3.1.1. Climate.

Given that most VHFs are arthropod-borne zoonoses, climatic drivers highly influence their transmission patterns [11,33,63], by affecting host and vector diversity, abundance, genetics and the pathogen infection load in the host and vector [11].

3.1.1.1. Temperature: Temperature is the most critical factor influencing the risk of VHFs. It affects mosquito reproduction and virus replication [60], impacting breeding site availability, longevity and their competence to microorganisms [20]. In Cameroon, temperature higher than mean temperature range [24.2–25.5 °C] reduce RVF by decreasing seropositivity in cattle [39], while intermediate temperatures promote stable vector populations and RVF maintenance in the African tropical moist forest [30]. Ae. aegypti larvae cannot survive below 10 °C and adults below 5 °C, limiting the distribution of diseases transmitted by this vector, like dengue and YF in tropical and sub-tropical regions [84]. Global warming is a likely contributor to the re-emergence of YF virus in endemic and non-endemic areas in Nigeria [33], and alter tick populations [11].

Additionally, temperature is a predictor of EVD outbreaks and MVD [38]. “Temperature Annual Range” and “Mean Diurnal Temperature Range” are key spatial variables determining the distribution of EVD in Africa [34] with small annual temperature ranges being significantly and negatively associated with the areas of high environmental favourability for Ebola virus presence [36].

Land surface temperature significantly predicts MVD zoonotic transmission [26] and had the second most important effect on global CCHF transmission risk to humans [15]. Night-time land surface temperature had an impact on the EVD distribution in an Africa-wide environmental suitability model [35].

3.1.1.2. Precipitations and humidity: Heavy rainfalls is a significant risk factor for mosquito-borne VHF’s infection, particularly RVF in cattle and camels in Nigeria [46,85] as well as YF and dengue transmission in Nigeria and Cameroon respectively [33,86]. High rainfall increases RVF occurrence in pastoral cattle herds in Nigeria [47]. In DRC, short periods of heavy rainfall [60] led to multiple flooding events followed by early droughts which increase breeding sites and consequently the density of mosquitoes [20,24,79]. Excessive seasonal rain, especially after drought, significantly contributes to RVF outbreaks in Nigeria [40]. In Cameroon, rare heavy rainfall and flooding are critical entomological risk factors for RVF in cattle [39,45]. An increase in the number of wet months raises the odds of Ae. albopictus (primary vector of RVF virus) presence by seven times in Cameroon [87], and intermediate rainfall can establish stable vector populations for RVF maintenance [30]. Increasing rainfall intensity is also a driver for YF re-emergence in Nigeria [33].

Redding et al. [41] found a strong association between LF cases and rainfall in southern Nigeria, with LF peaking in areas with 1500–2000 mm of annual rainfall [42] and declining sharply in the more arid northeast [16]. In Nigeria, it has been shown that the incidence of LF in humans correlates negatively with rainfall [88]. Rodents will forage near humans only when rainfall levels drop [88,89]. Mean monthly rainfall is a key spatial variable influencing EVD distribution and outbreak dynamics in parts of Republic of the Congo and Gabon [34,37,43], with spillover intensity peaking with intermediate rainfall [44].

Absolute humidity also affects the development of VHFs vectors, particularly ticks, as it is crucial for their water balance [48]. In Cameroon, higher absolute humidity is positively linked to lower odds of CCHF virus antibody seropositivity in pastoral cattle [48]. Additionally, potential evapotranspiration is important for predicting EVD outbreak and MVD zoonotic transmission [26,38].

3.1.2. Land cover and land use.

Around 34.2% of the reviewed papers identified land-use and land cover changes (Fig 5) as favouring factors for VHFs in the study area, including RVF, LF and EVD.

3.1.2.1. Vegetation: Vegetation plays an important role in harbouring mosquito breeding sites. Bushy vegetation has been found to significantly influence RVF’s emergence in cattle [47] and occurrence in camels of Nigeria [46]. The presence of bush around the house is also a potential factor increasing the risk for LF infection in humans in Nigeria [50,90].

