Determining the cellular targets and resulting pathology of rift valley fever virus infection of the rat CNS and reproductive system using microscopy

Rift Valley fever virus is an emerging zoonotic pathogen that infects domestic ruminants and causes fatal hepatic necrosis and “abortion storms” that result in loss of up to 90-100% of pregnancies in animals. Human infection occurs among individuals in contact with affected animals and typically manifests as a febrile illness that can advance to hemorrhagic fever or neurological disease. In patients that develop encephalitis, there is a 50% chance of death while survivors experience long-term complications. Human cases of vertical transmission during pregnancy resulted in fetal viremia, abnormalities, and death; however, the rates of congenital RVFV are misunderstood and understudied. Rodent encephalitic and congenital models of RVF have recently been developed and are crucial for elucidating the mechanism of infection separating febrile illness from severe outcomes effecting the central nervous and reproductive systems. Whole, homogenized tissues from these models have been analyzed for viral titers, infiltrating immune cells, and resulting pathologies; however, specific infection events occurring within precise tissue structures required immunofluorescence and chromogen staining for microscopic image analysis and scoring. Thin sections of tissues from both models were stained for viral and cellular markers and imaged in order to determine specific disease events observed in our lab’s previous studies, Albe et al. (2019) and McMillen et al. (2018). Immunofluorescence micrographs supported the presence of vRNA and inflammatory leukocyte infiltration in the olfactory bulbs and cortexes as early as 1 day post-infection (dpi), and massive neuronal infection and necrosis beginning by 3 dpi. End-stage neurological disease, beginning at 5 dpi, was represented by a massive blood brain barrier breakdown event and influx of inflammatory cells. Histological analysis of pregnant RVFV-infected rat tissues showed the highest amount of vRNA in the placental tissues rather than the liver, and moderate levels of virus in the uterus and ovaries. Within the placenta, RVFV targeted the maternal decidual layer and fetal basal and labyrinth zones. Viral signal was also detected in the placentas of dams that survived infection. Visual immunohistochemistry data, as developed and presented here, can help highlight structural and cellular targets of RVFV that should be focused on for therapeutic efforts. RVFV is in the Federal Select Agent program for pathogens that have the potential to post a severe threat to public, animal, or plant health, on the World Health Organization’s list of Blueprint priority diseases requiring an urgent need for accelerated research and development, and a category A high-priority pathogen of the National Institute of Allergy and Infectious Diseases, which are biological agents that post the highest risk to national security; therefore research into therapeutics for RVF is of major public health significance