Vectors

A vector is any animal or insect that is capable of transmitting pathogens to people or harming them. Common vectors around the world are rodents, mosquitoes, ticks, and fleas. In our District, mosquitoes are the primary vectors of public health concern.

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Mosquitoes

Roughly 50 mosquitoes species can be found in California. Only 15 of these mosquito species are considered to be of public health importance in Tulare County because of their nuisance biting or their potential to transmit pathogens through their bites. By understanding the general mosquito life cycle, we can reduce the overall mosquito population and bites.

Life Cycle

Mosquitoes have four life stages: egg, larva, pupa, and adult. All but the final, adult life stage takes place in water. By reducing or eliminating standing water, we can stop mosquito bites before they start. No standing water means no mosquitoes!

Eggs are the first stage of the mosquito life cycle. A single female mosquito lays about 100-300 eggs at a time. Anopheles mosquitoes lay single eggs at a time while Culex and Culiseta mosquitoes lay eggs in the form of rafts on top of standing water. Aedes mosquitoes lay their eggs along the waterline of containers, treeholes, and other water sources. Most eggs will hatch into larvae within 24 to 48 hours.

Eggs are the first stage of the mosquito life cycle. A single female mosquito lays about 100-300 eggs at a time. Anopheles mosquitoes lay single eggs (right) at a time while Culex and Culiseta mosquitoes lay eggs in the form of rafts on top of standing water (left). Aedes mosquitoes lay their eggs along the waterline of containers, treeholes, and other water sources (middle). Most eggs will hatch into larvae within 24 to 48 hours.

Larvae live in the water but breathe air at the water’s surface. They feed on microorganisms and organic matter in the water. When they sense movement or are disturbed, larvae dive to the bottom of their water sources for safety. Larvae shed their exoskeleton four times, in a process called molting, to grow. The stages between molts are called instars. After the fourth molt, the larva becomes a pupa.

Photo Credit: James Gathany, 2003, CDC

Pupae are the final developmental stage before a mosquito becomes an adult. Like larvae, pupae need to breathe air at the water’s surface and dive to the bottom to avoid predators. Unlike larvae, pupae do not eat. Instead, they are rapidly developing into an adult mosquito. After a day or two, the pupal skin splits open and the adult mosquito emerges.

Photo Credit: Alberto Garcia 2004.

It takes about 5 to 7 days for eggs to develop into adult mosquitoes. Warmer weather speeds up development while cooler weather slows it down. Both male and female mosquitoes feed on nectar. Only adult female mosquitoes bite people or animals. A combination of factors including carbon dioxide and smell influence biting or blood feeding. Female adult mosquitoes use protein from the blood meal to produce eggs. Over the course of her lifespan, a female mosquito can lay about four batches of eggs.

The lifespan of an adult mosquito depends on the species, gender, and environmental factors. Female mosquitoes live about 2 to 4 weeks. Male mosquitoes typically have shorter lifespans.

Culex Mosquito Species 

Culex quinquefasciatus (Southern House Mosquito)

Season: Spring to fall

Prefers to Bite: Birds, large mammals, and humans

Bites During: Dawn, dusk, and at night

Breeds in: Ponds, catch basins, neglected pools, and other residential sources

Public Health Importance: Capable of transmitting West Nile Virus and St. Louis Encephalitis virus.

Significance: High pest significance and seasonal disease risk

Photo Credit:James Gathany, 2001, CDC

Culex tarsalis (Western Encephalitis Mosquito)

Season: Spring to fall

Prefers to Bite: Birds, mammals, and humans

Bites During: Dawn, dusk, and at night

Breeds in: Natural and man-made fresh water sources

Public Health Importance: Capable of transmitting West Nile Virus, St. Louis Encephalitis virus, and Western Equine Encephalitis virus

Significance: High pest significance and seasonal disease risk

Photo Credit: Pablo Cabrera, 2014, SGVMVCD

Culex stigmatosoma (Foul Water Mosquito)

Season: Spring to fall

Prefers to Bite: Birds

Bites During: Dawn and dusk

Breeds in: Polluted water, dairy ponds, sewer ponds, log ponds

Public Health Importance: Capable of transmitting St. Louis Encephalitis virus

Significance: Low pest significance

Culex erythrothorax (Tule Mosquito)

Season: Spring to fall

Prefers to Bite: Birds, humans, and small mammals

Bites During: Dawn and dusk

Breeds in: Ponds and lakes with tules and cattails

Public Health Importance: Localized pest

Significance: Moderate pest significance

Aedes Mosquito Species

Aedes aegypti (Yellow Fever Mosquito)