Shrub density is positively associated with CCHF virus antibody seropositivity in pastoral cattle in Cameroon [48]. Tick abundance correlates strongly with tree species composition and shrub cover. Areas with higher proportion of grass and shrub account for almost 62% of predicted CCHF virus emergence or re-emergence in humans, based on global CCHF virus distribution model [15]. The standard deviation of mean EVI (Enhanced Vegetation Index) (8%), and mean EVI (5%) are also critical factors influencing global CCHF transmission risk to humans [15].

Adamu et al. [46] found that RVF occurrence in one-humped camels in Nigeria was significantly associated with mosaic vegetation, corroborated by Linthicum et al. [51] using a vegetation index. Vegetation is also a key determinant in the spatial distribution of MVD [26] and EVD cases [35].

The tropical, subtropical moist broadleaf forest of our 7 countries of interest comprises about 54.9% of the continent’s total forest area [91]. Forest cover is a significant covariate in model to produce maps of areas suitable for Ebola virus spillover in the Republic of the Congo and Gabon [43]. In Gabon, high prevalence regions were mainly located within forest ecosystems as it is the primary home of the zoonotic cycle of EVD [35]. This confirmed that the forest, particularly the deep forest, is the environment most at risk for Ebola viral infection in Gabon [36], as it harbours susceptible hosts such as great apes and bats [92]. Forest cover is a spatial variable that also influence other VHFs such as the emergence of LF [16] and those which are mosquito driven. In the forest area, the RVF seroprevalence was significantly higher than the savannah area in cattle raised by smallholder farmers in of DRC [54]. Increase RVF virus seroconversion was noted in animals after forest exposure, even far from endemic areas [30]. Moreover, non-human primates, the primary host of dengue virus are abundant in forests, supporting the sylvatic cycle of the disease [52].

3.1.2.2. Water bodies, rivers and streams: Rivers, streams, and water bodies are breeding grounds for mosquitoes [24]. Some mosquito species, including those transmitting RVF, dengue and YF (genus Aedes and Culex) were abundantly collected from rivers in Ngaoundere, Cameroon [93]. In pastoral herds of Nigeria, the presence of rivers and streams in grazing fields was eleven times more likely to influence RVF occurrence [47,49].

In Cameroon, Ngadvou et al. [93] collected the greatest number of mosquitoes (genus Aedes and Culex) in gutters and lakes. Children living in stagnant water free environment were less likely to contract dengue in Yaoundé [55]. In the same country, animal access to water bodies significantly influenced seroprevalence of RVF virus in domestic small ruminants, with herd locations along the Benue river posing risks for RVF transmission [24]. A Nigerian study identified water bodies as key factors for camel exposure to RVF virus [46], while surveys across Africa linked proximity to water points with inter-epidemic RVF transmissions [30]. In Cameroon, areas near water banks like Kismatari and Pitoa showed significant RVF antibody seroprevalence in domestic small ruminants [24].

3.1.2.3. Agriculture: Agriculture influences VHFs occurrences in two ways: by providing breeding habitats for vectors and by increasing human exposure to vectors, such as Ae. aegypti species [33]. Agricultural practices, like irrigation and fishponds, are linked to increased mosquito abundance and diversity. In Nigeria, dams and irrigated rice fields significantly contribute to emergence and occurrence of RVF in herds [46,47,49]. Serological surveys have identified irrigation as a risk factor for inter-epidemic RVF transmissions and its maintenance in the environment [24,30]. Fishponds serve as breeding sites for mosquitoes, increasing the density of certain species more than natural water bodies [20]. Nomadic pastoral communities in Nigeria have recognized ponds as risk factors for RVF occurrence [49]. In Cameroon, the presence of ponds may also favour sporadic RVF transmission among livestock [57]. Rainfed croplands have been identified as a risk factor for RVF occurrence, and in [46] one-humped camels in Nigeria, they were significantly associated with RVF virus antibodies [46].

