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  • An organism that lives on or within another organism (host)

  • A parasite causes harm to the host. This distinguishes parasitism from commensalism, in which the host derives no benefit but is not injured, and mutualism, where the relationship benefits both organisms

  • Host, in addition to providing a steady food source, provides warmth and shelter

  • Definitive host are those in which parasite becomes sexually mature and undergoes reproduction

  • Reservoir hosts are those in which parasites that are pathogenic to other animals or to humans reside

  • Vector are agent by which a parasite is transmitted to the host (e.g., arthropod, mollusk)


  • Bites usually result in localized, cutaneous reactions and pruritus

  • Some of these organisms are medically important: fleas, lice, and ticks can transmit lethal epidemic disorders

  • Many of these vector-transmitted diseases are endemic in various regions of the world

  • Four classes of arthropods are of dermatologic interest and are covered in this chapter:

    • Chilopoda: including centipedes

    • Diplopoda: including millipedes

    • Insecta: including caterpillars, moths, bedbugs, lice, flies, mosquitoes, beetles, bees, wasps, hornets, fire ants, and fleas

    • Arachnida: including ticks, mites, scorpions, and spiders

  • Organisms from the arthropod classes Arachnida and Insecta have a hard-jointed exoskeleton and paired, jointed legs

  • Class Insecta: a group of organisms with 6 legs and 3 body segments: head, thorax, and abdomen. Includes the following orders:

    • Siphonaptera: fleas

    • Anoplura: head and body lice

    • Pthiridae: crab louse

    • Diptera: 2-winged flies, mosquitos, midges

    • Hemiptera: true bugs

    • Lepidoptera: butterflies, moths, and their caterpillars

    • Hymenoptera: ants, wasps, and bees

  • Class Arachnida: a group of organisms with 8 legs and 2 body segments: cephalothorax and abdomen

    • Ixodidae: hard ticks

    • Argasidae: soft ticks

    • Araneae: spiders

  • Centipedes and millipedes


Siphonaptera (Fleas)

  • Wingless, laterally compressed insects with a hard, shiny integument

  • The body has 3 regions: head, thorax, and abdomen

  • Mouthparts are modified (paired maxillary palpi) for piercing and sucking

  • Survive months without feeding

  • Order Siphonaptera contains 2 flea families of medical importance

    • Pulicidae: (human, cat, dog, and bird fleas)

    • Sarcopsylidae (also called Tungidae): the sand flea

  • Fleas jump, on average, about 20 cm

  • One flea can bite 2 to 3 times over a small area

    • Bites produce irregular, pruritic, red wheals up to 1 cm in diameter

  • Patients may present with a surrounding halo with a central papule, vesicle, or bulla or with hemorrhagic macules, papules, vesicles, or bullae

  1. Pulex irritans (human flea) (Fig. 16-1)

    • Farms, urban areas, predominant flea on dogs in portions of the Carolinas

  2. Tunga penetrans (chigoe flea)

    • Tropical and subtropical regions of North and South America, Africa

    • Intense itching and local inflammation

    • Causes tungiasis

      • – Female sand flea, which burrows into human skin at the point of contact, usually the feet

      • – Head is down into the upper dermis feeding from blood vessels

      • – Caudal tip of the abdomen is at the skin surface

      • – Nodule (usually ...

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