Mosquitoes

Synopsis

Mosquitoes are arthropods of the class Insecta, order Diptera and family Culicidae. They are a large and diverse group of flying insects. Females feed on vertebrate blood, necessary for egg development, Males are nectar feeders and do not acquire blood meals. Their life cycles are complex and consist of four distinct stages (egg, larva, pupa, and adult). They are native to many different habitats, although all require water at one time or another to complete their life cycles. Mosquitoes are important because of the irritation that they cause during feeding, but also because they are formidable vectors of disease. Mosquitoes are rivaled only by ticks in the number of agents that they transmit.

Species

There are about 200 species of mosquitoes in the United States, all of which live in specific habitats, exhibit unique behaviors and feed on different vertebrate hosts. Important mosquito species generally belong to one of three genera: Anopheles, Culex, and Aedes. Despite differences in feeding preference, breeding habitat and other unique behaviors, all mosquitoes share common structural features, and a four-stage life cycle.

Overview of Life Cycle

See Image Below

Stages

• Egg – eggs are laid by female mosquitoes either singly or in clusters in water or moist surfaces at the water’s edge (flood plain mosquitoes). After an initial period of development, they produce a serosal cuticle that renders them resistant to dessication, although resistance varies with the species. Aedes spp. are particularly resistant to dessication, which makes them better able to survive in temporary water sites such as salt marshes, tidal zones, rock pools, tree holes, bird feeders and other containers. Mosquito larvae emerge from eggs using an egg tooth present on the head.

• Larva - (plural: larvae) ("wrigglers") have a well-defined head, thorax and abdomen. They breathe through spiracles placed at the water surface. Some have an elaborate siphon on the last segment of the abdomen that allows them to hang upside down and feed while they breathe. The larvae molts several times and develops to the pupa.

• Pupa - (plural: pupae) ("tumbler") - pupae are the last developing stage prior to emergence of the adult mosquito. Pupae do not feed. The head and thorax are joined (cephalothorax) in this stage and are attached posteriorly to a segmented abdomen. Respiration occurs through a pair of extensions (“trumpets”) on the cephalothorax. Pupae remain quiescent unless they are disturbed. Pupae have been observed to dive rapidly and return to the surface, probably a defense mechanism against predation. Pupae may complete their development in water or on dry substrata.

• Adult - Adults are active flying insects. They are 4-10 mm long, with long legs, long piercing mouthparts and 15-segmented antennae. Most male mosquitoes have fuzzy (some say plumose) antennae. A colleague once referred to them as bushy eyebrows. Females have antennae with many fewer and shorter bristle-like hairs. Interestingly, this is of behavioral significance since receptors on male antennae possess receptors that sense female wing beat patterns, allowing the male to locate the female. Only the female mosquito feeds on the blood of humans or other animals. The complete life cycle of mosquitoes requires about 4 days under ideal conditions, but can take much longer. Some species can undergo hibernation or diapause in one or more mosquito stages.

Prevalence

Mosquitoes are distributed throughout the world. Their specific locations are determined by the presence of suitable hosts and developmental habitats. Habitats include permanent waterbodies rich in plant life, riparian woodlands, swampy lowland forests, ponds, ditches, tree holes, and artificial containers (i.e. tires, bird baths, rain barrels, gutters). At least 3520 species were recognized in 2010, although the status and validity of some species are questioned.

Host Associations and Transmission Between Hosts

Only female mosquitoes are found on or around humans and other animals. Males do not require blood meals and have no attraction to vertebrate hosts. Adult female mosquitoes are highly mobile flying insects that can move freely from one host to another. They may feed on multiple hosts (both in number and kind) and once infected are capable of transmitting diseases to multiple hosts.

Environmental Factors

The prevalence of the different mosquito species and their activities are determined largely by the environment. Among these determinants are availability of water, vegetation, access to hosts (females), temperature, humidity and other climatic conditions. Mosquitoes also differ in their daily feeding patterns. Some species are nocturnal, others are crepuscular, while some feed intermittently throughout day depending on shade requirements, rainfall or inclement weather.

