Mosquitoes is a bugs can transmit serious, potentially fatal organisms causing such diseases as encephalitis, dengue fever, malaria and yellow fever. Until a first frost, and generally from April through October, they are a threat to people and can spread disease organisms to livestock, pets and other animals. Beyond transmitting debilitating diseases, the delicate and piercing mosquito attacks and annoys humans, pets and farm animals.
Associated with a parasite that causes malaria, a rare disease in North America, Anopheles mosquitoes are present in the tropics and subtropics.
The genus Aedes conveys dengue fever, one of the most rapidly expanding diseases in the world with a small but significant risk in the U.S., and yellow fever, which occurs in Africa and South America.
Mosquitoes of the genus Culex include the only ordinary house mosquito and species associated with St. Louis Encephalitis and dog heartworm.
Adult mosquitoes are small, fragile insects with slender bodies; one pair of narrow wings and three pairs of long slender legs. They vary in length from 3/16 to 1/2 inch. Mosquitoes have an elongated “beak” or piercing proboscis with which the female bites and feeds on blood. Usually a blood meal is required before producing each batch of eggs. Male mosquitoes feed upon plant nectar. Eggs of most mosquitoes hatch in two to three days. Larvae, which hatch from eggs, feed on organic matter in water. Larvae mature and change in seven to ten days. Pupae, shaped like commas, usually transform into adults, two to three days later. In another one to two days, females are ready to bite.
The time required from egg to adult may be as short as 10 days or as long as months, depending on environmental conditions. Cool weather delays development. Some mosquitoes have one generation each year, while others may have four or more generations, building up to large populations by later summer.
Human Health Impact
Global commerce, travel, population and climate changes are making parasitic diseases-including mosquito-borne diseases-an emerging or re-emerging public health threat, according to the national Centers for Disease Control and Prevention. CDC believes that available reports seriously underestimate the true incidence of mosquito-borne diseases because of under funding of state vector control programs and failure to diagnose some diseases. The following is what are reported about mosquito-caused illnesses.
West Nile Virus – A flavivirus that is spread by the bit of an infected mosquito and can affect people, horses, many types of birds and some other animals. The virus has been present for years in Africa, West Asia, and the Middle East. It was first detected in the United States in 1999, but has been spreading rapidly since that time. Experts believe that West Nile Virus is now established as a seasonal epidemic in North America that flares up in summer and continues into the fall.
West Nile Virus is a potentially serious, even fatal, illness. Most people who become infected with the disease will have only mild symptoms or none at all. However, on rare occasions, West Nile virus infection can result in a severe and sometimes fatal illness known as West Nile encephalitis (an inflammation of the brain). The risk of severe disease is higher for persons 50 years of age and older. There is no evidence to suggest that West Nile virus can spread from person to person or from animal to person.
For more information about West Nile Virus, please visit Center for Disease Control (West Nile Virus).
Encephalitis – A virus transmitted to humans by an infected mosquito, encephalitis cannot be treated with antibiotics. It also cannot be transmitted from human to human. The word encephalitis means inflammation of the brain. A general name for this mosquito-transmitted virus is an arbovirus, for ar(thropod) + bo(rne) + virus. Arthropoda is the phylum name for invertebrate organisms that include insects, crustaceans, arachnids and myriapods.
A potentially serious illness of the brain and central nervous system, encephalitis is characterized by seizures, coma, paralysis and permanent neurological damage. Sine 1964, between 150 and 3,000 cases have been reported annually in the U.S., including the St. Louis, LaCrosse, Eastern Equine and Western Equine Encephalitis.
St. Louis Encephalitis – The virus causing St. Louis Encephalitis is the most common mosquito-borne human pathogens in the U.S. Occurring in every state, St. Louis Encephalitis is found particularly in Florida, the Gulf Coast Region, Ohio, the Mississippi Valley, and Western states. Outbreaks are most likely from mid-summer through the early fall. Since 1964 the CDC has confirmed an average of 193 cases yearly. In 1990, 226 people were infected with the virus and 11 died in Florida. Fearing an outbreak in 1997, state health officials issued an encephalitis alert, prompting Disney World and other tourist parks to limit night activity.
