ENTO-040 06-15

Mosquitoes and the Diseases They Transmit

Sonja L. Swiger, Assistant Professor and Extension Entomologist The Texas A&M University System

osquitoes affect the health of people and more than any other pest M Figure 1. worldwide. Biting female mosquitoes transmit Egg life many disease-causing organisms, including raft cycle, including encephalitis, malaria, and yellow fever. the egg, larval, pupal, and adult Mosquito populations can occur anywhere Pupa stages. Source: in Texas because the habitats for immature mos- Molly Keck quitoes are almost everywhere in the state. To control mosquitoes the most effectively and Adult economically, you need to: • Understand their life cycle • Scales on the wing veins and margins • Be able to identify the mosquito in At least 85 species of mosquitoes occur in your area Texas. They vary considerably in larval breed- • Know the management steps that provide ing sites, time of day when they bite, and flight the best control for different species and at distances of the adults. Table 1 summarizes this specific locations information for the most common Texas species. Identifying mosquitoes It is difficult to identify species of larval or adult mosquitoes. To determine the species in Adult mosquitoes are small, long-legged your area, send samples to an identification lab that have two wings. They are distinguished or contact a mosquito control district, university, from all other flies by three characteristics: or pest control operator. • Long, many-segmented antennae • A piercing and sucking mouthpart system Life cycle elongated into a distinctive beak or pro- The mosquito life cycle has four distinct boscis, at least in the females stages: egg, larva, pupa, and adult (Fig. 1). The Table 1. Biological data on 20 common species of mosquitoes found in Texas.

Mosquito species Larval habitat(s) Biting time Flight range

Aedes aegypti AC C, D under 100 yards Aedes albopictus AC, TH C, D 100–300 yards Aedes atlanticus WP C, D 0.25–0.5 mile Aedes canadensis WP, DD, FS C 0–0.25 mile Aedes sollicitans SM C, N, D 5–40 miles Aedes taeniorhynchus SM C, N, D 5–40 miles Aedes triseriatus H, AC D 0.5–1 mile Aedes vexans FW, GP, IP C, N 10–25+ miles Anopheles punctipennis WP C, N 0–0.25 mile Anopheles quadrimaculatus FW, GP, LM C, N 0.5–1 mile Culex erraticus WP N 0–0.25 mile Culex nigripalpus GP, FW, DD C 0.5–1 mile Culex quinquefasciatus AC, SCB, GRP C, N 0.25–0.5 mile Culex restuans WP, GRP, DD C, N 1–2 miles Culex salinarius GP, LM, FS, SM C, N 0.25–5 mile Culiseta melanura FS, WP C, N 0.5–1 mile ciliata IP, RF, GRP C, N 5–10 miles IP, RF, GRP C, N 5–10 miles WP C, N 1–2 miles WP, coastal pools C, N 1–2 miles

AC: Artificial containers GP: Grassland pools RE: Rooted emerged vegetation DD: Drainage ditches GRP: Ground pools RF: Rice fields FS: Freshwater swamps IP: Irrigated pastures SCB: Sewage catch basins FW: Flood waters LM: Lake margins SM: Salt marshes WP: Woodland pools TH: Tree holes C: Crepuscular (dusk and dawn) D: Day N: Night

2 Figure 2. Mosquito eggs. Source: Mike Merchant Figure 3. Mosquito larvae. Source: Mike Merchant

adult stage can and lives on land; the other suspended in the water. They filter the food stages are aquatic. particles from the water with their brush-like Under favorable conditions, some mosqui- mouth parts. The larvae of some mosquito spe- toes can develop from egg to adult in 8 to 10 cies feed on other mosquito larvae. days. The amount of time to complete the life Most mosquito larvae mature in 4 to 10 cycle varies according to the mosquito species, days, passing through four growth stages food availability, and weather conditions. (instars) before transforming into pupae. The length of the larval development period depends Egg on the species, temperature, and availability of Eggs identify a mosquito breeding site, food. which may be laid: While feeding or breathing, mosquito larvae • In clusters called rafts on the water surface assume distinctive positions in the water. For (Fig. 2) most species, the larva breathes through an air • Singly on the water surface tube near the end of the abdomen, projecting it • In dry areas that are flooded periodically through the water surface and hanging its head Although white when first laid, within a few down at an angle to the surface. Only the tip of hours the eggs become dark brown to black. the breathing tube contacts the water surface. Their shape and size vary, with most being foot- An exception is the Anopheles larvae, which ball-shaped or boat-shaped and 0.02 to 0.04 inch lack air tubes and tend to lie flat against the long. water surface. In warm water, the eggs may hatch in 2 to 3 days. Some mosquito eggs can remain dormant Pupa in dry conditions for many months to 2 years. The pupal stage is the transitional stage between the larvae and the adults. Mosquito Larva pupae are sometimes called tumblers because of Mosquito eggs hatch into larvae called wig- the tumbling motion they exhibit in water when glers, which are seldom more than ½ inch long. disturbed. Mosquito pupae do not eat. Most of Wigglers have three body sections: a small head, the time they lie at the water surface and tend to an enlarged middle section (the thorax), and a move only when disturbed. long, cylinder-shaped abdomen (Fig. 3). The pupae are comma-shaped and, like the Wigglers live only in water and feed on larvae, breathe through air tubes at the water microscopic plants, animals, and organic debris surface. The front of the pupa’s body is greatly

3 Figure 4. Mosquito pupa. Source: Mike Merchant Figure 5. Mosquito adult. Source: Winfield Sterling enlarged, consisting of a fused head and thorax Adult female mosquitoes typically live for (Fig. 4). A pair of breathing tubes, or trumpets, about a week to a month, depending on the extends from the back of the thorax. The pupal environment. Some species spend the winter as abdomen or tail consists of several segments that adults that may live 6 months or more. move freely. Inactive females rest in protected areas that The pupal stage may last from 1 to 10 are typically dark or shaded, humid, and cool in days or more, depending on the species and the summer or warm in the winter. temperature. When a mosquito takes a blood meal, it uses its mouthparts to puncture the host’s skin Adult and injects a bit of saliva into the wound before Adult mosquitoes are small to medium-sized drawing blood. The saliva makes penetration with wings, long legs, and elongated easier and prevents the blood from clotting abdomens (Fig. 5). The coloration varies. Male during feeding. It is during saliva delivery that mosquitoes have feathery antennae; the females’ infected mosquitoes transmit disease agents antennae have short and sparse hairs. such as viruses. Male mosquitoes feed only on nectar, plant In most cases, the itching and swelling juices, and other sources of liquid carbohydrates. caused by the saliva subside within a few hours. They usually emerge a few hours up to a few The adult mosquitoes around your home days before the females emerge. The males rest may have come from a breeding site near or far in the vegetation surrounding the emergence away, depending on the species, wind patterns, site, waiting for the females to emerge. and the flight habits of the females: Female mosquitoes also feed on nectar, plant • Aedes aegypti and Aedes albopictus breed sap, and other sources of plant carbohydrates primarily in and around human habita- for energy. However, the females of most species tions and fly short distances, usually only must have a blood meal as a source of protein about 200 yards. before they can produce eggs. • Most Anopheles mosquitoes have a flight Mating usually occurs quickly in the air near range of about 1 mile. Psorophora species the emergence site. All of the eggs can be fertil- have flight ranges of at least 5 miles. ized after a single mating because the females • Some salt-marsh mosquitoes in the genera can store the sperm internally. Male mosquitoes Aedes can disperse with the prevailing usually die soon after mating.

4 Figure 6. Artificial containers that can serve as mosquito egg-laying sites: (from top left) a garbage can lid, birdbath, plastic bucket of water, flowerpot, plastic container, and a clogged rain gutter. Sources: Mike Merchant and Michael Sanders

winds for 20 to 40 miles or more away • Artificial containers and tree holes from the larval development sites. Table 2 lists the groups, breeding habitats, Knowing these flight distances can help and general management approaches of the you find the source of mosquito problems and genera and/or species that cause the most choose the appropriate management strategies. problems. If the mosquitoes originate away from your property, you may need to cooperate with others Managing mosquitoes to control the insects. In some cases, you may To manage mosquitoes effectively long-term, not be able to control them at all. use several complementary management tech- niques, including: Mosquito groups • Sanitation: Remove mosquito food, water, Mosquito species are divided into groups and shelter. based on where the females lay their eggs and • Habitat disruption: Drain the water where the larvae develop. The control strategies where mosquitoes breed (Fig. 6). differ for each group. The Centers for Disease • Biological control: Use mosquito fish, Control and Prevention (CDC) divides mosqui- nematodes, and Bacillus thuringiensis toes into four groups according to the habitats israeliensis toxin and Bacillus sphaericus. where the larvae generally develop: • Mechanical control: Maintain window • Permanent pools screens and alter building designs. • Transient water • Personal protection: Wear protective, • Floodwater light-colored, loose-fitting clothing; use

