Role of GI Microbiome in Clinical Signs of Giardiasis

In the Literature

Kuzi S, Zgairy S, Byrne BA, et al. Giardiasis and diarrhea in dogs: does the microbiome matter? J Vet Intern Med. 2024;38(1):152-160. doi:10.1111/jvim.16894

The Research …

Giardia duodenalis is an intestinal protozoan parasite that infects a variety of vertebrate hosts worldwide.^1,2 Infective stages are found in the environment, and new hosts are typically infected via ingestion of cysts in contaminated food or water.³ G duodenalis is divided into assemblages with varied infectivity and known hosts.^2,3 Assemblages C and D are dog specific and may be underdiagnosed due to subclinical infections, intermittent shedding of cysts, and misidentification as a result of sole reliance on microscopic fecal examination.³

Although subclinical G duodenalis infections are common, some hosts experience chronic diarrhea. Exact reasons for this are unknown; however, research indicates clinical signs may be associated with physical stress or changes in the gut microbiome.¹

This study examined associations between fecal microbiome and diarrhea in dogs naturally infected with G duodenalis. Fecal samples were collected from 131 dogs; 42 were diagnosed as infected via antigen dot ELISA testing. Of those infected, 23 had diarrhea, and 19 were nondiarrheic. Samples were further analyzed via PCR to determine fecal microbiome. Results suggested that alteration in fecal microbiome in dogs naturally infected with G duodenalis does not differ based on presence of diarrhea, and other host or parasite factors may play a larger role in clinical outcome. Infected dogs did not appear to experience a high level of gut dysbiosis; however, differences were found among specific groups, including an abundance of Proteobacteria spp in diarrheic dogs <1 year old compared with older dogs, and significantly more abundant Clostridium spiroforme in male dogs with diarrhea compared with nondiarrheic male dogs.

… The Takeaways

Key pearls to put into practice:

• Why some G duodenalis infections are associated with clinical signs and others are not is unknown; a combination of factors (eg, parasite virulence, host characteristics, concurrent pathogenic infections) may play a role. Other studies indicate that the gut microbiome cannot be ruled out as an important factor and G duodenalis may benefit some hosts.^1-3

• When treating dogs with giardiasis, awareness that many factors—not just presence of the parasite—play a role in clinical signs is key. Other host characteristics (eg, stress level, presence of immunocompromise, concurrent pathogenic infections) should be considered.

• The gut microbiome does not appear to be the main factor contributing to clinical signs of giardiasis but can affect clinical outcome. Maintenance of the microbiome during treatment is important, and overtreatment (eg, treatment of subclinical patients with a positive test result) should be avoided, especially when drug efficacy is low.

• Dogs with subclinical infections may not require treatment. In addition to a positive G duodenalis test, the decision to treat should include consideration of other dogs and humans the infected dog may encounter, lifestyle, and previous treatment history.