Additionally, the destruction of natural habitats for agricultural activities has driven multimammate rats (Mastomys natalensis) to seek new homes, often invading residential areas and increasing the risk of LF emergence [16] and infection [50].

3.1.2.4. Built-up land: Both occurrence and incidence models of LF in Nigeria show a strong positive association with built-up land [41]. LF is a neglected endemic zoonosis primarily found in rural West African regions [41,50]. Redding et al. [41] observed that LF tends to invade urbanized areas, which may be attributed to increased reporting and medical access or to rodent synanthropy, as urban environments provide favourable conditions for rodents [41]. Additionally, higher population densities in urban areas may facilitate human-to-human transmission and affect the distribution of rodent reservoirs and their interactions with humans [58]. Similarly, seropositivity rates for the dengue virus in Cameroon were higher near major urban centres compared to more remotely located areas, suggesting an association between dengue cases and urban areas [59].

3.1.3. Topography.

Higher altitudes have been associated with a significant decrease of the odds of RVF seropositivity in cattle in Cameroon, RVF occurrence in one-humped camel in Nigeria [39,46], and YF seropositivity in DRC [60]. Canelas et al. [87] further confirm that, in Cameroon, lower altitudes are linked to higher odds of Ae. albopictus (vector of RVF, YF and dengue) presence. Additionally, elevation is recognized as an environmental covariate driving the outbreaks [38] and cases distribution of EVD [35].

Landforms, such as mountain chains between Nigeria and Cameroon, may function as natural barriers to animal movements, including infected rodents. This could account for the lack of reported LF outbreak in Cameroon, while the neighbouring Nigeria has experienced several large-scale outbreaks [11,16], even if the Lassa virus reservoir and host M. natalensis is also found in Cameroon [90].

3.2.1. Arthropod vectors.

Mosquito vector density and diversity is confirmed as a spatial factor involved in the emergence and maintenance of mosquito-borne arboviruses in Africa [20]. Specific mosquito species are key to maintaining the RVF virus, influencing both enzootic and epizootic cycles [62]. In Nigeria, studies show that pastoralists perceived the presence of mosquitoes as a major risk factor for RVF in cattle [47,49]. Additionally, mean matrix scores indicated that the entry pathway for the RVF agent into the nomadic pastoral herds was primarily through RVF virus-infected mosquitoes [49]. Statistical analysis indicated that the presence of RVF virus-infected mosquitoes in pastoral environments increased the likelihood of RVF occurrence in pastoral herds of Nigeria by eight times [47]. In Cameroon, the presence of primary (Aedes) and secondary (Culex, Anopheles, and Mansonia) mosquito vectors poses a risk of RVF transmission to susceptible hosts [45]. Although certain species of mosquito have been associated with the maintenance of the RVF virus in Kenya through vertical transmission from adult mosquitoes to their progeny via eggs, this transmission mechanism has not been reported in Western and Central Africa [30].

CCHF is transmitted through tick bites (Hyalomma marginatum) or direct contact with the body fluids of infected people or livestock [67,94]. Regardless of the region, a higher risk of CCHF virus infection is generally associated with exposure to ticks (either through bites or handling them without gloves) or to animal (particularly among herders, agricultural works, abattoir staff, veterinarians) [22].

3.2.2. Domestic vertebrate hosts.

High cattle density is identified as a risk factor for the occurrence and emergence of RVF in nomadic pastoral herds in Nigeria [47,49]. A stable, dense population of susceptible livestock acts as a major amplification hosts for RVF maintenance in endemic countries [30]. In Cameroon, the presence of various domestic vertebrate hosts (including small ruminants, cattle, dogs, cats, horses, donkeys, and poultry) should be considered when defining RVF risk areas [45]. Additionally, a large serological survey during the 2001–2002 EVD outbreak in Gabon indicated that dogs might be asymptomatically infected with Ebola virus, likely due to consuming infected carcasses or contact with body fluids, potentially enabling transmission [63]. While bats and non-human primates are significant in filovirus transmission, other species like pigs and dogs may also be involved [63]. Moreover, sheep appear to be at higher risk for RVF compared to other domestic animals, showing significantly higher seroprevalence [65]. For CCHF, individuals living or working near livestock are considered to be at the highest risk for infection [15].

