Introduction

Carpets and companion animals are frequently seen in modern living rooms. However, the combination of the two gives a perfect condition for flea infestation in dogs and cats in a residential house. It is important to control flea infestation in companion animals and larval stages on the ground to prevent severe allergic dermatitis and the transmission of flea-borne pathogens that can cause serious disease in animals and/or in humans.

Understanding the biology and ecology of fleas is critical, as it affects what strategies will be effective to control flea infestation. A wide range of flea control products is available in the form of sprays, spot-ons, collars, oral tablets, and injections. Correct usage and understanding the mechanism of kill will ensure safe and highly effective flea control.

Flea Biology

Adult fleas live on animals. Unless they’re dislodged through grooming, fleas become permanent residents of their acquired host. Out of over 2500 distinct species in Siphonaptera, the two species of most importance for veterinary practitioners are the cat flea Ctenocephalides felis and the dog flea C. canis. Fleas have a shiny, glossy appearance because they have strongly sclerotized and chitinized bodies. Adults are laterally flattened, appearing as if they’ve been compressed side-to-side. The flattened body is an adaption that helps them move forward through host fur. Fleas, unlike most other insects, don’t have wings.¹

Ctenocephalides felis felis is the most common species associated with domestic dogs and cats in the USA and most parts of the world. The term “cat flea” is the common name of C. felis and doesn’t refer to all fleas recovered from cats. The species C. felis felis is characterised by an acutely angled frons, and head appears ‘pointier’ than C. canis.²

Unlike larval stages on the ground, both male and female adult fleas rely exclusively on host blood for living, so the mouth parts of a flea are specialized for blood feeding.

While imbibing blood, salivary ducts open to introduce anticoagulant saliva to the wound. Flea legs are long and well-adapted for jumping. The hind legs are much longer than the others, and are connected to specialized internal structures for leaping. A highly elastic protein called resilin enables their incredible jumps, not leg muscles.³

Adult fleas imbibe much more blood than they can use. As a result, fleas produce large amounts of feces, consisting largely of undigested blood. Feces from adult fleas, which is flea dirt, is the primary food source of flea larvae.⁴ A bit weird, but somewhere in nature close to our living, mother’s poop is baby’s food, and is bloody red instead of milky white. Fleas excrete feces in two different forms: Spherules and coils. Spherules are round and 0.07–0.25 mm in size. They can also be diarrheic. Coils are long with an average length of 0.84 mm. Newly emerged adults produce spherules. By the 10th day of feeding, their poop is mostly in the form of coils. Finding flea dirt on a dog or cat is one of the best ways to diagnose an infestation. If it’s truly flea feces, the black speck will smear red or dark yellow when rubbed on a wet paper towel. Flea dirt dries in irregular shapes and gets embedded into pet fur. The dry blood dislodges when animals scratch and groom themselves. As a result, it’s most common to find feces (and eggs) in areas where pets commonly rest and groom. Coils contain 33% more protein than spherules. The spherules are thought to be preferred by young larvae, while the protein-rich coils are more suitable for 3rd instars.

Adult fleas always stay on their host, and females lay eggs directly on their host after mating with male fleas. Roughly one flea egg is produced per hour. The eggs are initially wet and sticky, but they dry quickly and become non-adherent. Around 60% of eggs fall from the host within two hours of being laid. Flea eggs measure 0.5 mm in length and 0.3 mm in width. To survive, flea eggs must fall onto substrates with a warm, humid microclimate, such as carpeting. If the microenvironment is too dry, eggs will shrink and die. Flea eggs develop rapidly in warm, humid environments. They’ll hatch within one and a half days when conditions are optimal. Ideal conditions occur at temperatures near 89.6°F (32°C), and humidity between 75–92%. A relative humidity below 50% is often lethal, while 80% of flea eggs survive when relative humidity exceeds 50%.

In homes, it takes 2 to 3 days before flea eggs hatch. Most eggs and larvae live in carpeting, because the temperature and humidity are well secured. The microclimate within the carpet fibers is near ideal for developing fleas. A sharp spine projects from the front of the head. It helps a larva rupture its egg shell and hatch. The egg-burster spine is only found on first instars. It’s lost in the first molt. Newly hatched flea larvae are 2 mm long, growing to a final length of 5 mm. The larvae resemble worms or maggots. Dog or cat flea larvae develop through three stages, taking the form of 1st, 2nd, and 3rd instars. They molt once between each stage. Dried blood feces from adult fleas are the primary food source of larvae. As a larva feeds on flea dirt, its gut turns a dark red to purple color.

A mature larva will move to an undisturbed location to spin a cocoon. The cocoon’s sticky silk collects debris from the environment, such as carpet fibers, and becomes camouflaged. Mature flea larvae are 4–5 mm long and 0.5 mm wide. While forming cocoons, the larvae fold themselves in half. Flea cocoons are envelopes of thin, white, silk-like material. The fibers are soft, moist, and sticky. They’re wrapped loosely around the larvae. As a result, environmental debris easily adheres to cocoons. This detritus may consist of sand, dust, soil, carpet fibers, or any other small fragments of dry material. The larvae purposefully collect and integrate these particles into their cocoons with a few silk threads. Flea cocoons are difficult to detect. Adhering debris originates from the same environment where the cocoons rest, giving them a near-perfect camouflage. Within homes, larvae pupate at the base of carpeting. Once adulthood is reached, fleas can remain in a motionless, dormant state inside their cocoons. This quiescent period lasts up to 5 months, but it ends when a nearby host triggers their emergence. The presence of heat or pressure causes them to exit the cocoons within 5 seconds. After emerging, a flea will climb atop nearby objects, such as carpet fibers, where it’s able to jump onto a passing host. There, the flea orients itself toward sources of light. As a result, fleas tend to gather near openings where light enters, such as vents, crawl spaces, window sills, and other entrances.

The complex flea life cycle means that there are a number of challenges for veterinarians who help clients control fleas in their homes: the physical resilience and rapid multiplication in flea numbers; their ability to find alternative hosts; and the presence of environmental conditions in the home that are conducive to flea survival. In addition, there is a need to select an appropriate form of treatment from among the many options available and to manage the client’s expectations of what is possible to achieve. Clients need to be aware which strategies will and will not be effective: e.g., neither removing pets from a flea-infested house nor leaving the house vacant (to starve fleas) is likely to work, since immature stages live for a year or more, can survive winter temperatures, and will only hatch when they feel vibrations (from a passing human, animal, or vacuum cleaner) or sense the CO₂ given off by a potential host.⁵

Flea-borne Diseases

Fleas are important clinically as causes of pruritus and flea bite dermatitis, and in young animals with severe infestations anemia. Fleas are intermediate hosts for filarial nematodes and the tapeworm Dipylidium caninum and vectors for various pathogens, including Bartonella henselae, Rickettsia felis, Haemoplasma species, and Yersinia pestis. Cat fleas can also transmit Rickettsia typhi, a causative agent for murine typhus, normally transmitted by the rat flea Xenopsylla cheopis.⁶

The cat flea, C. felis, is the most frequently encountered parasite on both cats (98%) and dogs (93%) and is also the flea species with the most potential for transmitting zoonotic diseases. Yersinia pestis causes both bubonic (abscesses and lymphadenitis) and pneumonic forms of plague, with clinical signs of fever, inappetence, lymphadenopathy, and subcutaneous abscesses. Cats seem to be more clinically susceptible than many species. In feline patients, the abscesses can be difficult to distinguish from cat-bite abscesses. In around 10% of human cases, the disease is transmitted by cats, through passing on infected fleas, or directly via scratches, bites, or air droplets.

Cats are infected with the tapeworm D. caninum, which has a worldwide distribution, via C. felis. Fleas ingest tapeworm eggs, and the tapeworm is transmitted when an infected flea is swallowed by a cat or dog, after which there is a prepatent period of 2–3 weeks. In most infected pets, the condition is asymptomatic, although the proglottids may be detected in the feces or on the perianal area. Tapeworms may cause anal pruritus, which may lead the affected animal to ‘scoot’ along the ground, rubbing its anal area, which can embarrass dog owners. Other signs can include diarrhea, weight loss, and failure to thrive. Control is achieved through anthelmintic dosing with praziquantel, along with flea treatment to prevent reinfection and hygiene measures to remove the contaminated feces.

