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Author: Daniel Jun Yi Wong, Medical Student, University of Melbourne. Updated by Vanessa Ngan, Staff writer, 1 February 2014.
Fleas are insects from the order Siphonaptera. They are parasites that survive from sucking the blood from their hosts. Fleas that primarily feed on humans include the human flea, Pulex irritans, and dog and cat fleas, Ctenocephalides canis and Ctenocephalides felis.
Flea infestations can be difficult to eradicate.
The life cycle of the flea consists of four main stages: egg, larva, pupa, and adult.
The adult flea survives exclusively through blood meals from its host. During feeding, the flea has a specialized mouthpiece that punctures intact skin and syphons blood. It simultaneously secretes saliva, which can be very antigenic (allergy-inducing). The saliva helps to prevent the host's blood from clotting, which facilitates their feeding.
A flea has three pairs of legs, with long hind legs ideal for jumping. They are wingless. They can jump up to 18 cm in height. Adult fleas of domestic animals tend to live on the animal for easy access to blood meals.
The female fleas lay white, round eggs that tend to fall off the animal onto bedding, floor and soil. The emerging larva feeds on organic material in a moist environment including the faeces of adult fleas. After sufficient growth and development, the larva spins itself a cocoon and pupates. Cocoons are found in soil, carpets, under furniture and on animal bedding. With the right stimuli, such as warm temperatures, high humidity or vibrations from movement, the pupa emerges from its cocoon as an adult flea. 
The life cycle of the flea can range from 2 weeks to up to 2 years in duration, and is usually within 8 weeks. Survival depends on adequate temperature and high humidity.
Adult fleas move toward a light source, then jump when the light is shadowed e.g. by a suitable warm-blooded host. The flea starts feeding immediately and is estimated to consume 13.6µL of blood per day. They soon die if removed from the host, and rarely colonise another animal.
Fleas can be hard to see. They are dark and wingless. They are more obvious if they are jumping onto a person wearing white socks. Flea faeces (‘flea dirt’) appear as black pepper grains, and, when seen on a damp towel, are red, as a result of blood in the faeces.
Risk factors for flea bites include:
Species that primarily infest cats and dogs may bite humans if they have been starved since their last blood meal. Vibrations, e.g. from footsteps when entering a house, can also stimulate adult fleas to emerge from cocoons (pupal stage) and result in human bites .
Prevention of human flea bites is best achieved by:
Flea eggs can survive for several weeks and pets are readily reinfestated, so maintain these strategies long term to prevent perpetuating infestation. Difficult infestations may require the help of a professional exterminator. 
The main symptom of flea bites is intense itching (or pruritus). The bite may also be painful. Scratching can lead to secondary infection.
Some individuals mount an excessive immune response (allergy) to flea saliva resulting in a more pronounced or persistent bite reaction called papular urticaria, a form of prurigo. The site of the bite may become red and swell, sometimes with a central blister, which may break becoming oozy and crusted. Old bite marks may reactivate. Anaphylaxis has not been reported. 
Flea bites are most often found on the lower legs, where the fleas can have access to the skin from jumping height, or at the waistband where there is a gap between shirt and trousers or skirt.
A flea can act as a transporter (called a vector) for other diseases caused by bacteria and viruses. For example:
The tropical species of sand fleas (Tunga penetrans) burrow into human skin and cause an inflammatory reaction known as tungiasis. Tungiasis typically appears as inflamed papules on the feet, especially in the webspaces between the toes, causing pain, itch and secondary staphylococcal skin infection .
It is important to attempt elimination of the fleas from the environment – including from flooring and bed linen (where eggs are lain) and from household pets.
Treatment for flea bites may include:
Pain, redness, oozing, crusting and pustules at the site of a bite indicates secondary bacterial infection, often requiring topical or oral antibiotics.
Several measures have been suggested to reduce fleas in the home environment.
Pets carry about 5% of the total flea population and are the main reservoir of adult fleas. To minimise the use of potentially toxic insecticides:
Ask your veterinarian's advice about which insecticides should be used for your pet. Prescription chemicals will be different in dogs and cats. Some methods of eliminating fleas from cats and dogs are listed below. Many of these agents are only available from a registered veterinarian.
Combination products are the most effective insecticides but are more expensive.
The following traditional flea-control agents are toxic to humans and pets and are best avoided (with the exception of permethrin, which may be used in dogs). In addition, insect resistance is increasing to these chemicals.
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