Small pests can cause big skin problems

Denver - Reactions to arthropod attacks can range from minor skin manifestations to life-threatening situations, an expert says.

Some arthropods’ bark is bigger than their bite, says Julian Trevino, M.D., professor of dermatology at Boonshoft School of Medicine at Wright State University, Dayton, Ohio. Dr. Trevino spoke at the annual meeting of the American Academy of Dermatology.

Due to misidentification and misinformation, he explains, the number of encounters, bites and deaths attributed to brown recluse spiders (Loxosceles reclusa) and other arthropod encounters is greatly exaggerated.

Brown recluse bites generally resolve in one to two months with proper wound treatment, he says, although 10 to 15 percent of cases result in severe scarring. Additionally, Dr. Trevino says, emergency department physicians commonly misdiagnose ulcerating or necrotic wounds from many other sources such as insects, infections or physical trauma as the bites of Loxosceles species.

However, he says, these brown spiders (females bear a violin-shaped pattern on the cephalothorax) are generally unaggressive, biting only when handled, trapped or pinned in garments or linens. Bites of the Loxosceles species are a major cause of necrotic araenism, which is marked by a red, white and blue targetoid lesion that develops 24 to 48 hours post-bite, Dr. Trevino says. By 72 hours, it ulcerates in an eccentric pattern, followed by eschar formation and slow healing and scar formation over weeks to months.

Next: How to treat severe cases

Treating severe cases

Severe cases of necrotic araenism or loxoscelism (a rare systemic reaction usually in children) may require hospitalization, Dr. Trevino saus. In treating necrotic araenism, he says, oral leukocyte inhibitors such as dapsone and colchicine are unsupported by randomized, controlled trials, and they may pose significant toxicity risks.

“Use of hyperbaric oxygen is also unsupported by evidence. Early excision and intralesional corticosteroids also are contraindicated,” he says. No antivenoms for Loxosceles bites are universally available, he adds, except in South America.

A neurotoxic component of Latrodectus (black widow) spider venom can cause latrodectism, a systemic reaction that results from massive presynaptic release of neurotransmitters (e.g. acetylcholine, norepinephrine), according to Dr. Trevino. Latrodectism occurs within 30 minutes to a few hours after a stinging bite and is characterized by generalized (especially back and leg) pain, he says. It usually resolves over three to seven days, he adds, but rarely can result in respiratory arrest, seizures and death. Severe cases may require latrodectus antivenom, Dr. Trevino says, though supportive care such as wound cleansing, ice packs, oral or parenteral analgesics and tetanus prophylaxis usually suffice.

Stinging sensations

Much like the brown recluse, Dr. Trevino says, “Scorpions are shy and sting only with in self-defense.” Generally, scorpions hide under stones, bark or other debris during the daytime. Stings occur when a victim walks barefoot in scorpion-infested areas, or dons shoes or clothing that a scorpion has found its way into. Additionally, he says, scorpions often cling to the underside of tables - attempting to move such a table may result in a sting.

Along with local wounding, a scorpion sting in rare cases can lead to serious respiratory and cardiovascular complications. Of particular concern in the United States is Centruroides exilicauda (formerly Centruroides sculpturatus), a small scorpion whose sting is potentially fatal, Dr. Trevino says. C. exilicauda possesses a powerful neurotoxin capable of producing muscle spasticity, excessive salivation, nystagmus, blurred vision, respiratory distress and slurred speech, he says.

“Any child stung by a scorpion - especially one identified as C. exilicauda – should be admitted to a pediatric intensive care unit, where respiratory, cardiac and neurologic status can be monitored closely.”

For severe envenomations, Dr. Trevino says, after life-supporting measures are instituted, specific antivenin is the treatment of choice.

“Untreated stings in infants and young children may be fatal,” he says, “while death is uncommon in adults.”

Next: Other common threats

More common threats

Other biting and stinging insects range from mosquitoes - the most common arthropod vector of infectious disease worldwide - to bees, ants, chiggers, flies and even caterpillars, Dr. Trevino says. Among these, the imported fire ant, Solenopsis invicta, now documented in at least 12 states, brings a relatively new threat. Because of this species has a tendency to swarm and inflict multiple stings, a single person can commonly experience up to 3,000 stings.

“Fire ants may be the arthropod which poses the greatest risk for anaphylaxis to adults who live in endemic areas. Immunotherapy with fire ant whole-body extract is effective and safe for treatment of fire ant hypersensitivity,” Dr. Trevino says.

Bees, wasps and ants belong to the genus Hymenoptera. Generalized systemic reactions to Hymenoptera stings occur in up to 3 percent of victims, he says. Symptoms can include generalized urticaria, angioedema and bronchospasm. Treatment requires administering subcutaneous epinephrine as soon as possible, along with oral or parenteral diphenhydramine and, as needed, oxygen and systemic steroids.

When removing a bee, ant or other insect stinger from the skin, he says, “Be careful not to break it off or cause more venom to be released.” That’s why Dr. Trevino recommends gentle removal by scraping with a fingernail or knife-edge, or perhaps applying a glue or adhesive tape over the area to stick to and lift out the stinger.

The latter technique also applies to the setae (specialized hairs) of caterpillars, moths and butterflies, he says. Up to 150 Lepidoptera species are believed to produce irritant and allergic reactions known as lepidopterism, Dr. Trevino says. Mechanisms implicated include mechanical irritation from pointed setae, cell-mediated hypersensitivity to the setae, and toxin injections through hollow setae. Treatment is generally symptomatic, he says. It includes systemic antihistamines, topical menthol or camphor-containing products to quell pruritus and topical steroids (or systemic steroids in more severe cases).

Of particular concern is Lonomia obliqua, a venomous caterpillar that lives in South American rainforests and causes a handful of deaths annually, Dr. Trevino says.

“Most incidents occur when a traveler leans against a tree and brushes against one or several of these caterpillars, which release a very powerful anticoagulant venom,” he says.

Symptoms of Lonomia obliqua poisoning include severe internal bleeding, renal failure and hemolysis.

Among insect repellents, Dr. Trevino says, much of the evidence behind botanical agents is anecdotal. However, oil of lemon eucalyptus has been shown to be effective against mosquitoes, biting flies and gnats.

“I encourage dermatologists to stick with the established products that have shown efficacy in trials,” he says. These include diethyltoluamide (DEET, various manufacturers) permethrin and picaridin. Additionally, he cautions that applying sunscreens and DEET simultaneously can increase DEET absorption and diminish the sunscreen’s effectiveness.

Disclosures: Dr. Trevino reports no relevant financial interests.