Fuchsin fluorescence and autofluorescence in Cryptosporidium, Isospora and Cyclospora oocysts

Cryptosporidium parvum and Isospora belli oocysts stained with carbol–fuchsin, as in a modified Ziehl–Neelsen technique, fluoresce bright red under green light (546 nm). Cryptosporidium oocysts tend to fluoresce more brightly the less intensely stained they appear under transmitted light; this is not the case with Isospora. Fuchsin-stained Cyclospora cayetanensis oocysts fluoresce rather dimly, but those not taking the dye retain their typical autofluorescence. Cryptosporidium and Isospora oocysts are also autofluorescent, appearing violet under u.v. light (365 nm), and green under violet (405 nm) and blue–violet light (436 nm). Their autofluorescence does not survive the staining procedure.