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Class Insecta
Order Siphonaptera

Fleas are highly specialized bloodsucking parasites belonging to the order of insects called Siphonaptera, which means "wingless siphon". They have a formidable reputation of claiming more victims than all the wars ever fought, as a result of the "bubonic" (Black Death) plague they spread throughout the world in the 14th century causing the deaths of over 200 million people. Now, these insects are better known for their irritation and pest status worldwide.

Fleas are light brown to mahogany in color and roughly oval shaped. Their laterally flattened appearance enables them to quickly move through the host's hair. Measuring 2-8 mm in length, the adults are entirely covered with a series of bristles and combs that assists them in clinging to the host. The small head is equipped with sawing and sucking mouthparts, and two tiny simple eyes. To aid in the detection of a host, fleas possess two short antennae on the head that are sensitive to stimuli including heat, vibration, traces of carbon dioxide and change in air currents and shadows. The hind pair of legs that are well developed for jumping enable fleas to be propelled 10-30cms, either to make contact with a host or avoid a threatening situation.

Both female and males fleas rely on blood for their nutrition, but can survive for several months without it. When a flea blood feeds, it will crouch low to penetrate the host's tissue with a sawing motion of the mouthparts. A small amount of anti-coagulant is injected with the saliva, to permit easy siphoning of the blood. Fleas will bite only accessible parts of the body and clustered bites on the lower limbs are diagnostic. Blood feeding maybe interrupted, and fleas will often probe several times before repletion which can increase their total body weight by 30%. Each female flea uses her blood to nourish developing eggs, and will deposit up to 4 eggs after each blood meal; most females will lay at least 100 eggs within a life cycle of several months. The eggs are oval, white to cream in colour and measure 0.5mm in length; they can hatch within 1 week, but this will be dependent on prevailing conditions as larvae are extremely sensitive to desiccation. When the maggot-like larvae emerge, they are sparsely covered in hair and have no legs but are capable of moving rapidly in search of food, which consists mainly of skin scales or undigested blood excreted by the adults. Within a 1-3 week period, the larvae will grow and undergo 4 moults prior to pupating in a silken cocoon which they spin. The adult fleas emerge from the pupal case in 1-2 weeks but can remain dormant in their cocoons for several months depending on the availability of food and conditions. Often the emergence of adults from the pupal stage is triggered by vibrations, which occasionally happens on entering an unoccupied home of previous pet owners.

Some fleas can attack a range of hosts, and their ability to transfer from one host to another allows for the possible transfer of pathogens including viral, bacterial and parasitic diseases. The main flea species that attack humans include the cat flea Ctenocephalides felis, the dog flea C. canis, and the human flea Pulex irritans. The latter two species are relatively rare. The common cat flea is found on both cats and dogs, and is the species usually identified in attacks on humans and usually responsible for flea plagues. Cat fleas are the intermediate host for the dog and cat tapeworm (Dipylidium caninum) which is easily transmitted to humans.

Murine typhus is transmitted from rats to humans by particular rat fleas, typically Xenopsylla cheopis, and although it has been widespread, it is uncommon.

The continual biting activity of fleas alone causes a great deal of irritation and distress to humans, especially during flea plagues. Reactions to the flea's saliva are often delayed, with the formation of a wheal surrounding each puncture site within 5-30 minutes of the bite, accompanied by intense itching. Within 12-24 hours each wheal may progress to a small lesion or vesicle. The onset of symptoms in sensitized individuals often develops much later, and the initial reaction may become apparent only after 12-24 hours. Fleas are the major cause of papular urticaria, particularly on the legs of children, and continual scratching may lead to secondary infections.

The Human Flea

Human Flea

Close up of the head of a preserved human flea, Pulex irritans.

The Cat Flea

Cat flea

A preserved specimen of the Cat flea, Ctenocephalides felis.

Live Cat Flea

Live Cat Flea

A live specimen of the Cat flea, Ctenocephalides felis, showing actual color.

Cat Flea Larva

Cat Flea larva

The larva of a Cat flea. The larvae feed on human skin scales or undigested blood excreted by the adults.

The Rat Flea

Rat flea

The Rat flea, Xenopsylla cheopis, capable of transmitting Murine typhus.