5 repellents; and avoid activities outside expensive, requiring the cooperation of indi- when mosquitoes are active. vidual homeowners as well as such groups as • Chemical suppression: Use insecticides industry, agriculture, state agencies, and local against adults and/or larvae. governments. Mosquito management is often complex and Some communities may have to take an

Table 2. Mosquito groups, their breeding sites, and management suggestions for each group.

Mosquito Genera and/or Breeding sites General management group species

Permanent Anopheles, some Standing water that Biological control—using pool Culex, Culiseta, seldom dries, edges of mosquito fish, Bacillus Coquillettidia, ponds, lakes and smaller thuringiensis israeliensis Mansonia impoundments toxin, and Bacillus sphaericus Habitat disruption— draining the water or removing plants

Transient Culiseta, some Roadside ditches, Biological control—using pool Culex, occasionally excavations, canals, mosquito fish, Bacillus Anopheles, espe- ground pools, catch thuringiensis israeliensis toxin cially Anopheles basins, storm sewers, and Bacillus sphaericus punctipennis clogged streams, irrigated land Sanitation—removing food, water and vegetation

Floodwater Aedes, Psorophora Flood plains, salt Habitat disruption— marshes, smaller sites, draining the water where even footprints mosquitoes breed

Chemical suppression— using insecticides against adults and/or larvae

Artificial Most Aedes, espe- Artificial containers, Sanitation—removing food, container cially Aedes aegypti, discarded tires, tin cans, water and shelter and tree-hole Aedes albopictus, flower pots, cemetery and Aedes triseriatus vases, roof gutters, tree- Habitat disruption— holes, water caught in draining the water in small bromeliads and orchids containers and other plants

6 area-wide approach to mosquito management in the United States; others appear occasion- and hire permanent control personnel. These ally because of international travel. The most programs can provide workers and expertise common of these diseases now in Texas are that are usually unavailable to homeowners. and Chikungunya. In these programs, trained personnel con- Mosquitoes also commonly transmit duct mosquito surveys to identify the species, heartworms to dogs and cats, particularly track the population levels, and decide how to in the humid areas of Texas. They rarely kill manage them. Area-wide management can pro- the animals directly, but they can do so by a vide relief from mosquito problems that develop combination of effects, including exhaustion, miles away from your home. suffocation, toxemia, and blood loss. Laws have been enacted in Texas enabling various groups to form mosquito control dis- Mosquito-borne diseases tricts. The Texas Department of State Health Mosquito-borne diseases can become a Services in Austin monitors the mosquito-borne problem when these elements are present: diseases that affect people and horses. •• Pathogens, the organisms that cause the Homeowners can help reduce mosquito disease populations by looking for larval breeding sites •• Reservoir, the animals in which the and taking appropriate action to minimize these pathogen lives and which serve as the locations (Table 3). source of the pathogen for the mosquitoes that transmit it How mosquitoes affect people •• Susceptible hosts, the people and/or and animals other animals that can be infected by the pathogen Mosquitoes can affect people and animals •• Dead-end hosts, animals that get infected, directly or indirectly. sometimes have disease, but cannot infect Direct effects: Mosquito species that feed on mosquitoes blood can annoy people, birds, mammals, and •• Vectors, the organisms that can trans- other vertebrates. They disrupt outdoor work mit the pathogen, either mechanically or and recreational activities. If enough mosquitoes biologically, from its reservoirs to the sus- are in an area, they can cause severe blood loss ceptible hosts and slow the growth of livestock. For a mosquito-borne disease to continue The bites often cause mild allergic reactions to occur and cycle in a geographic location, the such as swelling and itching, which may con- pathogen, reservoir, susceptible hosts, and vec- tinue to affect the hosts long after the female tors must all be present. mosquitoes have gone. Although some people may react more strongly to the bites, severe reac- Encephalitis tions are uncommon. Encephalitis is an inflammation of the brain Indirect effects: Mosquitoes indirectly affect caused by certain viruses transmitted by mos- people and animals when they transmit disease quitoes. Human cases of encephalitis range from organisms to them. Each year worldwide, mos- unapparent or mild cases to very severe illnesses quitoes affect millions of people by transmitting that can permanently damage the central ner- the disease-causing agent (pathogen) of several vous system or, in some instances, cause death. serious diseases. Symptoms include high fever, convulsions, Some of these diseases were once common