During the dry season, transhumance addresses seasonal shortages of pasture and water [48]. This practice increases the risk of zoonotic disease transmission, as it brings domestic and wild animals into contact at water source and in pasture, facilitating the spread of viruses like RVF and CCHF [45]. For example, in Cameroon, cattle that participated in transhumance had double the odds of being seropositive for CCHF compared to those that are not involved in the practice [48]. In Nigeria, nomadic pastoral cattle exhibited a higher prevalence of RVF (7.4%) than agro-pastoral animals (3.8%), with animal movements significantly influencing RVF occurrence [47]. Livestock movement could also influence the risk of CCHF virus transmission [64], with potential introduction to a non-endemic area through legal or illegal trade of infected animals or ticks [64].

Contacts between susceptible hosts and mosquitoes at animal watering sites contribute to RVF virus maintenance [30]. An extensive husbandry system significantly influences RVF occurrence in herds while sharing a common water source during nomadic movements was increases the likelihood of RVF occurrence in Nigerian herds in Nigeria by fifty-three times (OR: 24.94, 95% CI: 13.54, 45.93) [66]. In the same region, cattle often graze alongside sheep and goats, which also impacts RVF occurrence in nomadic herds [66]. Sheep are more susceptible to RVF, experiencing more severe clinical outcomes than other ruminants [65,95]. This mixed grazing raises the risk of cross-infection with the RVF virus through infected aerosols [66].

3.2.3. Wildlife.

Wildlife plays a crucial role as amplification hosts and reservoirs in the epidemiology of infectious diseases such as EVD, MVD, RVF and CCHF [34]. The presence of wildlife (monkeys, warthogs, rodents, reptiles, etc.) should be considered when defining RVF risk areas [45]. Confirmed wildlife reservoirs for the RVF virus include African buffalos (Syncerus caffer), giraffes (Giraffa camelopardalis), desert warthogs (Phacochoerus aethiopicus), elephants (Loxodonta africana), rhinoceroses (Diceros bicornis), and possibly bats [30].

3.2.3.1. Rodents: Exposure to rodents has been significantly linked to seropositivity for EVD in DRC [61]. The presence of multimammate rats in/around the houses is the potential risk factor for LF infection [50]. Additionally, rodent species richness serves as a spatial variable that influences LF emergence events, showing a significant negative association with LF emergence events. This suggests that a dilution effect may apply to the spatial spread of this diseases [16].

3.2.3.2. Bats: Fruit bats are suspected of being reservoirs in the EVD transmission cycle [38]. They may transmit the virus to other wildlife (e.g., duikers, non-human primates) and humans [53]. Modelling studies indicate that the bat distribution significantly impacts EVD distribution and spillover [35,43]. This is supported by Moyen et al. [68], who found that “contact with bats” was significantly associated with Ebola virus antibody detection in blood donors in the Republic of Congo [68]. In DRC, individuals with a bat contact had 1.64 times higher odds of EVD seropositivity compared to those without such contact [96]. Additionally, the Old World rousette fruit bats (Rousettus spp.), which serve as reservoirs for Marburg virus and the Ravn virus, contribute to pandemic risk [11].

3.2.3.3. Antelopes and buffalo: Contact between cattle and antelopes increases the risk of cattle becoming seropositive for RVF. While several studies report high seroprevalences in wildlife species, such as various antelopes and buffalo, their epidemiological significance remains unclear [39]. In DRC, exposure to duikers was significantly associated with EVD seropositivity [61].

