Stephanofilaria stilesi

Stephanofilaira stilesi

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.

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.

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.

Pelodera strongyloides:
- Rhabditid parasite, adults 1-1.5mm long, found in follicles
- uteri contain eggs, not microfilaria.
Dermatophytosis (Trichophyton verrucosum is most common)
Mange (Chorioptes bovis)
Fly bite dermatitis
Contact dermatitis
Zn-responsive dermatitis
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).