Temporal range: Middle Jurassic–Recent 
|Scanning electron microscope (SEM) picture of a flea|
Fleas are the insects forming the order Siphonaptera. They are wingless, with mouthparts adapted for piercing skin and sucking blood. Fleas are external parasites, living by hematophagy off the blood of mammals and birds.
Some flea species include:
- Cat flea (Ctenocephalides felis)
- Dog flea (Ctenocephalides canis)
- Human flea (Pulex irritans)
- Moorhen flea (Dasypsyllus gallinulae)
- Northern rat flea (Nosopsyllus fasciatus)
- Oriental rat flea (Xenopsylla cheopis)
Over 2,000 species have been described worldwide.
Morphology and behavior
Fleas are wingless insects (1/16 to 1/8-inch (1.5 to 3.3 mm) long) that are agile, usually dark colored (for example, the reddish-brown of the cat flea), with tube-like mouth-parts adapted to feeding on the blood of their hosts. Their legs are long, the hind pair well adapted for jumping: a flea can jump vertically up to 7 inches (18 cm) and horizontally up to 13 inches (33 cm), making the flea one of the best jumpers of all known animals (relative to body size), second only to the froghopper. If humans had the jumping power of a flea, a 1.8 m (6 ft) person could make a jump 90 m (295 ft) long and 49 m (160 ft) high.
Researchers with the University of Cambridge in England found that fleas take off from their tibiae and tarsi (the insect equivalent of feet) and not their trochantera, or knees. It has been known that fleas do not use direct muscle power but instead use the muscle to store energy in a protein named resilin before releasing it rapidly (like a human using a bow and arrow), with researchers using high-speed video technology and mathematical models to discover where the spring action actually happens.
Their bodies are laterally compressed, permitting easy movement through the hairs or feathers on the host's body (or in the case of humans, under clothing). The flea body is hard, polished, and covered with many hairs and short spines directed backward, which also assist its movements on the host.
The tough body is able to withstand great pressure, likely an adaptation to survive attempts to eliminate them by mashing or scratching. Even hard squeezing between the fingers is normally insufficient to kill a flea. However, rolling them back and forth a dozen times disables their legs, resulting in death.
Fleas lay tiny white oval-shaped eggs better viewed through a loupe or magnifying glass. The larva is small and pale, has bristles covering its worm-like body, lacks eyes, and has mouth-parts adapted to chewing. The larvae feed on various organic matter, especially the feces of mature fleas. The adult flea's diet consists solely of fresh blood. In the pupa phase, the larva is enclosed in a silken, debris-covered cocoon.
Life cycle and habitat
Fleas are holometabolous insects, going through the four life cycle stages of egg, larva, pupa, and imago (adult). Adult fleas must feed on blood before they can become capable of reproduction. Flea populations are evenly distributed, with about 50% eggs, 35% larvae, 10% pupae, and 5% adults.
The flea life cycle begins when the female lays after feeding. Eggs are laid in batches of up to 20 or so, usually on the host itself, which means that the eggs can easily roll onto the ground. Because of this, areas where the host rests and sleeps become one of the primary habitats of eggs and developing fleas. The eggs take around two days to two weeks to hatch.
Flea larvae emerge from the eggs to feed on any available organic material such as dead insects, feces, and vegetable matter. In laboratory studies, some dietary diversity seems necessary for proper larval development. Blood only diets allow only 12% of larvae to mature, whereas blood and yeast or dog chow diets allow almost all larvae to mature. They are blind and avoid sunlight, keeping to dark places like sand, cracks and crevices, and bedding.
Given an adequate supply of food, larvae will pupate and weave silken cocoons within 1–2 weeks after 3 larval stages. After another week or two, the adult fleas are fully developed and ready to emerge. They may remain resting during this period until they receive a signal that a host is near - vibrations (including sound), heat, and carbon dioxide are all stimuli indicating the probable presence of a host. Fleas are known to overwinter in the larval or pupal stages.
