Chigger Bites

Chiggers are the six legged larvae of trombiculid mites (Trombicula, Neotrombicula, Eutrombicula, Leptotrombidium). Only the larval stage feeds on vertebrates. Nymphs and adults are free living predators.

• The larva attaches to skin and injects digestive saliva that dissolves epidermal and dermal cells. The host reaction forms a feeding tube (stylostome). The larva feeds on this liquefied tissue and does not suck blood, and it does not burrow or live in the skin.
• Exposure occurs in grass, scrub, woodland, gardens, fields and damp vegetation, mainly in warm months. Hikers, campers, farmers, soldiers and children are most affected.
• The itch is a delayed hypersensitivity reaction to the saliva and often starts after the larva has already dropped off, so the bite is rarely found.
• In Asia, the western Pacific and northern Australia, Leptotrombidium larvae transmit Orientia tsutsugamushi (scrub typhus), which causes a serious febrile illness.

• Intensely itchy red papules, papulovesicles or urticarial weals, 1 to 3 mm or larger, beginning 3 to 24 hours after exposure, peaking at 24 to 72 hours and lasting 1 to 3 weeks. Itch may last longer with scratching.
• Sites are where clothing is tight or skin is thin and warm: ankles, lower legs, popliteal fossae, waistband, groin, genitals, axillae and under bra straps. Lesions are clustered, with many bites in a small area.
• Genital involvement in men can cause marked penile swelling and itch (summer penile syndrome).
• Bullous or nodular reactions occur in sensitised patients. Secondary impetigo and excoriation are common.
• Scrub typhus: fever, headache, myalgia and a painless black eschar with regional lymphadenopathy 6 to 21 days after exposure, then rash, cough and, in severe cases, pneumonitis, meningoencephalitis, hypotension and multiorgan failure. Untreated mortality is 6 to 30%.
• Differential diagnosis: scabies, flea or bed bug bites, other insect bites, contact dermatitis, folliculitis, papular urticaria, dermatitis herpetiformis and cercarial dermatitis.

• Clinical, from the exposure history, season and the distribution at tight clothing margins. Mites are rarely found.
• Scabies is excluded by looking for burrows in the finger webs and wrists and by household itch pattern.
• In any febrile traveller or resident of an endemic area, look for an eschar, and test for scrub typhus by PCR of eschar swab or blood, or serology. Do not delay treatment while waiting. Full blood count (thrombocytopenia), liver enzymes (raised) and chest X-ray guide severity.
• Swab impetiginised lesions for bacterial culture.

• Wash the skin with soap and water and change clothes after exposure, which removes any remaining larvae. This does not reduce itch once the reaction has started.
• Symptom control:
  • Mometasone furoate 0.1% or betamethasone valerate 0.1% cream once or twice daily for 5 to 7 days (not on the face or genitals for more than a few days).
  • Calamine lotion, menthol 0.25 to 1%, or pramoxine 1% lotion 3 to 4 times daily.
  • Cetirizine 10 mg orally once daily (child 2 to 5 years 2.5 mg, 6 to 11 years 5 mg), with hydroxyzine 10 to 25 mg at night in adults for sleep.
  • Cool compresses and short nails.
• Severe or bullous reactions: prednisolone 0.5 mg/kg per day (30 to 40 mg in adults) for 3 to 5 days.
• Secondary infection: mupirocin 2% ointment 3 times daily for 5 to 7 days, or flucloxacillin 500 mg orally 6 hourly for 7 days (child 12.5 to 25 mg/kg 6 hourly).
• Scrub typhus (treat on clinical suspicion):
  • Doxycycline 100 mg orally or IV 12 hourly for 7 days (child under 45 kg 2.2 mg/kg 12 hourly). Defervescence usually occurs within 24 to 48 hours. Shorter courses of 3 to 5 days are used in mild disease but evidence is limited.
  • Azithromycin 500 mg orally once daily for 3 days (child 10 mg/kg once daily, maximum 500 mg) in pregnancy and in children who cannot take doxycycline.
  • Severe disease: doxycycline 100 mg IV 12 hourly, or azithromycin 500 mg IV daily, with organ support. In regions with doxycycline resistant strains, add rifampicin 600 mg orally daily for 7 days (evidence limited).
• Prevention:
  • Wear long trousers tucked into socks and closed shoes in vegetation.
  • DEET 20 to 30% on exposed skin and clothing edges, or picaridin 20%. Apply permethrin 0.5% to clothing.
  • Shower and wash clothes in hot water after outdoor exposure.
  • Keep lawns short and clear brush near living areas.
• Special groups:
  • Pregnancy: calamine, mild topical steroid, cetirizine or loratadine. Use azithromycin for scrub typhus, since doxycycline carries a risk to the fetus.
  • Children: weight based antihistamine, short courses of mild steroid, and short nails.
  • Immunosuppression and diabetes: treat secondary infection early.
• Follow up: bites settle in 1 to 3 weeks. Review if fever, an eschar, spreading redness or persistent lesions beyond 3 weeks occur. Refer to infectious diseases for suspected scrub typhus with organ involvement.