Giardiasis is caused by the flagellated protozoan Giardia duodenalis, also known historically as G. lamblia or G. intestinalis, and it ranks among the most frequent enteric parasitic diseases worldwide. Infection occurs predominantly through the fecal-oral route, via contaminated water, food, fomites, or direct contact with infected people or animals. Clinical outcomes range from asymptomatic carriage to persistent diarrhea, intestinal malabsorption, and impaired child development, making the parasite a particular concern in settings with inadequate sanitation and socioeconomic vulnerability. What complicates the epidemiological picture is that Giardia is not simply a waterborne human pathogen. Domestic animals, livestock, and wildlife can all harbor the parasite, and environmental reservoirs sustain its transmission, creating an interconnected web that the One Health framework was designed to untangle.

From a molecular standpoint, G. duodenalis is classified into eight assemblages, designated A through H, which differ in host range and zoonotic potential. Assemblages A and B are the most commonly reported in humans but have also been detected in numerous animal species, while the remaining assemblages tend to be host-specific, though occasional cross-species infections have been documented. This genetic diversity makes molecular characterization a powerful tool for tracing transmission pathways and identifying reservoirs. Yet the review found that assemblage-level genotyping was available in only 17 of the 114 included studies, or 14.9 percent, a proportion the authors caution should temper any strong molecular inferences drawn from their synthesis.

The human evidence assembled in the review paints a picture of broad geographic distribution paired with deep social stratification. In the North region, a low-income community in the Brazilian Amazon recorded a prevalence of 26.0 percent, with Giardia identified as the most prevalent parasite. The Northeast showed prevalences ranging from 1.21 to 35.5 percent, concentrated among children, rural communities, and peri-urban populations with limited sanitary infrastructure. In the Central-West, prevalences between 3.9 and 21.7 percent were documented, with notable findings among Indigenous populations, including rates of 13.5 to 21.7 percent in the Tapirapé people of Mato Grosso and 9.3 percent in the Haliti-Paresí, in contexts where water treatment was absent. The Southeast and South regions reported ranges of 1.2 to 51.4 percent and 1.44 to 13.1 percent respectively, with strikingly high figures in daycare centers, including 51.4 percent among asymptomatic children at a São Paulo public daycare when microscopy was combined with PCR.

Across these studies, a consistent set of socio-environmental determinants emerged: lack of treated water, consumption of unsafe water, inadequate waste disposal, contact with contaminated soil, household overcrowding, and low maternal education. A high frequency of asymptomatic infections, particularly in children and collective settings, further complicates control efforts, because silent carriers maintain transmission chains that clinical diagnosis alone cannot interrupt. The Brazilian pattern mirrors findings from other low- and middle-income countries. A meta-analysis of African children estimated a pooled giardiasis prevalence of 18.3 percent, and a separate review reported a pooled prevalence of 31.9 percent across the continent, both strongly associated with deficient water, sanitation, and hygiene conditions. In Latin America, molecular reports remain concentrated in Brazil and Colombia, with the highest prevalences recorded in Brazil.

Among animals, the review identified 43 studies spanning companion animals, livestock, and wildlife. Dogs were the most frequently investigated category, with prevalences ranging from 0.75 to 50.0 percent, the highest figures appearing in shelters and veterinary hospitals where population density and environmental contamination favor transmission. Cats showed prevalences of 2.02 to 24.8 percent, while cattle and buffaloes ranged from 2.0 to 26.75 percent, with young animals and intensive dairy systems most affected. Wildlife studies, though mostly descriptive, recorded prevalences from 2.94 to 59.0 percent across non-human primates, bats, sloths, aquatic mammals, wild carnivores, and birds. One study of non-human primates in southern Brazil found 59.0 percent overall prevalence, rising to 83.3 percent in captive animals compared with 20.0 percent in free-living ones, suggesting that confinement and intensified interspecies contact facilitate parasite spread.

