Giardia lamblia is an intestinal flagellated protozoan parasite. It was first observed and described by Van Leewenhock in 1681 and more fully described morphologically by Lambl in 1859.
Interest in this group of protozoa began only in the 70th years of the XX century, when Giardia organisms were isolated from mammal, bird, and amphibian hosts. G. lamblia causes intestinal malabsorption and diarrhea (giardiasis) in humans and other mammals worldwide. The clinical effects of Giardia infection range from an asymptomatic carrier state to severe malabsorption syndrome with chronic diarrhea. Different factors are thought to contribute to the wide variation in clinical symptoms, including the virulence of the Giardia strain, the number of cysts ingested, the age of the host, and the state of the host’s immune system at the time of infection.
Human giardiasis has been reported from all five continents and most countries in the world. Prevalence rates of infection vary between <1 and 50 % depending on the population sampled, being higher in under-developed than developed countries and in urban rather than rural areas. In many developing regions where basic sanitation is lacking, Giardia infections are almost universal by two years of age. On the contrary, in developed countries the prevalence of giardiasis is only 3–7%, and it is associated mainly with travelling and waterborne outbreaks.
Giardia’s simple, two-stage life cycle is central to its success as a parasite. After the host ingests cysts, which are the infective stage, the trophozoites emerge from the cysts in the duodenum and attach to the small intestinal mucosa. They undergo mitotic division in the intestine; some will encyst.
Multiple facts suggest the involvement of humoral immune responses in elimination of G.lamblia. In humans, patients with hypogammoglobulinaemia have prolonged infections, which are difficult to cure. As it was shown in a number of studies, the level of serum antibodies in patients with giardiasis increased significantly in comparison with control. The increase was less pronounced in patients with chronic giardiasis.
The dynamics of antibody response was different for IgM, IgG and IgA. As it was shown on the human experimental infection model, the level of IgM increased significantly by days 14-21 after infection; levels tended to fall after therapy. On the contrary, the levels of IgG remained elevated after successful treatment. IgA response was more similar to that of IgM.
A number of proteins were identified as highly immunoreactive during acute giardiasis, the most important being variable surface proteins (VSPs) and various giardins. VSPs are highly variable as well as lacking antigenic conservation and crossreactivity. On the contrary, α1-giardin is one of the most common conserved antigens recognized by humans with giardiasis, and it has no homolog in mammals. Some giardins (including alpha-1-giardin) are among the most highly expressed proteins in Giardia and show a plasma membrane association and/or localisation to different flagella. This makes the alpha-giardins useful in the diagnostics of giardiasis. In one study it was shown that treatment of infected patients quickly reduced antibody titers, but treatment failure or chronic infection/reinfection resulted in higher antibody titers.
Despite the importance of this parasite in causing disease, clinical methods most widely used today for diagnosing human infection with G. lamblia are still relatively primitive. The diagnosis of giardiasis is traditionally based upon clinical history, symptoms, presence of cysts in feces or trophozoites in material retrieved from the small intestine by duodenal aspiration or duodenal biopsy. A single microscopic examination of stools from giardiasis patients will generally miss detecting the infection about 20 to 50% of the time even if concentration methods are employed, chiefly because of the extreme variability with which the parasite is excreted in both symptomatic and asymptomatic infections.
Alternative methods to the routine microscopic examination are detection of G.lamblia antigen in feces and measurement of level of specific anti-Giardia antibodies in patients’ serum. Fecal immunoassays are valuable tools for the routine clinical microbiology laboratory because of their speed and simplicity, especially when microscopical examinations of stools are not performed in the laboratory; they are highly specific and sensitive. Serologic testing is now regarded as a useful complement in the diagnosis of giardiasis. Besides contributing to the aid of clinical diagnosis, it could help in understanding of the status of immune response for each individual and for epidemiological purposes. Outbreaks of giardiasis are almost always investigated retrospectively, and many patients either have cleared the infection or have been treated when the investigation starts. Symptomatology and fecal examinations are, therefore, not useful, but serologic evidence of a recent infection can be useful in a country where the population immunity to Giardia infection is low.