Causes of Giardia Infection
Giardia are found world-wide. They have what is known as a direct life cycle. Dogs become infected when they ingest the cyst (interchangeably called the “oocyst”) form of this parasite by drinking contaminated water, eating contaminated food, licking contaminated fur or otherwise coming into contact with contaminated feces in the environment. The cysts lodge in the upper part of the dog’s small intestine, called the duodenum. There, each cyst produces several motile larvae, known as trophozoites. Giardia trophozoites reproduce asexually inside the dog’s small intestine through an interesting process called “binary fission.” Basically, mature trophozoites replicate by splitting into two separate parts, which then each become individual Giardia organisms.
These parasites irritate and damage the dog’s intestinal lining, which disrupts digestion, reduces the absorptive surface area, causes abdominal pain and uses up nutrients that are essential to the dog’s health. Eventually, before they leave the dog’s intestinal tract, the trophozoites encyst (transform into cysts), and are shed intermittently in that form in the infected dog’s feces. They rarely are shed on a consistent basis in the stool, which tends to make checking for them in fecal samples a bit of a hit-or-miss exercise. In any event, the cysts are very hardy and can survive for weeks to months free in the environment, especially in cool, moist climates. Oocysts are immediately infective to animals that ingest them. Once that happens, the cycle of Giardia infection begins again.
Prevention of Giardia
Since Giardia are transmitted through direct contact with the cyst form of the parasite in contaminated water, food or feces, the best way to prevent infection is to remove dogs from situations where they can come into contact with contaminated substances. Areas where dogs or other domestic animals defecate should be thoroughly and routinely disinfected, and preferably avoided by dogs that don’t normally frequent those areas. Giardia cysts can be inactivated by steam, boiling water and most ammonia-based disinfectant solutions. Regular veterinary check-ups, including fecal examinations, can usually identify Giardia infections. These examinations are especially important at the end of summer, after the hot season.
A Giardia vaccine has been developed for dogs and cats and is available in North America. Unfortunately, according to most reports, this vaccine has not been particularly helpful in preventing or treating Giardia-related disease. It may help to reduce fecal shedding of Giardia cysts, but it has apparently been largely ineffective in preventing or treating actual infection by the parasites.
Giardia are the most common intestinal parasites of people in North America. Whether Giardia can be transmitted from domestic dogs to people, or vice versa, is controversial. However, the organism does not appear to be particularly host-specific. Unless and until medical science proves otherwise, it probably is prudent to assume that an infected dog might pass Giardia to people in the household, and that people may pass the parasites to their pets. Like dogs, people can get Giardia infection directly from contaminated water supplies, usually due to less-than-optimal hygienic conditions (including human sewage effluents). In rural settings, beavers have been blamed for contaminating water supplies with infected feces, leading to the occasional nick-name for this disease, “beaver fever.” However, this association is not a significant contribution to Giardiasis among companion animals today.