Chapter 16: Cutaneous Infestations

PARASITE

• 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)

ARTHROPODA

• 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

INSECTA

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 ...