St. Louis Encephalitis is transmitted among birds primarily by Culex pipiens mosquitoes. Only being bitten by an infected mosquito infects humans. There is no person-to-person transmission. Symptoms include fever, headache, nausea, vomiting, convulsions and lethargy. The state of symptoms may be abrupt onset of fever, nausea and vomiting with severe headaches. These symptoms develop within five to seven days after a bite by an infected mosquito. In its severe form, it can cause seizures, coma and paralysis. The mortality rate is 10% with most fatalities among people 60 and older. Treatment is administered for symptoms as they occur.
LaCrosse Encephalitis – Occurring primarily in upper Midwestern states of Minnesota, Wisconsin, Iowa, Illinois, Indiana and Ohio, LaCrosse Encephalitis also has been reported in the mid South Atlantic states of West Virginia, Virginia, and North Carolina and in the Southeastern states of Alabama and Mississippi. Some 75 cases have been reported annually in the U.S. since 1964.
Most cases occur among children and teens under 16. Symptoms are fever, headache, nausea and vomiting, convulsions and lethargy, with severe cases including seizures, coma and paralysis. Some 1% of cases result in death. Treatment is administered for symptoms as they occur.
Eastern Equine Encephalitis – A viral infection maintained in nature by a bird-mosquito cycle similar to St. Louis Encephalitis, Eastern Equine Encephalitis is found in freshwater swamp areas of coastal states of the Gulf of Mexico and the Atlantic Ocean, and in some inland mid-western locations. CDC has reported an annual average of five cases since 1964. Mosquitoes associated with the disease include Culex quiquefasciatus, Aedes sollicitans and Aedes vexans.
Eastern Equine Encephalitis can infect people of any age, but young children and infants suffer most from the disease. Symptoms begin 4-10 days after the bite of an infected mosquito, with sudden onset of fever, general muscle pain and increasingly severe headache. Serious cases have seizures and coma. About 35% of those infected die, usually within three to five days from the onset of symptoms. Children and infants who survive are frequently afflicted with varying degrees of mental retardation and paralysis. Up to half of survivors will suffer permanent brain damage, many requiring permanent institutional care. Treatment is administered for symptoms as they occur.
Western Equine Encephalitis – Found in every state west of the Mississippi River, in southwestern Canada and northwestern Mexico, Western Equine Encephalitis may be increasing because of expanded irrigated agriculture in the North Platte River Valley, which has fostered habitats and conditions favoring mosquitoes that transmit disease. On average, 19 cases have been reported annually by the CDC since 1964.
Culex tarsalis is the most important vector in the West. East of the Mississippi, Culex quinquefasciatus is the suspected vector. Birds are the major host. Causing death in about 3% of cases, epidemics are cyclical, roughly every 10 years. The last, of 41 cases, was in 1987. Symptoms include infections. Severe cases exhibit fever, headache, nausea, vomiting, anorexia and malaise, followed by weakness and altered mental functioning. Treatment is administered for symptoms as they occur.
California Encephalitis – Most cases are reported in the Midwestern states of Ohio, Indiana and Wisconsin. Rather than birds, the natural hosts of California Encephalitis are small animals-rabbits, hares, and squirrels. Vectors are mostly woodland mosquitoes of the Aedes genus.
Venezuelan Equine Encephalitis – Both humans and horses are susceptible to Venezuelan Equine Encephalitis, but unlike the other arboviral encephalitis, victims develop high levels of the virus in the blood. This allows mosquitoes to transmit disease to humans or equines without the intermediate host, birds. Vectors include species of Aedes, Culex and Anopheles.
