Fleas are small wingless insects of the order Siphonaptera. They are bloodfeeders and live as external parasites (ectoparasites) in the hair of mammals and the feathers of birds, and are thus nuisance pests. Furthermore, because they readily transfer from one host to another, they are very important vectors of human disease including plague, typhus, and tularemia.
Some people suffer more from flea bites, these can cause intense itching often resulting in secondary infection. The usual flea bite has a small red spot where the flea has inserted its mouthparts. Around the spot there is a red halo with very little swelling. Many people do not react to flea bites at all while others are sensitive and suffer severe allergic reactions.
Some 1,600 species and subspecies of fleas are known. All the adults feed avidly and repeatedly on blood. Some species of fleas, such as shrew fleas, are highly host specific, but others will parasitize a variety of animals, for example, the cat flea Ctenocephalides felis which is the most common flea, infests domestic cats (and dogs), leopards, mongooses, foxes, civets, dogs and opossums, and in their absence, will readily feed on human beings. Severe dermatitis and intense pruritus may follow infestation by fleas. Often the eruption resulting from flea bites in allergic persons is mistakenly diagnosed as hives and attributed to some kind of food. Species attacking man and his domestic animals include the human flea Pulex irritans, the cat flea, and the stickbight flea Echidnophaga gallinacea. In heavy flea infestations, animals may be severely damaged or killed by the effects of flea bites and by the considerable loss of blood. Poultry may be parasitized by the western chicken flea Ceratophyllus niger and the European chicken flea.
Two species of fleas, the stickbight flea and the sand flea Tunga penetrans penetrate the skin of the host and embed prior to egg deposition. The sand flea burrows into the soft skin of the feet of man. Intense itching accompanies the growth of the pregnant flea to pea-size. Itching and irritation may lead to secondary infections, and a number of deaths have occurred from gas gangrene and tetanus. The stickbight flea is similar to the sand flea, but attacks the heads of poultry, cats, and dogs. Certain fleas that primarily feed on rodents and birds will, if need be, attack man, as when hungry oriental rat fleas Xenopsylla cheopis abandon their hosts dying of bubonic plague, thereby transmitting the plague bacillus to man. Other species of fleas, such as Xenopsylla brasiliensis and Nosopsyllus fasciatus, also transmit plague, and are widespread among rodents and small mammals. Natural infection among animals with the plague cacillus has been demonstrated in more than 90 species of fleas, representing 45 genera and 9 families. Fleas are believed to be the principal vectors of murine typhus to man from rats and mice; of various enzootic infections among animals, including tularaemia and Russian spring-summer encephalitis; of myxomatosis, a virus disease of rabbits; of a filorial worm of dogs; and of a common tapeworm Diplyidium caninum of dog, cats and, occasionally, children.