I own two dogs with giardia, both of them infected before I got them. So far, no other dogs (and no humans) have caught giardia from my dogs--as far as I know. Can't tell unless they have a flare.
Giardia is a protozoan parasite. It forms a hard shell over itself (a cyst) and can survive a variety of conditions for a looooong time-- this is one tough pest. Once a dog has been infected, they are a carrier for life. It's sort of like demodex--a healthy dog's body keeps the parasite in check but being in check doesn't mean that the dog doesn't have it (and by the way, most or all dogs probably have demodex).
I live in Iowa and giardia is found in some (not all) of the surface water, including creeks. As I understand it, giardia is found in any moderately temperate and wet climate (desert areas and Arctic conditions are two areas where giardia doesn't thrive). Anywhere there are large numbers of mammals will have giardia. Here in Iowa, the culprit carrier is probably the white-tailed deer, which thrive in the sort of mixed eco-systems we have here (little bits of forest, lots of farms, etc--deer really like suburban conditions).
During a flare, the giardia is multiplying out of control. They attach to the lining of the intestines, causing great distress to the dog. The dog has diarrhea (often green in the beginning), gas, abdominal tenderness. As the flare progresses, the diarrhea will often turn bloody and full of mucous.
Diagnosing giardia is not easy. Diagnosis can be done in one of two ways: via fecal sample or via educated evaluation of clinical findings.
Via fecal sample is not straightforward. Even when a flare is at it's worst, the cysts will not be shedding in every single stool. The most thorough way to assess is to collect a sample from every single stool produced for 48 to 72 hours. FWIW, my own observation is that stools that are bloody and full of mucous are most likely to contain cysts.
Via clinical findings is the other way to diagnose. This means the vet takes a look at the dog, evaluates the history and symptoms, and treats on that basis. If the treatment is successful, the diagnosis was accurate. The vet should know the area--is there giardia in the area and what places are most likely to be infested? And is it possible that the dog was in one of the affected areas? The dog doesn't have to swim or drink to be infected--just walking across boggy ground infected with giardia can transfer it to the dog's feet, and then the dog can lick it off in self grooming.
The treatment of choice is with metronidazole (brand name Flagyl). Metronidazole has two interesting properties--the action is largely confined to the gut and it also seems to stimulate the local immune system. Metronidazole kills off the giardia and reduces the numbers to the level the dog's immune system can handle.
It cannot kill all of the cysts, however. A certain number of them burrow into the lining of the intestines and go dormant. They can stay dormant for years. Due to the hard shell protecting the cysts, it is impossible to kill them when they are encysted in the lining of the intestines. During times of stress, the cysts may re-activate and start to reproduce, causing a flare.
A healthy dog may have been infected years before and never shown any symptoms (asymptomatic carrier). They may occasionally shed very low numbers of cysts in stools--evaluating every stool (the WHOLE stool) for something like six months is supposed to be the conclusive way to rule out an asymptomatic carrier (someone did this with a couple dogs for a study, as I recall). This would cost literally thousands of dollars! Not exactly a practical way to test.
The amount of stress needed to cause a flare seems to be highly variable with different dogs. My male Belgian Sheepdog hasn't had a flare since he was a year old, not even last year when we fought a prostate infection for four months (and finally gave up and neutered him). My Belgian Sheepdog bitch has had several flares over the years, but she has lupus, which means her immune system is whacko anyway. She had so many flares when she was younger that I bought metronidazole by the 1000 count at the wholesale price from my vet. As she has gotten older, she's having fewer flares (both lupus AND giardia).
She'd been in remission for over a year before I started making their food myself and her remission has continued almost unbroken since then. The only exception was a minor lupus flare following the legally required rabies shot (I don't give her any other type of vaccination, since it triggers terrible lupus flares in her). I do a complete blood screen (counts, types, proportions and chemistries) at least once a year--her blood tests indicate that she continues to have lupus (her types and proportions are way out of whack). But clinically she's healthy!
As an experienced dog owner of two dogs with giardia, the following is what works for me. I'm not a vet and I'm not an expert of any type, just an experienced dog owner--please consult your vet before doing anything to treat/manage giardia!
First of all, I do a LOT of poop picking. My dogs pretty much poop twice a day (they eat twice a day) and I pick up after each time. I have built an outdoor litterbox, so the rest of the yard is (pretty much) clean. If a dog is having diarrhea, I go out with them each time and IMMEDIATELY scoop it (yep, I take a flashlight at night).
After scooping, I wash my hands with soap and warm water. The warm water doesn't do anything to kill organisms--it just makes the soap more effective. It's important to wash your hands long enough to sluice off organisms--the rule of thumb is to lather long enough to sing HAPPY BIRTHDAY, to make sure you're doing it long enough.
I have coated dogs, so it's more obvious, but this applies to smooth coated dogs as well. When they are having a flare, they have diarrhea. When they have diarrhea, a certain amount of fecal material gets on the hair around the hindquarters. After each bout of diarrhea, I use a sprayer bottle filled with a solution of 2 cups of plain water and one teaspoon of Orvus (you can use dog shampoo or DAWN dishwashing liquid) to spray off the dog's tail area and pants, then paper towels to wipe dry. Once a day (or more often if it's really bad), I put the dog in the tub and suds up the tail and pants area, then rinse thoroughly. I use an el cheapo dust mask (looks like a surgical mask, can be had for less than a dollar from the hardware store) to protect my nose and mouth from flying spray (I wear glasses, so my eyes are protected--someone who doesn't might want to get goggles).
It's also important to wash hands after grooming. If a dog is having a flare, I make sure to wash my hands after each and every contact with the dog. Yes, especially in cold weather, my hands do get chapped (I'm not about to give up touching my dogs!). One thing that works for me is to use hand lotion after each wash (best thing I've found is UDDER CREAM). Once or twice a day, instead of hand lotion alone, I use a dab of first aid cream containing cortisone (mixed in with a small squirt of hand lotion). Keeps the chapping down to a bearable level.
The most important thing is prevention--keep that dog as healthy as possible and the chances of having a flare are lower. I believe that one important factor in my own dogs' continuing health is feeding homemade food (YMMV).
I've become very familiar with the symptoms of giardia. The diarrhea caused by giardia is pretty distinctive (it's green). I start treatment as soon as I see it, rather than waiting to see my vet. She knows and approves of this! The sooner you start treatment, the better.
The author M. Shirley Chong is the list administrator for the K9-Cuisine List and can be reached via email at firstname.lastname@example.org.