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Overview of Fleas and Flea Allergy Dermatitis

By Michael W. Dryden, DVM, PhD, DACVM, Kansas State University

There are >2,200 species of fleas recognized worldwide. In North America, only a few species commonly infest dogs and cats: Ctenocephalides felis (the cat flea), Ctenocephalides canis (the dog flea), Pulex simulans (a flea of small mammals), and Echidnophaga gallinacea (the poultry sticktight flea). However, by far the most prevalent flea on dogs and cats is C felis. Cat fleas cause severe irritation in animals and people and are responsible for flea allergy dermatitis. They also serve as the vector of typhus-like rickettsiae and Bartonella sp and are the intermediate host for filarid and cestode parasites. Cat fleas have been found to infest >50 different mammalian and avian hosts throughout the world. In North America, the most commonly infested hosts are domestic and wild canids, domestic and wild felids, raccoons, opossums, ferrets, and domestic rabbits.

Transmission, Epidemiology, and Pathogenesis:

Cat fleas deposit their eggs in the pelage of their host. The eggs are pearly white, oval with rounded ends, and 0.5 mm long. They readily fall from the pelage and drop onto bedding, carpet, or soil, where hatching occurs in ~1–6 days. Newly hatched flea larvae are 1–5 mm long, slender, white, segmented, and sparsely covered with short hairs. Larvae are free-living, feeding on organic debris found in their environment and on adult flea feces, which are essential for successful development. Flea larvae avoid direct light and actively move deep in carpet fibers or under organic debris (grass, branches, leaves, or soil).

Larvae are susceptible to desiccation, with prolonged exposures to relative humidity <50% being lethal. The areas within a home with the necessary humidity are limited, and suitable outdoor sites are even rarer. Flea development occurs outdoors only where the ground is shaded and moist (1%–20% soil moisture content) and where the flea-infested animal spends a significant amount of time so that adult flea feces will be deposited into the larval environment. In the indoor environment, flea larvae probably survive only in the protected microenvironment deep within carpet fibers, in cracks between hardwood floors in humid climates, and on unfinished concrete floors in damp basements. The larval stage usually lasts 5–11 days but may be prolonged for 2–3 wk, depending on availability of food and climatic conditions.

After completing its development, the mature larva produces a silk-like cocoon in which it pupates. The cocoon is ovoid, ~0.5 cm long, whitish, and loosely spun. Flea cocoons can be found in soil, under vegetation, in carpets, under furniture, and on animal bedding. Once the pupa has fully developed (1–2 wk), the adult flea can emerge from the cocoon when properly stimulated by physical pressure, carbon dioxide, substrate movement, or heat.

The preemerged adult (which is a fully formed adult flea) residing in the cocoon is the stage that can extend the longevity of the flea. If the preemerged adult does not receive the proper stimulus to emerge, it can remain quiescent in the cocoon for several weeks until a suitable host arrives. Emergence can be delayed up to 350 days if preemerged adults are protected from desiccation. Newly emerging fleas move to the top of the carpet pile or vegetation, where they are more likely to encounter a passing host. Under ideal conditions of temperature (27°C [80.6°F]) and relative humidity (90%), a newly emerged cat flea can survive ~12 days before requiring a blood meal; at 50% relative humidity, this interval drops to ~3 days. It is these newly emerged unfed fleas that infest animals and bite people. There is generally minimal inter-host movement of cat fleas. However, it has been documented that before C felis reaches reproductive status, there can be some limited movement on and off hosts. Cat fleas that have found a preferred host (eg, dog, cat, opossum, etc) and have initiated reproduction generally do not leave their host unless forced off by grooming or insecticides.

Depending on temperature and humidity, the entire life cycle of the cat flea can be completed in as little as 12–14 days or can be prolonged for up to 350 days. However, under typical household conditions with normal pet and human activity, cat fleas complete their life cycle in 3–8 wk.

Adults begin feeding almost immediately once they find a host. Female cat fleas can consume 13.6 μL of blood daily, which is 15 times their body weight. After rapid transit through the flea, the excreted blood dries within minutes into reddish black fecal pellets or long tubular coils (flea dirt). Fleas mate after feeding, and egg production begins within 24–48 hr of females taking their first blood meal. Female cat fleas can produce up to 40–50 eggs/day during peak egg production, averaging 27 eggs/day through 50 days. Individual females may continue to produce eggs for >100 days.

Cat fleas are susceptible to cold. No stage of the life cycle (egg, larva, pupa, or adult) can survive exposure to <3°C (37.4°F) for several days. Therefore, cat fleas survive winters in north temperate climates as adults on untreated dogs and cats or on small wild mammals (eg, raccoons or opossums) in the urban environment. As these animals pass through yards in the spring or set up nesting sites in crawl spaces or attics, the eggs laid by surviving female fleas drop off and subsequently develop to adults. Cat fleas may also survive the winter as preemerged adults in microenvironments protected from the cold.

Fleas can cause iron-deficiency anemia in heavily infested hosts, particularly in young animals. Fleas in the genus Ctenocephalides have been reported to produce anemia in poultry, dogs, cats, goats, calves, and sheep.

Cat fleas are also involved in disease transmission. Murine typhus, caused by Rickettsia typhi and Rickettsia felis, is a mild to severe febrile disease of people characterized by headaches, chills, and skin rashes, with infrequent involvement of the kidneys and CNS. The disease is seen in people and many small mammals along the southeastern, southwestern, and Gulf coasts. In the USA, the principal transmission cycle involves opossums and cat fleas. Cat fleas also serve as the intermediate host of the nonpathogenic subcutaneous filarid nematode of dogs, Dipetalonema reconditum. Dipylidium caninum, the common intestinal cestode of dogs and cats (and rarely children), develops as a cysticercoid in C felis, C canis, and Trichodectes canis. Flea larvae ingest the eggs of the tapeworm, which develop into cysticercoids in the body of the flea. When grooming themselves, dogs and cats may ingest infected fleas, and the cysticercoids are released.

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