Ectoparasite Infestations and Arthropod Injuries¶
Chapter 472 | Part 14: Poisoning, Drug Overdose, and Envenomation
KEY CLINICAL POINTS¶
- Scabies, lice, ticks, and other ectoparasites cause significant dermatological and systemic complications.
- Treatment varies by parasite type, with topical agents like permethrin and systemic therapies like ivermectin being common.
- Arthropod bites and stings can lead to allergic reactions, infections, and severe systemic envenomation.
- Delusional infestations (Ekbom syndrome) require psychiatric evaluation and antipsychotic management.
- Proper identification of the arthropod and prompt treatment are critical to prevent complications.
1. DEFINITION & OVERVIEW¶
Ectoparasites are arthropods and other organisms that infest the skin, hair, or mucous membranes of animals. They include mites, lice, ticks, fleas, flies, and spiders. Arthropod injuries encompass bites, stings, and infestations that can cause mechanical, allergic, or infectious complications. Human ectoparasites are classified as obligate (e.g., scabies mites) or facultative (e.g., lice).
Table 1: Common Ectoparasites and Their Clinical Associations¶
| Parasite | Clinical Presentation | Treatment |
|---|---|---|
| Scabies (Sarcoptes scabiei) | Pruritic papules, burrows, secondary infections | Permethrin, ivermectin |
| Head Lice (Pediculus humanus capitis) | Itching, nits on hair | Permethrin, malathion |
| Body Lice (Pediculus humanus corporis) | Pruritic rash, vagabond’s disease | Hygiene, permethrin |
| Pubic Lice (Pthirus pubis) | Pruritic lesions in pubic hair | Permethrin, lindane |
| Ticks (Ixodes, Amblyomma) | Localized pain, rash, systemic infections | Tick removal, doxycycline prophylaxis |
1.1 Ectoparasite Classification¶
Ectoparasites are categorized by their host preference and life cycle. Obligate parasites (e.g., scabies mites) require humans for survival, while facultative parasites (e.g., lice) can survive in non-human hosts. Some, like ticks, are vectors for pathogens (e.g., Lyme disease).
1.2 Arthropod Injury Mechanisms¶
Arthropods cause injury through mechanical trauma, enzymatic action, or venom release. Bites and stings may lead to local inflammation, allergic reactions, or systemic envenomation. Pathogens transmitted by arthropods (e.g., tick-borne diseases) contribute to global morbidity.
2. EPIDEMIOLOGY¶
Scabies affects ~250 million people globally, with outbreaks in institutional settings. Lice infestations are common in children and crowded environments. Ticks are prevalent in temperate and tropical regions, with Ixodes spp. causing Lyme disease. Fleas and bed bugs are urban pests, while scorpions and spiders are more common in arid regions. Delusional infestations (Ekbom syndrome) are rare but increasingly reported in psychiatric populations.
2.1 Risk Factors¶
Crowding, poor hygiene, travel to endemic regions, immunocompromise, and psychiatric disorders increase risk. Children, the elderly, and immunosuppressed patients are particularly vulnerable to severe complications.
2.2 Geographic Distribution¶
Scabies is global, while tick-borne diseases are endemic to temperate zones. Chiggers (Leptotrombidium spp.) are common in tropical and subtropical regions. Scorpions and spiders are prevalent in arid and semi-arid areas.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Ectoparasites cause disease through direct mechanical injury, allergic reactions to saliva/secretions, or transmission of pathogens. Scabies mites trigger a hypersensitivity response to their secretions. Ticks and mosquitoes transmit pathogens via saliva. Venom from spiders and scorpions causes neurotoxic or hemotoxic effects. Delusional infestations arise from psychiatric dysfunction and sensory misinterpretation.
3.1 Scabies Pathogenesis¶
Sarcoptes mites burrow into the stratum corneum, triggering a Type IV hypersensitivity reaction. Eggs, feces, and mites release antigens that cause pruritus and inflammation.
3.2 Tick-Borne Pathogens¶
Ixodes ticks transmit Borrelia burgdorferi (Lyme disease), Anaplasma phagocytophilum (anaplasmosis), and Babesia spp. Salivary secretions prevent coagulation and facilitate pathogen transmission.
4. CLINICAL FEATURES¶
Symptoms vary by parasite: pruritus, rash, fever, and systemic toxicity. Scabies presents with burrows and excoriation. Tick bites may cause erythema, lymphadenopathy, or neurologic symptoms. Spider bites may lead to necrosis or neurotoxicity. Delusional infestations manifest as persistent pruritus and self-inflicted lesions without identifiable parasites.
4.1 Scabies Presentation¶
Burrows (3–15 mm), papules, and excoriations on wrists, genitalia, and intertriginous areas. Severe pruritus worsens at night.