3.2.3.4. Birds: Birds seem to be resistant to CCHF viral infection but can act as mechanical vectors by transporting infected ticks [94]. Migrating birds provide blood meals for immature ticks, facilitating the spread of infected vectors to remote areas [97].

3.2.3.5. Non-human primates (NHP): In the transmission cycle of the YF virus, NHPs (including chimpanzees, gorilla, and other monkey species) serve as the reservoir hosts of the virus in the forest [33,77]. Besides, NHPs contribute to the sylvan-to-urban spillover of the dengue virus [52]. These wild animals are also affected by Ebola virus, with studies showing serological evidence of exposure in chimpanzees in countries without recorded EVD outbreaks, such as Cameroon [73]. Notably, EVD epidemics in Gabon and Congo have followed significant declines in NHP populations due to EVD infection [98], indicating spillover with the human index cases [99,100]. During human outbreaks, carcasses of western gorillas (Gorilla gorilla) and chimpanzees (Pan troglodytes) have been discovered in nearby forests. Chimpanzees were identified as the source of an outbreak in Gabon in 1996 [100]. Several studies have found that consumption or contact with NHPs is meaningfully associated with increased odds of Ebola virus seropositivity [101], as well as with MVD transmission [102].

3.3.1. Demography and population.

3.3.1.1. Human population density and growth: Several studies have shown that population density is a significant factor in the risk of zoonotic EVD transmission. Specifically, it was found that higher population density correlates with spillover risk [8]. A literature review further identified human population density as a key risk factor for Zaire ebolavirus spillover [43]. According to Redding et al. [34], human population size has the most significant impact on EVD emergence and epidemic potential in Africa [34]. Furthermore, regions experiencing rising population density are associated with an increased risk of EVD spillover [44].

The spread of dengue is also attributed to population growth [70]. Research has shown that population growth serves as a spatial variable affecting the emergence of LF in Nigeria [16]. In addition, increasing human population may accelerate the risk of EVD transmission in Africa [35].

3.3.1.2. Urbanization: In low- and middle-income countries, demographic growth is associated with poor environmental hygiene and high urbanization levels. This likely favours Aedes mosquito productivity and might partly explain the frequent outbreaks of mosquito-borne zoonoses in the city, like YF [60]. Urbanization also contributes to rural exodus, which has been shown to facilitate the spread of dengue [70].

In Cameroon, urbanization levels can indicate the presence of Aedes albopictus. Specifically, a 1% increase in peri-urban area raises the likelihood of this vector’s presence by 1.3 times. Aedes mosquitoes are known vectors for RVF, YF and dengue. Human exposure to Ae. aegypti increases during urban development [33]. Seropositivity rates for dengue and YF, carried by Ae. aegypti and Ae. albopictus, are higher near major urban centres in Cameroon [59]. In Nigeria, RVF occurrence in one-humped camels is significantly linked to urban areas [46]. LF incidence is positively associated with poverty and urbanization, influencing effective human–rodent contact [41].

Proximity to waste dumpsites is a risk factor for dengue virus infection. Vectors lay eggs in artificial water containers found in homes and waste sites [81]. A significant association between dengue prevalence and residence near waste dumps has been found in Abuja (Nigeria) [81]. Another study confirmed that living near refuse dumps is a significant risk factor for dengue seropositivity [52].

