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Giardia in Dogs and Cats: Understanding this Protozoan Parasite

Giardiasis is an intestinal infection with the protozoan parasite Giardia spp. Giardia infection occurs worldwide in most domestic and wild mammals and many birds. In some animals, particularly ruminants but also wild mammals, Giardia infection commonly occurs without overt or apparent clinical signs.

In general, when veterinarians and physicians refer to Giardia or giardiasis, they are referring to G. duodenalis. Surveys conducted in different parts of the world have reported estimates of Giardia infection of around 10%–30% for dogs and somewhat lower for cats, with higher rates of infection in younger animals. However, there are wide variations between studies.

Dogs shedding Giardia cysts frequently have no clinical signs, and the relevance of continually treating dogs that seem to be permanently infected, but apparently healthy, remains a subject of debate. Various other small pet animals (such as rabbits and guinea pigs) have also been shown to be susceptible to Giardia infection. Among rodents, chinchillas are particularly notable for a high occurrence of infection, with clinical signs, and the predominance of assemblage B infections which, at some commonly used typing genes, are identical to the assemblage B infections reported in humans. Thus, it appears that although zoonotic transmission of Giardia to humans is generally rare, it may be more likely to occur from infections in chinchillas.

In small ruminants (sheep and goats), data tend to indicate an occurrence of around 20%–25%, with similar data for cattle. On farms where Giardia infection has been diagnosed in ruminant stock, a cumulative occurrence of close to 100% may be expected. In general, very young animals (neonates) are less likely to be infected, although animals less than 6 months of age tend to be more susceptible to infection with signs of disease. Frequently, however, even infections with very high cyst excretion are not associated with clinical signs. Nevertheless, diarrhea, weight loss, and even death have been associated with Giardia infections. In pigs, the occurrence of Giardia tends to be lower, at around 15%, but is age related, with low infection rates (around 5%) recorded from piglets and sows and higher rates reported from starter pigs and fatteners (15%–30%). Many infections in pigs seem to be asymptomatic. In horses, occurrence rates of less than 10% (from around 0.5% to 20%) seem usual, with infection more likely in foals, but with symptomatic giardiasis only rarely reported.

Humans and other species can become infected with Giardia; however, the type of Giardia that normally infects humans differs from the type that infects dogs and cats. (Dogs and cats are also affected by different types of Giardia). For this reason, it is rare for humans to get infected from a strain of Giardia that pets may carry and vice versa. Regardless, washing hands thoroughly with soap and water after handling pets or infected feces is always important.  

The life cycle of Giardia is relatively simple. The flagellate, vegetative stage (trophozoites) of Giardia inhabits the lumen of the small intestine, where they may attach to the brush border of epithelial cells via the ventral disk, absorb nutrients, and multiply by binary fission. There are no intracellular stages. The trophozoites usually live in the proximal portion of the small intestine and migrate down the intestine during the course of infection, encysting in the small or large intestine. The prepatent period is generally 3–10 days.

The cysts are passed in the feces and are immediately infectious. Cyst shedding may be continual over several days and weeks but is often intermittent, especially in the chronic phase of infection. The cyst is the infective stage and can survive for several weeks, or even months, in an appropriate (cool and damp) environment. Although trophozoites may be passed in the feces, especially during bouts of severe diarrhea, they do not survive for long in the environment and, if ingested, they are not able to infect a new host.

Transmission occurs via the fecal-oral route, either by direct contact with an infected host that is contaminated with infective cysts or through a contaminated environment. Characteristics that facilitate infection include the high and prolonged excretion of environmentally robust cysts by infected animals and the low infectious dose. Giardia cysts are infectious immediately after excretion and are very resistant, resulting in a gradual increase in environmental infection pressure. High humidity facilitates survival of cysts in the environment. Overcrowding favors transmission. Stagnant water contaminated with feces from infected animals is also a very common vessel for cysts, and the ingesting of the infested water while playing can create a larger than normal collection within the host animal.

Giardia infections cause an increase in epithelial permeability, increased numbers of intraepithelial lymphocytes, and activation of T lymphocytes. Trophozoite toxins and T-cell activation initiate a diffuse shortening of brush border microvilli and decreased activity of the small-intestinal brush border enzymes, especially lipase, some proteases, and disaccharidases. The diffuse microvillus shortening leads to a decrease in overall absorptive area in the small intestine and thus an impaired intake of water, electrolytes, and nutrients. In addition, the proteins secreted by Giardia trophozoites contribute to degrading the intestinal mucous barriers and disrupting the intestinal intracellular junctions. The combined effect of decreased resorption, brush-border enzyme deficiencies, and a lack of integrity between cells of the intestine results in dysregulation of the absorption and barrier functions of the intestinal epithelium, resulting in malabsorptive diarrhea.

Medications to get rid of Giardia are readily available and affordable. Metronidazole, an antibiotic, is most often prescribed. The infection may take multiple rounds of treatment because it can be hard to completely remove it from your home and prevent reinfection, but many times, the infection is cleared after one round. Additional medications may be prescribed depending on the severity of the infection and your dog’s condition. Dehydrated dogs and puppies may benefit from subcutaneous fluid therapy (fluids injected under the skin), administration of electrolytes, and injectable vitamins. If your dog is moderately to severely dehydrated, they could require hospitalization for intravenous fluid therapy.

Standard disinfectants and chlorine bleach can be used to treat areas soiled by your pet to kill cysts, thereby preventing the spread of the disease. Any pet positively diagnosed with the bacteria can have it eliminated from their fur with a standard bath. While being treated, infected dogs should be isolated in an area that is easy to disinfect.

If you live in an area where Giardia is common, routine fecal tests may be something to consider. Catching it early and treating your pet as soon as you have a diagnosis can often prevent symptoms from ever even developing.


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