Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Arthropoda (arthropods, segmented body, exoskeleton, jointed appendages)
Uniramia (with antennae, first mouthparts mandibles)
Insecta (insects, 3 body parts, 6 legs, many with wings)
Siphonaptera (fleas, wingless, laterally compressed, parasitic, complete metamorphosis)

Family: Pulicidae
Fleas are bilaterally-flattened wingless insects with three body parts, head, thorax and abdomen. The thorax has 6 legs arranged in 3 bilateral pairs, and the hindlimbs are enlarged and specially adapted for jumping (using elastic resilin pads rather than muscles). Fleas undergo complete metamorphosis whereby grub-like larvae form pupae from which adult fleas emerge. The larvae are not parasitic but feed on debris associated mainly with bedding, den or nest material, whereas the adult stages are parasitic and feed on host blood. This family contains several genera and species that are important parasites of humans, domestic and companion animals and wildlife, especially rodents.

Ctenocephalides spp. [these species cause dermatoses in domestic animals]

Parasite morphology: Fleas form four developmental stages: eggs, larvae, pupae and adults. The eggs are pearly-white ovoid bodies up to 0.5mm in size. Larvae appear as slender elongate brown grubs up to 5mm long, with each segment bearing a ring of bristles. Pupae appear as opaque ellipsoidal encysted stages surrounded by thin silk cocoons, often with detritus adherent to the external surface. Adult fleas vary in size according to gender, female fleas are larger measuring up to 2.5mm in length, while males are smaller, sometimes measuring less than 1mm in length. All adults have three distinct body segments; head, thorax, and abdomen. The head often bears genal ctenidium (spines), the dog flea C. canis and the cat flea C. felis have genal ctenidia with >5 teeth. The spacing of the spines is correlated to hair diameter. They are backward facing and used with setae to maintain position among the hair/fur of the host despite grooming.

Host range: Adult fleas attach to dogs, cats, humans, other mammals and occasionally chickens. Most fleas have promiscuous feeding habits and will try to feed on any available host. Most flea species are considered to be host-preferential rather than host-specific.

Site of infection: Adult fleas are blood-sucking ectoparasites living amongst the hair/fur on the skin of their hosts. They can also live off their hosts for extended periods in suitable micro-habitats (bedding, carpets, etc) awaiting the arrival of new hosts on which to jump.

Pathogenesis: Fleas have piercing mouthparts composed of cutting laciniae (back-and-forth action) and a stabbing epipharynx which enters small blood vessels. Saliva is ejected into the general area. Bite sites develop erythematous (reddened) papules or wheals, surrounding the central puncture site. Wounds may persist for days to several weeks and develop a crust of dried exudate. They are intensely itchy (pruritis) and may develop secondary infections if disturbed by scratching. Fleas are particularly annoying pests on dogs and cats, and can cause severe allergic reactions; especially in inbred strains. Flea-allergy dermatitis is a hypersensitive reaction to components of flea saliva injected into the skin. Severely-affected areas exhibit significant hair loss (alopecia), moist dermatitis (wet eczema) or the skin becomes hardened and thickened. Animals aggravate conditions by licking, biting and scratching and they exhibit restlessness, irritability, and weight loss. Fleas are blood-feeders (ingesting up to 10 ?l per day), so heavy infestations may also cause iron-deficiency anaemia, particularly in young animals. Fleas may act as vectors for a range of viral and bacterial infections and Ctenocephalides and Pulex fleas are intermediate hosts for the tapeworm Dipylidium caninum in dogs and cats.

Mode of transmission: Fleas undergo complete metamorphosis (egg-larva-pupa-adult). The female usually oviposits on the host but the eggs are not sticky and therefore drop off the host usually in den/lair/nest/bedding where there is a good supply of debris and flea faeces on which the larvae feed. The eggs hatch within 2-21 days releasing maggot-like larvae which are legless and eyeless. Larvae cannot close their spiracles and are sensitive to low humidity. There are usually 3 larval instars which moult over 9-15 days before forming a pupa. The pupa completes development over several days to several months. Low temperatures, however, can extend larval and pupal stages up to one year. Adults can survive long periods without food (up to 100 days at high humidity).

Differential diagnosis: Animals attempt to groom infested areas, and an ‘itch-and-scratch’ syndrome may develop, sometimes associated with intense inflammation or allergic reactions. Adult fleas can be found in infested areas by visual examination (manually parting hairs or using a fine -toothed comb).

Treatment and control: Many chemicals have been developed to kill fleas. These insecticides can be used as powders, washes, sprays, pour-ons or impregnated into collars. They are generally organophosphorous compounds, carbamates, or pyrethrum and its derivatives. Several new generation ectoparasiticides have also been developed as spray or spot-on formulations, including fipronil and imidacloprid. Treatments should be repeated regularly to avoid re-infestation and also to reduce environmental contamination by eggs. Drug efficacy should also be monitored as there are growing reports of insecticide resistance developing in flea populations. Corticosteroids are often used topically or systemically for palliative treatment of flea-bite allergy. Control measures should include environmental management such as the provision of clean bedding, efficient waste disposal and rodent control. Several methods of environmental decontamination have been developed including the use of light traps, indoor insecticides and flea bombs (diflubenzuron, pyriproxyfen, methoprene).