Papular urticaria describes multiple, sublime cutaneous lesions that presumably form due to a hypersensitivity reaction to insect or arachnid bites. Contrary to urticaria, skin alterations may persist for several days or even weeks.
Most PU patients are children; they typically present with multiple, clustered papules, which measure up to one, rarely two centimeters in diameter, and pruritus. In some cases, papules may turn into vesicles. PU is a chronic disorder characterized by spontaneous remission and spontaneous recurrence of skin lesions and in the course of several days or even weeks, papules emerge, merge, subside and emerge again. Adjacent skin regions may show variable degrees of erythema and edema.
While the above described lesions are often localized on exposed skin regions, e.g., face, neck, hands, arms and calves, they may also be observed anywhere else. It cannot be ruled out that papules develop at a certain distance of insect or arachnid bites and thus, distribution of lesions does not necessarily serve as a clue as to where and when patients were exposed to these parasites. Some time may have passed since the patient sustained arthropod bites and the lack of a close temporal connection further complicates recognition of a causal relation.
Skin lesions resolve upon application of topical antihistamines and corticosteroids, but recurrence is to be expected. Symptoms generally resolve in winter, but patients often return during warm summer months of the following year.
Of note, intense pruritus generally has patients scratching the affected areas of the skin and in some cases, deeper skin lesions, hemorrhages and eschar may dominate the clinical picture. Secondary bacterial infection is not uncommon and may lead to pyoderma.
PU is usually diagnosed during summer months and in geographical regions marked by warm and humid climates during that time of the year. Patients may not associate skin alterations with insect or arachnid bites, because papules often develop at a certain distance to the initial lesion. Also, one single bite may be sufficient to trigger PU and patients may not even consider it worth mentioning. However, they may report engagement in outdoor activities and exposure to parasites if questioned accordingly. Of note, exposure to parasites may also occur indoors, particularly if patients keep pets or maintain low hygiene standards. The latter may be associated with ongoing exposure to parasites and thus, clinical examination of the patient may reveal more recent bites.
Urticaria is a very common, unspecific symptom and may be evoked by a variety of conditions, e.g., administration of drugs, infectious diseases, infestation with intestinal parasites and physical stress [11]. Thus, anamnesis as well as subsequent general examination should aim at ruling out these differential diagnoses. With regards to drug administration, recently initiated or changed medication is most likely to trigger such a reaction. However, patients may present new side effects even after having tolerated a certain therapy for years.
Laboratory analyses of blood samples are not indicated unless previous findings point at an underlying primary disease or systemic involvement [12]. This also applies for histopathological analyses of skin biopsies.
Similarly, further diagnostic measures are recommended if papules persist for more than six weeks.
Treatment of PU is supportive. Topical application of antihistamines or corticosteroids may be considered and should induce involution of papules and provide rapid relieve from pruritus. In cases of extensive lesions, systemic use of such drugs may be indicated. However, neither of both types of administration is suitable for long-term use. Thus, it is of utmost importance to identify the source of insect or arachnid bites and to take appropriate measures to eliminate it.
Topical or systemic administration of antibiotics may be required to treat secondary bacterial infections.
Papules and pruritus may recur in warm summer months of several years during childhood, but long-term prognosis is very good. Patients finally develop tolerance to insect and arachnid bites, a process presumably mediated by desensitization, and symptoms rarely persist into adulthood.
PU-associated papules are generally thought to be the result of a hypersensitivity reaction to insect or arachnid bites. Whereas wheals developing at the site of mosquito or flea bites, for instance, are provoked by active injection of anticoagulants into the human skin, this is not necessarily the case in PU. The mere contact with the animal seems to be sufficient to induce hypersensitivity and PU has been described to occur after exposure to Dermestes maculatus and caterpillars [2] [3]. In rare cases, PU does not seem to be related to exposure to any arthropods [4].
The following animals have been suggested or proven to cause PU:
Fleas seem to be of particular importance in this context. One study describing 20 cases of PU found fleas to be responsible for all these cases [5].
Relapses of PU can best be prevented by identifying and eliminating or avoiding the trigger of the disease. However, this may not be an easy task and in some cases, general measures are more effective and the etiologic agent is not identified.
Because PU is triggered by exposure to insects and arachnids, there is a strong spatiotemporal correlation between PU incidence and parasite presence. In this context, it is little surprising to detect higher PU incidence rates in geographical regions marked by hot and humid climates and during seasons characterized by these same conditions [6]. Moreover, the climate phenomenon El Niño does not only account for elevated water and air temperatures but also for an increased incidence of PU [7].
No predilection regarding race or gender has been suggested.
The majority of PU patients are children aged less than 10 years. On the one hand, this may be due to little awareness of parasites in their surroundings and consequently increased exposure. On the other hand, there is evidence suggesting that children are generally more susceptible to PU than adults. An adult's immune system seems to provide certain protection against PU and the disease is rarely diagnosed in the respective age group.
