Medical Conditions Caused by Arthropod Stings or Bites
Arthropods may cause a variety of negative health effects by their stings and bites. Non-allergic reactions to stings and bites include erythematous and edematous lesions which resolve in a couple of hours, while allergic reactions may be characterized by much more serious large local phenomena and even systemic symptoms such as anaphylaxis. This chapter focuses on the medical conditions associated with arthropod bites and stings, such as those mentioned above, as well as secondary infections resulting from skin puncture. Lastly, clues to recognizing insect sting or bite reactions are presented and discussed.
KeywordsArthropods Morphology of mouthpart types Insects Stings Bites Allergic reactions Anaphylaxis Secondary infection Diagnosis Laboratory findings
10.1 Introduction and Medical Significance
Arthropods cause a wide variety of clinical conditions in humans, but especially skin lesions, because people are inevitably exposed to biting and stinging organisms in the urban and suburban environment [1, 2, 3, 4, 5]. Skin lesions resulting from arthropod exposure may arise via various pathologic pathways, such as direct damage to tissue, hypersensitivity reactions to venom or saliva, or infectious disease. Direct injury can occur from mouthparts or stingers piercing human skin  and/or blisters or stains resulting from exposure to arthropods [7, 8]. In some cases, proteins in venom or saliva may cause direct mast cell degranulation, leading to urticaria . In addition, secondary infections may result from bacteria entering the skin via the bite/sting punctum. This is especially likely if the bite/sting site is scratched extensively. As discussed in Part II, many vector-borne infectious diseases can also produce skin lesions such as rash, ulcers, or eschar.
10.2.1 Mouthpart Types
Biting and chewing
Piercing–sucking (Fig. 10.1)
10.2.2 Sting Apparatus
10.2.3 Direct Damage to Tissue
Some lesions are the result of direct tissue damage from stings or bites. Arthropod mouthparts puncture skin by various mechanisms (siphoning tube, scissorlike blades, and so on) leading to skin damage. In this case, damage may be a small punctum, dual puncta (from fangs), or lacerations. By far, most lesions on the human skin are produced by host immune reactions to the offending arthropod salivary secretions or venom. Arthropod saliva is injected while feeding to lubricate the mouthparts on insertion, increase blood flow to the bite site, inhibit coagulation of host blood, anesthetize the bite site, suppress the host’s immune and inflammatory responses, and/or aid in digestion. Stingers are needlelike structures that may puncture and damage human skin as well. Venom from certain spiders may directly affect the human skin, causing tissue death (necrosis). In the United States, violin spiders are primarily responsible for necrotic skin lesions, although sac spiders (Cheiracanthium spp. ) and hobo spiders may also cause necrotic arachnidism [10, 11]. Brown recluse spider venom contains a lipase enzyme, sphingomyelinase D, which is significantly different from phospholipase A in bee and wasp venoms. This specific lipase is the primary necrotic agent involved in the formation of the typical lesions. It is possible that neutrophil chemotaxis is induced by sphingomyelinase D. The subsequent influx of neutrophils into the area is critical in the formation of the necrotic lesion (see Chap. 12).
10.2.4 Infectious Complications
Increasing erythema, edema, or tenderness beyond the anticipated pattern of response of an individual lesion suggests infection.
Regional lymphadenopathy can be a useful sign of infection, but it may also be present in response to the primary lesion without infection.
Lymphangitis is the most reliable sign and suggests streptococcal involvement.
10.3 Clues to Recognizing Insect Bites or Stings
If a patient recalls no insect or arachnid exposure, arthropod bites or stings may pose difficulty in diagnosis. No physician or entomologist can accurately determine what insect caused a particular bite or sting lesion; however, there might be helpful clues. Alexander  described a typical hymenopteran sting (excluding ants) as a central white spot marking the actual sting site surrounded by an erythematous halo. Generally, the entire lesion is a few square centimeters in area. Of course, allergic reactions may result in much larger lesions. He also described an initial rapid dermal edema with neutrophil and lymphocyte infiltration . Plasma cells, eosinophils, and histiocytes appear later.
Arthropod bites should be considered in the differential diagnosis of any patient complaining of itching. Bites are characterized by urticarial wheals, papules, vesicles, and less commonly, blisters. After a few days or even weeks, secondary infection , discoloration, scarring, papules, or nodules may persist at the bite site. Complicating the picture further is the development of late cutaneous allergic responses in some atopic individuals. Diagnosis may be especially difficult in the case of biopsies of papules or nodules. Biopsies may reveal a dense infiltrate of a mixture of inflammatory cells, such as lymphocytes, plasma cells, histiocytes, giant cells, neutrophils, and eosinophils. Lesions containing a majority of lymphocytes could be mistaken for a lymphomatous infiltrate. If the infiltrate is predominantly perivascular and extending throughout the depths of the dermis, the lesion might be confused with a lupus erythematosus. Eosinophils are commonly seen in papules or nodules from arthropod bites. There may be a dense infiltration of neutrophils, resembling an abscess. Occasionally arthropod mouthparts may still be present within the lesion, and there may be a granulomatous inflammation in and around these mouthparts. Scabies mites occur in the stratum corneum and can usually be seen on microscopic examination. New lesions from scabies, such as papules or vesicles are covered by normal keratin, whereas older lesions have a heaped-up parakeratotic surface. There may also be a perivascular infiltrate of lymphocytes, histiocytes, and eosinophils. Histopathologic studies of late cutaneous allergic responses have revealed mixed cellular infiltrates, including lymphocytes, polymorphonuclear leukocytes, and some partially degranulated basophils. A prominent feature of late cutaneous allergic reactions has been fibrin deposition interspersed between collagen bundles in the dermis and subcutaneous tissues.
Maintaining a proper index of suspicion in this direction (especially during the summer months)
A familiarity of the insect fauna in one’s area
Obtaining a good history
It is very important to find out what the patient has been doing lately, e.g., hiking, fishing, gardening, cleaning out a shed, and so forth. However, even history can be misleading in that patients may present with a lesion that they think is a bite or sting, when in reality the correct diagnosis is something like urticaria, folliculitis, or delusions of parasitosis (see Chap. 14). Physicians need to be careful not to diagnose “insect bites” based on lesions alone and should call on entomologists to examine samples.
10.4 Summary and Conclusions
A human’s first line of defense against invasion or external stimuli is the skin. It may react in a variety of ways against all kinds of stimuli—physical or chemical—including arthropods and their emanations. Lesions may result from arthropod exposure, although not all lesions have the same pathological origin—some are owing to mechanical trauma, some owing to infectious disease processes, and some result from sensitization processes. Physicians and other healthcare providers are frequently confronted with patients having skin lesions attributed to a mysterious arthropod bite or sting. Diagnosis is difficult but may be aided by asking the patient numerous questions about the event and any recent activity that might have led to arthropod exposure. The following questions might provide useful information: “Did you see the offending arthropod?” “Was it wormlike?” “Did it fly?” “Where were you when these lesions occurred?” Most treatments (except in cases of infectious diseases) involve counteracting immune responses to venoms, salivary secretions, or body parts using various combinations of antihistamines and corticosteroids. Infectious diseases often require antibiotic/supportive care.
- 3.Frazier CA. Diagnosis of bites and stings. Cutis. 1968;4:845–9.Google Scholar
- 4.Goddard J. Physician's guide to arthropods of medical importance. 6th ed. Boca Raton: Taylor and Francis (CRC); 2013. p. 412.Google Scholar
- 10.CDC. Necrotic arachnidism -- Pacific northwest, 1988-1996. CDC. MMWR. 1996;45:433–6.Google Scholar