source of protein to produce her eggs. Once she has digested the blood she can lay over 250 eggs per brood and depending on the species, she can seek subsequent blood meals several times per season. She withdraws blood using specialized piercing and sucking mouthparts and is attracted to hosts by body heat, exhaled carbon dioxide and other complex physiological processes. Every human has their own unique body chemistry and may be more or less attractive to mosquitoes. Also, not all mosquitoes bite humans and, in fact, are fairly specific in their feeding preference. For example, some bite only birds or amphibians, such as toads, while others bite mammals, including humans. Males, in addition to females, feed on flower nectar and plant juices for energy.

The mosquito's life cycle includes egg, larva, pupa and adult stages. This type of development is referred to as "complete metamorphosis" by biologists and is similar to the development of butterflies. Mosquitoes, however, are aquatic, with most of their development occurring in or near water. Depending on the species, mosquito eggs are deposited either singly or clustered in egg “rafts” on the water surface, or individually on moist soil, the latter hatching when flooded by rain water or lunar tidal events. Eggs deposited on moist surfaces can withstand drying and can remain viable for several years. Certain species, such as Connecticut's saltmarsh mosquitoes, are very prolific breeders and can lay from 1,000 to 10,000 eggs per square foot on the moist mud found in a saltmarsh habitat. These moist depressions are usually found in the higher elevations of the saltmarsh which are dominated by salt hay grasses.

Once the eggs hatch, mosquitoes develop through four larval stages (instars) as they feed and grow larger. They then progress to a non-feeding pupal stage and finally emerge as adults. The larvae and pupae breathe oxygen at the water's surface by means of specialized, snorkel-like air tubes. These air tubes are called "siphons" in the larvae and "trumpets" in the pupae. One species of mosquito obtains its oxygen by piercing the fleshy underwater roots and stems of aquatic plants, such as cattails, and actually breathing through the plant. Mosquito larvae actively feed by filtering bacteria and organic matter in the water. The development from egg to adult depends on the water temperature and can take as little as seven to 10 days in warmer weather or up to two months in cool, spring weather.

Many Mosquito Species (genus Aedes, Ochlerotatus and Psorophora) in Connecticut deposit eggs in the fall which will not hatch until the following spring. With these mosquitoes, larval development usually begins in early March and egg deposition can extend into October, with adults most abundant from May through September. A few species (genus Culiseta and Coquillettidia) will overwinter in the larval stage, while adults of other species (genus Culex and Anopheles) will enter into buildings or sheltered areas and remain dormant until warmer weather arrives. (This is why adult mosquitoes are occasionally seen during a midwinter warm spell.) In Connecticut, some species of mosquitoes can produce up to 4 broods per year. Most have an average adult life span of about two weeks, excluding those which overwinter as an adult.

Mosquitoes can develop in a wide variety of aquatic habitats, with most species requiring a specific habitat. Stagnant water isolated from predators is ideal mosquito habitat. Examples of preferred mosquito habitats include saltmarshes, freshwater swamps, woodland pools, roadside ditches, catch basins and backyard habitats, such as clogged rain gutters, discarded tires or un-maintained bird baths. Some species, such as Culex pipiens remain close to their development sites and have a limited flight range of less than one-quarter mile. Other species, such as the common saltmarsh mosquito (Ochlerotatus sollicitans), can fly up to 40 miles from their saltmarsh habitats in search of an animal to feed on (Connecticut Mosquito Management Program, 2012).

Title: Mosquito Transmitted Diseases
Date: 2012
Connecticut Mosquito Management Program

Abstract: It has been known for over a century that certain mosquitoes can harbor and transmit the microorganisms that cause diseases. These disease pathogens are often viruses which are contracted or “picked up” by the mosquito when it bites (feeds on) an infected host including domestic or wild animals or people. The virus develops and multiplies inside the mosquito. The infected mosquito then transmits the virus through its saliva when it bites another, uninfected host. By passing on the virus, the mosquito acts as a “vector” of the disease.

Yellow fever, WNV and EEE are some examples of mosquito-borne diseases or “arboviruses” (arthropod-borne viruses). Although these viruses have probably been around for centuries, only within the past 100 years has it been determined that they were spread by mosquitoes. Mainly because of organized mosquito control efforts, better sanitation and vaccines, many of these diseases are kept in check and not usually thought of in today’s modern society.

However, the diseases are still prevalent in some parts of the world. For example, malaria, another mosquito-borne disease, kills 2.7 million people each year and infects 300 to 500 million others. The vast majority of the mosquito-borne diseases occur in Africa and the tropics of southeast Asia and South America.

There are 9 arboviruses that have been isolated from mosquitoes collected in Connecticut among which 6 are known to cause human disease.

West Nile virus can cause infection in animals and people. The virus is similar to the St. Louis encephalitis virus and produces similar symptoms. Like EEE, WNV is spread to humans by the bite of infected mosquitoes. A mosquito is infected when it bites a bird that is carrying the virus. The virus is not spread from person to person or directly from birds to people under normal circumstances.