Season/Active: Mid to late summer and fall

Prefers to Bite: Humans

Bites During: Daytime and near dusk and dawn

Breeds in: Small, man-made containers like pet water bowls and plant trays

Public Health Importance: Capable of transmitting Zika, Dengue, Chikungunya, Yellow Fever

Significance: High pest significance, currently no local disease transmission

Photo Credit: James Gathany, 2006, CDC

Aedes sierrensis (Western Tree-Hole Mosquito)

Season: Spring

Prefers to Bite: Small mammals and humans

Bites During: Daytime and dusk

Breeds in: Treeholes, tires, and other containers

Public Health Importance: Capable of transmitting Dog heartworm

Significance: High pest significance

Photo Credit: R. Berg, 2011, bugguide.net

Aedes vexans (Inland Floodwater Mosquito)

Season: Spring and summer

Prefers to Bite: Humans and large mammals

Bites During: Day and night

Breeds in: Irrigated pastures and woodland watercourses

Public Health Importance: Capable of transmitting Western Equine Encephalitis virus

Significance: High pest significance, low disease risk

Photo Credit:Perry Babin, 2006, bugguide.net

Aedes nigromaculis (Irrigated Pasture Mosquito)

Season: Spring to early fall

Prefers to Bite: Large mammals and humans

Bites During: Dusk and daytime

Breeds in: Irrigated pastures and alfalfa fields

Public Health Importance: Pest

Significance: High pest significance

Photo Credit: Joseph Berger, bugwood.org

Aedes melanimon

Season: Spring to early fall

Prefers to Bite: Large mammals and humans

Bites During: Dusk

Breeds in: Duck ponds, irrigated pastures and fields

Public Health Importance: Capable of transmitting California Encephalitis virus and Western Equine Encephalitis virus

Significance: High pest significance, low disease significance

Anopheles Mosquito Species

Anopheles franciscanus

Season: Late summer to early fall

Prefers to Bite: Small and large mammals

Bites During: Dusk

Breeds in: Shallow, sunlit pools along receding streams algal growth

Public Health Importance: Occasional pest

Significance: Low to moderate pest significance

Photo Credit: Don Loarie, 2015

Anopheles freeborni (Western Malaria Mosquito)

Season: Summer

Prefers to Bite: Large mammals and humans

Bites During: Dawn, dusk, and night

Breeds in: Vegetated pools, algal mats, and agricultural areas

Public Health Importance: Capable of transmitting malaria

Significance: Low to moderate pest significance. Malaria is no longer transmitted locally.

Photo Credit: James Gathany, 2004, CDC

Anopheles punctipennis (Woodland Malaria Mosquito)

Season: Late spring and summer

Prefers to Bite: Large mammals and humans

Bites During: Day and dusk

Breeds in: Clear seepage, water in sunlit algae laden pools

Public Health Importance: Capable of transmitting malaria

Significance: Moderate pest significance. Malaria is no longer transmitted locally.

Photo Credit: Mike Quinn, 2009, TexasEnto.net

Culiseta Mosquito Species

Culiseta incidens (Cool Weather Mosquito)

Season: Spring and fall

Prefers to Bite: Large mammals and humans

Bites During: Dawn and dusk

Breeds in: Shaded, clear, natural or man-made sources, rivers, and artificial containers

Public Health Importance: Localized pest

Significance: Low pest significance

Photo Credit: Shawn McCann, 2007

Culiseta inornata (Winter Mosquito)

Season: Spring and fall

Prefers to Bite: Large mammals and humans

Bites During: Dawn and dusk

Breeds in: Sunlit pools or man-made sources, duck ponds, and irrigation ditches 

Public Health Importance: Localized pest

Significance: Low pest significance

Culiseta particeps

Season: Spring and fall

Prefers to Bite: Large mammals and humans

Bites During: Dawn and dusk?

Breeds in: Shaded, clear pools with algae

Public Health Importance: Localized pest

Significance: Low pest significance

Ticks

Ticks are ectoparasites that feed on blood from reptiles, birds, mammals, and occasionally amphibians. Unlike mosquitoes, ticks do not need water to complete their life cycle and both male and female ticks bite. Ticks require a blood meal at each life stage to survive. Like mosquitoes, infected ticks can also transmit pathogens to people through their bite.

Life Cycle

Ticks have 4 life stages: egg, larva, nymph, and adult. Both male and female ticks must have a blood meal during each life stage to survive. Common hosts for blood meals include reptiles, rodents, birds, dogs, cats, deer, and other mammals. Some ticks, such as the brown dog tick, prefer to bite the same host for their entire life cycle, while others may prefer to bite a different host at each stage. The entire tick life cycle, from egg to adult, can take up to 3 years.

