Fleas are small (2 mm), dark, reddish brown, wingless, bloodsucking insects. Their bodies are laterally compressed (i.e., flattened side to side) and covered with many hairs and short spines directed backward, permitting forward movement through hairs on the host’s body. Their hind legs are long and well adapted for jumping. Adult flea mouthparts are equipped for sucking blood from the host.
In North America, the cat flea, Ctenocephalides felis felis . is the most common ectoparasite of dogs and cats. In tropical areas it is a year-round pest, whereas in temperate climates its season varies. Because they attack a range of warm-blooded hosts, including humans and pets, cat fleas are both a veterinary problem and a household pest.
SIGNIFICANCE OF FLEAS
Fleas are important to humans because of their potential as disease vectors, in addition to the annoyance they produce merely by biting. Pathogen transmission is facilitated by their habit of feeding sequentially on several hosts. The best known disease associated with fleas is bubonic plague; the plague bacterium, Yersinia pestis, is transmitted almost exclusively by rodent fleas. Murine typhus is another disease for which cat fleas have been implicated in the transmission cycle. The flea-borne typhus causative agent, Rickettsia typhi, is transmitted from its rodent reservoir by several flea species, including C. felis. Fleas probably play a role in maintenance and transmission of several other disease organisms such as Bartonella henselae, causing cat scratch disease. The cat flea is the intermediate host for the dog tapeworm, Dipylidium caninum, which can affect small children as well as dogs and cats.
Pets infested with fleas bite and scratch themselves repeatedly. In situations in which flea numbers are high, veterinarians occasionally see kittens and puppies near death from flea-produced anemia. Sensitized people suffer from flea bites, which can cause intense itching, with scratching opening the skin to infection.
Flea allergy dermatitis (FAD) is a severe condition found primarily in dogs, but also occasionally seen in cats. In a flea-allergic animal, flea salivary antigens initiate a cascade of symptoms, resulting in intense pruritus accompanied by scratching, biting, and self-inflicted trauma. An affected animal typically displays obsessive grooming behavior, with accompanying depilation, leaving the skin with weeping sores, often resulting in secondary infection. FAD is treated with cortico-steroids, which possess undesirable side effects, especially when continuous use is required as in chronic FAD cases. Until development of FAD immunotherapy, successful treatment involves flea elimination from the animal’s environment and flea bite prevention.
Fleas and their associated diseases can constitute over half a veterinary practice’s caseload in some areas of the country. More energy and money are spent battling these insects than any other problem in veterinary medicine.
LIFE HISTORY
The cat flea is a cosmopolitan, eclectic species, having been recorded from more than three dozen species, including opossums, raccoons, kangaroos, and even birds. This wide host range explains this flea’s ability to repopulate domestic animals after suppression efforts. Because it lacks host specificity and tends to feed on humans, the cat flea is a pest of both companion animals and humans with whom they share their abode.
Adults
Once adult cat fleas locate a host, they tend to remain on that animal unless dislodged. They feed readily and mate on the host. Female fleas lay eggs while on the host and because the eggs are not sticky, they readily fall off into the host’s environment, with large numbers accumulating in areas frequented by the animal. Each female flea can produce more than two dozen eggs per day. Adult fleas are about 1-4 mm in length and are strongly flattened from side to side. They are equipped with relatively long legs armed with strong outwardly projecting spines. Cat fleas have a collar of spines (ctenidium) on the back and another row of spines above the mouth. These characteristics allow for rapid movement through the host’s hairs and also serve to resist removal from the fur.
Once the adult flea finds a host, it begins to feed. Typically the female mates and begins oviposition within a couple of days. On the host, a female flea averages about one egg per hour and, as a female flea can live on the host for several weeks, potential production can amount to hundreds of eggs in her lifetime. Only the adult stage is parasitic; all other life stages develop off the host (Fig. 1 ).