7 delirium and other central nervous system prob- Louis encephalitis virus (SLEV). These viruses lems. If these symptoms occur, seek medical are normally transmitted from bird to mosquito assistance quickly. to bird and sometimes from bird to mosquito to These diseases cause concern to the people human. EEEV, WEEV, and WNV can also be and horses in Texas: West Nile virus (WNV), transmitted from bird to mosquito to horse. eastern equine encephalitis virus (EEEV), west- When the incidence of any of these viruses ern equine encephalitis virus (WEEV), and St. increases in the bird populations, it becomes

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

Mosquito sources How to reduce mosquitoes

Ponds Stock the pond with fish. Use Bacillus thuringiensis israeliensis (such as Mosquito Dunks®). Remove excess vegetations. Swimming pools Keep water off the cover. Maintain water quality at all times.

Tree holes Fill the holes with sand, or drill a drain hole. Plastic pools Drain the water when not in use or cover the pool to prevent mosquitoes from laying eggs in the water. Containers Empty the water. Store the containers in an inverted position. Dispose of the containers. Cover the containers so mosquitoes cannot lay eggs in them. Bird baths Change the water at least once a week. Standing water Eliminate it by draining it. Fill in low areas. Watering troughs Stock the trough with fish. Change the water weekly. Cooler drains Prevent water from standing in the drain.

Street gutter or catch basins Keep litter and garden debris out of the gutter. Do not overwater the yard. Cesspool or septic tank Seal and cover it so mosquitoes cannot lay eggs in it.

Roof gutters Clean them regularly to remove debris.

Irrigated lawns or fields Avoid over irrigation. Drain standing water.

Source: Modified from UC Pest Management Guidelines; http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7451.html

8 more likely that people and horses will be infected. Birds serve as reservoirs for the viruses that cause WNV, EEEV, WEEV, SLEV, and some less-common diseases. These diseases become a problem when transmitted to humans, horses, or other equines such as donkeys or mules. In most cases, the human or equine host is a “dead-end host” for the virus, meaning that Figure 7. the disease probably will not be transmitted Diagram showing how West Nile virus from these hosts because they cannot infect is transmitted among Culex quinquefasciatus, birds, mosquitoes. and humans and horses. Source: Gabriel Hamer Similarly, horses may have mild to severe or even fatal infections with WNV, EEE, or WEE nausea, vomiting, abdominal pain, back pain, viruses. Horses with the SLE virus show no out- suppressed appetite, and muscle aches. ward sign of infection. West Nile neuroinvasive disease is much Birds may die of infection caused by some more severe. It affects the nervous system, encephalitis viruses but not others. For example, requires medical intervention, and can be fatal. deaths from the EEE virus have been reported The early symptoms may be the same as those of in emus, pheasants, house sparrows, and red- WNF but usually do not include a rash. winged blackbirds. The SLE virus, however, Within a few days of developing WNF produces no outward sign of infection in birds. symptoms, a person infected with WNND can WNV virus: West Nile virus threatens birds also develop encephalitis, meningitis, weakness (wild and domestic), horses, and people. Wild or paralysis of muscles (generally on one side birds are the primary reservoirs of this disease, of the body), inflammation of the lining of the and the pathogen can move with migratory retina, or a combination of these. birds. The largest-ever U.S. outbreak of WNV Although about 40 percent of horses that occurred in the summer of 2012, with 5,674 contract WNV die, the disease is usually much cases and 286 deaths, according to ArboNET, less severe in humans. Direct contact with a national arboviral surveillance system man- infected individuals does not appear to spread aged by the CDC and state health departments. the virus from animals to humans or from In Texas that year, 1,868 cases of WNV and 89 human to mosquito to human (Fig. 7). deaths were reported by the Texas Department The vector of WNV in urban Texas areas of State Health Services. is the southern house mosquito, Culex quinq- EEE virus: A mosquito that breeds in fresh- uefasciatus. Two forms of the virus can affect water swamps, Culiseta melanura, is typically humans: West Nile fever (WNF) and West Nile involved in the bird-to-mosquito-to-bird cycle of neuroinvasive disease (WNND). eastern equine encephalitis virus. West Nile fever causes symptoms that are like Because this mosquito rarely bites humans those of many other viral illnesses—most people or horses, other mosquitoes, such as Coquille- have a fever, and 20 to 50 percent will develop a tidia, probably play a role as a “bridge vector” mild rash on their arms, chest, and back. Other in transmitting the EEE virus to humans and symptoms include diarrhea, fatigue, headache, horses.