According to current knowledge, a type I hypersensitivity reaction accounts for PU. This type of allergy requires repeated exposure to certain antigens and sensitization of mast cells. With regards to insect and arachnids inhabiting the surroundings of PU patients, this situation is easily conceivable. Antigens may be introduced into the patient's circulation during an insect or arachnid bite, although plasma cells and mast cells are less likely to encounter antigens of parasites that don't bite. The fact that papules often form at a considerable distance from insect bites supports the theory of systemic pathomechanisms. Specific antigens that trigger PU have not yet been identified.
Histopathological analyses confirm the hypothesis of an immune-mediated process. Mononuclear and polynuclear cell infiltrates predominate in dermal layers, but inflammatory cells may also migrate into the epidermis. Further, characteristic, but not specific findings are mild acanthosis, spongiosis and subepidermal edema [8]. While immunoglobulin deposits could not be detected in the previously cited study, different findings have been reported several years ago [9].
Such deposits would indicate a type III hypersensitivity reaction. The latter may induce urticarial vasculitis, but this disease generally comprises larger areas of the skin than PU and instead of small papules, more extensive lesions develop.
Type I and type III hypersensitivity additionally differ regarding involved subclasses of immunoglobulins. While type I hypersensitivity reactions are mainly mediated by IgE, immunocomplexes formed during a type III reaction principally contain IgG. A recent publication reports antibody profiles to vary with the duration of the disease, but found a general predominance of IgE in patients suffering from PU [10]. These results further support the hypothesis of a type I hypersensitivity reaction to provoke skin lesions in PU.
Exposure to insects and arachnids can be significantly reduced by applying the following measures:
Papular urticaria (PU) is a common disorder mostly diagnosed in children. PU patients typically present with multiple papules and pruritus that are thought to be the result of a hypersensitivity reaction to insect or arachnid bites [1]. Of note, PU-associated sublime lesions do not correspond to insect bites themselves and may form at a considerable distance from the initial lesion. Unlike true urticaria, a condition provoked by exposure to allergens, nettles etc., PU persists for several days or even weeks.
Patients, particularly pediatric patients, may be greatly bothered by pruritus, may scratch papules and cause deeper injuries. Secondary bacterial infection of the latter may aggravate skin damage. This may require specific treatment and lesions may not heal without leaving scars. In general, PU-associated lesions don't break, don't pose a risk of infection and undergo spontaneous remission.
Because in the vast majority of cases, PU is associated with arthropod exposure, prevention of the latter may contribute to avoid the former. Thus, appropriate measures should be taken to reduce parasitic infestation of pets, to diminish parasitic loads in bedding and clothes, and to prevent exposure to those parasites that typically accompany outdoor activities. Indeed, analysis of epidemiological data revealed an increased incidence of PU during summer months, the time of the year at which people are mostly engaged in outdoor sports and recreation.
Although recurrent PU is frequently observed, the disease is generally self-limiting. Topical application of antihistamines or corticosteroids may provide relieve from severe pruritus. Otherwise, pharmacological treatment is only required if deep scratches aggravate skin damage.
Urticaria is the medical term used to describe itching wheals that generally develop upon exposure to an allergen, contact with nettles etc. Such dermatological alterations usually subside after short periods of time. Patients suffering from papular urticaria (PU) show similar sublime, reddened skin lesion that do, however, persist for days or even weeks.
Causes
According to current knowledge, PU is triggered by a type I hypersensitivity reaction to parasitic antigens, i.e., papules form due to an immune reaction similar to that one observed during an allergy test. A wide variety of parasites, namely insects and arachnids, have been suggested or proven to cause PU:
Consequently, in order to avoid PU, pets should be regularly treated with antiparasitic drugs, an appropriate hygiene should be kept regarding bedding and clothing, and preventive measures should reduce the risk of insect bites while engaging in outdoor activities.
Symptoms
Multiple, clustered papules and pruritus are characteristic for PU. While these lesion may form in close proximity to an insect or arachnid bite, they may also develop at a considerable distance from it. Exposed skin is affected more frequently, but PU may affect any area of the body. Skin alterations persist for several days and in the course of disease, papules emerge, merge, subside and emerge again.
Most PU cases are diagnosed in summer months in geographical regions marked by warm and humid climates. Skin lesions often resolve in winter, but may reappear the following year.
Diagnosis
Anamnesis and dermatological findings are the basis of PU diagnosis. Additional exams are rarely required, but may be realized if doubts remain and differential diagnoses need to be ruled out. Such exams may comprise laboratory analyses of blood and urine samples as well as skin biopsies.
Treatment
Symptoms associated with PU may be treated with antihistamines or corticosteroids; both may be applied topically or systemically. They induce involution of papules and provide rapid relieve from pruritus. However, they are not suitable for long-term therapy. It is therefore of utmost importance to identify the source of insect or arachnid exposure and to eliminate it.
Of note, antibiotics may be required if deep skin lesions, usually resulting from scratching due to intense pruritus, show signs of secondary bacterial infection.