An unusually long Rift valley fever inter-epizootic period in Zambia: Evidence for enzootic virus circulation and risk for disease outbreak

PLoS Negl Trop Dis. 2022 Jun 2;16(6):e0010420. doi: 10.1371/journal.pntd.0010420. eCollection 2022 Jun.

Abstract

Rift valley fever (RVF) is a mosquito-borne disease of animals and humans. Although RVF outbreaks are usually reported at 5-15-year intervals in sub-Saharan Africa, Zambia has experienced an unusually long inter-epizootic/-epidemic period of more than three decades. However, serological evidence of RVF virus (RVFV) infection in domestic ruminants during this period underscores the need for comprehensive investigation of the mechanisms of virus perpetuation and disease emergence. Mosquitoes (n = 16,778) captured from eight of the ten provinces of Zambia between April 2014 and May 2019 were pooled (n = 961) and screened for RVFV genome by a pan-phlebo RT-PCR assay. Aedes mosquito pools (n = 85) were further screened by nested RT-PCR assay. Sera from sheep (n = 13), goats (n = 259) and wild ungulates (n = 285) were screened for RVFV antibodies by ELISA while genome detection in pooled sera (n = 276) from domestic (n = 248) and wild ungulates (n = 37) was performed by real-time RT-PCR assay. To examine the association between the long inter-epizootic period and climatic variables, we examined El Niño-Southern Oscillation indices, precipitation anomalies, and normalized difference vegetation index. We then derived RVF risk maps by exploring climatic variables that would favor emergence of primary RVFV vectors. While no RVFV genome could be detected in pooled mosquito and serum samples, seroprevalence was significantly high (OR = 8.13, 95% CI [4.63-14.25]) in wild ungulates (33.7%; 96/285) compared to domestic ruminants (5.6%; 16/272). Retrospective analysis of RVF epizootics in Zambia showed a positive correlation between anomalous precipitation (La Niña) and disease emergence. On risk mapping, whilst northern and eastern parts of the country were at high risk, domestic ruminant population density was low (< 21 animals/km2) in these areas compared to low risk areas (>21 animals/km2). Besides evidence of silent circulation of RVFV and the risk of disease emergence in some areas, wildlife may play a role in the maintenance of RVFV in Zambia.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antibodies, Viral
  • Culicidae*
  • Disease Outbreaks / veterinary
  • Mosquito Vectors
  • Real-Time Polymerase Chain Reaction
  • Retrospective Studies
  • Rift Valley Fever*
  • Rift Valley fever virus* / genetics
  • Ruminants
  • Seroepidemiologic Studies
  • Sheep
  • Zambia / epidemiology

Substances

  • Antibodies, Viral

Grants and funding

This study was supported in part by the Japan Program for Infectious Diseases Research and Infrastructure (JP21wm0125008; H.S) from Japan Agency for Medical Research and Development (AMED); and grants for the AMED and Japan International Cooperation Agency (JICA) within the framework of the Science and Technology Research Partnership for Sustainable Development (SATREPS) (JP21jm0110019; H.S); Grants-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (JP16H06429, JP16H06431,JP 16K21723; H.S); Japan Society for the Promotion of Science (JSPS) KAKENHI (JP16H05805; H.S, JP20K21298; Y.O); Japan Science and Technology Agency (JST) Moonshot R&D (JPMJMS2025; Y.O); Department of Veterinary Services in the Ministry of Fisheries and Livestock, Government of the Republic of Zambia (H.M.C). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.