• Home
• SERVICES
  • skeeterNOW
  • skeeterSTOP
  • Bite Back! Program
  • skeeterMISTER
• ABOUT US
  • Vision
  • The Squad
  • The Buzz
• NEWS & FAQ's
  • skeeterstats & Myths
  • West Nile FAQ's
  • Community News
    • W. Nile Confirmed in Local Resident
    • Human Case of West Nile is Year's First
    • BRAZOS COUNTY RESIDENT DIES OF WEST NILE
  • ARTICLES
    • Mosquitos and the Diseases They Transmit
      • Mosquito Diseases Continued
    • Do Backyard Mosquito Sprays Work?
• CONTACT US

Mosquitos and the Diseases They Transmit

(continued from page 1)

To control mosquitoes effectively long-term, you need to use several complementary management techniques, including:

• Sanitation — removing food, water, and shelter
• Habitat disruption — draining the water where mosquitoes breed
• Biological control — using mosquito fish, nematodes, and Bacillus thuringiensis israeliensis toxin and Bacillus sphaericus
• Mechanical control — maintaining window screens and altering building designs
• Personal protection — wearing protective, lightcolored, loose-fitting clothing; using repellents; and avoiding activities in areas when mosquitoes are active
• Chemical suppression — using insecticides against adults and/or larvae

Mosquito control is often complex and expensive, requiring the cooperation of individual homeowners as well as such groups as industry, agriculture, state agencies and local governments.

Some communities may have to take an areawide approach to mosquito control and hire permanent control personnel. These areawide programs are sometimes the only solution to a mosquito problem. Areawide mosquito control programs can provide manpower and expertise that are usually unavailable to homeowners.

In these programs, trained personnel conduct mosquito surveys to identify the species, track the population levels and determine appropriate management alternatives. Areawide management can provide relief from mosquito problems that develop miles away from the affected area.

Laws have been enacted in Texas enabling various groups to form mosquito control districts. The Texas Department of Health in Austin can provide information on how to establish a control district. The Texas Animal Health Commission monitors the mosquito-borne animal diseases in the state.

Homeowners can reduce their local program in many cases. More specific management actions are provided by location in Table 3. Evaluate your area to recognize the likely larval breeding sites for mosquito larvae then take appropriate action.

Table 3. Possible mosquito sources around the home and other property.

How Mosquitoes Affect People and Animals

Mosquitoes can affect people and animals directly or indirectly.

Direct effects: Mosquito species that feed on blood can annoy people, birds, mammals and other vertebrates.  They disrupt outdoor work and recreational activities. If there are enough mosquitoes in an area, they can cause severe blood loss and slow the growth of livestock. When they feed, mosquitoes inject salivary fluids into their hosts. These fluids contain chemicals that can prevent blood from clotting and break down red blood cells.
The bites often cause mild allergic reactions such as swelling and itching. These reactions may continue to affect the hosts long after the female mosquitoes have taken their blood meals. Although some people may react more strongly to the bites, severe reactions are uncommon.

Indirect effects: Mosquitoes indirectly affect people and animals when they transmit disease organisms to them. Each year worldwide, mosquitoes affect millions of people by transmitting the disease-causing agent (pathogen) of several serious diseases, including encephalitis, dengue, yellow fever, malaria and filariasis.

Although most of these diseases were once common in the United States, none causes serious human problems in the United States today. The most common of these diseases now in Texas is encephalitis, which affects a few people each year.

Mosquito-Borne Diseases

Mosquito-borne diseases can become a problem when these four living elements are present:

• Pathogen — the organism that causes the disease
• Reservoir — the animals in which the pathogen lives and which serve as the source of the pathogen for the mosquitoes that transmit it
• Susceptible hosts — the people and/or other animals that can be infected by the pathogen
• Vectors — the mosquito species that can transmit the pathogen, either mechanically or biologically, from it reservoirs to the susceptible hosts

All four of these living elements must be present for a mosquito-borne disease to continue to occur and cycle in a geographic location.

Encephalitis
Encephalitis is an inflammation of the brain caused by certain viruses transmitted by mosquitoes. Human cases of encephalitis range from unapparent or mild cases to very severe illnesses that can permanently damage the central nervous system or, in some instances, cause death. Symptoms include high fever, convulsions, delirium and other central nervous system problems. If these symptoms occur, seek medical assistance quickly.

Birds serve as the primary reservoirs for the viruses that most often cause the disease. Encephalitis occasionally becomes a problem when it is transmitted to humans, horses or other equines such as donkeys or mules.

Public health officials often check local and migrating bird populations to see if they have the virus and whether there is a potential for its transmission to humans and animals. Several types of encephalitis cause concern in Texas: eastern equine encephalitis (EEE), western equine encephalitis (WEE) and St. Louis encephalitis (SLE). The viruses causing EEE, WEE or SLE are normally transmitted from bird to mosquito to bird and sometimes from bird to mosquito to human. EEE and WEE can also be transmitted from bird to mosquito to horse. When the incidence of any encephalitis virus increases in bird populations, it becomes more likely that humans and equines can become involved.

In most cases, the human or equine host is a “deadend host” for the virus, meaning that the disease probably will not be transmitted from these hosts because they cannot infect mosquitoes.

Similarly, horses may have mild to severe or even fatal infections with EEE or WEE viruses. Horses with the SLE virus show no outward sign of infection.

Birds may die of infection caused by some encephalitis viruses but not by others. For example, deaths from the EEE virus have been reported in red-winged blackbirds, house sparrows, pheasants and emus. SLE virus, however, produces no outward sign of infection in birds.

The various types of viruses are transmitted by different mosquito species:

EEE virus: Culiseta melanura, a mosquito that breeds in freshwater swamps, is typically involved in the bird to mosquito to bird cycle of the eastern equine encephalitis virus.

Because this mosquito rarely bites humans or horses, other mosquitoes, such as the eastern salt marsh mosquito, Ochlerotatus (=Aedes) sollicitans, probably transmit EEE virus to humans and horses.

WEE virus: Culex tarsalis and Ochlerotatus (=Aedes) dorsalis are the main mosquito vectors for western equine encephalitis virus, particularly west of the Mississippi River. Other insects, such as the swallow bug (Cimicidae), may also serve as overwintering hosts of the WEE virus.

SLE virus: Members of the Culex pipiens complex, Cx. p. pipiens (the northern house mosquito) and Cx. p. quinquefasciatus (the southern house mosquito), are the main urban vectors of the St. Louis encephalitis virus,with the latter species causing concern in Texas.

Culex tarsalis is the chief vector of SLE virus in rural areas of the western United States. Culex nigripalpus has been identified as the primary vector of SLE virus in Florida and is a potential vector in Texas.

VEE and CE viruses: The Venezuelan equine encephalitis (VEE) virus complex and the California encephalitis (CE) virus complex also have been recorded in Texas. These virus complexes can cause encephalitis in humans and sometimes in horses. They differ from the other mosquito-borne encephalitis viruses in that their reservoirs are small mammals such as rodents rather than birds, and in the case of VEE sometimes horses serve as reservoirs.

The dark rice field mosquito, Psorophora columbiae, is the only mosquito that has been confirmed to transmit VEE from horse-to-mosquito-to-horse in the epidemic cycle in Texas. The tree-hole mosquito, Ochlerotatus (=Aedes) triseriatus, is the primary vector of CE, and tree squirrels serve as the primary reservoirs. The La Crosse type of California encephalitis is the strain that occurs most often in humans, but only a few cases have been confirmed in Texas.

WNV virus: West Nile virus (WNV) is a relatively recent concern in the eastern United States and continues to spread toward Texas. It threatens birds (both wild and domestic), horses and people. Wild birds are the primary reservoirs of this disease, and the pathogen can move with migratory birds.

WNV can also be spread by mosquitoes to horses and humans, causing encephalitis that may result in death.  Although about 40 percent of horses that contract encephalitis die, the disease is much less severe in humans. There is no evidence that it can be spread from human to mosquito to human or from animals to humans through direct contact with infected individuals.

Since 1999, WNV has been detected in 12 states and the District of Columbia — from as far north as Vermont down to Florida along the east coast and as far west as Wisconsin in the upper midwestern states and Louisiana in the south central states.

During the summer of 2000, the Texas Department of Health (TDH) cooperated with several agencies and research groups in an intense national monitoring survey for WNV. The department monitored the possible reservoirs, incidental hosts and mosquito species likely to feed on birds migrating from the northeastern United States through Texas to the Gulf Coast.

As of 2001, the mosquito vectors of WNV had been reported in Texas, but no human, veterinary or wild bird cases of WNV had yet been reported here.

Dengue
Another virus-caused disease transmitted by mosquitoes is dengue, or breakbone fever. The more serious manifestations of this disease are called dengue hemorrhagic fever and dengue shock syndrome. It is transmitted from infected humans to susceptible humans by mosquitoes.

A dengue outbreak occurred in Texas in 1999, with 62 cases reported to the TDH, including one death. Dengue is usually a severe but nonfatal disease. Symptoms include the sudden onset of high fever, severe headache, backache, joint pains and a rash that appears on the third or fourth day of illness.

In Texas, the mosquito primarily responsible for transmitting dengue to people is Aedes aegypti. Mosquitoes obtain dengue virus from the blood of infected humans during the period from the day before the initial fever occurs in the person through the third or fourth day of illness.