Anyone can become infected with WNV. Most people who are infected with the virus will have no symptoms (80%) or may experience mild illness (20%), before fully recovering. However, a small portion of people (1%), particularly the elderly or persons with compromised immune systems, become seriously ill when infected. In some individuals, the virus affects the central nervous system - the brain and spinal cord. At its most serious, it can cause permanent neurological damage and can be fatal. Onset of symptoms generally occurs 3 to 14 days following the bite of an infected mosquito.  Symptoms may range from a slight fever, headache, body aches, rash, nausea and swollen lymph nodes to the rapid onset of severe headache, high fever, stiff neck, disorientation, muscle weakness, coma and, rarely, death. There is no vaccine, or cure for WNV or St. Louis encephalitis though treatment can reduce the severity of the symptoms.

The chance of getting sick from the WNV is very small. In areas where mosquitoes carry the virus, only about one out of 500 mosquitoes are infected. Furthermore, if bitten by an infected mosquito, the chance of a person developing the illness are roughly one in 300. Therefore, the chance of being bitten by an infected mosquito and developing disease symptoms from that bite is very small. This does not mean, however, that people should be complacent. The best way for people to protect themselves in areas where mosquito-borne viruses are present is to take personal protective measures to prevent mosquito bites.

WNV has been isolated from 18 different species of mosquitoes in Connecticut, but 5 species have been implicated as the most important vectors: Culex pipiensCulex restuans, Culex salinarius, Culiseta melanura and Aedes vexansCx. pipiens, Cx. restuans and Cs. melanura principally feed on birds and are largely involved in perpetuating the virus among wild bird populations in nature, while Cx. salinarius and Ae. vexans readily feed on mammals including humans and are believed to be involved in transmission of WNV to horses and humans. Cx. pipiens, a peridomestic species that develops in water with high organic content and is particularly abundant in urban centers, is the most frequently infected mosquito species in the state.

Although WNV has been detected throughout the State of Connecticut, the large majority of infected mosquitoes and human cases have occurred in densely populated urban and suburban communities in lower Fairfield and New Haven Counties and the greater Hartford area which have been identified as regions of the State where the risk of becoming infected is greatest.

The WNV season in Connecticut extends from July through early October but the greatest risk of human infection occurs from August through September.

Eastern equine encephalitis is a rare but serious disease caused by a virus that is spread by adult mosquitoes. On average there are 5 cases each year in the United States. There has never been a documented human case of EEE in Connecticut, but the virus is found in birds and bird-biting mosquitoes that live near wetland habitats along the eastern seaboard from New England to Florida. In some years, high numbers of birds get infected favoring spread to the types of mosquitoes that bite both mammals and birds.  These mosquitoes can then infect people and horses.  EEE is not spread by people and horses with the disease.  The risk of getting EEE is highest from late July through September.

The virus responsible for EEE attacks the central nervous system of its host. Horses are particularly susceptible to the infection and mortality rates approach 100%. Onset is abrupt and horse cases are almost always fatal. Signs of the disease in horses include unsteadiness, erratic behavior, loss of coordination and seizures. There is no effective treatment and death can occur within 48 to 72 hours of the horse's first indications of illness. Horses can and should be inoculated against this disease especially in areas where EEE is known to circulate.

In humans, symptoms of EEE appear from three to 10 days after being bitten by an infected mosquito. Some infected people may not develop illness.  For those who become ill, the clinical symptoms may include high fever (103 to 106 degrees F), stiff neck, headache and lack of energy. Inflammation of the brain, encephalitis, is the most dangerous. The disease gets worse quickly and some patients go into a coma within a week. Once symptoms develop, treatment for EEE is supportive and aimed at reducing the severity of the symptoms.  As many as one-third of people who get the disease die from it and of those who survive approximately one-half will have permanent neurologic damage. Presently, there is no available vaccine for use in humans (Connecticut Mosquito Management Program, 2012).

Title: West Nile Virus Found In More Maine Mosquitoes
Date: August 23, 2012
USA Today

Abstract: The Maine Center for Disease Control and Prevention says 
West Nile virus has been found in a second mosquito population, this time in Cumberland County.

Mosquitoes are sorted Aug. 16 at a Dallas County lab. There has been a significant increase in the number of U.S. West Nile cases.

Mosquitoes are sorted Aug. 16 at a Dallas County lab. There has been a significant increase in the number of U.S. West Nile cases.

Officials said Thursday it's the second mosquito pool from Maine to test positive this year. The first, confirmed last week, was found in York County.

There has never been a human case of West Nile in Maine.

State epidemiologist Dr. Stephen Sears said the state continues to investigate two separate cases for the possible human exposure to West Nile and EEE. Testing is being conducted by a federal lab.

Both viruses are transmitted by infected mosquitoes. EEE has historically been found in late-summer in Maine, and West Nile has been found in dozens of mosquito pools in neighboring New Hampshire (USA Today, 2012)