Fleas are small, wingless insects with mouth parts that are specifically adapted to piercing the skin and sucking blood. There are greater than 1,600 species of fleas present worldwide. Roughly 95% of flea species will live on mammals and about 50% will live on birds. Most flea infections in the US are due to Ctenocephalides felis which is more commonly known as the cat flea. C. felis affect more than 50 different mammalian and avian hosts throughout the world. In the U.S., the most common hosts are domestic and wild mammals including cats, dogs, cattle, and man.

Fleas are harmful to mammals in three capacities:
1. Anemia due to extensive blood loss.
2. Transmitters of disease: fleas that have fed on rodents may transmit plague and murine typhus. Other flea transmitted diseases include rickettsia, tapeworms, and cat scratch fever.
3. Flea bites produce an irritating dermatitis called Flea Allergy Dermatitis (FAD) which accounts for more than half of all dermatological conditions requiring veterinary assistance.

Flea metamorphosis consists of an egg, larval stage, pupa, and adult stages. The entire life cycle of the flea may be completed in as little as 12 to 14 days or may be prolonged for up to a year. This is dependent on the environmental conditions that are present. Flea eggs are deposited a few at a time on the skin of the affected host by the female flea after she has fed on the host. A female flea consumes 15 times her own body weight in blood daily. Followed by insemination, the eggs usually drop to the ground where depending on the humidity and temperature, they hatch to the elongated larvae form. Flea feces in the environment are the principal food source for the three larval stages since it contains an abundance of dried blood. The most likely place to find flea larvae is in infested pet bedding. The mature third-stage larva spins a cocoon around itself and pupates for as short as one week or as long as one year. Once the pupa stage is complete, the adult will emerge. Unfed fleas may survive for two months to one year while they wait for a host to happen by.

Fleas display little tendency to leave their dog or cat host until the population on a particular host approaches about 200. Once critical population counts have been reached, the fleas will jump to a less parasitized host. Fleas characteristically prefer pets to people but once the population becomes too extensive for the pets to handle, the fleas will attack humans. The hind pair of legs is especially long and is adapted for jumping to a new host. Fleas may leap vertically up to seven inches and horizontally up to 13 inches. The equivalent hop for a human would be 250 feet vertically and 450 feet horizontally.

Unfed fleas may survive for months while they wait for a host to happen by.

Adult fleas cannot survive temperature <3°C (37.4°F) for more than several days. They survive in cold environments as adults on untreated wild or domestic animals or in protected micro-environments.

Americans spend about $9 billion a year controlling fleas. This is one of the biggest expenses for pet owners.

References:
Kahn, Cynthia. “The Merck Veterinary Manual”. 9th Edition. pp 710-715.
Faust, Ernest; Russell, Paul and Rodney Jung. “Clinical Parasitology”. 8th edition. Lea and Febiger: Philadelphia. Pp 733-747.
Georgi, Jay and Marion Georgi. “Parasitology for Veterinarians”. 5th Edition. W.B. Saunders. 1990. pp.38-43.
Fun Flea Facts. Advertisement by Bayer Animal Health. 2007.