Bartonellosis is the main flea-borne bacterial disease circulating in pets; Bartonella henselae or B. clarridgeiae are the usual variants in cats, while B. vinsonii subsp. berkhoffii is the most common in dogs, along with B. henselae. The prevalence is much lower in dogs than in cats, and there is some suggestion that it is an opportunistic pathogen in this host. Bartonellosis is normally transmitted through skin contact with the contaminated flea dirt, and the disease has a worldwide distribution. Testing of healthy animals is currently not recommended. Clinical signs may appear when the bacteria are released from the infected cells into the circulation and will consist of endocarditis in cats and dogs, and other inflammatory diseases such as recurrent pyrexia, uveitis, and immune-mediated polyarthritis. The bacterium may also cause vascular proliferative disease, particularly in human patients. This pathogen appears to have co-evolved with the feline host, which may show no obvious clinical signs, but due to its regular release from infected cells, it may have a role in various chronic disease conditions.

Another emerging zoonotic disease is flea-borne spotted fever caused by the Rickettsia felis or R. typhi. The pathogen has a worldwide distribution and is transmitted via bites from infected fleas, which are capable of vertical transmission through the egg. Cats are rarely symptomatic and are not known to be reservoirs. Dogs may be a source, based on limited evidence from countries such as Australia. Humans may be infected through bites or contact with infected flea feces on skin abrasions and will show nonspecific signs of fever, rash, headache, and myalgia.

Fleas may also be a source of mycoplasmal disease, with three Mycoplasma species (M. haemofelis, M. haemominutum, and M. turicensis) present in cats and two in dogs (M. haemocanis and M. haematoparvum). Of these, M. haemofelis is the most pathogenic, causing severe haemolytic anaemia, especially in young animals. The other species are only likely to produce anaemia in cats with concurrent disease, or in immunocompromised or splenectomised patients. Haemoplasmas are known to be ingested by C. felis while feeding and may be detected in flea feces, which is one possible mode of transmission, and there is recent experimental evidence of M. haemofelis transfer by fleas during feeding.

Flea-borne Human Diseases

Rickettsia typhi and Rickettsia felis are two major flea-borne rickettsiae of humans that are distributed throughout the world. Rickettsia typhi is acquired by fleas while feeding on rickettsemic rats. The organism infects the midgut epithelium of the flea and is shed in the feces, where it is transmitted to humans by the inoculation of R. typhi-laden flea feces onto flea-bite wounds or mucous membranes.⁶ Murine typhus is characterized by the abrupt onset of fever with accompanying headache, chills, myalgia, and malaise. Rash, which is the sign that often prompts a clinician to consider a rickettsiosis, is absent in 50% and may be present in as few as 20% of those with darkly pigmented skin. Since the mid-1990s, it has been increasingly recognized as a cause of human infection throughout the world. The primary reservoir and vector of R. felis is thought to be C. felis.

References

1.  Flea Encyclopedia; 2018. https://fleascience.com

2.  Lawrence AL, Hii S-F, Jirsová D, Panáková L, Ionică AM, Gilchrist K, Modrý D, Mihalca AD, Webb CE, Traub RJ. Integrated morphological and molecular identification of cat fleas (Ctenocephalides felis) and dog fleas (Ctenocephalides canis) vectoring Rickettsia felis in central Europe. Vet Parasitol. 2015;210:215–223.

3.  Sutton G, Burrows M. Insect jumping springs. Current Biol. 2018;28:R142–R143.

4.  Moser BA, Koehler PG, Patterson RS. Effect of larval diet on cat flea (Siphonaptera: Pulicidae) developmental times and adult emergence. J Econ Entomol. 1991;84:1257–1261.

5.  Bourne D, Craig M, Crittall J, Elsheikha H, Griffiths K, Keyte S, Merritt B, Richardson E, Stokes L, Whitfield V. Fleas and flea-borne diseases: biology, control & compliance. Compan Anim. 2018;23:204–211.

6.  Blanton LS, Walker DH. Flea-borne rickettsioses and rickettsiae. Am J Trop Med Hyg. 2017;96:53–56.