Once the flea reaches adulthood, its primary goal is to find blood and then to reproduce. Its total life span can be as short as one year, but may be several years in ideal conditions. Female fleas can lay 5000 or more eggs over their life, allowing for phenomenal growth rates. Average 30–90 days.Template:Contradiction-inline
A flea might live a year and a half under ideal conditions. These include the right temperature, food supply, and humidity. Generally speaking, an adult flea only lives for 2 or 3 months. Without a host for food a flea's life might be as short as a few days. With ample food supply, the adult flea will often live up to 100 days.Template:Contradiction-inline
Newly emerged adult fleas live only about one week if a blood meal is not obtained. However, completely developed adult fleas can live for several months without eating, so long as they do not emerge from their puparia. Optimum temperatures for the flea's life cycle are 21 °C to 30 °C (70 °F to 85 °F) and optimum humidity is 70%.Template:Contradiction-inline
Adult female rabbit fleas, Spilopsyllus cuniculi, can detect the changing levels of cortisol and corticosterone hormones in the rabbit's blood that indicate it is getting close to giving birth. This triggers sexual maturity in the fleas and they start producing eggs. As soon as the baby rabbits are born, the fleas make their way down to them and once on board they start feeding, mating, and laying eggs. After 12 days, the adult fleas make their way back to the mother. They complete this mini-migration every time she gives birth.
In the past, it was most commonly supposed that fleas had evolved from the flies (Diptera), based on similarities of the larvae. (Some authorities use the name Aphaniptera because it is older, but names above family rank need not follow the ICZN rules of priority, so most taxonomists use the more familiar name). Genetic and morphological evidence indicates that they are descendants of the Scorpionfly family Boreidae, which are also flightless; accordingly it is possible that they will eventually be reclassified as a suborder within the Mecoptera.
Their evolution continued to produce adaptations for their specialized parasitic niche, such that they now have no wings and their eyes are covered over. The large number of flea species may be attributed to the wide variety of host species they feed on, which provides so many specific ecological niches to adapt to. In any case, all these groups seem to represent a clade of closely related insect lineages, for which the names Mecopteroidea and Antliophora have been proposed.
Flea systematics are not entirely fixed. While, compared to many other insect groups, fleas have been studied and classified fairly thoroughly, details still remain to be learned about the evolutionary relationships among the different flea lineages.
- Suborder Pulicomorpha
- Superfamily Pulicoidea
- Superfamily Malacopsylloidea
- Family Malacopsyllidae
- Family Rhopalopsyllidae—hosts
- Family Vermipsyllidae—hosts: carnivores
- Superfamily Coptopsylloidea
- Family Coptopsyllidae
- Superfamily Ancistropsylloidea
- Family Ancistropsyllidae
- Suborder Pygiopsyllomorpha
- Superfamily Pygiopsylloidea
- Family Lycopsyllidae
- Family Pygiopsyllidae
- Family Stivaliidae
- Superfamily Pygiopsylloidea
- Suborder Hystrichopsyllomorpha
- Superfamily Hystrichopsylloidea
- Family Hystrichopsyllidae—hosts: rats and mice. Includes Ctenopsyllidae, Amphipsyllidae
- Family Chimaeropsyllidae
- Superfamily Macropsylloidea
- Family Macropsyllidae
- Superfamily Stephanocircidoidea
- Family Stephanocircidae
- Superfamily Hystrichopsylloidea
- Suborder Ceratophyllomorpha
- Superfamily Ceratophylloidea
- Family Ceratophyllidae
- Superfamily Ceratophylloidea
Relationship with host
Direct effects of bites
Fleas are a nuisance to their hosts, causing an itching sensation which in turn may result in the host attempting to remove the pest by biting, pecking, scratching, etc. in the vicinity of the parasite. Fleas are not simply a source of annoyance, however. Flea bites generally cause the formation of a slightly raised, swollen itching spot with a single puncture point at the center (similar to a mosquito bite).: 126
The bites often appear in clusters or lines of two bites, and can remain itchy and inflamed for up to several weeks afterwards. Fleas can also lead to hair loss as a result of frequent scratching and biting by the animal, and can cause anemia in extreme cases.: 126
As a vector
Besides the problems posed by the creature itself, fleas can also act as a vector for disease. Fleas transmit not only a variety of viral, bacterial and rickettsial diseases to humans and other animals, but also protozoans and helminths.: 72–73
- bacteria: Murine or endemic typhus.: 124 Fleas have helped cause epidemics by transmitting diseases such as the bubonic plague between rodents and humans by carrying Yersinia pestis bacteria. Fleas can transmit Yersinia pestis, Rickettsia typhi, Rickettsia felis, and Bartonella henselae.
