What are the possible causes of a non-blanching rash over the lower limb after travel to a mangrove forest?

Non-Blanching Rash on Lower Limbs After Mangrove Forest Travel

The most critical immediate consideration is meningococcemia or other life-threatening vasculitic/purpuric conditions requiring urgent evaluation, though the travel history to a mangrove forest makes infectious and environmental causes more likely if the patient is otherwise stable.

Immediate Life-Threatening Causes to Exclude

A non-blanching rash indicates petechiae or purpura from vascular damage or platelet dysfunction. Before considering tropical infections, you must urgently exclude:

• Meningococcemia - requires immediate blood cultures, lumbar puncture if safe, and empiric antibiotics
• Rickettsial infections - particularly if fever present; African tick-bite fever can present with eschars and rash 1
• Dengue fever with hemorrhagic manifestations - leading cause of febrile exanthema in travelers from endemic areas 1
• Acute schistosomiasis (Katayama syndrome) - can present with urticarial rash and systemic symptoms 2

Tropical/Travel-Related Causes Specific to Mangrove Exposure

Most Likely Diagnoses:

Cercarial dermatitis (Swimmer's itch) is highly probable given mangrove water exposure:

• Presents as itchy maculopapular rash within hours of freshwater/saltwater exposure 2
• Caused by schistosome species that infect birds penetrating skin 2
• Worldwide distribution, often occurs in outbreaks 2
• Self-limiting, resolves over days to weeks 2
• May respond to topical corticosteroids 2

Acute schistosomiasis (Katayama syndrome) if systemic symptoms present:

• Can present with urticarial or non-blanching rash 2
• Associated with fever, headache, dry cough 2
• Incubation 5-12 weeks for S. haematobium 2
• Requires serology and terminal urine microscopy 2

Other Helminthic Causes with Lower Limb Predilection:

Onchocerciasis (though less likely from single mangrove visit):

• Diffuse pruritic dermatitis usually over legs and buttocks 2
• Transmitted by Simulium black fly near fast-flowing rivers 2
• Incubation 8-20 months 2
• Travelers typically present with pruritus and limb swelling 2

Strongyloidiasis (Larva currens):

• Itchy linear urticarial rash on trunk, upper legs, buttocks 2
• Moves 5-10 cm per hour 2
• Associated with subcutaneous larval migration 2

Critical Diagnostic Approach

Immediate Assessment:

• Check if truly non-blanching - apply pressure with glass slide
• Assess for systemic toxicity - fever, hypotension, altered mental status
• Examine for eschars - suggests rickettsial infection 1
• Document rash distribution and morphology - localized vs generalized 3, 1

Key History Elements:

• Exact water exposure - swimming, wading in mangrove waters 2
• Timing - hours suggests cercarial dermatitis; weeks suggests helminthic infection 2
• Associated symptoms - fever, pruritus, systemic symptoms 1
• Duration of travel and activities - barefoot walking, freshwater exposure 1

Laboratory Investigations:

• Complete blood count with differential - eosinophilia suggests helminthic infection 2
• Blood cultures if febrile - exclude bacterial sepsis
• Schistosomiasis serology - though may be negative early 2
• Strongyloides serology - recommended for all returning travelers with eosinophilia 2
• Concentrated stool microscopy - for helminth ova 2

Management Algorithm

If Patient is Systemically Unwell:

1. Admit immediately for sepsis workup
2. Empiric broad-spectrum antibiotics if concern for meningococcemia
3. Infectious disease consultation urgently

If Patient is Stable with Localized Rash:

For cercarial dermatitis (most likely with acute mangrove water exposure):

• Reassure regarding self-limiting nature 2
• Topical corticosteroids for symptomatic relief 2
• No specific antimicrobial therapy needed 2

For suspected schistosomiasis with systemic symptoms:

• Serology and terminal urine microscopy 2
• Treatment with praziquantel 40 mg/kg if confirmed 2
• Do NOT treat acute Katayama syndrome with praziquantel initially - use corticosteroids alone to avoid neurological complications 2

For eosinophilia with skin manifestations:

• Concentrated stool microscopy and strongyloides serology mandatory 2
• Consider empiric albendazole 400 mg daily if prolonged tropical exposure 4

Critical Pitfalls to Avoid

• Never assume tropical etiology without excluding life-threatening causes - meningococcemia can kill within hours
• Do not rely on urine dipstick for schistosomiasis - has low sensitivity 2
• Avoid treating acute Katayama syndrome with praziquantel alone - requires corticosteroids first 2
• Remember most helminth infections are asymptomatic - eosinophilia may be the only clue 2, 4
• Serology for schistosomiasis may be negative early - seroconversion takes 4-8 weeks (up to 22 weeks) 2