Fleas

Distribution

Adult fleas live exclusively as parasites of warm-blooded animals, especially mammals, although birds may also be attacked. Whilst they show a certain degree of host preference, fleas are by no means specific, and will feed on other animals in the absence of the normal host. In fact, they tend to be more nest than host specific, for while the adults may feed on the blood of a variety of animals, the larvae require more precise conditions which are associated with the habitats and nesting habits of the hosts rather than the characteristics of their blood.

Cat fleas are responsible for many flea infestations, the remainder being attributable to a variety of bird and animal species. Pulex irritans (human flea) infestations are rare in this country. The significance of Ctenocephalides felis (the cat flea) is explained by the increased number of pets being kept and the tendency for their beds to be neglected during cleaning. Wall-to-wall carpeting also provides a relatively undisturbed environment for flea larvae to develop, while the spread of central heating has served to ensure ideal temperature conditions.

Significance

Fleas may be vectors of disease or may transmit parasitic worms. The most serious infection which they can spread is bubonic plague, transmitted to man by rodent fleas (Xenopsylla cheopis) which carry the causative bacillus from infected rats. In the past, rodent fleas have been responsible for serious epidemics of the disease, notably the Great Plague of London in 1665.

Rodent fleas may also carry murine typhus and, because of their readiness to attack humans as well as rats, are probably the major flea vector of disease.

The dog flea (Ctenocephalides canis) is an intermediate host of the dog tapeworm (Dipylidium caninum), whose vertebrate host is usually the dog, but occasionally the cat. It can sometimes be transmitted to humans.

In Europe, fleas are not generally responsible for the transmission of disease. However, they are still objectionable because of the bites they inflict and the deep-rooted social stigma attached to people with flea infestations. Flea bites are identified as a tiny dark red spot surrounded by a reddened area. The bite persists for one or two days, and is intensely irritating. First bites are not generally liable to cause serious reactions, but they may lead to hypersensitivity. Reactions are usually delayed following regular biting over a long period. There will then follow a period when reactions are immediate. The cycle then repeats until a state of non-reactivity - immunity - is achieved.

Occasionally psychological problems arise with the induction of delusory parasitosis, in which the victim imagines himself to be infested with ectoparasites.

Life-Cycle

Flea eggs are about 0.5mm long, oval, pearly-white in colour, and laid indiscriminately in the fur or feathers of the host or in its nest or bedding. They do not stick to the host, but readily fall from the animal, are shaken or scratched off. The same applies to the dark-coloured faeces of the adult fleas. This gives rise to the black-and-white / salt-and-pepper effect associated with a flea infestation. Four to eight eggs are laid after each blood meal, and a single female may produce 800 - 1,000 eggs during her lifetime, which may be as long as two years.

The eggs hatch in about one week, producing white, thread like, legless larvae 1.5 mm long, which may be recognised by:

• an identifiable, usually brownish, eyeless head
• jaws designed for biting
• three thoracic segments and ten abdominal segments, all equipped with bristles
• peg-like processes on the terminal abdominal segment

The larvae thrive in dark, humid places, such as animal bedding and carpet fluff, and feed on organic debris and adult flea excrement. The latter forms a valuable part of the diet as a source of blood, which some larvae require for their development. Larvae may also be predacious, living on small and weak arthropods. Cats' bedding may support a flea population of 8,000 immature and 2,000 adult forms. A typical flea infestation may comprise 5% adults, 35% larvae, 10% pupae, and 50% eggs.

After 2-3 weeks, by which time they will have moulted twice and be about 5 mm long, the larvae spin silken cocoons, incorporating debris, in which to pupate. The cocooned larvae then moult within three days to produce the pupae, which are initially creamy-white, but change to dark brown as they mature to become adults. This phase is the quiescent stage, and the flea may over-winter in this state. The adult flea will then be stimulated to emerge by the vibrations set up by a passing host. This explains the occasional mass attacks which occur in deserted premises.

The development cycle from egg to adult is normally completed in 4 weeks, but at low temperatures will take much longer.