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Rift Valley fever virus (RVFV) causes an acute Rift Valley fever (RVF) disease in humans and domestic ruminants whose occurrence assumes an epidemic pattern in most cases. Understanding the maintenance of RVFV in susceptible hosts and potential vectors during inter-epidemic periods (IEP) is vital for better understanding of disease transmission dynamics. The aim of this study was to investigate the transmission dynamics of RVFV in Ngorongoro district, Northern Tanzania.
This study involved Rift Valley fever virus-Immunoglobulin G (RVFV-IgG) seropositive samples (n=160) from cattle, sheep, goats and humans that were collected from previous cross-sectional survey conducted in parallel with mosquito sampling in Malambo, Meshili, Osinoni, Endulen and Nainokanoka villages. The selected seropositive samples were tested for the presence of RVFV using the real- time Reverse Transcription polymerase chain reaction (qRT-PCR) and conventional RT-PCR.
Adult mosquitoes were collected outdoor using the Centres for Disease Control and Prevention (CDC) light traps baited with carbon dioxide. The traps were set in proximity to potential breeding sites and cattle kraals. The mosquitoes were identified using conventional morphological keys, in addition, Anopheles gambiae complex species were identified using Conventional PCR.
Viral RNA was extracted directly from serum samples and mosquitoes using a QIAamp Viral RNA Mini Kit (QIAGEN), the molecular detection of RVFV from serum samples was first performed using qRT- PCR, positive samples were then rescreened using conventional RT-PCR. A total of 96 Pools each containing 10 monospecific potential mosquito vectors were also tested for RVFV RNA using qRT-PCR.
The qRT- PCR detected 2 (5%), 4 (10%) and 1 (2.5%) positive samples in cattle, sheep and goat sera respectively. These 7 positive samples following screening using qRT-PCR were also found to be positive after retesting using conventional RT-PCR. No RVFV was found in human sera. The detection of RVFV was found in domestic ruminants of Meshili (6 cases) and Malambo (1 case) villages.
A total of 2,094 adult mosquitoes belonging to 3 genera and 9 species were collected. Most of them were collected in Meshili village (87.5%, n= 1832), followed by Malambo (8.2%) and Osinoni (4%) villages. No mosquitoes were caught in Nainokanoka and Endulen villages. The nine species collected were C. pipiens complex, C. antennatus, C. tigripes, C. annulioris, C. cinereus, An. arabiensis, An. squamosus, An. pharoensis and Ma. uniformis. With the exception of C. cinereus, all the other eight species have been implicated in RVFV transmission. None of the 96 mosquito pools tested was found to be positive for RVFV.
This study has demonstrated the maintenance of the RVF virus in domestic ruminants during the inter-epidemic period in the absence of reported cases in livestock or humans. There is a need of active surveillance system to monitor circulation of the virus especially during IEPs for better understanding of the mechanisms of disease transmission in these villages. In addition, various mosquito species potentially competent for RVFV transmission were collected. This demonstrates the local mosquito abundance that could propagate RVFV in the study villages. These vectors were heterogeneously distributed suggesting possible differences in disease transmission between the villages. However, comprehensive entomological study has to be undertaken during epidemic so as to identify mosquito species that can maintain and transmit the virus by bite. |
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