- protozoa: Trypanosome protozoans such as those of the subgenus Herpetosoma, use a variety of flea species opportunistically as vectors.: 74
Fleas that specialize as parasites on specific mammals may use other mammals as hosts; therefore humans are susceptible to the predation of more than one species of flea.
A misconception concerning the carrying/transmission of the HIV/AIDS virus by fleas has been debunked by the Centers for Disease Control and Prevention (CDC 2003). According to the CDC and other sources, it is highly unlikely or impossible for fleas to carry the virus or spread it to other humans.
Fleas can settle in a person's hair in less than ten minutes, causing soreness and itching. The itching associated with flea bites can be treated with anti-itch creams, usually antihistamines or hydrocortisone. Calamine lotion has been shown to be effective for itching.
Modern flea control is approached using Integrated Pest Management (IPM) protocols at the host (pet) level. IPM is achieved by targeting fleas during at least two separate life stages, with at least two separate molecules. This is typically achieved using an adulticide to kill adult fleas and an insect development inhibitor (IDI), like lufenuron, or insect growth regulator (IGR), like methoprene, to prevent development of immature stages.
Flea adults, larvae, or eggs can be controlled with insecticides. Lufenuron is a veterinary preparation (known as Program) that attacks the larval flea's ability to produce chitin, necessary for the adult's hard exoskeleton, but does not kill fleas. Flea medicines need to be used with care because many of them also affect mammals.
Flea treatments that are meant for dogs can be hazardous to cats. Flea and tick ointment is also hazardous to humans; the label of a commercial preparation warns: “First aid: If on skin or clothing, take off contaminated clothing, rinse skin immediately with plenty of water for 15 – 20 minutes; call a poison control center or doctor for treatment advice. . . Although (the product is) applied only between the shoulder blades and at the base of the tail, the dog’s skin and hair oils carry the product over the entire body . . . Wash thoroughly with soap and water after handling . . .”
Cedar oil, a non-toxic natural substance, has been proven effective in the eradication of infestations in pets.[dubious ] However, use of some essential oils can be hazardous to cats, especially those containing phenols.
Since more than three-quarters of a flea's life is spent somewhere other than on the host animal, it is not adequate to treat only the host; it is important also to treat the host's environment. Thorough vacuuming, washing linens in hot water, and treating all hosts in the immediate environment (the entire household, for example) are essential and if possible must be performed on a regular basis.
Contemporary commercial products for the topical treatment of flea infestations on pets contain pesticides such as imidacloprid, permethrin, and (S)-methoprene. All flea control products are recommended to be used at least half-yearly because the life cycle of flea and tick can last to up to 6 months, and by using one of the flea and tick control products for so long, the infestation is highly prevented and, in the end, stopped. Although all these products are effective in fighting against flea and tick infestations, they have different active ingredients and, because cats cannot metabolize some of the compounds of the product, care must be taken in their use.
For the home
Combatting a flea infestation in the home takes patience because for every flea found on an animal, there could be many more developing in the home. A spot-on insecticide will kill the fleas on the pet and in turn the pet itself will be a roving flea trap and mop up newly hatched fleas. The environment should be treated with a fogger or spray insecticide containing an insect growth regulator, such as pyriproxyfen or methoprene to kill eggs and pupae, which are quite resistant against insecticides.
Frequent vacuuming is also helpful. The vacuum must be used around everything the animal frequents, to pick up all the larvae and eggs. Traditional advice recommends disposing of the bag after each vacuuming. One very limited experiment (only one vacuum sweeper used on a single type of carpeted surface) found that vacuuming killed 96 percent of adult fleas and 100 percent of younger fleas. This preliminary study's applicability to other circumstances is unclear.
Diatomaceous earth can also be used as a home flea treatment in lieu of acetylcholinesterase inhibitory treatments or insecticides which carry with them a risk of poisoning for both humans and other animals. However, diatomaceous earth dust is harmful to pets and people when inhaled, so use of a dust mask is recommended when applying it. Application is effective on both the interior and exterior of one's property, but the efficacy of Diatomaceous earth is diminished when introduced to water. Diatomaceous earth is commonly available in (amorphous silica) food grade quality; the grade used for pool filters (crystalline silica) should never be used for flea control, as it is not as effective and causes silicosis when inhaled. It is also effective to simply leave it exposed in areas typically vulnerable to fleas and other insects.
