Ectoparasites: Lice

• Ectoparasites are parasites found on the skin or in its superficial layers.
• All ectoparasites are arthropods, characterized by jointed legs and a chitinous exoskeleton.
• Arthropods are invertebrates possessing a chitinous exoskeleton.
• Ectoparasites causing human disease are categorized into insects and arachnids.

Insects

• Insects have six legs, examples included lice, flies, and bedbugs.

Lice (Louce)

• Lice are the causative agent of Pediculosis.
• Pediculosis is caused by two louse species: Pediculus humanus (head and body louse) and Phthirus pubis (pubic louse).
• Phthirus pubis primarily affects the genital area, but can also involve the axilla and eyebrows.
• Pediculus humanus has two subspecies:
  • Pediculus humanus capitus (head louse) primarily infests the scalp.
  • Pediculus humanus corporis (body louse) primarily infests the trunk.

Important properties of lice

• Lice are easily visible, measuring roughly 2 to 4 mm long.
• Lice have six legs armed with claws for attachment to hair and skin.
• Pediculus has an elongated body, while Phthirus has a short body resembling a crab.
• Nits are the eggs of lice, typically attached to the hair shaft.
• Nits are white and visible to the naked eye.
• Body louse nits are often attached to clothing fibers.

Transmission of lice

• Head lice are mainly transmitted by fomites like hats, combs, and towels, especially among schoolchildren.
• Body lice are mainly found on clothing and are transmitted by clothing or personal contact.
• Body lice leave clothing to feed on blood.
• Pubic lice are primarily transmitted via sexual contact.
• Widespread infestations of body lice occur when personal hygiene is poor, such as in wartime or refugee camps.
• The body louse is a vector for several human pathogens, notably Rickettsia prowazekii, which causes epidemic typhus.
• Head lice and pubic lice are not vectors of human disease.

Pathogenesis and Clinical Findings of lice

• Adult lice bite and feed on blood, injecting saliva into the skin, which causes a hypersensitivity reaction and pruritus.
• Pruritus is the main symptom of lice infections.
• Excoriations, resulting from scratching, may lead to secondary bacterial infections.
• In Pediculosis capitis, adult lice are often difficult to see, but nits are easily visualized.
• In Pediculosis corporis, adult lice are primarily found in clothing rather than on the body.
• In Pediculosis pubis, both adult lice and nits can be seen attached to pubic hair.

Laboratory Diagnosis of lice

• Laboratory diagnosis is generally not involved; diagnosis is mainly clinical.
• Nits fluoresce under ultraviolet light from a Wood's lamp, which can be used to screen large numbers of people for lice.

Flies

• Myiasis is caused by fly larvae
• Fly larvae also are known as maggots
• Botflies are most known to cause myiasis
• Flies that cause myiasis are found worldwide and infest many animals and humans

Transmission of fly larvae:

• Adult flies deposit eggs in a wound, and the eggs hatch to produce larvae.
• Flies may deposit eggs in the nostrils, conjunctiva, or on the lips.
• Some flies deposit eggs on unbroken skin, and the larva invades the skin directly.
• Dermatobia deposits eggs on a mosquito, which then transmits the larvae to a human when it bites.

Pathogenesis and Clinical Findings of fly larvae

• Presence of larvae in tissue induces an inflammatory response.
• The characteristic lesion is a painful, erythematous papule resembling a furuncle.
• The lesion may also be pruritic.
• Often, the larvae are visible.
• Some patients report a sense of movement within the lesion.
• Cutaneous myiasis is the most common presentation, but ocular, intestinal, genitourinary, and cerebral forms can occur.

Bedbugs

• Cimex lectularius is a bedbug species characterized by an oval, brownish body, approximately 5 mm long.
• Bedbugs reside in mattresses and the crevices of wooden beds.
• At night, bedbugs emerge to take a blood meal from sleeping humans.

Clinical Findings of bedbugs

• The main symptom of a bedbug bite is pruritic wheals, caused by hypersensitivity to proteins in the bug's saliva.
• Bedbug bites are not known to transmit any human diseases.

Arachnids

• Arachnids are eight-legged arthropods that include mites, ticks, and spiders.

Mites

• Scabies is caused by the "itch" mite, Sarcoptes scabiei.
• The mite is not a vector for other human pathogens.

Important Properties of Mites:

• The adult female Sarcoptes mite is approximately 0.4 mm in length, with a rounded body and eight short legs.

Transmission of Mites

• Transmission occurs through personal contact or via fomites like clothing, especially under unhygienic conditions or during wartime.

Pathogenesis of Mites

• The mite resides within the stratum corneum of the epidermis.
• Pruritic lesions result from a delayed hypersensitivity reaction to the feces of the mite.

Clinical Finding and Diagnosis of Mites

• Typical lesions are either tracks or papules that are very pruritic.
• Common sites include the hands, wrists, axillary folds, and genitals.
• Areas of the body, where clothing is tight, such as along the waistband, are often involved.
• Itching is typically worse at night.
• In immunocompromised individuals, extensive crusted dermatitis (Norwegian scabies) can occur
• These patients may be infested with thousands of mites with little or no pruritus
• Excoriations may become infected with Staphylococcus aureus or Streptococcus pyogenes resulting in pyoderma.
• Microscopic examination of skin scrapings reveals the mites, their eggs, or fecal pellets.

Ticks

• Can cause Tick paralysis.
• Vector of diseases: rickettsial, spirochaetal, viral, bacterial and protozoal diseases.
• Ticks are commonly found in grassy woodland areas and are attracted by carbon dioxide and warmth from humans.

Important Properties and Transmission of Ticks:

• Female ticks require a blood meal for maturation of their eggs, and hence it is the female that causes tick paralysis as well as serves as the vector of diseases.
• A tick attaches to human skin by means of its mouth part (proboscis).

Pathogenesis of Tick paralysis

• Paralysis is mediated by a neurotoxin that blocks acetyl choline release at the neuromuscular junction, similar to botulinum toxin.
• The toxin is made in the salivary gland of the tick.
• The tick must remain attached for at least 4 days before the onset of symptoms.

Clinical Finding of Tick paralysis

• Ascending paralysis resembling Guillain-Barre syndrome occurs
• The paralysis is symmetrical and can ascend from the legs to the head within several hours.
• Respiratory failure and death can occur.
• Recovery typically occurs within 24 hours of removal of the tick.
• The tick is often found at the hairline at the back of the neck or near the ear.
• Children younger than 8 years are most often affected, as they play and stay in grassy woodland

Spiders

• Two species cause most of the significant disease: black widow spider(Latrodectus mactans) and brown recluse spider (Loxosceles reclusa)
• The black widow spider is about 1 cm in length with characteristic orange-red hourglass on its ventral surface.
• The brown recluse spider is also about 1 cm in length, but has a characteristic violin-shaped marking on its back.

Bites of Spiders

• Black widow spider bite

  • Causes neurologic symptoms primarily
  • Within one hour of the bite, pain and numbness spread from the site
  • Severe pain and spasms in the extremities and abdominal pain will occur
  • Fever, chills, sweats, vomiting, and other constitutional symptoms will occur.
  • Tissue necrosis doesn't occur
  • Recovery last several days but some die mainly children

• Brown recluse spider bite

  • Causes tissue necrosis symptoms primarily
  • Necrosis is due to proteolytic enzymes in the venom
  • Bites are initially itchy and painless, followed by vesicles then hemorrhagic bullae
  • The lesion ulcerates, becomes necrotic, and may not heal for weeks to months

Case study

• Suggests infestation of “Phthirus pubis”