Giardia 101: A crash course

What is Giardia and how do dogs get it?

Giardia is the most common intestinal parasite in dogs. Dogs are infected when they ingest the infective stage of the parasite Giardia cysts. This could happen if they eat or drink contaminated food or water. It can also occur if Giardia cysts get onto the dog’s paws or fur from a surface contaminated with feces from another infected animal, and the dog ingests the cysts while grooming itself. (4)

Excystation occurs after the cysts pass through the stomach and into the duodenum, and the Giardia enters the mobile trophozoite stage. The trophozoites attach to the small intestine, where they acquire nutrients and reproduce. Some trophozoites encyst again and can then be shed in the feces.

What are the most common signs of Giardia?

Malabsorptive diarrhea is the most common clinical sign. However, some dogs may have a “subclinical” infection, meaning they have no symptoms at all. Some dogs have more signs than others due to differences in the overall GI microbiome. The pathogenesis of Giardia remains uncertain and may involve flattening the villi where the organism attaches, changes in the intestinal microbiome, or proteases released by the organisms.

Giardia is more common in younger dogs, but can affect dogs of any age. Around 15% of dogs with gastrointestinal signs (diarrhea and/or vomiting) test positive for Giardia. (1,2,3)

How is Giardia diagnosed?

Giardia is diagnosed with a fecal sample. There are three main tests: (4)

• Fecal floatation for cysts (the infectious stage)
• Direct smear test looking for the mobile trophozoite stage (least sensitive test)
• ELISA antigen testing (most sensitive test)
  • In house SNAP antigen test
  • Lab based ELISA antigen testing

What is the treatment?

No treatment is currently FDA-approved for treatment of Giardia, but the Companion Animal Parasite Council recommends fenbendazole and metronidazole as first-line treatments. Fenbendazole is approved in Europe for treatment of Giardia. (4)

• Fenbendazole:  50 mg/kg, standard treatment is for 3-5 days although longer courses have been used.
• Metronidazole: 10-25 mg/kg twice per day, but caution must be used at high doses due to the risk of neurologic side effects.
• Febantel (Drontal Plus): dosed per label instructions for 3 days. Febantel metabolizes to fenbendazole in the body after ingestion.
• Many other medications have been used including azithromycin and ronidazole. (11,12)

I treated with the standard meds but the dog is still positive!

Treatment failure has been reported in both dogs and humans with Giardia infections.
If it is ELISA positive, but no cysts are seen, and the dog no longer has clinical signs, monitoring with no further treatment may be appropriate. (4)

If the dog is still shedding cysts, trying a different medication or a combination of medications may help. Additionally, the owner can bathe the dog periodically (especially on the last day of medication) to help eliminate cysts on the fur. The owner can also clean any surfaces like concrete kennels, patios, and dog crates and launder all bedding. (5,9)

Probiotics and prebiotics may also help due to the impact of Giardia on the microbiome. (10)

Benign neglect could be an option if a dog continues to test positive but has no clinical signs. However, keep in mind that the dog would still have the potential to infect other dogs and may not be allowed at doggy daycare or boarding facilities. (4,9)

Can I get Giardia from my dog?

Zoonotic transmission from dogs to humans is thought to be uncommon. Giardia comes in a variety of strains called “assemblages,” which are based on genotyping. Different species tend to be infected with different assemblages. Assemblages A and B, which are found in most human infections, can also be found in many other species, so transmission from dogs to humans and humans to dogs is possible, although rare. Dogs primarily are infected with assemblages C and D. (6, 7, 8)

Takeaways

• Giardia is one of the most common canine parasites, and dogs presenting with diarrhea should be tested for this organism by fecal float or ELISA.
• Fenbendazole and metronidazole are the most common treatments, and some patients may need multiple rounds of treatment in conjunction with environmental hygiene measures to eliminate the organism.
• Giardia causes malabsorptive diarrhea, although the mechanism is not fully understood.
• The risk of zoonosis is thought to be low, but standard measures such as washing hands after handling fecal material are recommended.

Resources for Veterinarians

• CAPC Giardia Guidelines
• Clinician’s Brief
• Clinician’s Brief, Recurrent Giardia with table of alternative medication options

Sources:

1. https://journals.asm.org/doi/full/10.1128/CMR.00024-19
2. https://pubmed.ncbi.nlm.nih.gov/25583357/
3. https://pubmed.ncbi.nlm.nih.gov/20808578/
4. https://capcvet.org/guidelines/giardia/
5. https://avmajournals.avma.org/view/journals/javma/249/6/javma.249.6.644.xml
6. https://link.springer.com/article/10.1007/s00436-021-07325-2
7. https://www.sciencedirect.com/science/article/abs/pii/S0304401717304776
8. https://link.springer.com/article/10.1186/1756-3305-7-412
9. https://www.cliniciansbrief.com/article/recurrent-giardia-infection-dog
10. https://pubmed.ncbi.nlm.nih.gov/34576757/
11. https://files.brief.vet/migration/article/17956/recurrent-giardia-infection-17956-article.pdf
12. https://pubmed.ncbi.nlm.nih.gov/22240238/