4.2 Tick Bite Complications¶
Localized pain, erythema, and systemic symptoms (e.g., fever, meningitis). Tick paralysis causes ascending flaccid paralysis due to neurotoxins.
5. DIFFERENTIAL DIAGNOSIS¶
Scabies must be differentiated from eczema, psoriasis, and fungal infections. Lice infestations should be distinguished from dandruff or seborrheic dermatitis. Arthropod bites may mimic bacterial infections or allergic reactions. Delusional infestations require exclusion of psychiatric conditions and secondary infections.
5.1 Scabies vs. Other Dermatoses¶
Scabies has characteristic burrows and symmetric distribution. Eczema lacks burrows and is more pruritic. Psoriasis has thickened, scaly plaques without burrows.
5.2 Tick Bite vs. Insect Bite¶
Tick bites often have a central punctum and erythematous ring. Insect bites (e.g., mosquitoes) cause wheals without central punctum.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis relies on clinical features, skin scrapings, and laboratory tests. Scabies is confirmed by finding mites, eggs, or feces. Tick-borne diseases require serology (e.g., Lyme disease ELISA) or PCR. Venom-induced envenomation is diagnosed by clinical presentation and exclusion of other causes.
6.1 Diagnostic Techniques¶
Skin scrapings for scabies, PCR for tick-borne pathogens, and histopathology for myiasis. Allergy testing may identify hypersensitivity to arthropod saliva.
6.2 Laboratory Tests¶
Complete blood count (CBC), inflammatory markers, and serology for tick-borne infections. Coagulation studies may be needed for scorpion envenomation.
7. MANAGEMENT & TREATMENT¶
Treatment is symptom-based, with topical agents for ectoparasites and systemic therapies for severe cases. Scabies requires permethrin or ivermectin. Tick removal is critical, with prophylaxis for Lyme disease. Venom-induced envenomation may require antivenin, antihistamines, and supportive care. Delusional infestations require antipsychotics and psychiatric intervention.
Table 2: Scabies Treatment Options¶
| Agent | Dosage | Application | Notes |
|---|---|---|---|
| Permethrin | 5% cream | Thoroughly apply to skin | Avoid eyes/mucosa |
| Lindane | 1% lotion | Apply to affected areas | Avoid in pregnancy/children |
| Ivermectin | 200 mg/kg | Oral | Repeat in 1 week for crusted scabies |
| Agent | Dosage | Application | Notes |
|---|---|---|---|
| Crotamiton | 10% cream | Apply nightly | Less effective than permethrin |
7.1 Scabies Treatment¶
Permethrin cream (5%) applied to entire body. Ivermectin (200 µ g/kg) for crusted scabies. Repeat treatment after 7 days. Antipruritics (e.g., antihistamines) relieve itching.
7.2 Tick Bite Management¶
Prompt removal with fine-tipped forceps. Doxycycline prophylaxis for Ixodes bites. Monitor for neurologic symptoms (e.g., tick paralysis).
8. PROGNOSIS & COMPLICATIONS¶
Scabies typically resolves within weeks, but secondary infections (e.g., Staphylococcus aureus) may occur. Tick-borne diseases can lead to chronic arthritis or neurological sequelae. Venom-induced envenomation may cause renal failure or death. Delusional infestations may result in social isolation and self-harm.
8.1 Complications of Scabies¶
Secondary bacterial infections, crusted scabies (Norwegian scabies), and systemic spread of mites in immunocompromised patients.
8.2 Tick-Borne Disease Outcomes¶
Lyme disease may progress to arthritis or neuroborreliosis. Anaplasmosis and babesiosis can cause hemolytic anemia or organ failure.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Scabies treatment with permethrin is safe, but lindane is contraindicated. Pediatrics: Head lice are common in children; avoid harsh insecticides. Elderly: Increased risk of severe tick-borne infections and delayed healing. Psychiatric patients: Delusional infestations require antipsychotics and psychiatric evaluation.
9.1 Pregnancy and Lactation¶
Permethrin is safe in pregnancy; avoid oral ivermectin. Lindane is contraindicated in pregnancy. Scabies may worsen in immunocompromised patients.
9.2 Pediatric Considerations¶
Head lice are common in school-aged children. Avoid harsh insecticides; use pediculicides with caution. Monitor for secondary infections.
10. KEY POINTS & CLINICAL PEARLS¶
- Scabies is diagnosed by finding mites, eggs, or feces in skin scrapings.
- Permethrin is the first-line treatment for scabies.
- Tick removal is critical to prevent systemic disease.
- Delusional infestations require psychiatric evaluation and antipsychotics.
- Venom-induced envenomation may require antivenin and supportive care.