Ticks find hosts by waiting on the edges of grass, shrubs, and other vegetation with their first pair of legs extended. This waiting position is known as questing. When a host brushes against their waiting spot, the tick can quickly latch onto the host. Ticks can recognize hosts through a variety of physical and chemical cues including body odor, heat, moisture, and vibration.

American Dog Tick (Dermacentor variabilis)

Season: Spring and summer

Prefers to Live: Outdoors in grassy areas with tall foliage, especially areas with a change in terrain like the edge of trails.

Public Health Importance: Capable of transmitting tularemia, Rocky Mountain Spotted Fever, and tick paralysis

Significance: Low pest and disease significance locally

Photo Credit: James Gathany, CDC

Brown Dog Tick (Rhipicephalus sanguineus)

Season: Year-round

Prefers to Live: Indoor spaces where dogs are present, yards

Public Health Importance: Capable of transmitting Rocky Mountain Spotted Fever

Significance: Low pest and disease significance locally

Photo Credit: James Gathany, 2005, CDC

Pacific Coast Tick (Dermacentor occidentalis)

Season: Cooler months, especially April & May

Prefers to Live: Shrublands, chaparral, and along trails

Public Health Importance: Capable of transmitting 364D rickettsiosis, tularemia, and tick paralysis

Significance: Low pest and disease significance locally

Photo Credit: University of Rhode Island, tickencounter.org

Western Black Legged Tick (Ixodes pacificus)

Season: Adults are active in late fall to early spring while nymphs are active in spring and early summer

Prefers to Live: Cool, moist areas and on wild grasses and low vegetation, leaf litter, and logs in both urban and rural areas

Public Health Importance: Capable of transmitting Lyme disease and human granulocytic anaplasmosis.

Significance: Low pest and disease significance locally

Photo Credit: James Gathany, 2006, CDC

Soft Ticks (Ornithodoros species)

Season: Year-round

Prefers to Live: Forested foothills and mountains. Cool dark places such as rodent nests, wood piles, and indoors between walls and under floorboards.

Public Health Importance: Capable of transmitting tick-borne relapsing fever

Significance: Low pest and disease significance locally

Photo Credit: CDC

Fleas

Fleas are common pests in domestic and wild animals. They can become a nuisance to people who come into contact with infested animals. Although rare in California, fleas can also serve as vectors for several pathogens including murine typhus and plague.

Life Cycle

Fleas have four life stages: egg, larva, pupa, and adult. Eggs are typically laid in the host animal’s nest or sleeping area where they hatch into worm-like larvae. Flea larvae feed on organic matter or adult flea feces found in the host’s sleeping area. After three instars or molts, the larva creates a cocoon and molts into the pupal stage. Adults emerge from the cocoon and are ready to feed within a day. Both male and female fleas feed on blood. The entire life cycle, from egg to adult, can take as little as 2 weeks to as long as a year depending on the species and environment.

Dog Flea (Ctenocephalides canis)

Season: Year-round but most active in summer months

Prefers to Bite: Domestic dogs and sylvatic carnivores

Public Health Importance: Localized pest capable of transmitting urban plague, murine typhus

Significance: Low pest and disease significance within District

Photo Credit: Ken Walker, 2007, Parasite and Diseases Image Library, Australia

 

Cat Flea (Ctenocephalides felis)

Season: Year-round but most active in summer months

Prefers to Bite: Any warm-blooded animal

Public Health Importance: Localized pest capable of transmitting urban plague, murine typhus

Significance: Low pest and disease significance within District

Photo Credit: Ken Walker, 2007, Parasite and Diseases Image Library, Australia

 

Human Flea (Pulex irritans)

Season: 

Prefers to Bite: Humans, domestic animals particularly pigs, and large carnivores

Public Health Importance: Localized pest capable of causing allergic dermatitis

Significance: Low pest and disease significance within District

Photo Credit: Ken Walker, 2007, Parasite and Diseases Image Library, Australia

 

Oriental Rat Flea (Xenopsylla cheopis)

Season:

Prefers to Bite: Rats, ground squirrels, and lagomorphs

Public Health Importance: Localized pest capable of transmitting urban plague and murine typhus

Significance: Low pest and disease significance within District

Photo Credit: James Gathany, 2017, CDC

 

Ground Squirrel Flea (Oropsylla montana)

Season: Cool weather

Prefers to Bite: Ground squirrels

Public Health Importance: Localized pest and sylvatic plague vector

Significance: Low pest and disease significance within District

Rodents

Rodents are a diverse group of mammals that includes mice, rats, squirrels, chipmunks, and marmots. Like other vectors, rodents can be both pests and disease vectors.