Ectoparasites

Three species of lice infest humans, each adapted to a specific body site: Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Pthirus pubis (pubic or "crab" louse). All are obligate ectoparasites, unable to survive for more than a short period away from a human host.

Transmission occurs through direct head-to-head or body contact, or via shared fomites such as combs, hats, bedding, and clothing. Fomites matter particularly for the body louse, which lives and lays its eggs (nits) in clothing seams rather than on the body itself, moving onto skin only to feed. Pubic lice are transmitted predominantly through close physical or sexual contact, reflecting their preferred site of infestation.

The head louse infests the scalp and hair, attaching eggs (nits) firmly to hair shafts close to the scalp.

The pubic louse infests coarse body hair, principally the pubic region but also eyelashes, axillary hair, and other coarse hair in heavy infestation.

The body louse, uniquely, resides and breeds in clothing rather than on the skin, visiting the body only to feed.

Disease from lice infestation itself is generally limited to pruritus from the host's hypersensitivity reaction to louse saliva injected during feeding, together with secondary bacterial skin infection from scratching.

The body louse carries additional and greater medical significance as a biological vector. It transmits Rickettsia prowazekii (epidemic typhus), Bartonella quintana (trench fever), and Borrelia recurrentis (louse-borne relapsing fever). These infections are historically associated with crowded, unsanitary conditions such as war and displacement, where body lice thrive on infrequently changed clothing.

Sarcoptes scabiei is a microscopic mite (not visible to the naked eye) responsible for scabies. It is transmitted primarily through prolonged direct skin-to-skin contact, and to a lesser extent via contaminated bedding or clothing.

Fomite transmission is far more significant in crusted (Norwegian) scabies, where the extremely high mite burden makes it far more important than in ordinary scabies.

The fertilised female mite burrows into the superficial epidermis (the stratum corneum), creating a characteristic thin, thread-like burrow in which she lays eggs as she progresses, typically two to three eggs per day over her several-week lifespan.

Classic sites include the finger webs, wrists, elbows, axillae, and genital area. The face and scalp are generally spared in adults, though infants and young children can have facial and scalp involvement.

The intense pruritus of scabies, often worse at night, results not from the mite's physical presence alone but from a delayed-type hypersensitivity reaction to mite antigens (saliva, eggs, and faecal pellets deposited within the burrow).

For this reason, symptoms typically take several weeks to develop after a first infestation but appear far more rapidly on reinfestation, once the host is already sensitised.

Crusted scabies is a distinct, severe variant occurring in immunocompromised, elderly, or neurologically impaired individuals unable to mount an adequate scratching or immune response. Mite numbers can reach into the millions rather than the dozens typical of ordinary scabies.

This form is highly contagious and presents with thick, crusted, hyperkeratotic skin plaques rather than typical burrows. It poses a significant transmission risk to caregivers and close contacts, including via brief contact or contaminated surfaces.

Fleas are small, wingless, laterally flattened, blood-feeding insects with powerful hind legs adapted for jumping, allowing rapid movement between hosts. The most medically significant species affecting humans include Pulex irritans (human flea), Ctenocephalides species (cat and dog fleas, which readily bite humans), and Xenopsylla cheopis (the Oriental rat flea).

Transmission of flea bites requires direct contact with an infested host or environment (bedding, carpets, pet resting areas). Fleas can persist in the environment for extended periods between blood meals, so infestation can be sustained even after removal of the primary animal host, a common practical issue in household flea control.

Flea bites typically occur on exposed lower limbs and produce intensely pruritic, small papular lesions, often in a linear or clustered pattern reflecting repeated feeding along an accessible skin area. Direct disease from the bite itself is generally limited to this local reaction and secondary infection from scratching.

The far greater medical significance of fleas lies in their vector role. Xenopsylla cheopis is the principal vector of Yersinia pestis (plague). It is transmitted when infected fleas, having fed on an infected rodent, regurgitate bacteria-laden material into a subsequent human bite after their gut becomes blocked by a bacterial mass (a "blocked flea").

Fleas also transmit Rickettsia typhi (murine typhus) and serve as an intermediate host for Dipylidium caninum (the dog tapeworm, occasionally infecting humans, particularly children, who accidentally ingest an infected flea).

Ticks are blood-feeding arachnids divided into two families of medical importance.

Hard ticks (Ixodidae) have a rigid dorsal shield and typically attach for prolonged periods (days) to complete a blood meal.

Soft ticks (Argasidae) lack this shield and feed more rapidly (often within an hour). They tend to inhabit animal burrows and rustic shelters rather than open vegetation.

Ticks acquire infection by feeding on an infected reservoir host (commonly rodents, deer, or other wild mammals) and transmit pathogens to a subsequent host, including humans, during feeding.

Hard ticks generally require a substantial attachment period, often many hours to over a day, before pathogen transmission occurs. This is the biological basis for the practical recommendation to remove attached ticks promptly to reduce infection risk.

Tick bites are usually painless due to anaesthetic and anticoagulant compounds in tick saliva. The bite itself is often unnoticed, and the tick may remain attached and feeding for an extended period before being found. This increases the effective transmission window for tick-borne pathogens compared to more immediately noticed insect bites.

Ticks are vectors for an unusually broad range of pathogens spanning bacteria, viruses, and protozoa, reflecting their prolonged feeding and wide host range.

• Ixodes species transmit Borrelia burgdorferi (Lyme disease), Anaplasma phagocytophilum, and Babesia species.
• Dermacentor species transmit Rickettsia rickettsii (Rocky Mountain spotted fever) and can cause tick paralysis through a salivary neurotoxin.
• Various tick species transmit tick-borne encephalitis virus and other regional pathogens depending on geography.

This broad vector role, rather than the bite itself, accounts for the great majority of clinically significant tick-associated disease.