Bathing can dramatically reduce the flea population on a badly infested animal, especially when in combination with a mild detergent or shampoo, and brushing or combing.
Baking soda can be used to kill fleas via dehydration. It is available in large amounts as a food grade material and is safe for family and pets when used inside the home on carpets and floors. A layer can be sprinkled onto a carpet and worked into the fibers down to where the larvae and eggs are, and will dehydrate and kill them. The soda can be easily vacuumed up afterwards on the condition of safe disposal. Often multiple weekly treatments will be required to remove an infestation completely. Table salt can also be used inside the home in the same way as, or in combination with, baking soda as a low cost and safe method of breaking their life cycle. Pulverizing or grinding the salt with a coffee grinder will make it more effective as it will stick to the flea, killing it quicker through dehydration.
Borax is sold as a "Natural Laundry Booster" and can also be used as another home treatment for flea infestations. Borax contains sodium borate which kills fleas by dehydrating them, but its safety for pets is untested.
Fleas cannot withstand high temperatures, so a turn through the dryer on medium or high will kill the fleas.
To collect living fleas from a room or space, eliminate lighting as much as possible while focusing a single source of light just above the floor and directing the light downward. This is best done at night or by covering windows. A sticky bug pad can be placed under the light, or a plate of water mixed with typical soap for the fleas to fall into. The fleas will be drawn to the light and die in the water or be trapped on the sticky pad just below the light.
Vacuuming, temperature and humidity
A combination of controlled humidity, temperature, and vacuuming should eliminate fleas from an environment. Altering even one of these environmental factors may be enough to drastically lower and eliminate an infestation.
Vacuuming on a frequent basis, not only the places where the pet lies, but extensively, is particularly effective. A laboratory study done at the University of California showed that vacuuming catches about 96% of adult fleas.
In arid areas, less than 5% of flea eggs complete the life-cycle. Because humidity is critical to flea survival, eggs need relative humidity of at least 70–75% to hatch, and larvae need at least 50% humidity to survive. In humid areas, about 20% of the eggs survive to adulthood. Dehumidifiers with air conditioning and vacuuming all may interrupt the flea life cycle.
Lower temperatures slow down or completely interrupt the flea life-cycle. Fleas thrive at higher temperatures, but need 21° to 32 °C (70° to 90 °F) to survive.
Fleas can be drowned. One of the best references on the subject is Forensic Entomology: An Introduction By Dorothy Gennard; John Wiley & Sons, Apr 30, 2013; section 4.3 which reflects a 1985 work by Simpson K. (*Journal of the New York Entomological Society 76: 253-265, not finding this online*). Gennard is using the fleas present on a human body at death to determine how long the body had been submerged. Findings -
- A flea submerged for up to 12 hours will appear to be dead, but can revive in about 60 minutes after being removed from the water.
- A flea submerged for 18 – 20 hours will appear to be dead, but can revive in 4 – 5 hours after being removed from the water.
- It takes 24 hours of full submersion to fatally drown a flea.
- Side note; lice can be fatally drowned in about 12 hours.
Soap can facilitate the death of fleas in bathing. The work Medical and Veterinary Entomology by Gary R. Mullen, Lance A. Durden Academic Press, Apr 22, 2009 suggests the process of washing the flea (and the pet) removes integumental waxes on the flea's body and it dies from desiccation (dehydration). There does not seem to be a lot of solid research specific to fleas (Siphonaptera) and soap (surfactants), but there are many general insect works. There are a number of pesticides which include surfactants (soap) in their make-up to increase effectiveness but these seem to be an aid in the delivery of the pesticide and are not added because the surfactants have any additional killing power.
"A surfactant may affect the efficacy of an insecticide by its influence on wetting, spreading and run-off rather than by its influence on cuticular penetration."  There is some science suggesting soaps can help break down cell membranes but they seem to be most effective on soft bodied insects (which fleas are not) "Soaps... kill insects by disrupting the exoskeleton and breaking down cell membranes. Soaps generally work best against small soft bodied insects such as aphids, scale crawlers, meatybugs, and young caterpillars as well as spider mites."
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