Cat flea eggs are approximately 1mm in length, with little surface structure other than aeropyles (permitting gaseous exchange for
FIGURE 1 Cat flea life cycle: adult (top), eggs, larva, and cocoon (enclosing pupa).
the developing embryo) and micropyles (for sperm entrance during fertilization). Typically, the larvae hatch within 24-48h after oviposi-tion, with more rapid hatching at warm temperatures.
Larvae
From the eggs emerge small, white, eyeless, legless larvae with chewing mouthparts. Because they seldom travel far from where they hatch, cat flea larvae are usually found in furniture, carpeting, or outside in areas frequented by flea hosts. Flea larvae have three instars that, under favorable conditions, can be completed in as little as 10 days. Larvae will develop only in protected microhabitats in which the relative humidity exceeds 75%. Cool temperatures, food shortages, or other unsuitable environmental conditions may extend larval developmental time to several weeks or a month. The third instar voids its gut approximately 24 h before initiation of cocoon construction. The white prepupa wanders until it locates an appropriate site for pupation and then begins to spin a silk cocoon. Frequently, environmental debris is incorporated into the cocoon, adhering to the sticky silk fibers, so that the cocoon may appear as a small dirt clod or lint ball.
Pupae
Within its cocoon, the prepupa molts to the pupa and continues metamorphosis to the adult flea within about 4 days, under favorable conditions. Length of the preemerged adult stadium is the most variable in the flea life cycle, ranging from less than a day to several months (or perhaps over a year). The mechanisms are not completely understood, but it appears that some individuals are programmed to delay emergence. Likely, this is an evolutionary strategy whereby offspring emerge over an extended interval, ensuring that some successfully achieve hosts. Stimuli such as pressure, carbon dioxide, and warmth (triggers associated with mammalian hosts) serve as releasers, causing the adult flea to emerge from the cocoon. Upon emergence, if the flea does not locate a host immediately, it can survive for approximately 7-10 days (or longer under high-humidity and low-temperature conditions).
FLEA SUPPRESSION
Because fleas must have blood from a mammalian host to survive, treating host animals is the most efficient and effective means of suppressing fleas. There are several on-animal products that are effective for flea control. Many contain pyrethrins, which are safe, effective products but kill only fleas on the animal at the time of treatment and do not provide residual control. Other over-the-counter compounds include spot-on permethrin products, which are limited to canine use because they can be lethal to cats.
Veterinarians can recommend products that provide several weeks of control with a single application. Products prescribed by veterinarians for on-animal flea control are applied in a small volume (a few milliliters) on the back of the animal’s neck. The material distributes over the body surface in skin oils. In addition to spot-on formulations, some products are available as sprays. These adulticides kill fleas on the animal within a few hours, then provide residual flea suppression for several weeks.
To forestall flea infestations, pets can be started on flea developmental inhibitors early in the season. Products containing insect development inhibitors can be applied topically, given orally (once monthly as a pill for dogs or a liquid added to a cat’s food), or given as a 6-month injectable formulation for cats. Female fleas that feed on blood of treated animals subsequently are unable to reproduce.
Once pets have been treated, it will take a while for fleas in the environment to die off. Meanwhile, as they emerge, fleas will hop onto the animal; the host will continue to “harvest” fleas from the surrounding environment until they have been killed and no more are emerging. Insect growth regulators can be used to break the flea life cycle. Although these compounds do not kill adult fleas, they do prevent eggs and larvae from completing their development, ensuring that any fleas brought into the area will not establish a sustaining population.
Sanitation is an important flea-suppression tactic; by eliminating larval developmental sites and destroying immature stages before they develop to the pestiferous adult stage, pets and people can be protected from fleas. Areas frequented by pets accumulate flea eggs and larval food, so these microhabitats should be vacuumed and treated to prevent flea infestations. These might include areas under furniture, animal bedding and sleeping quarters, and utility rooms or other areas where the pet spends time.
