logo Arthropods and parasites

Skin infections

Arthropods and parasites


  • Be able to identify common dermatoses due to arthropods.

Key points

  • Arthropod bites may cause hypersensitivity reactions, secondary infection and spread viral, rickettsial and parasitic diseases.
  • Papular urticaria presents as crops of itchy red papules sometimes topped by vesicles.
  • Spider bites may be initially unnoticed but can cause large painful areas of tissue necrosis.
  • Scabies is characterised by intense itch, particularly at night. Findings may include burrows on fingers and wrists, penile papules, nodules in axilla and groin and widespread erythematous rash.
  • Head lice cause scalp and neck pruritus. Nits are diagnostic of present or past infestation, but may be confused with follicular casts and scaling from dermatitis or psoriasis.
  • Cutaneous larva migrans presents as itchy creeping tracks, most often found on the feet.

Arthropod bites

Mosquitoes and sandflies account for most bites in New Zealand and rarely cause serious harm. Hypersensitivity reactions to bee stings may result in localized oedema or anaphylaxis and precipitate discoid eczema or vasculitis. Secondary infection may result in impetigo or cellulitis.

Elsewhere in the world, numerous crawling and flying arthropods cause bites and stings. They are also responsible for the spread of other contagious diseases, including:

Papular urticaria

Papular urticaria describes clusters of irritable urticated papules, most often due to bites by mosquitoes, sandflies, fleas or sometimes bed bugs. Each spot is erythematous but may have a pale centre, and elevated (urticated). There is often a central punctum at the site of penetration of the insect bite or sting and a vesicle may appear. Lines of bites are characteristic: "breakfast, lunch and tea". A few individuals may have bullous lesions, especially tourists exposed to new insects. Older papules tend to be excoriated with scabs and crusts. Children with papular urticaria may present with numerous lesions on exposed areas, particularly the lower legs but also arms, cheeks and waistline. Papules may persist for weeks, and older lesions may reappear with new crops presumably in response to new bites. The eruption may be seasonal, dying down in mid-winter to reappear in spring for two or three seasons until immune tolerance occurs.

Management should emphasise identification of the source of bites and prevention, using protective clothing and DEET (diethyltoluamide) insect repellents.

Spider bites

The majority of medically important spider bites can be attributed to widow spiders (Latrodectus spp.) and recluse spiders (Loxosceles spp.). Spider bites may arise painlessly but later result in painful erythematous plaques and subsequent tissue necrosis due to toxins that cause neutrophilic destruction of endothelial cells, and systemic toxicity. In New Zealand, the rare katipo and imported red back spiders may be responsible and antivenom is available; there is debate whether white tailed spiders are capable of serious damage. Ladybird bites rarely may cause superficial necrotic papules.


Scabies is common in all communities and is responsible for a great deal of misery because of intractable pruritus and secondary bacterial pyoderma. The human mite Sarcoptes scabiei is responsible, and resides only in the stratum corneum. Animal mites (mange) may cause papular urticaria but do not infest humans.

In most cases the infestation predominantly affects hands, wrists and genitals. Fine grey irregular 5-10 mm burrows are most easily identified on the sides of the fingers or volar aspects of the wrist. Using magnification, the mites can be extracted from the distal end of their burrows using a fine needle and examined under low power light microscopy to confirm the diagnosis.

Within days to weeks of infestation an allergic erythematous non-specific papular rash and/or generalised pruritus may appear, sparing the head and neck. Frequently the rash is masked by numerous excoriations due to intense pruritus, particularly at night. Later, papules may be found on the shaft of the penis and dermal nodules in the axillae and/or inguinal regions.

Scabies mite

Scabies burrows along and between the digits

Generalised hypersensitivity eruption - note pruritus

Penile and scrotal papules are pathognomonic of scabies

Inguinal nodules in a patient with longstanding scabies

Vesicles and pustules on sole of infant with scabies

Innumerable burrows in a patient with crusted scabies

Insecticides currently used to eradicate scabies include malathion lotion and permethrin cream (a synthetic pyrethroid) applied for 6-10 hours to the entire body surface below the hairline and chin, including under the nails and genitals. Unfortunately, treatment failure is common. Reasons may include:

Severe infestation resulting in psoriasis-like hyperkeratosis and scalp involvement (Norwegian scabies) is most likely in immunosuppressed or debilitated elderly patients who are unable to scratch out the arthropods. This form of scabies is highly infectious due to the large number of mites and can infest hundreds of individuals living or working in affected rest homes or hospitals. Off-label oral ivermectin is the most convenient treatment in this situation.

Scabies may cause hypersensitivity reactions including urticaria, nummular dermatitis and vasculitis. These may persist after eradication of the responsible mite.


Body lice are a very rare cause of papular urticaria in New Zealand but can be expected to reappear in times of war or deprivation. The eggs can be found along the seams of intimate clothing.

Pubic lice ("crabs") are transmitted sexually. Occasionally they will infest axillary hair or even eyelashes. Public lice are now rare, possibly due to prevalence of pubic hair removal

On the other hand, despite excellent hygiene, head lice are very prevalent in school children in most societies. The egg cases ("nits") are found firmly attached to hair shafts. Empty egg cases are easier to see because they are white and further away from the scalp than grey nits containing live eggs. Scurrying live mites are most easily found on the occiput or behind the ears. They cause irritable crusted papules and sometimes, secondary dermatitis, impetiginisation and lymphadenopathy.

Dermatitis due to headlice

Head louse

Pubic louse

Treatment should include:

Larva migrans

Endemic larva migrans (creeping eruption) is very rare, but travellers to South East Asia regularly present with cutaneous larva migrans. Affected areas are usually the feet and lower legs, or any part of the body that has been in contact with soiled ground. The eruption is due to penetration of the skin by dog hookworm larvae (usually Ankylostoma species), which are unable to complete their life cycle in humans as they cannot penetrate the basement membrane. The larva travels several centimetres under the skin causing an irritable 3-5mm track that persists for one to three months. It may be destroyed using anthelmintics such as thiabendazole, albendazole, mebendazole or ivermectin.

Larva migrans on the soles of travellers who had recently visited Bali (left image) and Thailand (right image)