Human skin is exposed to the environment on a constant basis and encounters multiple threats, including arthropods of many varieties. Each species of arthropod can inflict its own type of damage to the skin; some bites are mild and barely noticeable, and others can be life-threatening. The most common bites are those of mosquitoes, fleas, bedbugs, mites, ticks, and spiders. Not only can these bites cause direct damage to the skin, but these organisms may have the ability to transmit infectious diseases such as Lyme disease, leishmaniasis, and rickettsial diseases.
Clinical Findings: Mosquitoes are prominent insects in the spring, summer, and early fall seasons. In warmer climates, they can be seen year round. Their bite is often not noticed until after the mosquito has gone. The recently bitten person is left with a pruritic urticarial papule that typically resolves by itself within an hour or so. Some individuals are prone to severe bite reactions and develop warm, red papules and nodules that can last for a week or two and can be associated with regional lymphadenopathy. Mosquitoes are essentially a nuisance for the most part, but in some areas of the world they are the major vectors for transmission of malaria and encephalitis viruses. Sand flies are similar, but they are the major vector for leishmaniasis.
Fleas have been around since before the beginning of human civilization and were responsible in the Middle Ages for helping transmit the bubonic plague, which killed millions of people. Fleas are most commonly seen in households with pets. Individuals can be bitten after the pet transfers the fleas to bedding, carpeting, or clothing. Characteristic bites occur in groups of three, referred to as “breakfast, lunch, and dinner.” Flea eggs can lay dormant for years, only to reactivate in response to movement and vibration that indicate a meal is likely to be nearby. Many flea bites occur around the ankles of adults; the fleas jump from the carpeting to the ankles, take their meal, and leave. The typical skin lesion is a small papule with a central punctum. It is self-resolving. Fleas have been known to carry organisms responsible for infectious diseases, including Yer-sinia pestis (bubonic plague) and Rickettsia typhi (murine typhus).
Bedbugs (Cimex lectularius) have made a resurgence in the United States. The are ubiquitous insects that can live in any area of the country. Households, hotels, and other sleeping quarters become infested with colonies of bedbugs. They emerge in the night, typically 1 to 2 hours before dawn, and search for a blood meal. They find their victim asleep and feed for a few minutes before retreating back to the nest. The nest is almost never in the bed; it is most likely to be located within the baseboard molding or floor boards. In the morning, the afflicted individual awakens with one to hundreds of bites. Most are small papules with a central punctum. Depending on the species of bedbug, a more inflammatory response may occur, causing vesiculation and bullae. Bedbugs have been reported to transmit hepatitis B virus.
Encounters with the large mite family of organisms are more likely to occur in the summer months in northern latitudes but can occur at any time of the year in the southern regions. The term chigger refers to the larval phase of the Trombiculidae family of mites; it is one of the most common and well-recognized causes of human bites. Chiggers are small red mites, so small that they are not felt, and they bite quickly. They usually leave pinpoint red papules that can be numerous and can cause severe pruritus. Many other mites are present in the environment and can cause similar reactions.
Most ticks bite and feed for up to 24 hours before falling off after receiving their blood meal. They can leave a tick bite granuloma, which is a small red papule with a central punctum, at the site of the bite. Many methods have been used to remove ticks; most can result in more skin damage than the actual tick bite.
These methods include burning the end of the tick with a cigarette or a match, an approach that is more likely to cause a skin burn than it is to remove the tick. The best method of removal is to grab the tick as close to the surface of the skin as possible and gently pull in a direction perpendicular to the skin. If the mouthparts are left embedded in the skin, a small punch biopsy can be performed to remove the remaining parts. Ticks are well known to transmit many infectious diseases, including Lyme disease and Rocky Mountain spotted fever.
Most spider bites are caused by jumping spiders. As with all spiders, bites frequently occurs after the spider’s web or nesting location is disturbed. The bites can be painful and can leave erythema and a papule or nodular reaction. On occasion, these bites develop secondary cellulitis. Two spiders are unique in their potential to cause severe human disease: the black widow spider (Latrodectus mactans) and the brown recluse spider (Lox-osceles reclusa).
The black widow spider is a web-weaving spider that paralyzes its prey with a potent neurotoxin called latrotoxin. The venom causes massive release of acetylcholine from nerve endings. In humans, this can lead to pain, fever, and symptoms of an acute abdomen.
The brown recluse spider is a solitary stalking spider that lives in dark, hidden locations. It is not aggressive and typically bites only when a human accidentally disturbs its location. The toxin released in its venom contains a mixture of sphingomyelinase-D, hyaluronidases, proteases, and esterases. Sphingomyelinase-D is the major component that is believed to be responsible for most of the tissue damage caused by the spider’s bite. It can cause severe pain and aggregation of platelets and red blood cells, resulting in intravascular clotting with resultant necrosis of the skin. The characteristic pattern seen on the skin is a central bluish region with necrosis and coagulation, a surrounding vasoconstricted area that appears to be blanched white and a peripheral rim of erythema. This has been termed the “red, white, and blue” sign of a brown recluse bite. Some bites can progress rapidly and cause severe necrosis of the skin requiring surgical debridement.
Histology: Most bite reactions are not biopsied, because they are typically diagnosed clinically. The histological findings for most bug bites are very similar. There is a superficial and deep inflammatory infiltrate with many eosinophils. Superficial necrosis of the epidermis may be seen at the site of the bite. Occasionally, tick mouth parts are located in the biopsy specimen. Brown recluse spider bites show intravascular thrombosis and necrosis of the skin.
Treatment: The treatment of most bites is supportive. Pruritus can be treated with a potent topical corticosteroid and an oral antihistamine. Avoidance is the most important preventive measure. Areas of standing water provide breeding grounds for mosquitoes and should be drained routinely. Pets should be groomed and treated with preventive tick and flea medications. Flea and bedbug infestations should be treated by a professional exterminator. Proper use of bug sprays containing DEET (N,N-diethyl-m-toluamide) and staying in the center of wooded trails can help decrease one’s chance of being bitten. In endemic areas, any patient with a deer tick bite that has lasted longer than 24 hours should be considered for prophylactic therapy for Lyme disease.