1 Introduction
Rift Valley fever virus (RVFV) (family Phenuiviridae , genusPhlebovirus ) is a mosquito-borne virus that causes periodic
epizootic outbreaks across Africa and the Arabian peninsula(Al-Afaleq &
Hussein, 2011; Nguku et al., 2010). In ruminants, primarily sheep,
goats, camels and camelids, RVF is often characterized by sudden
epizootics marked by near universal fetal death at all stages of
gestation, congenital malformations(Coetzer, 1982) and significant adult
animal deaths often due to acute virus induced hepatic and renal
pathology(Wichgers Schreur et al., 2021). Though most human cases are
typically self-limiting with mild to moderate symptoms (McElroy, Harmon,
Flietstra, Nichol, & Spiropoulou, 2018), cases of delayed onset
encephalitis, kidney and/or eye damage, severe anemia, hemorrhagic fever
and miscarriage can occur(Baudin et al., 2016; Coetzer, 1982; Madani et
al., 2003; Oymans, Wichgers Schreur, van Keulen, Kant, & Kortekaas,
2020).
Over 40 species of mosquitoes have been demonstrated as competent
vectors for RVFV(reviewed in(Lumley et al., 2017)), some of which range
on multiple continents. Following periods of heavy rainfall, which
stimulate rapid increases in vector mosquito populations, RVFV
re-emerges periodically in explosive epizootics(Al-Afaleq & Hussein,
2011; Nguku et al., 2010). In the absence of humans and livestock, RVFV
cycles between mosquitoes and wild ruminants(Britch et al., 2013; Clark,
Warimwe, Di Nardo, Lyons, & Gubbins, 2018). Between epizootics, there
is also support for low level maintenance of RVFV in livestock in
inter-epidemic periods(Lichoti et al., 2014).
Due to these health implications and the potential to cause a public
health emergency, in 2018 the World Health Organization listed RVFV as a
research and development blueprint priority pathogen(Mehand,
Al-Shorbaji, Millett, & Murgue, 2018). Availability of a safe and
effective human vaccine against RVFV is essential to protect the health
of people in endemic regions and a preparatory measure for the
anticipated cross-border spread and establishment of RVFV into new
geographic areas. A number of vaccine candidates have been developed for
RVFV, including formalin inactivated (Pittman et al., 1999; Randall,
Gibbs, Aulisio, Binn, & Harrison, 1962) and live attenuated
strains(Ikegami et al., 2015; Smithburn, 1949). However residual
teratogenic effects in animals or the need for boosters to maintain
protective immunity(Bird, Ksiazek, Nichol, & Maclachlan, 2009; Botros
et al., 2006) present challenges for further development of these
earlier candidates. To date, there is currently no commercially
available and fully FDA-approved RVFV human vaccine.
To meet this critical health need, a human vaccine candidate (DDVax), a
double deletion construct of the parental wild-type strain ZH501 was
generated using a reverse genetics approach wherein both the NSs
(non-structural, S segment) and NSm (non-structural, M segment)
virulence genes were removed(Bird et al., 2008). NSs is expressed in an
ambisense fashion from the viral S segment(Ikegami et al., 2009) and is
a multi-functional protein that antagonizes host cell interferon
responses(Le May et al., 2008). The viral M segment encodes 2 major
glycoproteins and multiple open reading frames in the NSm coding
regions, which is required for efficient dissemination in
mosquitoes(Crabtree et al., 2012). Neither NSs nor NSm are required for
viral replication in interferon-deficient cell culture, and the
attenuated DDVax vaccine candidate has proven to be safe and immunogenic
in a variety of animal species with the added benefit of inhibited
replication and transmission in mosquitoes(Bird et al., 2008; Bird et
al., 2011; Crabtree et al., 2012; Kading et al., 2014).
The objective of this study was to confirm that the newly rescued
version of DDVax produced for development under Good Manufacturing
Practices (GMP) behaved as previously described and exhibited a highly
favorable environmental safety profile by not being transmitted by
potential mosquito vectors. Here, we describe characterization of RVFV
DDVax in mosquitoes using both in vitro and in vivoapproaches. These vector assessments were divided into two experimental
phases: 1) mosquito oral challenges via artificial feeding and 2)
mosquito feeding on DDVax inoculated goats. Features of vector
competence were measured in two competent mosquito species, Culex
tarsalis Coquillett and Aedes aegypti L., to determine
infection, dissemination and transmission potential, using reverse
transcriptase- quantitative PCR (RT-qPCR) and infectious virus plaque
assay. Vertebrate-to-vector transmission from DDVax-inoculated goats to
mosquitoes was also measured. Collectively, these experiments provided
important comparison of vector competence of mosquitoes exposed to
DDVax(Bird et al., 2008), ZH501, the parental wild-type virus and MP-12,
an existing vaccine virus strain(Turell & Rossi, 1991).