Dengue Fever – One of the most rapidly expanding diseases in the world, with tens of millions of cases annually in the tropics, dengue fever is a small but significant risk in the United States because the two transmitting mosquito species, Aedes aegypti and Aedes albopictus, are found here. The CDC reports 100 to 200 cases in the United States each year. In 1996, 181 cases were reported and 45 diagnosed, according to Texas State health officials. Heavy rains that year brought out swarms of mosquitoes at the Mexican border. Arizona State health officials are preparing for dengue after finding the Aedes aegypti in Tucson.
Sometimes called “breakbone fever,” dengue fever’s symptoms include high fever, headaches, bodyaches, sore throat and a rash appearing three to four days after symptoms start. The rash may not be visible in persons with darker skin tones. Symptoms lasting five to seven days may be followed by several weeks of fatigue and weakness. More severe cases, in addition to fever and headaches, may show coughing and abdominal pain. Without treatment, fatalities may reach 15%. Treatment is administered for symptoms as they occur.
Malaria – Caused by a parasite transmitted by infected Anopheles mosquitoes present in tropical and subtropical countries, malaria annually affects 300-500 million people and kills 1.5 to 2.5 million, mostly children, according to the World Health Organization. Caused by any of the Plasmodium species of protozoa, it remains the most important human disease transmitted by mosquitoes worldwide but is rare in the U.S. Thought to have been introduced on the North American continent during colonial days, malaria is not native to the U.S.
Travel-related outbreaks have occurred here after the domestic Anopheles mosquito population was infected. The CDC reports U.S. malaria transmission since 1986 among Hispanic immigrants and California residents. Since 1985, about 1,000 cases of imported malaria have been reported each year, with 66 deaths in 1986-1995. In 1996, the U.S. had 1,800 cases.
Early stages have flu-like symptoms, seven to eight days after infection, such as fever, chills, headache and muscle aches. If not diagnosed and treated rapidly, malaria may cause shock, renal failure, acute encephalitis and coma, and it can be fatal. If not treated properly, symptoms may reappear months or years after infection. Treatment includes antibiotics. Prevention when traveling in high-risk areas includes anti-malarial drugs.
Yellow Fever – Occurring in Africa and South America, yellow fever is a rare cause of illness in travelers. Most countries have requirements for yellow fever vaccination. Vaccinations are required of persons reentering the U.S. from yellow fever areas. All planes and ships are required to kill infected mosquito vectors.
The first recognized and documented importation of yellow fever into the U.S. since 1924, involves an American who contracted the virus while fishing on the Amazon and Rio Negro Rivers in 1996. Returning to Tennessee, he died after six days of hospitalization.
Caused by a virus closely related to the dengue virus, and transmitted by Aedes aegypti, yellow fever infections produce dengue-like symptoms in humans. But the effects of yellow fever are much more severe. Symptoms include fever, jaundice or yellowing of skin, and hemorrhaging. The fatality rate may exceed 50% in epidemics. A safe vaccine is available, but no treatment exists.
Animal Health Impact Mosquitoes harass and bite livestock, causing beef cattle and hogs to fail to gain weight, and dairy cattle to decrease milk production. Family pets and wildlife also suffer from the unrelenting attacks of mosquitoes.
Area-wide control of mosquitoes is generally the concern of professional services, including those obtained by mosquito abatement districts. Personal protection is usually the consumer’s area of control.
Breeding Site Management – Water management to prevent mosquito breeding is essential for effective control. Eggs cannot hatch without water. Locate standing water and eliminate. Drain or fill stagnant water pools, puddles, ditches or swampy areas. Fill tree holes with sand or mortar. Remove tires, buckets, tin cans, jars, toys and other receptacles so they will not collect rainwater or other moisture. Mosquitoes can breed in as little as a cup of water. Change water in birdbaths and wading pools once or twice a week. Clean out roof gutters. Check the water in flowerpots and other plant containers for mosquito larvae.
