Detection of Rabies IgG and IgM Antibodies Using the Rabies Indirect Fluorescent Antibody Test

The rabies indirect fluorescent antibody (IFA) test is used to detect various rabies-specific antibody isotypes in sera or cerebral spinal fluid. It is one of an arsenal of tests available for monitoring an antemortem rabies patient. It is especially useful for the early detection of antibodies to evaluate a patient’s immune response to rabies infection. When used in conjunction with other tests, case history, and the patient’s vaccination status, the IFA test can assist in determining exposure to rabies virus or a vaccine¹. As the IFA test measures IgM and/or IgG, the values of the specific antibody can indicate an approximate time frame from exposure to the antigen¹. This test may be useful in the listed applications or others not yet explored.

There are several rabies serological assays available. The rapid fluorescent focus inhibition test (RFFIT), fluorescent antibody virus neutralization (FAVN) test, or modifications of these are the primary methods for measuring rabies virus neutralizing antibodies (RVNAs)¹. However, these tests do not differentiate IgM and IgG antibodies. When differentiating antibody isotype is important in monitoring the rabies immune response, the rabies IFA and the rabies enzyme-linked immunosorbent assay (ELISA) tests are used, but they do not measure RVNAs. Although the IFA and ELISA tests can be used to determine the presence of rabies-specific antibodies in a sample, there are some differences in how they are executed. The IFA test utilizes a cell-cultured live virus as its antigen substrate, whereas a typical ELISA for rabies detection uses one or more of the viral proteins. In a laboratory setting where the rabies virus can be cultured, the IFA test may be more easily performed instead of purchasing or cultivating individual viral proteins for the ELISA. The purpose of testing and the information garnered from the results of any rabies serological assay should be considered when determining which to choose².

IgM is the first to respond, increasing until class switching is observed at around day 28, at which point IgG becomes the predominant circulating antibody³. Hence, IgM would only be expected for a limited amount of time following exposure to rabies virus or vaccination. Testing both serum and cerebrospinal fluid (CSF) can indicate if the exposure was through vaccination, in which antibodies would be seen only in sera, or from a viral infection, which would potentially show antibodies in CSF¹.

It has been established that rabies antibodies persist for several years following pre-exposure prophylaxis (PrEP)⁴. The IFA test can be a useful tool to demonstrate this at different time points following vaccination or exposure.