Here's what the merck veterinary manual has to say about giardiasis in pet birds:
This intestinal protozoal disease is seen most often in cockatiels. Adult birds may be latent carriers. Transmission is presumably direct (ingestion of infective cysts). Affected cockatiels occasionally exhibit feather pulling in the axillary and inner thigh regions, along with vocalization. A true causal relationship between giardiasis and these clinical signs has not been proved. Droppings of affected cockatiels may be voluminous and aerated (a “popcorn” appearance).
Giardia psittaci, Gram's stain, parakeet small intestine
Metronidazole (50 mg/kg, sid for 5–7 days) is the recommended treatment.
It sort of makes sense the vet would be shocked to see giardia; it's not a common diagnosis in parrots and the little "clown face" guys there can be tough to see, especially if you're not looking for them.
Here's some notes from my veterinary parasitology course (keep in mind this refers to the parasite in dogs and cats, but the life cycle info is the same):
-The "clown face" guys are called trophozoites; these are the guys that like to hang out in the intestine (in dogs and cats, usually in the duodenum)
-Cysts are the infective form, they're even tinier than the cysts!
-The life cycle begins when the cyst is ingested.
-When inside the animal, the trophozoite comes out of the cyst and migrates to the small intestine
-Trophozoites are the active feeding stage and while most infections are asymptomatic, acute inflammation (diarrhea, etc) is possible
-Trophozoites multiply asexually by binary fission. After a few replication cycles, the trophozoite encysts
-Cysts come out in the feces, and the cycle begins again
-Cysts are hardy and can survive for 2-4 weeks in the environment
-Sources of transmission are usually either fecal-oral (eating infected poo) or from contaminated drinking water.
-To remove the possibility of water transmission, high-quality water filters (you need them to be able to filter things about 12 micrometers in size, which is the size of the cysts)
-Trophozoites can't survive long outside the host but may emerge in runny stool; they are not infective
-Giardia is a zoonotic disease and care should be taken to avoid fecal-oral transmission
-Treatment can be from 5-15 days because reinfection from cysts can easily occur
I was at school on a caribbean island and I got giardia from the tap water; I'll spare the details of the symptoms, but you can imagine. I was treated with a drug which the doctor said would make me nauseous, especially without food and water to accompany it. The drug was strong tasting and did indeed make me woozy. Unfortunately, there's not many drugs out there that are effective and safe that don't case the nausea or have the bad taste.