Disease

Mosquitoes may cause skin reactions as a result of their bites and mild to severe allergic reactions to their salivary secretions (the red bump recognized universally). Humans and animals have varied responses to the salivary components transferred during mosquito feeding. Some suffer an intense pruritic reaction while others tend to have a very mild reaction. There is the potential for a secondary bacterial infection to occur at the bite site. Anaphylactic reactions are uncommon. Unusually large population of mosquitoes may impact the health of entire herds of animals. However, animal and human health is impacted most severely by diseases transmitted by mosquitoes. They may transmit pathogenic protozoans, nematodes, and viruses. More than one million people worldwide are killed by mosquito-borne diseases every year. They also transmit diseases to dogs, cats, horses and other hosts. Selected important mosquito-borne diseases are discussed below and are presented below.

Selected Mosquito-Borne Diseases

Agent: Yellow Fever Virus

• Primary Host: Humans
• Common Vectors: Aedes spp. particularly A. aegypti
• Reservoir Host: Non-human primates
• Geographic Distribution: Africa, Central America, South America

Agent: Dengue Virus

• Primary Host: Humans
• Common Vectors: Aedes spp. particularly A. aegypti
• Reservoir Host: Non-human primates
• Geographic Distribution: Cosmopolitan

Agent: Japanese Encephalitis Virus

• Primary Host: Humans
• Common Vectors: Culex spp.
• Reservoir Host: Aquatic Wild Birds
• Geographic Distribution: Cosmopolitan

Agent: St. Louis Encephalitis Virus

• Primary Host: Humans, Horses
• Common Vectors: Culex spp.
• Reservoir Host: Wild Birds
• Geographic Distribution: North America, Central America

Agent: West Nile Meningoencephalitis Virus

• Primary Host: Humans, Horses
• Common Vectors: Culex spp.
• Reservoir Host: Wild Birds
• Geographic Distribution: Africa, Southern Europe, Middle East, India, North America, Central America, South America

Agent: Zika Virus

• Primary Host: Humans
• Common Vectors: Aedes spp.
• Reservoir Host: Non-human primates, Humans(?)
• Geographic Distribution: Cosmopolitan

Agent: Chikungunya Virus

• Primary Host: Humans
• Common Vectors: Aedes spp.
• Reservoir Host: Non-human primates
• Geographic Distribution: Sub-Saharan Africa, Asia

Agent: Eastern Equine Encephalomyelitis Virus

• Primary Host: Horses, rarely humans
• Common Vectors: Culex spp., Culiseta spp., Aedes spp., Coquillettidia spp.
• Reservoir Host: Wild Birds
• Geographic Distribution: USA, Caribbean, Central and South America

Agent: Western Equine Encephalomyelitis Virus

• Primary Host: Horses, rarely Humans
• Common Vectors: Culex spp., Aedes spp., Culiseta spp.
• Reservoir Host:
• Geographic Distribution: North, Central and South America

Agent: Venezuelan Equine Encephalitis Virus

• Primary Host: Horses, rarely Humans
• Common Vectors:
• Reservoir Host:
• Geographic Distribution: USA, Central and South America

Agent: Dirofilaria immitis

• Primary Host: Canids
• Common Vectors: Aedes spp., Anopheles spp., Culex spp.
• Reservoir Host: Domestic and wild canids
• Geographic Distribution: Cosmopolitan

Agent: Malaria (Plasmodium spp.)

• Primary Host: Humans (species also infect birds and reptiles)
• Common Vectors: Anopheles spp. for humans and nonhuman primates; Bird malaria is transmitted by many genera of mosquitoes
• Reservoir Host: Humans and non-human primates, birds and reptiles (humans are also infected with non-human primate species)
• Geographic Distribution: Cosmopolitan

Agent: Wucheraria and Brugia spp. (lymphatic filariasis)

• Primary Host: Humans
• Common Vectors: Culex spp., Anopheles spp., Mansonia spp.
• Reservoir Host: Humans (other animals?)
• Geographic Distribution: Africa, Indo-Pacific region, South America

Canine and Feline Heartworm Disease

In 2016 over 110,000 cases of canine heartworm were diagnosed in the United States. The Southeastern US is the region of greatest prevalence (see CAPC prevalence and forecast maps). Heartworm infections are a significant health risk to dogs as even light infections are capable of producing profound pulmonary vascular, airway and interstitial disease. Despite improved diagnostic methods, effective preventives and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to be diagnosed in high numbers. Heartworm infections are becoming more prevalent in areas previously considered to be at a low risk (see Heartworm disease in CAPC guidelines). Cats are also at risk for heartworm infection. The profile of heartworm disease in cats is quite different from dogs. See feline heartworm disease in CAPC guidelines.

