Introduction
Before knowing
the driving forces for the emergence and re-emergence of Rift Valley fever (RVF) in
Africa, we should know about the disease, transmission cycle, and epidemiology.
RVF is a mosquito-borne zoonosis caused by a Phlebovirus of the Bunyaviridae
family. The virus affects both livestock and humans, with severe implications
for public health, food security, and trade. Most importantly, the virus is
carried by more than 53 species of
mosquitoes in 8 genera within the family Culicidae in regions of epizootics
(Linthicum et al., 2016). Since its first description in Kenya in 1931, RVF has
exhibited episodic outbreaks across Africa and occasional expansion into the
Arabian Peninsula, highlighting its persistent and dynamic epidemiological
profile (Nanyingi et al., 2015; Himeidan et al., 2014). Its transmission
involves complex interactions between vectors, hosts, and environmental
conditions, making it a quintessential One Health challenge. Based on the
literature provided in the study materials, the important driving forces were
critically examined and assessed its potential for spread beyond Africa. In
addition, it also evaluates current control strategies and identifies key
unresolved challenges.
The drivers of
RVF emergence and re-emergence in Africa are ranked based on the criteria,
including 1. Magnitude of impact on outbreak initiation and scale, 2. Strength
and consistency of empirical evidence across outbreaks, 3. Geographic and
temporal generalizability, and 4. Potential for intervention (modifiability)
1. Climatic and environmental
variability (primary driver)
The top-ranked driver for the emergence of RVF from the different literature identifies abnormal rainfall and
flooding as the most critical driver of RVF outbreaks. These conditions promote
hatching of dormant Aedes mosquito eggs and amplification of vector
populations (Linthicum et al., 2016). The massive increased of infected
mosquitoes leads to high epizootic potential. In addition, a couple of studies
on satellite-based vegetation indices and rainfall anomalies have consistently
predicted outbreaks in East Africa (Anyamba et al., 2002). Nanyingi et al.
(2015) demonstrate that nearly all major RVF outbreaks correlate with El
NiƱo–Southern Oscillation (ENSO) events. Floodwater Aedes species act as
primary vectors, initiating transmission cycles, while Culex species
amplify epidemics.
The climatic and
environmental variability was considered a primary driver because it was the
strongest predictive evidence. It is also directly related to outbreak
initiation (Bird
and McElroy,
2016).
2. Livestock dynamics
and husbandry practices
Another important driver
of the emergence of RVF is livestock dynamics. It plays a central role as an amplification host. The high density of infected mosquitoes amplifies virus
activity among the livestock within 6-8 weeks. High-density herds, animal
movement, and trade networks facilitate rapid viral spread (Balenghien et al.,
2013). Consequently, the intensification of livestock production and
transboundary trade increase exposure risks to humans. A review of studies from
African countries emphasizes socioeconomic practices such as communal grazing,
seasonal migration, and informal markets, which enhance virus dissemination (Muga
et al., 2015). Additionally, susceptible livestock populations lacking immunity
can trigger explosive outbreaks.
Livestock dynamics is
considered another important risk factor since it is essential for epidemic
amplification and strongly linked to outbreak severity; however, it can be
overcome with strict policy and vaccination of livestock.
3. Weak surveillance
and health systems
Besides climatic factors
and livestock dynamics, the health surveillance system has a direct role in the emergence of RFV. The poor health system is responsible for delayed detection
and response that significantly exacerbate RVF outbreaks. Bird and McElroy
(2016) highlight gaps in diagnostic capacity, underreporting, and fragmented veterinary-public
health coordination as important causes of outbreaks. In addition, limited
early warning systems and inadequate integration of animal and human health
surveillance delay interventions, allowing outbreaks to expand uncontrolled.
The surveillance health
system is critical for outbreak control rather than initiation. The system
can be modified and improved; however, it is inconsistently implemented in
African countries.
Interconnection
between the drivers
These drivers operate
synergistically rather than independently. The climatic factor amplifies vector proliferation, which ultimately leads to livestock infections.
Consequently, the increase in livestock infections spills over to the human
population, causing outbreaks. Likewise, weak surveillance delays detection
of climate-driven outbreaks. Lastly, livestock movement spreads infection
beyond environmentally suitable zones. For example, heavy rainfall may
initiate an outbreak, but livestock trade determines its geographic spread, while
poor surveillance allows it to escalate.
Alternative ranking
and counterargument
An alternative
perspective places socioeconomic and cultural factors as the primary driver
(Muga et al., 2015). Human behavior—such as slaughter practices and consumption
of raw animal products—directly influences transmission. Sociocultural factors
shape transmission intensity but are less predictive of outbreak initiation.
2. Conditions for
RVF spread beyond Africa
The conditions for RVF to spread
and become established on other continents require three distinct stages,
including introduction of virus, local transmission, and establishment.
2.1. Introduction
of virus
The introduction of the virus
to the human population may be via infected livestock trade (legal or informal)
and the movement of infected humans or vectors. The livestock trade is a main
cause of the 2000 outbreak in Saudi Arabia and Yemen (Himeidan et al., 2014). The
lack of proper regulation and surveillance during trade with endemic regions and
weak border security intensifies the introduction of the virus from one region to
another or one country to another. International
trade plays a crucial role in establishing national animal health policies
because it requires countries to maintain and demonstrate high standards of
animal health in order to safely import and export animals and animal products.
