What is Giardia?
Giardia is a microscopic, single-celled protozoan parasite that lives in the small intestines of humans and animals. The species Giardia duodenalis (also known as G. intestinalis or G. lamblia) is responsible for Giardia infections in dogs.
The parasite cannot be seen with the naked eye and can cause gastrointestinal upset, including diarrhoea, particularly in puppies, older dogs and those with weakened immune systems. However, some infected dogs may show no obvious symptoms.
Giardia duodenalis has zoonotic potential, meaning certain genetic types can be transmitted between animals and humans. Eight genetic assemblages (A–H) are recognised. Dogs are most commonly infected with assemblages C and D, while assemblages A and B are associated with human infections and can also occur in animals. Although transmission from dogs to humans is possible, the overall risk is considered low.
The parasite has two main forms:
- Trophozoite: The active, mobile form that lives and multiplies in the small intestine.
- Cyst: The environmentally resistant, infectious form that is shed in faeces. Cysts can survive for extended periods, particularly in cool, moist conditions, and are responsible for transmitting infection.
Infection occurs when a dog ingests Giardia cysts through contaminated water, food, surfaces or contact with infected faecal material.
Dogs pick up infection by swallowing cysts from contaminated sources. Common routes include:
- Drinking from puddles, ponds, or streams
- Licking contaminated fur or paws
- Eating food or objects soiled with faeces
- Contact with infected dogs in kennels, parks, or shelters
Cysts survive well in cool, damp areas, so dogs that spend time outdoors or in shared spaces are at higher risk.
Giardia infection may cause no visible signs or range from mild to severe illness. Typical symptoms include:
- Intermittent or chronic diarrhoea (often soft, greasy, or foul smelling)
- Weight loss despite normal or good appetite
- Vomiting
- Bloating or abdominal discomfort
- Lethargy
- Dehydration
- Dull or poor coat condition
Some dogs appear healthy but still carry the parasite, so testing is the only way to confirm infection.
Accurate testing is essential because cysts are shed intermittently. The most reliable diagnostic methods are:
- Antigen testing: detects Giardia proteins in faeces even when cysts are not visible
- qPCR (quantitative PCR): identifies and measures Giardia DNA for early, precise detection
Microscopic checks can still be used but may miss infections. Antigen and qPCR testing provide consistent results and help confirm clearance after treatment.
Treatment targets the parasite and supports recovery. The most commonly prescribed medicines are Fenbendazole and sometimes Metronidazole.
Always complete the full course, even if your dog seems better before it ends. Stopping early can lead to relapse or resistance. After treatment, your vet may recommend a follow up test to confirm the infection has cleared.
Maintain good hygiene throughout and after treatment to reduce the risk of reinfection.
You can lower the chance of infection by:
- Providing clean, fresh water and avoiding natural water sources
- Picking up faeces promptly and disposing of them safely
- Bathing your dog after exposure to communal areas
- Cleaning bowls, bedding, and living spaces regularly
- Avoiding overcrowded areas where outbreaks have been reported
Giardia can occasionally pass between dogs and humans, though this is uncommon. Most human infections come from contaminated water or food.
Wash hands after handling pets or cleaning up faeces. Families with young children, elderly members, or anyone with reduced immunity should take extra care.
If your dog is diagnosed with Giardia:
- Begin treatment as advised by your vet
- Wash hands thoroughly after contact or cleaning
- Clean and disinfect bedding, bowls, and surfaces daily
- Bathe your dog at the start and end of treatment
- Retest after treatment to confirm clearance
Reinfection can occur, especially in multi dog homes, so keep hygiene routines consistent.
Giardia is a common intestinal parasite but is manageable with accurate testing, effective treatment, and careful hygiene. If you notice signs such as diarrhoea or weight loss, arrange testing promptly. Early action helps protect your dog and others from reinfection.