3.3.1.3. Population movements: It is likely that the active and uncontrolled migration of humans [60] and animal population might have played a key role in spreading RVF virus in the DRC. The role of population movement in the emergence and spread of mosquito-borne viruses is highlighted by a recent YF outbreak that began in Angola and spread to the DRC through transboundary trade [60]. The influence of population mobility and transportation on VHFs epidemiology was predominant in the dispersal of EVD cases [35,69]. “Human mobility” encompasses a broader range of movements than “migration”, including tourists who do not typically engage in migration [53]. Local and global connectivity played a major role in disease importation in Nigeria, where the first case of EVD was recorded in July 2014 after an infected Liberian-American landed in the densely populated town Lagos [35,53]. Population movements increase the risk of EVD and MVD transmission by rising contacts rates between humans and infected individuals or animals [63]. Porous borders facilitated secondary transmission during the 2014–2016 West Africa EVD outbreak [72]. Countries lacking adequate health screening at entry points experienced introduction and dispersal of arboviral zoonosis like RVF [30] and YF, especially if imported cases reach areas conductive to outbreaks [11]. Proximity of infected individuals to main roads is linked to EVD dispersal [37], with a significant correlation found between Ebola virus locations and distance to roads [36]. In Nigeria, Sokoto and Borno States, which border countries with reported RVF virus infections (Niger, Chad, Cameroon), show relatively higher RVF virus seropositivity rates among residents [23]. These borders are porous and poorly monitored, facilitating the movement of potentially infected domestic ruminants [103].

3.3.2. Socio-cultural and behavioural risk factors.

3.3.2.1. Deforestation and human forest activities: Human encroachment into previously uninhabited areas, through forest fragmentation and habitat destruction, contributes to the spillover of zoonotic pathogens to humans [63], including LF [16,90]. In Cameroon, the increased risk of Ebola virus infection has been linked to logging activities such as clearing and burning [76]. Logged forests provide adequate aeration, humidity, and temperature for mosquito reproduction, enhancing their density and diversity [20]. Intensive deforestation in north-western DRC, Cameroon and CAR has further increased mosquito density and diversity, facilitating the emergence of YF and RVF [60,79].

3.3.2.2. Ethnicity: In 1993, ethnic background, particularly among the Congo basin forest ethnic groups, was identified as a significant factor influencing filovirus antibody activity among the forest communities [75]. Subsequent studies confirmed that ethnic background remains an important risk factor for Ebola virus exposure in many communities, as it shapes various community behaviours (such as feeding, funerals, and health seeking) among inhabitants, including the Baka Community of the Tropical Rainforest in Cameroon [73]. Specifically, feeding behaviour, particularly the consumption of uncooked fresh bush meat, is a crucial risk factor for Ebola virus exposure [73,76].

3.3.2.3. Water storage: Farmers in the Sudan savannah in Nigeria commonly build small dams to store water at the start of the wet season. In Cameroon, practices such as using temporary walls materials, storing water in containers, keeping old tires in yard, and having uncovered water containers have been independently linked to anti-dengue IgG positivity [104]. These practices create favourable conditions for mosquito breeding, which in turn facilitates the spread of RVF virus and dengue [40,104].

3.3.2.4. Animal slaughtering techniques: People involved in animal slaughtering during events such as Eid-al Adha face risks of infection from VHFs such as CCHF [67] and RVF [74]. High-risk practices, including slaughtering, evisceration, and processing raw meat without protective equipment, as well as using bare mouth to enhance flaying contribute to RVF transmission in abattoirs and slaughterhouses [23]. Significant anthropogenic factors, such as contact with aborted animal tissue, birthing, skinning and/or slaughtering an animal, are significantly associated with RVF infection in Gabon [74]. Abattoirs and slaughterhouses environments may also influence vector dynamics, such as the creation of standing water for mosquito breeding [30]. Additionally, animal handling and slaughtering are noted as risk factors for CCHF virus transmission [67].

3.3.2.5. Wildlife hunting and trade: Cultural practices often linked with poverty, like bushmeat hunting, increase human exposure to wildlife that may harbor several diseases, including EVD [35,69]. In the DRC, hunting, butchering/skinning of bushmeat are significantly associated with human seropositivity for EVD [61]. In CAR, filovirus antibody prevalence among hunter-gatherers is significantly higher than that among farmers who are not in contact with the forest [75].

