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.
- Without previous exposure, there’s no sensitization. A bite won’t react.
- Subsequent bites cause delayed reactions after 24 hours.
- With continued biting, both immediate and delayed reactions occur.
- More encounters with the insect diminish the intensity of the delayed reaction. Eventually only immediate reactions appear.
- 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.
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.
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 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.
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.