Giardia duodenalis is a gastrointestinal protozoon that infects a wide variety of mammals, including humans and farm animals (Dixon, 2021). The infection is acquired through ingestion of infective cysts either directly by contact with infected hosts or indirectly by water or food contamination (Ryan et al., 2019). The economic impact of Giardia infection in livestock is significant, causing gradual weight loss and decreased productivity, leading to substantial financial losses (Caccio et al., 2018). As reports of G. duodenalis in cattle have increased significantly in recent years (Cai et al., 2021, Heng et al., 2022, Taghipour et al., 2022), concerns have been raised about the potential of cattle to act as a reservoir for zoonotic G. duodenalis.

Molecular analyses have been widely used in genetic characterizations of G. duodenalis. Thus far, eight assemblages (A–H) of G. duodenalis have been identified, exhibiting distinct host ranges (Ryan et al., 2019). Among them, assemblages A and B are observed to infect humans and various animal species, and are considered zoonotic genotypes. In contrast, assemblages C–H appear to be largely host-specific (Feng and Xiao, 2011). In cattle, assemblage E is the dominant G. duodenalis genotype found in most areas worldwide (Dixon, 2021). Nevertheless, zoonotic assemblages A and B were occasionally identified in calves in China and New Zealand (Cai et al., 2021).

Despite an increasing number of studies using molecular techniques to determine the distribution of G. duodenalis genotypes in cattle, there is little information on the occurrence of genotypes in a particular group of animals by age. In a longitudinal cohort study of G. duodenalis in the USA, only assemblage E was found in pre-weaned calves, while assemblage A dominated in older calves at 18 weeks and six months of age (Santin et al., 2009). Similarly, a longitudinal study performed on adult dairy cattle in Canada revealed that assemblage E and assemblage A coexisted, with the infection rate of assemblage A being as high as 43.0% (Uehlinger et al., 2006). In contrast, in another study of G. duodenalis infections in pre-weaned calves in Australia, G. duodenalis infection rates peaked at four to seven weeks of age with exclusive occurrence of assemblage E (Becher et al., 2004). These findings indicate that the dynamic infection of G. duodenalis might vary among regions, and the infection rate of zoonotic assemblages in dairy cattle could be high in some areas.

In China, little is known about the age-associated occurrence of G. duodenalis genotypes and subtypes among calves. In the present study, cross-sectional studies were conducted to assess the occurrence of G. duodenalis on five dairy farms in Guangdong Province, China. One farm was then selected from them for longitudinal birth cohort study on the natural history of G. duodenalis infection in dairy calves during their first year. Genotyping, subtyping, and quantitative determination of cyst shedding were applied to identify infection patterns. The generated data will improve our understanding of transmission characteristics and infection dynamics of G. duodenalis in dairy cattle in China.