Mosquitos - The Merck Veterinary Manual

Mosquitos

Mosquitos are members of the family Culicidae. Important genera include Aedes , Anopheles , Culex , Culiseta , and Psorophora . Although they are tiny, fragile dipterans, mosquitos are perhaps some of the most voracious of the blood-feeding arthropods. About 300 species have been described worldwide, with ~150 species found in the temperate regions of North America. Mosquitos are found in such diverse areas as salt marshes of the coastal plains to snow pools above 14,000 ft (4,300 m) to the gold mines of India 3,600 ft (1,100 m) below sea level. The volume of water in which mosquitos will breed varies from that within a can or tree hole to large shallow pools of accumulated, standing water.

Mosquitos lay their eggs either on the surface of standing water (eg, Aedes and Psorophora spp ) or on a substrate (such as damp soil) where the eggs will hatch after inundation from rainfall, irrigation, snow melt, etc. Larval mosquitos are known as wrigglers, while pupal mosquitos are known as tumblers. These stages are always aquatic and are found in a wide variety of habitats. Large numbers of mosquitos can be produced from eggs laid in relatively small bodies of water. Some species have several generations per year. The flight habits of adult mosquitos vary with the species; some Aedes spp will migrate many miles for their aquatic, larval habitat. In strong winds, mosquitos may be carried great distances. Some species overwinter as eggs, while others overwinter as adults.

Pathology:

Only female mosquitos actively take a blood meal so that they can lay eggs. Males feed on nectar, plant juices, and other liquids. Mosquitos annoy livestock, cause blood loss, and transmit disease. Also, the toxins injected at the time of biting may cause systemic effects. Mosquitos have been known to plague humans and livestock and, like black flies, in swarms, they have been known to keep cattle from grazing or cause them to stampede. The feeding of large numbers of swarming mosquitos can cause significant anemia in domestic animals. Although they are known for spreading malaria, yellow fever, dengue, and elephantiasis in humans, mosquitos are probably best known in veterinary medicine as the intermediate host for the canine heartworm, Dirofilaria immitis , and as the vectors of the equine viral encephalitides, including West Nile virus.

Anopheles quadrimaculatus is the intermediate host for malaria ( Plasmodium spp ) in humans and other primates. Aedes aegypti is the yellow fever mosquito, transmitting this virus among people. Psorophora columbiae is a severe pest of both livestock and humans in the rice fields of Louisiana and Arkansas. Culex tarsalis is an important vector of western equine encephalitis and is found in the western, central, and southern USA. Aedes vexans is an important nuisance species found in the midwest. Aedes albopictus is a recently introduced Asian species that also spreads yellow fever, dengue, and equine encephalitis. Certain Mansonia spp are severe pests of livestock in Florida. In Central and South America, the adult female bot fly Dermatobia hominis fastens her eggs to a species of Psorophora mosquito, which then transmits them to the mammalian host during feeding.

Diagnosis:

Adult mosquitos are most often collected in the field and are not found on animals. Adults are 3-6 mm long and slender, with small, spherical heads and long legs. The wing veins, body, head, and legs are covered with tiny, leaf-shaped scales. The long, filamentous antennae have 14-15 segments and are plumose in the males of most species. They also have proboscides designed for lacerating tiny blood vessels and sucking up pooled blood. Identification of the plethora of mosquito species (adult, larval, and pupal stages) is probably best left to an entomologist.

Treatment and Control:

Area control of mosquitos usually involves the cooperation of many individuals and can be accomplished successfully by experienced personnel with proper equipment. Areas that can serve as breeding sites for mosquito larvae should be eliminated or reduced. In addition, area programs generally include extensive use of larvicides; however, mosquito larvicides can disrupt the normal ecologic balance within an ecosystem. Recently, the use of various species of fish as biologic controls has been successful. In massive emergence of adult mosquitos, particularly when disease transmission is a concern, application of an insecticide active against the adult may be necessary.

Caution is advised with area treatment programs because many nontarget organisms (eg, fish, shrimp, bees) may be exposed to insecticides. A local extension entomologist should be consulted regarding appropriate materials for use on animals or within premises. Large-scale programs usually are coordinated by mosquito abatement district or other government agencies.

It is difficult for individual producers to protect their animals; residual sprays on the animals do not prevent attachment, and currently available repellents do not confer adequate protection during massive emergence. Protection from adult mosquitos may be provided by ground and, in some cases, aerial application of an insecticide at the time of emergence. Depending on local conditions, this protection may be of short duration. Valuable animals should be housed in closed or screened buildings, and the mosquitos inside killed with a fog or aerosol formulation of an approved insecticide. Temporary relief may be afforded by a spray or “wipe on” of materials commercially available.

Walking pets in the early morning or early evening hours when adult mosquitos are most abundant should be avoided to reduce exposure to mosquito bites. Imidacloprid has been used as a topical prevention and treatment of ticks, fleas, and mosquitos on dogs and puppies 7 weeks of age and older, weighing > 2 lb (0.91 kg). The compound has been shown to repel adult female mosquitos for up to 4 wk. Unfortunately, it cannot be used on cats. Mosquitos are not attracted to light; thus, electrocution devices are not helpful in mosquito control and may actually be detrimental because they may destroy beneficial insects that prey on mosquitos.