Rift Valley fever virus live attenuated vaccine strain Smithburn can cause meningoencephalitis in alpacas

Rift Valley fever (RVF) is a zoonotic, viral, mosquito‐borne disease that causes considerable morbidity and mortality in ruminants during the rainy season in Africa. In June 2018, 4 alpaca inoculated subcutaneously with Rift Valley Fever (RVF) Smithburn live attenuated vaccine strain exhibited pyrexia, aberrant vocalization, anorexia, neurological signs (tremors, disorientation, overknuckling and ataxia leading to full recumbency), respiratory distress and sudden death (3 animals). One animal died the evening of inoculation, the other two at 2–3 weeks post inoculation. The fourth alpaca fully recovered. Due to significant concern that this was wildtype Rift Valley fever, vaccine strain reversion, or an atypical camelid response to Smithburn vaccination, a full necropsy and further workup were conducted on the latter 2 fatal cases including necropsy, histopathology, immunohistochemistry (IHC), RNAscope in situ hybridization (ISH), reverse transcription real‐time PCR (RT‐qPCR) and confirmatory sequencing. Necropsy and histopathology were conducted per standard procedures at the WCPVL. RT‐qPCR for RVFV L segment and sequencing of each of the three genome segments were conducted per standard procedures at the OVI. Anti‐RVFV glycoprotein IHC and ISH for RVFV L segment RNA were conducted per published methods in Ragan et al. , 2019 . Macroscopic lesions included cyanotic mucous membranes, severe pulmonary oedema and congestion and multifocal petechiae and ecchymoses throughout the left and right ventricular epi‐, endo‐ and myocardium. The most distinctive microscopic lesions were multifocal neuronal necrosis, gliosis and prominent histiocytic and lymphoplasmacytic perivascular cuffing of blood vessels in the cerebrum as well as a histiocytic and lymphoplasmacytic cerebral meningitis. Multifocally, in the cerebrum, viral antigen and RNA were present in the cytoplasm of neurons, including in the cell body, dendritic arborizations and axons (Figure 1 ) and brain samples tested positive for RVFV RNA by RT‐qPCR. The liver was negative for RVFV by both IHC and ISH. Additional genetic analysis of the brain tissue revealed that the virus clustered in Lineage K (100% sequence identity) with close association to published Smithburn vaccine sequences (sequence identity ranging from 99.1 – 100%). This is the first report of respiratory distress, neurological signs and sudden death in alpaca post inoculation with the Smithburn strain. It is also the first examination of RVF viral RNA distribution in alpaca tissues. RVF live attenuated Smithburn vaccine can cause meningoencephalitis in alpacas. Support or Funding Information WCPVL and Laboratory of Investigative Pathology, Department of Diagnostic Medicine/Pathobiology, KSU CVM Alpaca Rift Valley fever virus RNA positive encephalitisCerebral encephalitis in an alpaca vaccinated with Smithburn live attenuated vaccine strain labeled with pan‐RVFV (L segment probe‐set) ISH (brown), counterstained with hematoxylin. Bar is 50 microns. Figure 1