9 Eastern EEV has a 30 percent mortality rate Chikungunya virus in both horses and humans but is not typically seen throughout Texas every year. The Chikungunya (CHIK) virus is trans- WEE virus: The main mosquito vectors for mitted primarily by Aedes aegypti and Aedes the western equine encephalitis virus, partic- albopictus mosquitoes. In humans it causes a ularly west of the Mississippi River, are Culex severe fever and incapacitating joint arthritic tarsalis and Aedes dorsalis. Other insects such as pain. the swallow bug (Cimicidae) may also host the Humans are the primary reservoir and 72 WEE virus over the winter. to 97 percent of the population will be infected SLE virus: The primary urban vectors of with clinical signs. Symptoms will resolve in 7 the St. Louis encephalitis virus are the northern to 10 days, but the pain can linger for years and house mosquito (Culex p. pipiens) and the south- mortality is very rare. ern house mosquito (Culex p. quinquefasciatus), International travelers brought the first with the latter species causing concern in Texas. imported cases (patients contracting the dis- The chief vector in rural areas of the western ease while out of the country) of CHIKV to the United States is Culex tarsalis; in Florida and United States in 2014. By early 2015, Florida potentially in Texas, it is Culex nigripalpus. was the only state to have reported domestic- SLEV is becoming less common in the acquired cases. United States, and research has suggested that the presence of WNV has contributed to this Dengue displacement. Another virus-caused disease transmitted by VEE and CE viruses: The Venezuelan mosquitoes is dengue, or breakbone fever. The equine encephalitis (VEE) virus complex and more serious manifestations of this disease are the California encephalitis (CE) virus complex called dengue hemorrhagic fever and dengue also have been recorded in Texas. These virus shock syndrome. It is transmitted from infected complexes can cause encephalitis in humans and humans to susceptible humans by mosquitoes. sometimes in horses. A dengue outbreak occurred in Texas in They differ from the other mosquito-borne 1999, with 62 cases reported to the DSHS, encephalitis viruses in that their reservoirs are including one death. From 2003 through 2012, small mammals such as rodents rather than Texas recorded 154 cases, all imported. Sporadic birds, and in the case of VEE, sometimes horses outbreaks have occurred in Texas in the Gulf serve as reservoirs. Coast region and the extreme southern regions The dark rice field mosquito, Psorophora of the state. columbiae, is the only mosquito that has been Dengue is usually a severe but nonfatal dis- confirmed to transmit VEE from horse to mos- ease. Symptoms include the sudden onset of quito to horse in Texas. a high fever, severe headache, backache, joint The tree-hole mosquito, Aedes triseriatus, is pains, and a rash that appears on the third or the primary vector of CE, and tree squirrels are fourth day of illness. the primary reservoirs. The La Crosse type of In Texas, the mosquito primarily responsi- California encephalitis is the strain that occurs ble for transmitting dengue to people is Aedes most often in humans, but only a few cases have aegypti. Mosquitoes obtain the dengue virus been confirmed in Texas. from the blood of infected humans during the period from the day before the person has a

