Complex sentence: The most spectacular symptom of lymphatic filariasis is elephantiasis--stage 3 lymphedema with thickening of the skin and underlying tissues; this was the first mosquito-borne disease to be discovered.
Split into simpler sentences: The most spectacular symptom of lymphatic filariasis is elephantiasis, a stage 3 lymphedema with thickening of the skin and underlying tissues. This was the first mosquito-borne disease to be discovered.