Inter-epidemic Acquisition of Rift Valley Fever Virus in Humans in Tanzania

Abstract

BackgroundIn East Africa, epidemics of Rift Valley fever (RVF) occur in cycles of 5–15 years following unusually high rainfall. RVF transmission during inter-epidemic periods (IEP) generally passes undetected in absence of surveillance in mammalian hosts and vectors. We studied IEP transmission of RVF and evaluated the demographic, behavioural, occupational and spatial determinants of past RVF infection.MethodologyBetween March and August 2012 we collected blood samples, and administered a risk factor questionnaire among 606 inhabitants of 6 villages in the seasonally inundated Kilombero Valley, Tanzania. ELISA tests were used to detect RVFV IgM and IgG antibodies in serum samples. Risk factors were examined by mixed effects logistic regression.FindingsRVF virus IgM antibodies, indicating recent RVFV acquisition, were detected in 16 participants, representing 2.6% overall and in 22.5% of inhibition ELISA positives (n = 71). Four of 16 (25.0%) IgM positives and 11/71 (15.5%) of individuals with inhibition ELISA sero-positivity reported they had had no previous contact with host animals. Sero-positivity on inhibition ELISA was 11.7% (95% CI 9.2–14.5) and risk was elevated with age (odds ratio (OR) 1.03 per year; 95% CI 1.01–1.04), among milkers (OR 2.19; 95% CI 1.23–3.91), and individuals eating raw meat (OR 4.17; 95% CI 1.18–14.66). Households keeping livestock had a higher probability of having members with evidence of past infection (OR = 3.04, 95% CI = 1.42–6.48) than those that do not keep livestock.ConclusionThere is inter-epidemic acquisition of RVFV in Kilombero Valley inhabitants. In the wake of declining malaria incidence, these findings underscore the need for clinicians to consider RVF in the differential diagnosis for febrile illnesses. Several types of direct contact with livestock are important risk factors for past infection with RVFV in this study’s population. However, at least part of RVFV transmission appears to have occurred through bites of infected mosquitoes.

Inter-epidemic Rift Valley fever virus seroconversions in an irrigation scheme in Bura, south-east Kenya

“…Previous studies have detected RVFV seropositivity among domestic and wild animals during long IEPs in southern Africa and other parts of east Africa, with seroprevalences ranging between 15% and 23% (Anderson & Rowe, ; Evans et al., ; Lichoti et al., ; Sumaye et al., ). Similarly, two studies on human hosts in east Africa detected RVF seroprevalences of between 11% and 13% (LaBeaud et al., ; Sumaye et al., ). Most of these earlier studies were seroprevalence studies carried out within one local ecosystem and did not actually observe seroconversions.…”

“…The hazard ratio for RVFV seroconversion was 0.71 (CI = 0.26-1.91, p = .11). Sumaye et al, 2015). Most of these earlier studies were seroprevalence studies carried out within one local ecosystem and did not actually observe seroconversions.…”

“…Current knowledge suggests that the virus survives in mosquito eggs through transovarial transmission from female mosquitoes of the genus Aedes when temporary dambos dry up (Linthicum et al, 1985;Pepin et al, 2010). Even though several studies have detected RVFV activity in humans and animals during long IEP (LaBeaud et al, 2008;Lichoti et al, 2014;Sumaye et al, 2013Sumaye et al, , 2015 earlier studies into the inter-epidemic maintenance of the virus proposed that long-term persistence among mosquito species was more important for the maintenance of RVFV in a closed ecosystem, due to the small period of infectiousness in livestock (Favier et al, 2006;Fischer, Boender, Nodelijk, de Koeijer, & van Roermund, 2013;Gaff, Hartley, & Leahy, 2007;Pedro, Abelman, Ndjomatchoua, Sang, & Tonnang, 2014). More recent models however suggest that both vertical transmission in vectors and direct transmission between hosts may be responsible for persistence of the virus once introduced into an ecosystem (Manore & Beechler, 2015;Nicolas et al, 2014).…”

Serological evidence of rift valley fever virus among acute febrile patients in Southern Mozambique during and after the 2013 heavy rainfall and flooding: implication for the management of febrile illness

“…To our knowledge this is the first investigation of RVFV among febrile patients in Mozambique and we found that 5 % (10/200) of febrile patients seroconverted to IgG antibodies against RVFV between their acute and convalescent samples. Although novel for Mozambique, this finding is not surprising, based on the following arguments: i) A recent study conducted by our group in the same area showed that Aedes mosquitoes were abundant [ 22 ] ii) recent studies conducted in several districts in the surroundings of the study area found elevated frequencies of anti-RVFV IgG antibodies in cattle [ 10 , 11 ], iii) several outbreaks and sporadic cases have been reported in countries neighboring Mozambique such as South Africa and Tanzania [ 4 , 8 , 23 ], iv) patients were recruited during and after the severe heavy rainfalls that occurred in southern Mozambique in 2013 [ 8 , 15 ], v) people living in the study area travel regularly to the surroundings districts where elevated frequencies of antibodies were recently documented in cattle [ 10 , 11 ] and vi) cattle population is much more frequent than sheep in the study area [ 24 ]. Although the source of infection was not investigated in this study, the fact that most of cases occurred between February through May, peaking in April, suggest that heavy rainfall and floods that occurred in the same period might have influenced the occurrence of RVFV in Mozambique.…”

Crowdsourcing Vector Surveillance: Using Community Knowledge and Experiences to Predict Densities and Distribution of Outdoor-Biting Mosquitoes in Rural Tanzania

“…Lymphatic filariasis remains endemic in the country [ 20 ] and dengue fever viruses resulted in 1017 cases and 4 deaths in 2014 [ 21 ]. Other mosquito-borne infections such as Rift Valley fever [ 22 – 24 ] and Chikungunya have also been documented in some areas [ 25 – 27 ]. Ongoing mosquito control interventions such as long-lasting insecticidal nets (LLINs) and indoor spraying with residual insecticides (IRS) are having significant impact already [ 28 ], but the remaining residual transmission which occurs in spatially segregated areas must also be targeted to ensure elimination is achieved [ 29 , 30 ].…”