West Nile Virus (WNV)

Cats are more likely than dogs to show minor signs of WNV infection such as mildly elevated temperatures and lethargy. However, dogs and cats have not been shown to exhibit neurological problems. Both dogs and cats developed titers to WNV when bitten by infected mosquitoes or fed infected birds or mice; however, they are not considered to be viable reservoir hosts. Dogs are unlikely to serve as amplifying hosts since virus levels appear to be low (but may serve as a sentinel species). Cats, although not as effective as birds as reservoir hosts for WNV, do appear to be capable of infecting mosquitoes.

In 2004, a study indicated dogs and cats could be experimentally infected with WNV. Viremia of low magnitude and short duration developed in four dogs, but they did not display signs of disease. Four cats became viremic. Three of the cats showed mild, non-neurologic signs of disease. WNV was not isolated from saliva of either dogs or cats during the period of viremia. An additional group of four cats were exposed to WNV orally, through ingestion of infected mice. Two cats consumed an infected mouse on three consecutive days, and two cats consumed a single infected mouse. Viremia developed in all of these cats with a magnitude and duration similar to that seen in cats infected by mosquito bite, but none of the four showed signs of disease. These results suggest that dogs and cats are readily infected by WNV. The high efficiency of oral transmission observed with cats suggests that infected prey animals may serve as an important source of infection to carnivores. Neither species is likely to function as an epidemiologically important amplifying host, although the peak viremia observed in cats may be high enough to infect mosquitoes with low efficiency.

Zika Virus

Zika is an arthropod-borne virus placed in the family flaviviridae. It is related to the yellow fever, dengue, Japanese encephalitis, and West Nile viruses. Zika is spread by mosquitoes of the genus Aedes (principally A. aegypti and A. albopictus in the urban cycle). It occurs naturally in the primate populations (sylvatic cycle) where it is transmitted by other species of Aedes. Zika was first described in East Africa in 1948 and has spread globally. By 2015 it had spread to the new world, establishing principally in Brazil and Mexico. It is now present in 72 countries worldwide. By 2014 Zika had spread to 40 countries and territories in the Americas. Although mosquitoes are its primary route of transmission, additional routes include sexual intercourse and blood transfusions. Virus has also been detected in urine, semen, breast milk and saliva. Eighty percent of human Zika virus infections are either without clinical signs or include only mild symptoms. Severe sequelae include birth defects such as microencephaly and abnormalities in limb development. The true impact if Zika infections in infants is yet to be determined. There is neither an effective vaccine or proven chemotherapy available for Zika. At his point, there is no evidence that domestic animals such as dogs and cats display clinical disease or serve as suitable reservoir hosts. Experimental transmission studies only recently revealed that rabbits and pigs might serve as sentinel hosts. For those at risk of infection and potentially severe consequences such as pregnant women, personal protection from mosquitoes is the best mean of avoiding exposure. Urban mosquito control programs are being intensified in cities at increased risk of Zika transmission.

Diagnosis

Mosquitoes are recognized by visualization of life cycle stages in or near water (eggs, larvae, or pupae) or visualization of adult mosquitoes actively feeding on mammals or plants. Different structural characteristics can be used to differentiate different mosquito species. Intact specimens may be sent to vector control districts, university entomology or parasitology departments, or to cooperative extension service offices for identification. Evidence of mosquito feeding may be confirmed by visualization of mosquito bites on human or animal skin.

Treatment, Control and Prevention

Control of mosquitoes (and other ectoparasites) requires implementation of an integrated pest management (IPM) strategy. IPM employs the use of EPA-registered repellents/insecticides on the pet and humans, minimizing exposure, and altering the environment to discourage mosquito growth development.

