CONDITION
Giardiasis
Giardiasis is an infection of the intestine caused by a microscopic parasite called Giardia. The parasite lives in the small intestine and can interfere with the normal absorption of nutrients and water, though many infected dogs and cats show no outward signs at all. When signs do appear, they most often take the form of soft or watery stool, sometimes with a slightly greasy appearance or unusual odour. Owners typically arrive on this page because they have noticed a change in their pet's stool consistency that has persisted, or because a faecal test has returned a positive result for Giardia and they are trying to understand what that means. In young animals or those in multi-pet households, the question often centres on whether the infection explains ongoing digestive upset, and what the implications may be for other animals or people in the home. This page explores the signs that can accompany giardiasis, the mechanisms by which the parasite affects the gut, the ways in which infection is identified in practice, and the range of approaches used to address it. It also considers the contexts in which Giardia is found, the variability in how different animals respond, and the factors that shape decisions around testing and intervention.
Why this matters now
Giardiasis can affect animals of any age, though young puppies and kittens are often more susceptible. The condition may be acquired from contaminated water sources, infected faeces, or environments where many animals congregate. Outbreaks may occur in kennels, catteries, or shelters where transmission opportunities are greater.
The condition typically begins with mild digestive upset that may initially be attributed to dietary indiscretion. Some infected animals remain asymptomatic carriers whilst others develop persistent or intermittent diarrhoea. Without identification and treatment, chronic infection can lead to poor nutrient absorption and gradual weight loss, particularly in younger animals.
Signals & patterns
Early signals
Soft or loose stools
Early infection may cause subtly softer faeces before progressing to more obvious diarrhoea.
Increased flatulence
Gas production may increase as the intestinal environment becomes disrupted.
Mild appetite fluctuations
Some animals may show variable interest in food without complete appetite loss.
Occasional mucus in faeces
Stools may have a mucoid coating or visible mucus streaks.
Later signals
Watery, foul-smelling diarrhoea
Characteristic pale, greasy, and particularly malodorous stools may develop.
Intermittent or chronic diarrhoea pattern
Symptoms may wax and wane, with periods of normal stools interspersed with diarrhoea episodes.
Gradual weight loss
Chronic infection can impair nutrient absorption, leading to poor body condition despite adequate food intake.
Dull coat quality
Poor nutrient absorption may manifest as reduced coat lustre and condition.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis can be challenging as Giardia cysts are shed intermittently and may not be present in every faecal sample. Various testing approaches offer different sensitivity levels.
Faecal flotation
Faecal antigen test (ELISA or snap test)
Faecal PCR testing
Direct faecal smear
Options & trade-offs
Management typically involves antiparasitic treatment combined with environmental measures to prevent reinfection. Various approaches may be considered depending on the situation.
Antiprotozoal medication
Specific medications targeting Giardia are typically prescribed for a course lasting several days to weeks.
Trade-offs: Different medications exist with varying treatment protocols. Some animals may require repeated courses if initial treatment is incomplete.
Environmental decontamination
Thorough cleaning of living areas and washing of bedding to remove cysts.
Trade-offs: Essential for preventing reinfection. Cysts can survive standard cleaning, so specific disinfection protocols may be needed.
Bathing during treatment
Washing the animal to remove cysts from the coat, particularly around the hindquarters.
Trade-offs: Helps prevent self-reinfection through grooming. Most beneficial at the end of the treatment course.
Dietary support
Highly digestible diets may support recovery whilst intestinal function normalises.
Trade-offs: Can help reduce gastrointestinal signs during treatment and recovery. Gradual return to normal diet after resolution.
Testing and treating household pets
Evaluating and potentially treating other animals in the household.
Trade-offs: May help prevent cycling of infection between pets. Asymptomatic carriers can serve as sources of environmental contamination.
Common misconceptions
"Giardia always causes obvious symptoms"
Many infected animals are asymptomatic carriers, shedding cysts and potentially infecting other animals whilst appearing completely healthy themselves.
"One negative test rules out giardiasis"
Intermittent shedding means a single negative result does not exclude infection. Multiple samples or antigen testing may be needed for reliable diagnosis.
"Pet Giardia commonly infects humans"
Whilst giardiasis is technically zoonotic, the strains affecting dogs and cats are typically different from those causing human disease. Direct pet-to-human transmission appears uncommon, though good hygiene remains sensible.
Noting the pattern and character of any digestive upset, including whether symptoms are constant or intermittent, can help guide discussions with a veterinary professional. Considering potential exposure sources such as communal water bowls, dog parks, or boarding facilities may be relevant. Observing whether other household pets show similar signs could also provide useful context.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS