Answer: Cyclospora cayetanensis oocysts
Thanks again to Florida Fan for providing these colorful images. Idzi Potters mentioned that C. cayetanensis oocysts also produce beautiful autofluorescence. Instructions for observing their autofluorescence can be found HERE) Below is an image that Florida Fan gave me a while back which nicely highlights this phenomenon (oocysts are denoted by arrows):
Note that the oocysts of C. cayetanensis are in an unsporulated (immature) stage when passed in feces and so you won't see sporozoites within them like you sometimes can with Cryptosporidium spp. oocysts. Instead you may just see globular material like in the figure A (wet prep; arrows) from this case:
The immature nature of freshly-shed C. cayetanensis oocysts has an important impact on this parasite's epidemiology. Because the oocysts are shed in an unsporulated state, they must undergo further development in the environment before becoming infective. Once mature, soil or water containing the oocysts must then be allowed to contaminate food (e.g. fruits and vegetables) or drinking water, which is then consumed by humans. Humans are the only known host of C. cayetanensis and thus there are no animal reservoirs contributing to the environmental contamination.
The bottom line is that for human infection to occur, human feces must first contaminate the environment and subsequently get into food or water. Other humans must then eat this contaminated food or water. This explains why cyclosporiasis is seen primarily in settings with inadequate sanitation facilities. In resource-rich settings, most cases are seen in travelers, or are linked to consumption of imported fruits or vegetables from endemic countries. You can read about the current cyclosporiasis outbreaks in the United States, Canada, the United Kingdom, and Europe. As I mentioned in the case introduction - this is a very timely discussion!
Of note, the life cycle is a bit different for Cryptosporidium spp. Understanding the differences allows us to understand why Cryptosporidium is endemic in resource rich countries while C. cayetanensis is not. One of the most important differences is that the oocysts of Cryptosporidium spp. ARE infective when passed in stool. They do not need to enter the environment to mature. Instead, they can be passed directly from person to person on contaminated hands or fomites. An infected person can also contaminate water sources (e.g. swimming pools) by shedding even a small (~100) number of microscopic oocysts. Since the oocysts are immediately infective, other individuals can become infected if they accidentally swallow some of the contaminated water. The second important difference between life cycles is that there are animal reservoirs for some of the Cryptosporidium species that are infective to humans. Therefore, prevention is not as simple as controlling for human fecal material. These are important facts that I try - hopefully successfully - to impart to all of my medical students, pathology residents, and fellows when I teach them about human parasites.