Why Control Mosquitoes?
Throughout history, no insect has been a more significant contributor to human disease, discomfort and death than the mosquito. Mosquitoes are a serious threat to human health and comfort ranging from the transmission of arboviruses, like West Nile virus, to the annoying biting of the floodwater mosquitoes.
Mosquito control agencies provide early detection of disease threats to the public, in addition to reduction of the mosquito population. Integrated mosquito management practices currently employed by organized control agencies and endorsed by the CDC [Centers for Disease Control] and EPA [Environmental Protection Agency] are comprehensive and specifically tailored to safely confront each life stage of the mosquito. Larval control through source reduction and environmentally friendly EPA-registered larvicides is prudent. When source reduction or larval control measures are deficient or when disease is imminent, the EPA and CDC have emphasized in a published joint statement the need for considered applications of adulticides by certified applicators trained in handling these specialized products.
Mosquitoes are already here and we are continually finding new imported diseases associated with them. We must be primed to respond to their threat. Disease prevention through preparedness has to be our agency’s primary focus.
Mosquito Breeding Life Cycle
Over 40 different species of mosquito are found in Illinois. While each species has its own developmental habitat, all mosquitoes need water in which to pass their early life stages. Some mosquitoes lay their eggs directly on the surface of water. Other mosquitoes lay their eggs in areas that will eventually fill with water.
Once eggs come in contact with water, they hatch into larvae within several days. Mosquito larvae undergo four stages of growth and development called instars. Larvae feed on organic material and microorganisms in the water and return to the surface of the water to breathe. Larval development may be as rapid as 5-7 days in warm weather. After the larval stages are complete the larvae shed their skins and emerge as comma-shaped pupae. Pupae are very active and dive vigorously if disturbed. Pupae do not feed while they undergo metamorphosis to the adult stage. The adult mosquito emerges from the pupal skin and rests on the water’s surface until it dries.
Both male and female adult mosquitoes feed on plant nectar, but only the female bites to get the blood needed for the development of eggs. While some kinds of mosquitoes can live several months, the main nuisance mosquitoes we have in this area usually survive four weeks or less. Not all mosquitoes can carry disease, nor are all mosquitoes vicious biters. Some kinds of mosquito never bite humans. Mosquitoes also vary in the distances they travel from the water they developed in. While some species will not stray more than a block or two from their source, other species’ flight range can be 20 miles or more. The great diversity between different species of mosquitoes makes their control more complex, requiring a variety of approaches and methods.
- There are four stages in the life of a mosquito: egg, larva, pupa and adult
- Mosquito eggs are laid so that they hatch in water
- A larva emerges from the egg and feeds and grows in the water for about a week
- The larva then turns into a pupa, which is also found in water, but does not feed
- After about two days, the adult mosquito, which has developed inside the pupa, will emerge into the familiar flying form
- There are both male and female mosquitoes, but only the female bites because she needs the blood to provide nutrients for her eggs
- Female mosquitoes will lay 200-300 eggs each time they have a blood meal. She may lay eggs three or four times during the month that she is alive
Mosquitoes Transmit Diseases in Illinois
The most immediate danger from mosquitoes in central Illinois is West Nile Virus.
It’s not just an inconvenience, it can be debilitating or fatal.
West Nile Virus (WNV) was first discovered in the United States in New York City during 1999. It has since spread south and west with confirmation in Central Illinois during 2001. In 2002, an epidemic of WNV in Illinois resulted in 884 human cases of the disease with 66 deaths. WNV is caused by a virus that cycle between mosquitoes and birds, with occasional spill-over into humans and other mammals. The virus can have a high mortality in certain bird species while having little effect on other bird species. Crows and Blue Jays are extremely susceptible to the virus, as are eagles, hawks, and other raptors. Humans are considered a dead-end host for the virus, which cannot be transmitted from person to person.
Mosquitoes from the genus Culex, in particular the species Culex pipiens, are the vector of WNV from birds to birds and birds to humans. These mosquitoes deposit their eggs in raft clusters of 50-400 eggs directly on the water’s surface. They are most abundant in periods lacking rainfall, when areas of stagnant water prevail. They are well adapted to many habitats including curbside storm water catch basins, off-road storm water catch basins, discarded tires, buckets & other artificial containers, rain gutters, bird baths, unused swimming pools, ditches, ponds, etc. Anywhere that water can stand for more than a week can become a potential breeding source for Culex mosquitoes. The adult mosquito is a non-aggressive biter with feeding primarily confined to evening or night hours. The adult mosquito seldom travels more than 1/2 mile from its source. The Culex mosquitoes can have many over-lapping generations each season, and over-winter as adults.
The surveillance and control of Culex mosquitoes is very crucial in the efforts to reduce as much as possible the occurrence of WNV.
Signs & Symptoms
If you are bitten by a mosquito, you probably don’t need to see a doctor. Most people who are infected with West Nile virus will not have any symptoms and some will develop a mild flu-like illness. However, the virus can also cause encephalitis or meningitis which can be serious health threats. Symptoms generally appear three to fifteen days after exposure.
Serious Symptoms in a few people: About 1 in 150 people infected with WNV will develop severe illness. This may include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks and neurological effects may be permanent.
Milder Symptoms in some people: Up to 20% of people infected with WNV develop West Nile fever, symptoms of which may include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms may last a few days to several weeks.
No Symptoms in most people: Approximately 80% of people who are infected with WNV will not show any symptoms at all.
Who is at Risk?
While everyone is equally susceptible to WNV, people over the age of 50 are at greatest risk for contracting more severe forms of the disease. No vaccine for people exists at this time and only supportive treatment is available for symptomatic cases.
Other Mosquito related viruses and diseases
St. Louis Encephalitis (SLE)
As with West Nile Virus (WNV), SLE is caused by a virus that cycles between mosquitoes and birds, with occasional spill-over into humans and other mammals. Humans are considered a dead-end host for the virus, which cannot be transmitted from person to person. Unlike WNV, the virus has little effect on birds that are infected. SLE affects the central nervous system causing an inflammation of the brain. Most human infections are subclinical (mild and not diagnosed) exhibiting flu-like symptoms. More severe cases can exhibit high fever, nausea, headache, personality changes, paralysis, and in 2-20% of the severe cases, death. The elderly are most susceptible to SLE. An epidemic of SLE within Illinois occurred in 1975, with a few cases reported in the 1980’s and 1990’s. The mosquitoes that transmit (vector) SLE are the same as WNV, with description under that
Other Encephalitis Viruses
LaCrosse Encephalitis (LAC) is caused by a virus that cycles between mosquitoes and small mammals as chipmunks and ground squirrels. Humans are considered a dead-end host. LAC is endemic in Illinois with 5-15 cases per year, primarily occurring in local foci in central and northwestern regions of the state. The majority of LAC cases are mild and subclinical. Less than 1% fatality occurs in cases severe enough to be diagnosed. Children under the age of 16 are most susceptible to this virus. As with other mosquito-borne encephalitis, LAC cannot be transmitted from person to person. The eastern tree hole mosquito, Ochlerotatus triseriatus (formerly Aedes triseriatus) is the vector of this disease. This mosquito normally develops in water filled rot cavities in trees (tree holes), but has adapted well to many man-made habitats as discarded tires, buckets, and other artificial containers. The Ochlerotatus triseriatus adult mosquito lays its eggs singly on the inside wall of the tree hole or artificial container just above the waterline. The adult mosquito is an aggressive biter with feeding all day long, and generally stays within the vicinity of its source. The Ochlerotatus triseriatus has one generation each season, and over-winters in the egg stage.
Western Equine Encephalitis (WEE) is similar to SLE, cycling between birds and mosquitoes with rare spill-over to the human population. WEE primarily affects horses, and is typically found west of the Mississippi River, but has also been found in Illinois. Human cases severe enough to be diagnosed can have a 2-5% fatality. The primary vector of WEE is Culex tarsalis, and possibly Culex pipiens. The Culex tarsalis, like other Culex, lay eggs in rafts directly on the water’s surface. The mosquito can be found in sunlit sources with high organic content as ditches and artificial containers. The mosquito seldom travels more than 1 mile from its source,
however has been known to travel up to 10 miles. This species has continuous generations each season, and over-winters as adults.
Eastern Equine Encephalitis (EEE) is another virus that cycles between birds and mosquitoes with horses and humans as dead-end hosts. Human infections of EEE can range from mild to severe, with 50-75% of diagnosed severe cases resulting in death. Fortunately, human cases of EEE are rare with only 150 cases throughout the United States from 1964-1998. No human cases of EEE have been reported in Illinois, although the virus has been found regularly in bird populations. The primary vector of EEE is Coquillettidia perturbans, a mosquito found in wetlands. They lay their eggs in rafts directly on the water’s surface. The mosquito is an aggressive biter, strong flier, and feeds during the evening or night. This species usually has one generation each season, and over-winters as larvae attached to the stems of cattails or similar aquatic plants.
Other Mosquito-Borne Diseases
Malaria is a disease caused by a protozoan parasite transmitted from person to person via the mosquito. At one time, malaria was prevalent in Illinois, with concentration in the southern part of the state. Mosquito control efforts in the 1920’s have eliminated the risk of malaria in Illinois, although the mosquito which carries the disease, Anopheles quadrimaculatus, is still found in the area.
Dengue and Yellow Fever are both diseases caused by viruses that are transmitted by mosquitoes. They are common in the Caribbean, South America, Asia, and Africa, but are unlikely to occur in Illinois.
Canine (Dog) Heartworm is a disease affecting dogs that is transmitted by mosquitoes. It is caused by a roundworm, Dirofilaria imitus. The Culex pipiens and possibly Aedes vexans are the vectors in our area. The disease is ongoing and best controlled by prevention. Veterinarians typically prescribe drugs to prevent the roundworm larvae development in dogs.
HIV in Mosquitoes
Mosquito-borne viruses must be able to multiply and infect the salivary glands of the mosquito. This does not occur when HIV is ingested by mosquitoes. Studies have concluded that mosquitoes are not a factor in HIV transmission.