Stephanofilaria stilesi

Stephanofilaira stilesi

• Dermall lesions of cattle in USA

Lifecycle

• The infective larvae develop in the horn fly - Haematobia irritans
  • Flies ingest microfilaria as they feed on cutaneous lesions.
  • Microfilaria develop into L3 larvae in the hornfly over 18-21 days.
  • Infective L3 larva are deposited into the skin when the fly bites again, where they grow into adult worms.

Gross pathology

• Lesions begin as small circular patches on the ventral midline with serous exudate.
• These enlarge and coalesce producing lesions 25cm or more in diameter.
• Hemorrhage develops along the periphery while the older, central areas develop scabs or dry crusts.
• Healing lesions are alopecic, lichenified plaques.
• Lesions may also occur on the flank, udder, teats, face and neck.

Histopathology

• Adult parasites occur within cystic diverticula of hair follicles or free within the adjacent dermis.
  • Parasites may have lateral cords and lateral cuticular projections.
  • The thick intestine is distinctive.
  • Microfilariae within uteri is the key feature.
    • Microfilariae also occur free within the dermis, in dermal lymphatics, or in the surface exudate.
• There is little dermal reaction to adults in cystic hair follicles, but their presence within the dermis elicits eosinophilic and mononuclear inflammation.

Differential diagnosis

1. Pelodera strongyloides:
   • Rhabditid parasite, adults 1-1.5mm long, found in follicles
   • uteri contain eggs, not microfilaria.
2. Dermatophytosis (Trichophyton verrucosum is most common)
3. Mange (Chorioptes bovis)
4. Fly bite dermatitis
5. Contact dermatitis
6. Zn-responsive dermatitis
7. Other subcutaneous filarid parasites:
   • Parafilaria bovicola (adult worms 30-70mm long, found coiled in nodules in subcutaneous and intramuscular connective tissues; not seen in US)
   • Onchocerca gutterosa (adult worms 60mm long, found in nuchal ligament and subcutaneous connective tissues).

Stephanofilaria assamensis

• 'Humpsore'
• Bos indicus in India