The virus then multiplies in the mosquito and invades the mosquito’s salivary glands, making the mosquito infective to humans from 8 to 14 days after taking the infective blood meal. The mosquito then may remain infective for the rest of its life and able to transmit the disease during any subsequent blood feedings it takes on susceptible people.

Yellow fever
Historically, yellow fever is one of the most feared epidemic diseases in the United States because the mortality rate in humans can reach 85 percent. Although the last case originating in the United States occurred in 1911, it is still regarded as so dangerous that U.S. law requires that cases of yellow fever be reported immediately.

The symptoms are high fever, internal bleeding and jaundice. Illness from yellow fever may be acute and fatal, or so mild that it is unapparent. In the classical “urban type” of yellow fever, epidemics are the result of human to mosquito to human transmission of the virus by Aedes aegypti, the yellow fever mosquito. The virus is introduced into the blood stream via the saliva of the mosquito as it bites.

An extremely slight infection risk exists for tourists who visit countries where yellow fever exists. To enter many of those countries, visitors must take a highly effective and well-tolerated vaccine. Occasionally people who have contracted the yellow fever in other countries return to the United States infected with the virus.

Malaria
Although by the early 1950s, malaria had disappeared as a significant problem in the United States, it is still one of the most important communicable diseases affecting people worldwide.

Malaria in humans is an acute or chronic disease caused by any one of four species of microscopic protozoan parasites belonging to the genus Plasmodium. Malaria infections in people vary from a moderately severe to a highly fatal illness, depending on the species of Plasmodium involved and the condition of the human at the time of infection.

Malaria causes fever and flu-like symptoms that may include chills, headache, muscle aches and fatigue.  Nausea, vomiting, and diarrhea may also occur. The parasites are transmitted from human to mosquito to human by the bite of Anopheles mosquitoes.

Although at least 16 species of Anopheles occur in theUnited States, only two species are known to be significant vectors of malaria: Anopheles freeborni and Anopheles quadrimaculatus.

In the blood in humans, these parasites invade individual red blood cells eventually destroying these cells and reproducing asexually. The parasites leave these cells and invade new red blood cells as the blood-cycling phase progresses. If insufficiently treated, a malaria infection may persist in a human for many months or even years. During that time it can be continuously or or periodically able to infect mosquitoes.

People in areas where malaria is common may be infected repeatedly, which can result in them developing a “tolerance” for the parasite. Although this “tolerance” may prevent severe acute consequences, it does not prevent a chronic, often debilitating infection.

Dog heartworm

Another common mosquito-borne disease in Texas is heartworm in dogs, particularly in the humid areas of East Texas and the Coastal Plain. Mosquitoes rarely cause death directly, but they can do so by a combination of effects, including exhaustion, suffocation, toxemia and blood loss.

Dog heartworm is caused by a mosquito-borne filarial worm, a threadlike parasite called Dirofilaria immitis.  Adult stages of this worm amass in the heart cavities of dogs and cats, causing heart damage, blockages and eventually death if the infestation grows too large.

Heartworm can cause severe circulatory problems in dogs and produce symptoms such as coughing, labored breathing and general loss of vitality in advanced stages. D. immitis is normally transmitted from dog to mosquito to dog. We do not know definitely what the mosquito vectors of dog heartworm are in Texas, but several mosquito species are suspected from the genera Culex, Aedes, Ochlerotatus, and Anopheles. Mosquitoes can also transmit D. immitis to people, where the parasite usually migrates to the lungs and less often to the heart. It causes a condition known as eosinophilia. Some people also have allergic reactions to the filarial worms. Fortunately, human cases of D. immitis are rare.

Dog heartworm is fairly widespread in the United States, with the greatest prevalence being along the Atlantic and Gulf Coasts from Massachusetts to Texas and up the Mississippi River Valley inland to as far north as Minnesota.

Mosquito control is important for pet owners. Because it is very difficult to protect dogs from mosquitoes, the most effective way to control heartworm is to prevent the worms from reaching the adult stage. Veterinarians can prescribe drug treatment to protect pets during the mosquito season.

If you are concerned about symptoms that you have be sure to contact your physician. The current information on human diseases is tracked by the Center for Disease Control. Your veterinarian is the local contact to recommend management of pet and animal related diseases.

For more information on insect management, visit our Web site at http://insects.tamu.edu

Acknowledgments

Thanks to Doug Stevenson, who provided a manual that served as a basis for much of this information. Photos and figures were supplied by Jefferson Parish Mosquito Control, LA; Ruth Allen, Rutgers; and the Centers for Disease Control.

The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.

Educational programs of Texas Cooperative Extension are open to all people without regard to race, color, sex, disability, religion, age or national origin.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Chester P. Fehlis, Deputy Director, Texas Cooperative Extension, The Texas A&M University System.