Dogs:

There are several products available for use on dogs to repel and kill mosquitoes for an entire month. Products contain permethrin at relatively high concentrations (e.g. Effitix® [44.88%; Virbac], K9 Advantix® II [44%; Bayer], Vectra 3D® [36.08%; Ceva]; Activyl Tick Plus [42.5%; Merck]). Several of these products are labeled to control other ectoparasites as well. Recent research indicates that treatment of dogs with a combination of dinotefuran, permethrin and pyriproxyfen (Vectra 3D^®) inhibits uptake of heartworm microfilariae from infected dogs and prevents transmission of heartworm infective larvae from infected mosquitoes to non-infected dogs. These spot-on products are not labeled for use in cats, nor should they be used in cats. Long-lasting permethrin-impregnated clothing and bedding is also available for dogs (not cats) to repel mosquitoes. Mosquito control products approved for use on dogs should not be used on humans. A complete list of available ectoparasiticidal products is available at capcvet.org.

Cats:

Currently there are no mosquito control products with monthly residual activity approved for use in cats. There are some short-acting, over-the-counter products available for mosquito repellency. Only use products that are approved for use in cats. Keeping cats indoors will minimize exposure to mosquitoes.

Humans:

A number of EPA-registered repellents are available for use on humans. The following products can be used on exposed human skin and clothing: DEET, Picaridin, oil of lemon eucalyptus (OLE) or synthetic OLE (PMD [para-menthane-3,8-diol]) *IR3535. Mosquito avoidance strategies include wearing long pants, long sleeves, and shoes with socks. Repellents can be applied to clothing.

Permethrin can be applied to clothing and gear. Clothing may also be purchased with permethrin already impregnated. Use of permethrin on human skin is not recommended. Use of products labeled for human use should not be applied to animals.

Environmental Products:

Spatial repellents are available to assist in repelling mosquitoes from designated areas. These products often contain synthetic pyrethroids (e.g. metofluthrin, allethrin) that are chemically related to permethrin.

Products designed for broadcasting in larger spaces (such as a backyard) are also available and can be applied by the consumer or a pest-control specialist. Take caution to follow the label directions for application. Remove sources of standing/stagnant water that would provide breeding sites for mosquitoes. Birds, bats, and fish can feed on various stages of mosquitoes and assist in reducing mosquito populations. Mosquito traps also can be used as one part of an integrated pest management control program, but should not be implemented as the only means of mosquito control.

Public Health Considerations

D. immitis is occasionally reported as the cause of human dirofilariasis in the United States. Infections are usually observed as pulmonary “coin” lesions or dermal nodules. Only rarely have adult worms been recovered from the hearts of humans. In the United States, infection in dogs and humans is most common in the southeastern US. The definitive treatment of Dirofilaria infection in humans is surgical removal of lung granulomas and dermal nodules. In most cases, treatment with medications is not necessary.

Suggested Reading

Russell RC, Otranto D, Wall R. 2013. Mosquitoes (Diptera: Culicidae) In: The Encyclopedia of Medical and Veterinary Entomology CABI, Oxfordshire, UK, pp. 243-282.

Triplehorn CA, Johnson NF. 2005. Order Diptera. In: Borror and DeLong’s Introduction to the Study of Insects Brooks/Cole, Belmont, CA, USA, pp. 672-744.

Woodbridge AF, Walker ED. 2009. Mosquitoes. In: Medical and Veterinary Entomology (2^nd Edition) GR Mullen, LA Durden (eds.) Elsevier AP, San Diego, pp. 207-260.

Ledesma N, Harrington L. 2011. Mosquito vectors of dog heartworm in the United States: Vector status and factors influencing transmission efficiency. Topics in Companion Animal Medicine 26 (4): 178-185.

Abbitt B, Abbitt, LG. 1981. Fatal exsanguination of cattle attributed to an attack of salt marsh mosquitoes (Aedes sollicitans). J American Veterinary Medical Association 179 (12): 1397-1400.

www.epa.gov/mosquitocontrol/general-information-about-mosquitoes

McCall J, Hodgkins E, Varloud M, et al. Blocking transmission from dogs to mosquitoes and from mosquitoes to dogs, using repellents and ectoparasiticides (insecticides and macrocyclic lactone preventives) as part of a multimodal approach – shifting the paradigm in Dirofilaria immitis prevention. Proceeding of the 15th Triennial Symposium, American Heartworm Society, September 11-13, 2016, New Orleans, LA.