Description

Arthropods make up the largest division of the animal kingdom; 2 classes, insects and arachnids, have the greatest medical impact on humans. Arthropods affect humans by inoculating poison or irritative substances through a bite or sting, by invading tissue, or by contact allergy to their skin, hairs, or secretions. The greatest medical importance is transmission of infectious microorganisms that may occur during insect feeding. Sequelae to arthropod bites, stings, or contact may include:

• Local redness with itch, pain, and swelling: Common, usually immediate and transient
• Large local reactions increasing over 24–48 hours
• Systemic reactions with anaphylaxis, neurotoxicity, organ damage, or other systemic toxin effects
• Tissue necrosis or secondary infection
• Infectious disease transmission: Presentation may be delayed weeks to years

Epidemiology

• Difficult to estimate, as most encounters unreported
• ~50 deaths/yr in the US from fatal anaphylactic reaction to Hymenoptera stings
• Unrecognized anaphylactic reactions to Hymenoptera stings may be cause of 1/4 of sudden and unexpected deaths outdoors (1).

Prevalence
Widespread, with regional and seasonal variations

Risk Factors

• Previous sensitization is key to most severe allergic reactions, but exposure history may not be recalled.
• Although most arthropod contact is inadvertent, certain activities, occupations, and travel increase risk.
• Greater risk for adverse outcome in young, elderly, immune compromised, or those with unstable cardiac or respiratory status

Genetics
Family history of atopy may be a factor in the development of more severe allergic reactions.

General Prevention

• Avoidance of common arthropod habitats, where possible
• Insect repellents (not effective for bees, spiders, scorpions, caterpillars, bedbugs, fleas, ants):
  • DEET:
    • Most effective broad-spectrum repellent against biting arthropods (2)
    • Formulations with higher concentrations (20–50%) are first-line choice when visiting areas where arthropodborne diseases are endemic (2)
    • Apply to skin or outer clothing.
    • Concentrations >30% give longer duration of effect (5+ hours).
    • Appears safe for children >2 months of age at lower concentrations.
  • Icaridin (formerly known as picaridine):
    • Use of concentrations <20% may require more frequent application to maintain effectiveness.
    • Less toxic effects on humans than DEET
  • PMD: Component of lemon eucalyptus extract:
    • Recommended alternative repellent to DEET at concentrations >20% (2)
    • Not studied for use in children age <3 years
  • IR3535: Less effective in most studies
  • Citronella: Not for disease-endemic areas (2)
  • Other botanical oils: Less effective than DEET
  • Barrier methods: Clothing, bed nets:
    • Use of light-colored pants, long-sleeved shirts, and hats may reduce arthropod impact.
    • Permethrin: Synthetic insecticide derived from Chrysanthemum plant. Should not be applied to skin, but permethrin-impregnated clothing provides good protection against arthropods.
    • Mosquito nets: Insecticide-treated nets advised for all travelers visiting disease-endemic areas at risk from biting arthropods (2)
• Desensitization 75–95% effective for Hymenoptera-specific venom:
  • Skin tests are needed to determine sensitivity.
  • Refer to allergist/immunologist if candidate
• Fire ant control (but not elimination) possible:
  • Baits; sprays, dusts, aerosols; biologic agents
• Tickborne diseases prevented by prompt removal of ticks within 24 hours of attachment

Pathophysiology

4 general categories of pathophysiological effects: toxic, allergic, infectious, traumatic:

• Toxic effects of venom: Local (tissue inflammation or destruction) vs. systemic (neurotoxic or organ damage)
• Allergic: Antigens in saliva may cause local inflammation. Exaggerated immune responses may result in anaphylaxis, serum sickness.
• Trauma: Mechanical injury from biting or stinging causes pain, swelling, and portal of entry for bacteria and secondary infection. Retention of arthropod parts can cause a granulomatous reaction.
• Infection: Arthropods are vectors and can transmit bacterial, viral, and protozoal diseases.

Etiology

Arthropods: 4 medically important classes:

• Insects: Hymenoptera (bees, wasps, hornets, fire ants), mosquitoes, bed bugs, flies, lice, fleas, beetles, caterpillars, and moths
• Arachnids: Spiders, scorpions, mites, and ticks
• Chilopods (centipedes)
• Diplopods (millipedes)