The environmental evidence was equally sobering. Twenty-three studies examined water, sewage, sludge, soil, food, and recreational matrices, and the highest detection frequencies occurred in water resources intended for human supply. Raw water samples showed prevalences from 33.3 to 100 percent, and one monitoring program in the metropolitan region of São Paulo found positivity in 100 percent of surface water samples and 85.7 percent of reuse water samples, with molecular characterization revealing sub-assemblages AII, BIII, and BIV, indicating a predominantly anthropogenic origin of contamination. Persistence after conventional treatment was repeatedly demonstrated: treated water in Espírito Santo contained up to 20.0 cysts per liter, and a Quantitative Microbial Risk Assessment in southern Brazil estimated a maximum annual infection risk of 5.04 per thousand persons, up to fifty times higher than the United States Environmental Protection Agency recommended limit. Sewage studies found Giardia in 100 percent of raw samples and 70 to 76 percent of treated samples, with viable forms in up to 30 percent of final effluent, while biosolids from São Paulo treatment plants carried concentrations reaching 4,800 cysts per gram.

The nine integrated studies, though few, provided the strongest evidence of interconnected transmission. In semi-rural communities of Ilhéus, Bahia, researchers documented simultaneous circulation in children, dogs, and environmental samples, with prevalences of 37.3 percent in children, 16.8 percent in dogs, and 5.2 percent in environmental samples, and detected assemblages A, B, and E in humans alongside a predominance of assemblage E in canids, the first report of that genotype in dogs from northeastern Brazil. At dairy farms in Paraná, the parasite was found in cattle and in drinking water. In a rural area of Santa Isabel, São Paulo, G. duodenalis was detected in up to 47.8 percent of water samples from shallow wells used for human consumption and in 46.1 percent of bovine fecal samples collected concurrently, demonstrating cross-contamination between herds and household water sources. An outbreak investigation in Cascavel, Paraná, linked the parasite to an acute diarrheal disease event involving 12,223 cases, with detection in both human fecal samples and water treatment plant backwash.

The molecular synthesis confirmed that assemblages A and B predominate across Brazilian hosts and matrices, consistent with their recognized zoonotic and anthroponotic potential. Assemblage E, traditionally associated with production animals, was occasionally detected in human samples, hinting at a zoonotic interface that appears less evident in Africa, where assemblage B accounts for roughly 70 percent of genotyped cases and assemblage E has not been found in humans. The authors are careful to note, however, that shared genotypes alone do not prove direct zoonotic transmission; they indicate potential epidemiological connectivity rather than established direction or frequency of cross-species events. With genotyping available in fewer than 15 percent of studies, and with differences in genetic targets and amplification protocols limiting comparability, the molecular picture remains incomplete.

The review’s limitations are candidly acknowledged: substantial methodological heterogeneity precluded meta-analysis, the protocol was not prospectively registered, formal risk-of-bias assessment was not performed, and independent double screening was not applied. Diagnostic method alone likely shaped reported prevalence, since microscopy is less sensitive than immunological or molecular assays, particularly with intermittent cyst shedding. Nevertheless, the synthesis makes clear that giardiasis in Brazil cannot be understood as an isolated human infection but as the product of interconnected environmental, social, and sanitary determinants. The authors call for multidisciplinary studies integrating simultaneous human, animal, and environmental sampling with standardized molecular characterization, expanded investigation in underrepresented northern and central-western regions, and articulated strategies combining epidemiological surveillance, sanitation, veterinary control, and molecular monitoring. Until such integrated work becomes routine, the true architecture of Giardia transmission across Brazil’s human, animal, and environmental interfaces will remain only partially mapped.

Subject of Research: One Health systematic review of Giardia spp. occurrence and molecular diversity across human, animal, and environmental compartments in Brazil

Article Title: Giardia spp. in Brazil at human–animal–environment interfaces: a systematic review from a One Health perspective

Article References: Fonseca, M. A., Sarti, E. S. G., & Guedes, E. (2026). Giardia spp. in Brazil at human–animal–environment interfaces: a systematic review from a One Health perspective. Frontiers in Parasitology, 5, Article 1895842. https://doi.org/10.3389/fpara.2026.1895842

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1895842

Keywords: Giardia duodenalis, giardiasis, One Health, Brazil, systematic review, zoonoses, assemblages, water contamination, sanitation, public health, molecular epidemiology, neglected tropical diseases