Do flea bites show up right away?


Sometimes flea bites show up right away. Sometimes they don’t. It depends upon the individual’s level of sensitization. When flea bites appear immediately, they take the form of a wheal (raised area of skin). In almost all cases, an itchy papule forms within 24 hours of the bite.


Sensitization to Flea Bites

5 Stages of Sensitization

Regardless of the individual, humans (and animals) have similar reactions to insect bites. There are five stages of sensitization which correspond to the individual’s previous exposure to the insect.

  1. Without previous exposure, there’s no sensitization. A bite won’t react.
  2. Subsequent bites cause delayed reactions after 24 hours.
  3. With continued biting, both immediate and delayed reactions occur.
  4. More encounters with the insect diminish the intensity of the delayed reaction. Eventually only immediate reactions appear.
  5. In time, with more exposure, complete desensitization (immunity) is reached.


In humans, immediate reactions sometimes don’t appear, but there’s almost always a delayed reaction. Young children who are building immunity experience more intense reactions than adults. It’s rare for people to gain full immunity, because stage 2 lasts for at least 18 months of continued biting.


Sensitization in guinea pigs predictably follows the five stages. They gain immunity after 180 days of flea exposure. Sensitization in dogs is more random, and full immunity hasn’t been observed.

Immediate Reactions


Whealing is the most common immediate reaction to flea bites. A wheal is a raised section of skin, similar to the early stages of a mosquito bite. It’ll develop within 20 minutes of being bitten. The surrounding area may turn red (solar erythema). Wheals may be slightly itchy. However, those bit rarely complain because the lesions quickly fade.

Puncture Mark

There’s often a red pinpoint at the center of flea bites. This is where the skin and capillary were punctured by the flea. However, the mark (hemorrhagic punctum) may be absent if the flea didn’t find a blood vessel.

Delayed Reactions


Delayed reactions occur 12 to 24 hours after a flea’s blood meal. The most common lesion is a papule (hard bump), which often occur in clusters. A rash covered in tiny bumps (maculopapules) is also common. With both, the surrounding skin may turn red and swell up. People bitten by fleas often complain about the papules. They’re extremely itchy, reaching their maximum intensity in 12 to 24 hours.


It’s uncommon for flea bites to contain fluid. However, in extreme cases, delayed reactions can take the form of irregular-shaped bullous reactions (blisters). These lesions occur 48 to 72 hours after the bite.

Secondary Infections

Flea bites are itchy and often induce scratching, especially at night. Scratching can break open the skin, cause fluid release, and result in crusting. Secondary infections aren’t uncommon. When flea bites become infected, they’ll take the form of pus-filled lesions, such as pustules and boils.


People who are continually exposed to fleas sometimes aren’t affected by their bites. However, this is atypical. Immunity takes years to develop or may never occur at all. It’s mainly been observed in indigenous people of flea-infested regions. Desensitized individuals only experience slight skin reddening, and this passes within minutes. There’s no itching. A red puncture mark is the only visible sign of the bite, and it fades in 3 to 4 days.


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  • debi August 11, 2017, 12:42 pm

    I have had red dots from *fleas*. I have had major allergic reaction with secondary skin infections, I have been on and off Bactrim for a year now. I have dug out eggs, red larvae and black insects. no one believes me!!!! nights are horrible with itching and pain to the open lesions. I have a dog and obviously she has fleas with all the traditional meds given monthly and as needed. no exterminator believes me when I show them the specimens I have accumulated the past year. many different stages. they all look at me like I am a nut case. my doctor told me I had bed bugs and didn’t even look at the specimens. he actually told me to “volunteer” at something to occupy my time. I am desperate to find a name of these and a cure from my allergic reactions. I feel them digging at night and it hurts soooo bad. Please, help me or guide me to a prognosis and or prevention with a cure.
    thank you!!!

    • Adam Retzer September 29, 2017, 11:21 am

      Debi, I’m sorry to hear about your problems. I am uncertain what pest you are dealing with. Fleas don’t lay eggs in skin and larvae don’t live in skin. This doesn’t sound related to fleas. I am sorry I can’t offer more help, but this problem lies outside my field of expertise.

  • Patricia Humphries October 6, 2017, 4:29 am

    I had a cat in my house with fleas last night for a short time. How soon will i know if I have fleas?

    • Adam Retzer October 16, 2017, 2:16 pm

      The flea life cycle, from egg to adult, completes in 17-26 days in home environments. So if eggs fell in your home, then you should see new adult fleas emerging from the environment within 3-4 weeks.