The epidemiology of Rift Valley fever in northern Tanzania

Nyarobi, Makuru James (2020) The epidemiology of Rift Valley fever in northern Tanzania. PhD thesis, University of Glasgow.

Abstract

Rift Valley fever is a mosquito-borne viral disease of ruminants, camels and humans. In Tanzania, outbreaks have occurred at intervals of 10 - 20 years with major epidemics reported in 1977, 1997/98 and 2006/2007. Our ability to prevent future epidemics is limited by poor understanding of how the virus circulates between major epidemics. This study aimed to investigate the epidemiology of inter-epidemic RVFV infections in northern Tanzania.
This study involved (a) collection and characterisation of mosquitoes; (b) RVFV serological analysis of serum samples from cattle (n=3582), sheep (n=2586), goats (n=3303) and human populations (n=565) collected through cross-sectional household surveys; (c) analysis of risk factors for livestock and human seropositivity; (d) molecular detection of RVFV in mosquitoes and diagnostic materials collected during investigation of 190 livestock abortion events. Generalised Linear Mixed-Effects Models (GLMMs) were used to examine predictors of vector mosquito abundance, and risk factors for RVFV exposure in livestock and humans. Maximum Entropy (MaxEnt) algorithm was used to model vector mosquito habitat suitability and spatial distribution.
A total of 2224 mosquitoes were collected including Culex spp (n = 1123), Anopheles spp (n=1006), Mansonia spp (n=56), Aedes spp (n=34), and Coquillettidia spp (n=5) with significant variation in abundance with percentage difference in normalised difference vegetation index (NDVI). No RVFV infections were detected in any of the mosquitoes collected. RVFV seroprevalence was higher in cattle 4.4% (95% CI:3.7-5.1), than in sheep 2.6%, (95% CI: 2.0-3.3) and goats 1.4% (95%CI: 1.0-1.8), with seropositivity in young animals providing evidence of recent virus circulation. Seropositivity in livestock increased with age (OR=1.3, CI: 1.2 - 1.4, p<0.001) consistent with endemic circulation and was associated with a history of abortion in goats (OR=2.5, 95%CI: 1.1 - 5.4, P=0.023) and sheep (OR=2.7, 95%CI: 1.1 - 6.3, P=0.025). Human seroprevalence was 8.5% (95% CI: 6.4 - 11.2) and varied between villages and between households within villages. Handling of aborted material (OR=4.3, 95% CI: 1.7-10.8) and consumption of raw milk (OR=4.1, 95%CI: 1.8 - 9.3, P=0.001) were significant risk factors for human seropositivity. RVFV was detected in a cluster of 14 (7.4%) abortion cases including the milk of three aborting dams. This provides strong evidence for continuous RVFV circulation in livestock between major epidemics in Tanzania and that unboiled milk is an important potential source of infection for people.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Rift Valley fever, inter-epidemic, epidemiology, abortion, seroprevalence, pastoral, agro-pastoral, outbreak
• Subjects: Q Science > QL Zoology; Q Science > QP Physiology; Q Science > QR Microbiology; Q Science > QR Microbiology > QR180 Immunology; Q Science > QR Microbiology > QR355 Virology; S Agriculture > SF Animal culture; S Agriculture > SF Animal culture > SF600 Veterinary Medicine
• Colleges/Schools: College of Medical Veterinary and Life Sciences
• Funder's Name: Biotechnology and Biological Sciences Research Council (BBSRC)
• Supervisor's Name: Cleaveland, Prof Sarah
• Date of Award: 2020
• Depositing User: Mr Makuru James Nyarobi
• Unique ID: glathesis:2020-81309
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Apr 2020 07:52
• Last Modified: 27 Apr 2020 14:05
• URI: https://theses.gla.ac.uk/id/eprint/81309