Fleas are a relatively small but distinct group of insects known as the Siphonaptera. All fleas are parasitic as adults on warm-blooded animals (mammals or birds) and most are adapted to living on one type of animal (or close relations). Some species are able to feed on a range of hosts but may only be able to breed when feeding on their primary host. A number of species are important pests but the cat flea is by far the most common flea found in domestic premises. The cat flea is usually found in association with the domestic cat, but it is often found on dogs and will bite humans. Fleas cannot jump higher than 6 inches.

The adults spend only some of their time on the host animal, usually only to feed, the majority of the time is spent in the hosts bedding or carpets from where they can climb or hop back onto the host when they require a feed.

Fleas have a distinct larval (or grub) phase and an intermediate pupae stage between the larva and adult. The life cycle is therefore very different to the bed bugs and lice whose young are similar in appearance and habit to the adult and are called nymphs. The female flea may lay as many as 25 eggs per day with a total of about 800-1000 eggs during a lifetime, which may last over a year. The eggs may be laid in the host’s bed or lodge in the fur before dropping off onto the floor. They hatch after about 5 days and the small white larvae which emerge feed on organic debris and on adult flea faeces, rich in partly digested blood After about three weeks, during which time they molt twice, the fully grown larvae spin a silk cocoon covered in debris in which to pupate. They moult after three days and change into the pupae stage. A gradual transformation now takes place and after about 2 weeks the adult emerges from the pupa. It does not immediately leave the cocoon, however, unless stimulated by vibrations, which indicate the presence of a host. It can remain in this ‘waiting state’ for up to 1 year. Thus, a heavy flea infestation may appear in a building room, which has been unoccupied for some time. In good conditions in heated premises the life cycle takes about 4-6 weeks from egg too adult but longer at lower temperatures. Once they have emerged from the pupa, adults normally live for 2-4 months if feeding regularly. They can survive over a year at lower temperatures.

Both males and females require blood for nutrition and feed on nothing else. The female also requires blood from her preferred host in order to lay viable eggs, so an infestation where no domestic animals are present will not breed but will still need treatment as the fleas will feed of humans in the absence of domestic animals. Fleas will often prefer women & children to men, no one knows why.

Where conditions are favourable fleas will eat more than they need excreting more semi-digested blood than usual for their grubs to feed on. This ensures that the population expands when conditions are right.

It is often the case that a flea infestation will go unnoticed by the human inhabitants until the removal of the host animal, after which the fleas are forced to look for alternative food, i.e. humans. It is common that previously empty houses or houses where a cat was living will have a flea problem once the new occupiers move in. The original owners may have not realized there was a problem, as the fleas would have been feeding on the animal leaving the people alone.

The eggs of the flea are comparatively large, 0.5 mm in length and oval or round in shape, pearly white in colour and are sticky which attracts debris from the surrounding habitat, thus camouflaging their appearance.

For every flea found on the host there will probably be a hundred or so in the bedding (if there is any as most cats are allowed to sleep any where in the house) and carpets, hence the effectiveness of insecticidal collars on the animal without a full treatment is very limited. The cat flea commonly causes domestic infestations in Britain.