2.2. Local
Transmission
Local transmission is
directly associated with a competent mosquito population and suitable climatic
conditions as mentioned above. The massive increase in infected mosquito
spread virus to livestock, and ultimately spills over to humans. Several mosquito
genera are capable of RVF transmission (e.g., Aedes, Culex) and are
globally distributed (Elliott & Brennan, 2014). Thus, competent mosquito
vectors, suitable climatic conditions, and the presence of susceptible hosts are
responsible for local transmission of the disease.
2.3. Sustained circulation
(establishment)
Long-term establishment of
the infection requires persistent infected mosquito populations, vertical
transmission of virus in mosquitoes and continuous availability of susceptible
hosts. Bird and McElroy (2016) note that maintenance mechanisms outside Africa
remain uncertain, particularly regarding overwintering of the virus.
Are the same drivers
relevant outside Africa?
The same drivers may not be entirely relevant outside Africa. Although the climatic factor remains on the
top for emergence of RVF, the temperate regions (low temperature range) in many
countries may limit vector survival. Likewise, livestock systems differ, often
with stronger regulation and biosecurity in other countries playing a role in
preventing the spread of the disease. Lastly, stronger surveillance systems
are generally reducing outbreak scale, although other factors favor the
outbreak. Thus, while climate remains a key driver, trade and biosecurity may
become more critical determinants outside Africa.
3. Appraisal of current
RVF control strategies
3.1 Diagnostics
The diagnostic approaches
of RFV not only detect the disease in the early phase but also help to prevent further spread by breaking the transmission chain. Recent advances such as molecular
tools (RT-PCR) enable early and rapid detection within a few hours during the incubation phase, while serological assays with a focus on antibody detection
identify past exposure and seroprevalence among the livestock and human
population. However, the RT-PCR tool is limited in endemic regions, and serology
can’t differentiate between active vs past infection (Bird & McElroy, 2016).
Although culture is the gold standard method for diagnosis of viral infection, including RVF, only a few reference laboratories have this facility.
3.2 Surveillance
Integration of satellite
data for early warning (Linthicum et al., 2016) and increasing adoption of One
Health surveillance systems are key strengths of the surveillance system. However, underreporting
and delayed response may hinder control strategies.
3.3 Vaccination
Currently, recommended
vaccines (live-attenuated and inactivated vaccines) exist for livestock and
humans to control RVF. The new-generation vaccines are under development
(Kortekaas, 2014). However, there are a few challenges to using those vaccines.
The safety concerns (e.g., teratogenic effects) of new vaccines, their limited
availability, and logistical barriers for massive vaccinations are key hurdles in
preventive measures of RVF.
3.4 Integrated vector
control strategy
Integrated vector
control remains key in controlling the spread of infections. Integrated
vector control includes the use of insecticides, environmental management to
prevent vector breeding, farm sanitation to avoid mosquito bites to
livestock, etc. The limitation of vector control is difficulty in sustaining and
limited effectiveness during large-scale outbreaks (Balenghien et al., 2013).
3.5 Outbreak preparedness
and response
The outbreak preparedness and response team recognizes the importance of One Health approaches to timely
control of potential outbreaks and immediate management. It also improves
forecasting tools. Sometimes, poor coordination between sectors and insufficient
funding and infrastructure may be a barrier to the role of the team in control
measures.
4. Conclusion:
Unresolved challenges and priorities
Despite advances, several
critical challenges remain in controlling RVF. These are;
i. Predictive uncertainty:
Although climate-based models are useful, they cannot fully predict
outbreak timing or scale.
ii. Vaccine gaps:
Safe, affordable, and widely deployable vaccines remain limited. Many
vaccines are under development for livestock and humans. These vaccines are in
the clinical trial phases. Development costs and licensing costs are still
challenges for veterinary vaccines and human vaccines. Only a combined effort
from veterinary vaccines to develop and approve human vaccines can reduce costs
and licensing procedures.
iii. Poor surveillance
system: Persistent gaps in early detection hinder rapid response. In addition, political influences and lack of a single platform to coordinate between
stakeholders remain major obstacles in data sharing and policymaking.
iv. Socioeconomic
barriers: Although socioeconomic barriers don’t have a direct role in outbreaks and massive transmission, the cultural practices and economic
constraints limit intervention uptake. If such conditions are revealed in an endemic
region, it may cause further spread of infections and make control measures difficult.
Priority actions
Based on the above
information, the following urgent priorities are identified:
- We should strengthen integrated One Health surveillance systems that
collect data from humans, livestock, and the environment (climate data) for
prediction of potential outbreaks.
- Secondly, more research should focus on the development of safe and
effective livestock vaccination.
- Thirdly, the local authorities should focus on an integrated vector
control strategy.
- Lastly, based on integrated data, we should improve quick data
sharing and predictive modeling through interdisciplinary data.
These actions address
both biological and systemic drivers, offering the greatest potential impact.
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