Experimental Yellow Fever in the Squirrel Monkey (Saimiri spp.): Hematological, Biochemical, and Immunological Findings

Between 2016 and 2018, Brazil experienced the largest sylvatic epidemic of yellow fever virus (YFV). Despite to the magnitude and rapid spread of the epidemic, little is known about YFV dispersion. The study evaluated whether the squirrel monkey is a good model for yellow fever (YF) studies. Methods: Ten animals were infected with 1 × 10⁶ PFU/mL of YFV, with one negative control. Blood samples were collected daily during the first 7 days and at 10, 20 and 30 days post infection (dpi) for detection of viral load and cytokines by RT-qPCR; measurements of AST, ALT, urea and creatinine were taken; IgM/IgG antibodies were detected by ELISA, and hemagglutination inhibition and neutralization tests were performed. The animals exhibited fever, flushed appearance, vomiting and petechiae, and one animal died. Viremia was detected between 1 and 10 dpi, and IgM/IgG antibodies appeared between 4 and 30 dpi. The levels of AST, ALT and urea increased. The immune responses were characterized by expression of S100 and CD11b cells; endothelial markers (VCAM-1, ICAM-1 and VLA-4), cell death and stress (Lysozyme and iNOS); and pro-inflammatory cytokines (IL-8, TNF-α, and IFN-γ) and anti-inflammatory cytokines (IL-10 and TGF-β). The squirrel monkeys showed changes similar to those described in humans with YF, and are a good experimental model for the study of YF.