10 fever through the third or fourth day of illness. Malaria in humans is an acute or chronic The virus then multiplies in the mosquito disease caused by four species of microscopic and invades the mosquito’s salivary glands, parasites belonging to the Plasmodium. making the mosquito infective to humans from Symptoms vary from moderately severe to 8 to 14 days after taking the infective blood highly fatal, depending on the mosquito species meal. The mosquito then may remain infective and the person’s health when infected. for the rest of its life, able to transmit the disease Malaria causes fever and flu-like symptoms during any blood feedings it takes on susceptible that may include chills, headaches, muscle people. aches, fatigue, nausea, vomiting, and diarrhea. The parasites are transmitted by Anophe- Yellow fever les mosquitoes. Although at least 16 species of Historically, yellow fever is one of the most Anopheles occur in the United States, only two feared epidemic diseases in the United States species are known to be significant vectors of because the mortality rate in humans can reach malaria: Anopheles freeborni and Anopheles 85 percent. Although the last case originat- quadrimaculatus. ing in the United States occurred in 1911, it In the blood of humans, these parasites is still regarded as so dangerous that U.S. law invade individual red blood cells, eventually requires that cases of yellow fever be reported destroying them. The parasites leave these cells immediately. and invade new red blood cells as the blood-cy- The symptoms are high fever, internal cling phase progresses. bleeding, and jaundice. Illness from yellow If not treated properly, a malaria infection fever may be acute and fatal, or so mild that it is may persist in a human for many months or unapparent. even years. During that time, it can be continu- In the classical “urban type” of yellow ously or periodically able to infect mosquitoes. fever, epidemics are the result of transmission People in areas where malaria is common from human to mosquito to human. The virus may be infected repeatedly, which can result is spread by the yellow fever mosquito, Aedes in them developing a “tolerance” for the par- aegypti. asite. Although this “tolerance” may prevent An extremely slight infection risk exists for severe acute consequences, it does not prevent a tourists who visit countries where yellow fever chronic, often debilitating infection. is present. To enter many of those countries, visitors must take a highly effective and well-tol- Dog heartworms erated vaccine. Occasionally, people who have Dog heartworms are caused by a mosqui- contracted yellow fever in other countries have to-borne filarial worm (a threadlike parasite) returned to the United States infected with the called Dirofilaria immitis. Adult stages of this virus. worm amass in the heart cavities of dogs and cats, causing heart damage, blockages, and even- Malaria tually death if the infestation grows too large Although malaria had disappeared as a (Fig. 8). significant problem in the United States by Heartworms can cause severe circulatory the early 1950s, it is still one of the most seri- problems in dogs and produce symptoms such ous communicable diseases affecting people as coughing, labored breathing, and general loss worldwide. of vitality in advanced stages.

11 Although dog heartworms are fairly wide- spread in the United States, it is most prevalent along the Atlantic and Gulf Coasts from Mas- sachusetts to Texas and up the Mississippi River Valley to Minnesota. Pet owners should make mosquito control a high priority. Because it is very difficult to pro- tect dogs and cats from mosquitoes, the most effective way to control heartworms is to pre- vent the worms from reaching the adult stage. Veterinarians can prescribe drugs to protect Figure 8. Dog heart infected with dog heartworms. dogs during the mosquito season, which is year- Source: Lone Star Shih Tzu and Lhasa Apso Rescue round in Texas. D. immitis is normally transmitted from dog Contact your doctor if you develop to mosquito to dog. We do not know definitely symptoms of any of these diseases. Current what the mosquito vectors of dog heartworms information on human diseases is tracked by are in Texas, but several mosquito species are the Centers for Disease Control. Your veterinar- suspected from the genera Culex, Aedes, and ian can recommend management strategies for Anopheles. animal diseases. Mosquitoes can also transmit D. immitis to For more information: people, where the parasite usually migrates to http: ://livestockvetento.tamu.edu/ the lungs and less often to the heart. It causes a http://mosquitosafari.tamu.edu/ condition known as eosinophilia. Some people also have allergic reactions. Fortunately, human cases of D. immitis are rare.

Acknowledgments Gabriel Hamer of Texas A&M University and Wizzie Brown and Molly Keck of the Texas A&M AgriLife Extension Service reviewed the manuscript of this publication. John A. Jackman, former Extension Entomologist, and Jim K. Olson, formerly of Texas A&M University, contributed to an earlier manuscript of this publication.

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