Giardia intestinalis continues to be one of the most encountered parasitic diseases around the world. Although more frequently detected in developing countries, Giardia infections nonetheless pose significant public health problems in developed countries as well. Molecular characterisation of Giardia isolates from humans and animals reveals that there are two genetically different assemblages (known as assemblage A and B) that cause human infections. However, the current molecular assays used to genotype G. intestinalis isolates are quite controversial. This is in part due to a complex phenomenon where assemblages are incorrectly typed and underreported depending on which targeted locus is sequenced. The purpose of this review is to outline current knowledge based on molecular epidemiological studies and raise questions as to the reliability of current genotyping assays and a lack of a globally accepted method. Additionally, we will discuss the clinical symptoms caused by G. intestinalis infection and how these symptoms vary depending on the assemblage infecting an individual. We also introduce the host-parasite factors that play a role in the subsequent clinical presentation of an infected person, and explore which assemblages are most seen globally.

Patricia Zajaczkowski has completed multiple projects in gastrointestinal illness working closely with NSW Health and the University of Technology Sydney (UTS). Her interests lay in enteric protozoa such as Giardia intestinalis and Cryptosporidium spp., and the molecular epidemiology surrounding these parasites. Zajaczkowski has previously worked with the South-Western Sydney Local Health District Public Health Unit (SWSLHD PHU), conducting case-control studies to document G. intestinalis cases across metropolitan Sydney. Her papers are the first major epidemiological investigations done on these parasites in Sydney.