Epidemiologic and genomic characterization of an outbreak of Rift Valley fever among humans and dairy cattle in northern Tanzania

Background A peri-urban outbreak of Rift Valley fever virus (RVFV) among dairy cattle from May through August 2018 in northern Tanzania was detected through testing samples from prospective livestock abortion surveillance. We sought to identify concurrent human infections, their phylogeny, and epidemiologic characteristics in a cohort of febrile patients enrolled from 2016-2019 at hospitals serving the epizootic area. Methods From September 2016 through May 2019, we conducted a prospective cohort study that enrolled febrile patients hospitalized at two hospitals in Moshi, Tanzania. Archived serum, plasma, or whole blood samples were retrospectively tested for RVFV by PCR. Human samples positive for RVFV were sequenced and compared to RVFV sequences obtained from cattle through a prospective livestock abortion study. Phylogenetic analysis was performed on complete RVFV genomes. Results Among 656 human participants, we detected RVFV RNA in four (0.6%), including one death with hepatic necrosis and other end-organ damage at autopsy. Humans infected with RVFV were enrolled from June through August 2018, and all resided in or near urban areas. Phylogenetic analysis of human and cattle RVFV sequences demonstrated that most clustered to lineage B, a lineage previously described in East Africa. A lineage E strain clustering with lineages in Angola was also identified in cattle. Conclusion We provide evidence that an apparently small RVFV outbreak among dairy cattle in northern Tanzania was associated with concurrent severe and fatal infections among humans. Our findings highlight the unidentified scale and diversity of inter-epizootic RVFV transmission, including near and within an urban area.