Environmental Dermatology

What’s Eating You? Cat Flea (Ctenocephalides felis) Revisited

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Cat fleas (Ctenocephalides felis) often leave cats and dogs to bite humans, causing pruritic lesions of the lower extremities. Once flea bites are confirmed, various efforts can be made to eradicate these pests from the home or pets. Cat fleas also play a role in transmission of vector-borne diseases, especially endemic typhus.

Practice Points

  • Cat fleas classically cause pruritic grouped papulovesicles on the lower legs of pet owners.
  • Affected patients require thorough education on flea eradication.
  • Cat fleas can transmit endemic typhus, cat scratch disease, and bubonic plague.



Fleas of the order Siphonaptera are insects that feed on the blood of a mammalian host. They have no wings but jump to near 150 times their body lengths to reach potential hosts.1 An epidemiologic survey performed in 2016 demonstrated that 96% of fleas in the United States are cat fleas (Ctenocephalides felis).2 The bites often present as pruritic, nonfollicular-based, excoriated papules; papular urticaria; or vesiculobullous lesions distributed across the lower legs. Antihistamines and topical steroids may be helpful for symptomatic relief, but flea eradication is key.

Figure 1. Characteristic pronotal and genal combs in a cat flea (Ctenocephalides felis)

Figure 2. A, Male cat flea (Ctenocephalides felis). B, Female cat flea.


Ctenocephalides fleas, including the common cat flea and the dog flea, have a characteristic pronotal comb that resembles a mane of hair (Figure 1) and genal comb that resembles a mustache. Compared to the dog flea (Ctenocephalides canis), cat fleas have a flatter head and fewer hair-bearing notches on the dorsal hind tibia (the dog flea has 8 notches and the cat flea has 6 notches)(Figure 2).

Flea Prevention and Eradication

Effective management of flea bites requires avoidance of infested areas and eradication of fleas from the home and pets. Home treatment should be performed by a qualified specialist and a veterinarian should treat the pet, but the dermatologist must be knowledgeable about treatment options. Flea pupae can lie dormant between floorboards for extended periods of time and hatch rapidly when new tenants enter a house or apartment. Insecticidal dusts and spray formulations frequently are used to treat infested homes. It also is important to reduce flea egg numbers by vacuuming carpets and areas where pets sleep.3 Rodents often introduce fleas to households and pets, so eliminating them from the area may play an important role in flea control. Consulting with a veterinarian is important, as treatment directed at pets is critical to control flea populations. Oral agents, including fluralaner, afoxolaner, sarolaner, and spinosad, can reduce flea populations on animals by as much as 99.3% after 7 days.4,5 Fast-acting pulicidal agents, such as the combination of dinotefuran and fipronil, demonstrate curative activity as soon as 3 hours after treatment, which also may prevent reinfestation for as long as 6 weeks after treatment.6

Vector-Borne Disease

Fleas living on animals in close contact with humans, such as cats and dogs, can transmit zoonotic pathogens. Around 12,000 outpatients and 500 inpatients are diagnosed with cat scratch disease, a form of bartonellosis, annually. Ctenocephalides felis transmits Bartonella henselae from cat-to-cat and often cat-to-human through infected flea feces, causing a primary inoculation lesion and lymphadenitis. Of 3011 primary care providers surveyed from 2014 to 2015, 37.2% had treated at least 1 patient with cat scratch disease, yet knowledge gaps remain regarding the proper treatment and preventative measures for the disease.7 Current recommendations for the treatment of lymphadenitis caused by B henselae include a 5-day course of oral azithromycin.8 The preferred dosing regimen in adults is 500 mg on day 1 and 250 mg on days 2 through 5. Pediatric patients weighing less than 45.5 kg should receive 10 mg/kg on day 1 and 5 mg/kg on days 2 through 5.8 Additionally, less than one-third of the primary care providers surveyed from 2014 to 2015 said they would discuss the importance of pet flea control with immunocompromised patients who own cats, despite evidence implicating fleas in disease transmission.7 Pet-directed topical therapy with agents such as selamectin prescribed by a qualified veterinarian can prevent transmission of B henselae in cats exposed to fleas infected with the bacteria,9 which supports the importance of patient education and flea control, especially in pets owned by immunocompromised patients. Patients who are immunocompromised are at increased risk for persistent or disseminated bartonellosis, including endocarditis, in addition to cat scratch disease. Although arriving at a diagnosis may be difficult, one study found that bartonellosis in 13 renal transplant recipients was best diagnosed using both serology and polymerase chain reaction via DNA extraction of tissue specimens.10 These findings may enhance diagnostic yield for similar patients when bartonellosis is suspected.


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