Formaldehyde is routinely used to fix tissues in preparation for pathology studies, however concerns remain that treatment of tissues with cellular
fixatives may not entirely inactivate infectious virus particles. This concern is of particular regulatory importance for research involving viruses that are classified as select agents such as Rift Valley fever virus (RVFV). Therefore, the specific aims of this study were to (1) assay RVFV-exposed Aedes aegypti mosquitoes fixed in 4%
paraformaldehyde for the presence of infectious RVFV particles at various time points following
infection and (2) demonstrate the utility of immunofluorescence assay (IFA) for the detection of RVFV
antigen in various tissues of
paraformaldehyde-fixed mosquitoes. Mosquitoes were administered an infectious blood meal containing one of two strains of RVFV, harvested at various time points following
infection, intrathoracically inoculated with 4%
paraformaldehyde, and fixed overnight at 4°C. The
infection status of a subset of mosquitoes was verified by IFA on leg tissues prior to fixation, and infectivity of RVFV in fixed mosquito carcasses was determined by Vero cell plaque assay.
Paraformaldehyde-fixed mosquitoes harvested 14 days post
infection were also
paraffin-embedded and sectioned for detection of RVFV
antigen to particular tissues by IFA. None of the RVFV-exposed mosquitoes tested by Vero cell plaque assay contained infectious RVFV after fixation. Furthermore, incubation of mosquito sections with
trypsin prior to antibody staining is recommended for optimal visualization of RVFV
antigen in infected mosquito tissues by IFA.