Lee-Cruz et al. [43] identified anthropogenic factors like bushmeat hunting, trade and consumption as risk factors for Zaire ebolavirus spillover. In a spatio-temporal analysis of areas at risk for Ebola virus spillover in Central Africa, hunting areas and bushmeat trade were ranked as the most important risk factors [43].

3.3.3. Socio-economic and political risk factors.

3.3.3.1. Poverty: Poverty represents a major factor favouring EVD, MVD, LF, and dengue. EVD outbreaks have occurred in rural areas, among geographically isolated populations with poor housing conditions. Poverty-related issues, such as prostitution, violence and sexual exploitation, contribute to the spread of EVD [53,82]. In the tropical rainforest of Cameroon, the Baka community often avoids seeking medical treatment when ill, which drives EVD transmission [73]. Moreover, inadequate infrastructure, such as poor water and sanitation, is assumed to increase EVD transmission [71]. Positive Marburg virus antibodies are significantly associated with mining work in DRC, indicating that local mines may serve as sites of primary infection through exposure to zoonotic reservoirs [80]. In Nigeria, both occurrence and incidence models of LF showed that poverty prevalence was linked to its incidence [41]. Poor housing quality favours close contacts between rodents and humans [90]. Furthermore, gross domestic product per capita is a spatial factor influencing the LF emergence in Nigeria [16]. Socio-economic factors also contribute to the spread of dengue, as Aedes mosquito breeding is encouraged by the lack of drinking water and waste collection systems [70].

3.3.3.2. Access to health infrastructures: Insufficient health facilities per unit area and insufficient equipment in existing infrastructures are significant risk factors for disease transmission [105]. Proximity to healthcare facilities has been identified as a driver of EVD transmission within the Baka community in Cameroon [73], and it also affects outbreak dynamics in countries already experiencing the disease [37,83]. Poor healthcare infrastructure in Western and Central Africa has led to high rates of dengue virus infection from 2009 to 2020 [52]. Diagnostic laboratories enable rapid detection and early interventions in infectious disease outbreak [89] and EVD transmission [105]. However, they can also introduce bias, as incidence rates tend to be higher near these diagnostic laboratories. In Nigeria, for instance, travel time to LF diagnostic laboratory negatively impacted LF incidence in humans [41].

3.3.3.3. War and political instability: Conflict leads to the destruction of health resources and infrastructure, instilling fear, stress, distrust, and causing population displacement [82]. Countries like DRC [71] and Nigeria [89] have a long history of conflicts and insecurity [82]. Internally displaced persons camps can be the original site of an outbreak as closer human contacts and poor hygiene exacerbate the risk of infection. Besides, rebel takeover of certain areas prevents the supply of medical equipment and personnel [71,82]. Research indicates a strong positive association between the number of violent events and EVD cases and deaths in DRC [78]. Furthermore, impacts of socio-political crisis, such as population displacement, may increase the risk of emergence of an RVF epidemic in CAR [79].

3.3.3.4. Preventive health policies: Immunization coverage is a crucial spatial factor in the transmission of various VHFs. Re-emergences of diseases like YF in Nigeria and DRC from 2016 to 2019 are partly due to inadequate immunization [29,54]. In Cameroon, YF outbreaks occurred in 1990, 2006, 2007, and 2021, despite the availability of the vaccine. This pattern correlates with vaccination coverage, around 57% in Cameroon in 2022, and 27.2% in Nigeria in 2006, 60.1% in 2010 with a drop of 39% in 2016 [20,77]. The high number of individuals at risk can be linked to low vaccination rates [77].

Table 2 summarizes evidence related to various risk factors for the VHFs studied, indicating areas/diseases needing further investigation to enhance our understanding of these spatially relevant risk factors in African tropical moist forest. It suggests that exploring the effects of climatic and land cover/use variables on the incidence of YF, dengue and MVD would be particularly beneficial.

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Table 2. Summary showing, for each group of risk factors and for each VHF studied, where the evidence is strong and where it is rarer.

https://doi.org/10.1371/journal.pntd.0012817.t002