Saturday, January 01, 2011
Hypersensitivity Reactions to Arthropod
The bites of many arthropods can trigger hypersensitivity
reactions in humans. Exposure to penetrating body parts (setae) of
certain moths and caterpillars may cause similar reactions. This
chapter discusses those hypersensitivity reactions to arthropod
bites and exposures from species that do not live on humans
(scabies and lice are discussed elsewhere). By far the most common
are mosquito bites, followed by bites by fleas, chiggers, bedbugs,
and blackflies. Many of these arthropods have pets or other animals
as their primary host (eg, Cheyletiella, animal scabies, fleas).
All these exposures result in similar urticarial pruritic lesions,
usually 0.3-1.0 cm in diameter and lasting a few days to a week.
They may have a central punctum. These lesions are termed papular
urticaria, independent of the inciting organism. The therapeutic
strategy is to identify and eliminate the arthropod source and to
then suppress the resultant inflammatory reaction.
- The most difficult step in therapy is identifying the offending
arthropod; three factors are needed in this identification:
- Know the offending arthropods in your geographic region.
- Know the distribution pattern of bites (ie, lower leg: fleas,
chiggers; exposed areas of arms and face: biting flies,
- Take a careful history from the patient: Are there pets in the
home? Have arthropods or potential arthropod vectors been seen or
collected? Is there occupational, travel, or recreational exposure?
In difficult cases, this information is critically important and
aids in identifying unusual causes of papular urticaria (eg, rodent
- Exclude human scabies and lice infestation by careful
examination of the patient's body and clothing.
- Management involves three basic steps:
- Treat the patient's hypersensitivity reaction:
- Oral antihistamines (eg, hydroxyzine 25-50 mg before bed and
10-25 mg three times daily steadily for 1 week). The nightly dose
is not given as needed, but rather is given every night, so the
antihistamine is on board when a bite occurs in order to block the
allergic cascade induced by the bite.
- Apply a high- to superpotency topical steroid cream 2 to 3
times daily to each new papule until it resolves.
- Secondary infection is common; treat it with appropriate
antibiotics (e.g., oral dicloxacillin or cephalexin 250 mg 4 times
daily for 7 days).
- Make the patient less attractive to the biting arthropod:
- Instruct the patient to apply an insect repellant containing
diethyltoluamide (DEET) daily.
- Permethrin 5% may be used as an insect repellent. It repels
many biting arthropods (but not fleas) for about 1 week.
- Instruct the patient when outdoors to cover as much exposed
skin as possible.
- Instruct the patient to eliminate infestations of pets and/or
the environment: A veterinarian and a professional exterminator may
be helpful in this endeavor. The basic principles of flea control
are discussed here, as this is one of the most common problems.
- Treat all dogs and cats in the household. Cats especially may
harbor fleas without visible signs. Many of the oral agents
available on line or from a veterinarian are very effective.
- Spray or wash the bedding on which the pets sleep or lie for
- If the pet frequently lies in one area outdoors, treat this
area with an effective insecticide (eg, malathion, diazinon). It is
probably not necessary to treat the entire yard, although this can
be done and should be considered in situations of severe
- Use flea bombs (foggers) in all the rooms of the house,
especially in carpeted areas. Most effective are those containing
an agent that kills adult fleas (eg, pyrethrin) plus an insect
growth regulator to stop development of immature forms (eg,
fenoxycarb ormethoprene). Additional spraying may be required under
furniture and in crevices along the wall boards.
- Vacuum the entire house and all furniture to pick up eggs and
adult fleas. Dispose of the vacuum bags immediately after
- Retreatment of the house may be required several weeks after
the initial treatment to kill any fleas that may have hatched.
- Most patients who fail to respond to the above measures are
still being bitten. Additional effort to remove the cause of the
papular urticaria should be undertaken. The patient should consult
an exterminator and have pets examined by a veterinarian. In some
patients, the papular urticaria will continue for weeks to months
after the inciting agent has been removed.
- Rarely patients will require a short course of systemic
steroids for severe reactions.
- More powerful antihistamines (eg, doxepin 25-50 mg before bed)
may be helpful. They should be continued as suppressive therapy for
as long as lesions persist.
- Phototherapy (broadband UVB, narrowband UVB, PUVA, or
commercial tanning bed) 2 to 3 times weekly can be effective in
patients in whom topical steroids and antihistamines are
inadequate. Response is noted after about ten treatments.
Phototherapy can be continued as maintenance treatment.
- For flea control, the pets and environment may need to be
- Misdiagnosis is the major pitfall. Patients with papular
urticaria are frequently misdiagnosed as having human scabies.
Frequent applications of scabicides may exacerbate the
- Cercarial dermatitis (swimmer's itch), sea bather's
eruption/sea lice dermatitis, and moth and caterpillar dermatitis
all have similar morphologies and symptomatology to papular
urticaria. A careful history is required to identify these
- Occupational exposure to fiberglass produces a dermatitis very
similar to papular urticaria.
- Patients with delusions of parasitosis present with no primary
lesions, but a fixation regarding infestation.
- Doxepin is a tricyclic antidepressant, and if used in doses
above 75 mg daily, cardiac arrhythmias can result.
- Pesticides can be toxic. Repeated or incorrect use can produce
complications. Follow directions carefully.
- Do not rule out papular urticaria if only limited members of a
family are affected. Because papular urticaria is a
hypersensitivity reaction, only sensitized household members will
have clinical lesions, even though all family members are