Fresh infection and ways of getting it
Class M antibodies appear approximately two weeks after Giardia infection and persist for several months. Their detection indicates a relatively recent invasion, which is practically significant during examination after a trip, in case of group morbidity in a family or children's group, and in case of acute intestinal complaints.
Infection occurs through the fecal-oral route: through water that has not undergone proper purification, unwashed vegetables, fruits and herbs, as well as through contact and household contact through hands, dishes and toys. Giardia cysts are stable in the external environment and do not die during normal water chlorination, so outbreaks are often associated with tap water and open reservoirs.
As with other classes of antibodies, the result is assessed in a complex manner. If IgM is positive and there are characteristic complaints, the logical next step is to test the stool for Giardia antigen, which confirms the presence of the pathogen and serves as the basis for prescribing treatment. Examination of family members when giardiasis is detected is recommended, since intrafamily spread occurs easily.
What do IgM and IgG mean?
The immune system produces different classes of antibodies at different times. Immunoglobulins M appear first, usually in the first or second week after infection, last for several weeks or months and then disappear. Their detection indicates an acute or recent process.
Immunoglobulins G appear later, after a few weeks, and persist for years, and often for life. They mean that a meeting with the pathogen took place in the past, and in many cases indicate formed immunity. Therefore, positive IgG in itself is not a reason for treatment.
Why dynamics are important, not just a number
A single result is often ambiguous. Antibody levels vary from person to person, with borderline levels falling into what is known as a gray area. A reliable sign of active infection is a fourfold increase in IgG titer in paired sera taken at intervals of 10–14 days.
This is why serology is rarely prescribed as a single test. The doctor either combines IgM and IgG, or repeats the test, or confirms the result with a direct method - PCR, which detects the DNA or RNA of the pathogen itself.
Limitations of the method
Antibodies do not appear immediately: in the first days of the disease, serology can be negative if the infection has already begun - this is the period of the serological window. In case of immunodeficiency and against the background of immunosuppressive therapy, antibodies may not be produced at all.
Cross-reactions with related pathogens are also possible, giving a false-positive result. Therefore, treatment is never prescribed “based on analysis” - only on the basis of a combination of clinical data and research results.