Larval Control (Larviciding) – Attacking the mosquito problem through larval control is a logical approach to reducing public health concerns and annoyance. The larval stage is the only time in the insect’s life cycle when it is truly confined and concentrated. Larvae may be killed off when standing water is treated with larvacides in the form of briquettes, granules, liquid concentrates or other controlled released systems.
Adult Mosquito Control (Adulticiding) – Adult control programs are necessary in areas of disease epidemics such as encephalitis as may occur in localities with a high density of mosquitoes, Cutbacks in funding for local mosquito abatement districts are of concern, especially when weather conditions have encouraged greater mosquito populations.
These methods include: thermalfogging, effective as a space spray treatment when applied in late evening, at night or early morning when winds are calmest; ultralow volume application, with special nozzle adaptations that break up undiluted, specially formulated insecticides into microscopic droplets; mist spraying of very fine mist of insecticide in water by blowing machines; and residual spraying as a barrier zone to tall grasses, weeds and shrubs, with effectiveness of several days.
Because community controls cannot reduce mosquito populations to zero, people may need to protect themselves as well, particularly on evening outings. Consumers have a wide variety of products available for personal mosquito control, probably more than for any other type of insect.
These include indoor aerosol products for control of mosquitoes that do enter the residence and outdoor foggers to spray the patio area and areas near the patio for mosquito control. There are arrays of personal repellents for mosquito control that can be applied directly to the exposed skin or the clothing. At this time, it appears that chemical based mosquito control products are the most effective, while the mechanical and bug zapping devices are not very effective in controlling mosquitoes. The following focuses on flying insect killer aerosols, outdoor foggers, house and garden type products, and personal repellents.
Indoor Control – Flying insect space sprays can be used indoors for control of mosquitoes that come inside. Aerosols and pump sprays are effective.
Outdoor Area Control – Outdoor foggers, house and garden products, and flying insect space sprays can be used on the patio, and in the yard and environs for effective control against mosquitoes. These products appear to be much more effective in controlling mosquitoes spatially than mechanical devices such as bug zappers.
Repellents – Repellents can protect humans from mosquito bites from one to five hours, depending on the amount of perspiration and rubbing of skin and abundance of mosquitoes. Personal mosquito repellents can be applied directly to the exposed skin and/or to clothing, depending on label instructions. Other repellents for outdoor use, space repellents, include candles and torches.
Seasonal Use – Use insect repellents from April through October, until the first frost.
The Right Product – Use an appropriate repellent for the type of activity and location. Aerosol and pump spray products allow broad and even application of repellent on both clothing and skin. Liquid, cream, lotion and stick products allow consumers to apply insect repellents in exact locations where protection is desired. Match the concentration of repellent to the duration of outdoor exposure. Brief outdoor activities require the lowest concentration, while activities that keep people outside longer require higher concentrations. Lower concentration repellent products are appropriate for most situations when exposure to biting insects is limited.
With Children – Use a mosquito net over an infant’s buggy or stroller. Keep small children’s fingernails clipped short in summer months to prevent excessive scratching of bites. Take precautions to help children guard against scratching insect bites including advising them not to scratch any bites. Do not apply insect repellent to the hands of young children, who often put hands in their mouths.
Always keep insect repellents out of the reach of children. Adults should apply repellent to young children. Follow all child safety precautions on labels.
Safe and Effective Use – Read and follow product label directions and cautions. Apply repellent only to exposed skin and/or clothing as directed on the product label. Do not apply under clothing. Use only enough to cover exposed skin and/or clothing. Saturation of clothing or frequent reapplication to skin is unnecessary for effectiveness. Do not apply repellent to eyes or mouth, nor over cuts, wounds or irritated skin. On returning indoors, wash treated skin with soap and water. This is particularly important when repellents are used repeatedly. If any reaction to a repellent is suspected, wash the treated skin and seek medical attention. Show the product to a health professional for proper identification.