Filariasis is caused by a thread-like worm. They can easely be found in a thick blood smear, and even in a thin blood smear. Microfilariae can sometimes also be found in a fresly prepared drop of blood.
Timing of blood sampling is critical; microfilariae exhibit a window of the time circulating in the blood, depending on the species involved.
   >  Loa loa:                          midday
   >  Brugia or Wuchereria:   at night, after 8 PM
   >  Mansonella:                   any time
   >  Onchocerca:                  any time

Some freshly prepared blood samples: