Crimean-Congo hemorrhagic fever (CCHF) is endemic in Africa, eastern Europe the Middle East and in Asia and is the most widespread tick-borne disease in the world (Fanelli and Buonavoglia, 2021). CCHF is caused by an enveloped, segmented, negative sense, single-strand RNA virus (CCHFV) belonging to the Orthonairovirus genus, Nairoviridae family (Garrison et al., 2020). The viral genome consists of three RNA segments: small (S), medium (M), and large (L), which encode the viral nucleoprotein (NP), the glycoprotein precursor (GPC) which is matured in two structural glycoproteins (Gn and Gc), and the RNA-dependent RNA polymerase, respectively (Garrison et al., 2020). CCHFV is among the most genetically diverse arboviruses known currently. Many genotypes are distinguished based on the genomic segment considered and they show geographic segregation according to the origin of the virus. The phylogenetic analysis of the S segment described seven lineages named Africa 1, Africa 2, Africa 3, Asia 1, Asia 2, Europe 1, and Europe 2 while nine and six additional genetic lineages can be characterized for the M and L segments respectively, with however a certain congruent level (Deyde et al., 2006; Bente et al., 2013; Guo et al., 2017). Interestingly, segment reassortment are common with CCHFV, leading to differences in the phylogenetic three topology for the same isolate when the three genomic segments are analysed (Deyde et al., 2006; Chinikar et al., 2015).

CCHFV is a zoonotic virus maintained in an enzootic transmission cycle involving Ixodidae ticks and several vertebrate animals such as birds, small mammals, domestic animals, and wild ungulates (Kuehnert et al., 2021). Although the virus has been found in different species of ticks such as Amblyomma spp., and Rhipicephalus spp., Hyalomma spp. ticks are considered as the primary vectors and reservoirs of CCHFV. In fact, the area of endemicity of CCHFV closely mirrors that of the different species of Hyalomma ticks in Africa, Europe, and Asia with the involvement of local species (Turell, 2007; Okely et al., 2020). Hyalomma maintains the virus through transovarial, transstadial, and venereal transmission (Turell, 2007). CCHF infection in non-human vertebrates is usually asymptomatic or mild with a viremia lasting less than 14 days (Spengler et al., 2016). CCHFV can be transmitted to humans through the bite or crushing of an infected tick, by direct contact with blood or tissues of a CCHFV-infected animal or patient. Nosocomial transmission has been documented as mostly associated with unsuitable sterilization of medical equipment and contamination of medical supplies (Whitehouse, 2004; Turabi Gunes et al., 2009). Clinical presentation of CCHF in humans is variable, from a mild non-specific febrile illness to a fatal haemorrhagic fever characterized by disseminated intravascular coagulation, shock, and multiple organ failure. No commercial vaccines and treatments for humans or animals are available to date (Tipih and Burt, 2020). With a case fatality rate ranging from 10 to 40%, and only supportive care to control the disease symptoms in patients, CCHF is _with COVID-19, Ebola virus disease and Marburg virus disease, Lassa fever, Middle East respiratory syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS), Nipah and henipaviral diseases, Rift Valley fever, Zika and Disease X_ on the blueprint list of priority diseases on which most research and development efforts should be focused (Freitas et al., 2022; WHO, 2022a).

The epidemiology of CCHF has long been underestimated in Africa. Though not many efforts to understand CCHF have been made on the continent, reports show the evident activity of the virus in animals and ticks. Since the year 2000, at least nineteen African countries have described CCHF outbreaks in humans, showing the growing impact of this disease (Temur et al., 2021; WHO, 2022b). Indeed, Mali, Mauritania, Namibia, Nigeria, Senegal, South Africa, Sudan, and Uganda have each reported at least three human outbreaks from 2010 to 2021 (Zivcec et al., 2017; Boushab et al., 2020; Temur et al., 2021). Very little information is available on CCHF in Central Africa besides small-scale seroprevalence studies in cattle or humans (Guilherme et al., 1996; Sas et al., 2017; Sadeuh-Mba et al., 2018; González Gordon and Bessell, 2022). Grard and colleagues have however described in 2008 a human CCHF case in the Democratic Republic of the Congo (DRC) with the occurrence of CCHFV genotype II (Grard et al., 2011). The aim of this study was to gain greater insight into CCHFV activity by screening domestic ruminants and their attached ticks for the detection of either specific antibodies or the virus in two markets of Yaoundé in Cameroon.

Crimean-Congo haemorrhagic fever is included in the WHO list of top priority diseases in public health emergency contexts on which research and development should be focused (WHO, 2022a). With the same goal, we conducted a cross-sectional survey of CCHFV in livestock and ticks from various locations in Cameroon to evaluate the risk to the local human population. The contact of cattle, goats, and sheep with CCHFV was assessed using a double antigen sandwich ELISA test confirmed by a seroneutralization test using the tecVLP system while ticks were grouped in species-specific pools and screened by RT-PCR.

The CCHFV seroprevalence was high in all tested animals (61.77%) with the highest seroprevalence in cattle (98.18%) as compared to sheep (15.65%) and goats (6.55%). With such alarming and surprisingly high seroprevalences, cross-reaction, as previously described among orthonairoviruses, especially for CCHFV and Hazara virus (Vanhomwegen et al., 2012; Kalkan-Yazıcı et al., 2021) was a possible explanation. Hence, we selected some negative and positive da-ELISA samples for confirmation by seroneutralization, since da-ELISA was not validated for cross reactivity with other orthonairoviruses. However, CCHFV is a biosafety level 4 (BSL-4) pathogen that should be handled in high confinement settings, often not available or affordable by every institution (Weidmann et al., 2016; Organization, 2018). This limitation was overcome using the tecVLP system, an alternative valuable BSL-2 seroneutralization method (Zivcec et al., 2015; Sas et al., 2018). Interestingly, the tecVLP results were concordant with the da-ELISA results, in favour of the high CCHFV seroprevalence observed, even though cross-seroneutralization could occur in case of infection with a closely related virus. Moreover, two sera from the da-ELISA negative group were found seroneutralizing, which could reflect a better sensitivity of the CCHFV tecVLP system. Although da-ELISA and tecVLP seroneutralisation assay results were concordant, logistic and financial constraints preclude the testing of more if not all samples with tecVLP assay.

In Cameroon, the circulation of CCHFV has been documented in humans and cattle but never in small ruminants until now (Sadeuh-Mba et al., 2018; González Gordon and Bessell, 2022). Indeed, González Gordon and colleagues and Sadeuh-Mba and colleagues have described high seroprevalence in pastoral cattle and low seroprevalence in humans in Cameroon respectively (Sadeuh-Mba et al., 2018; González Gordon and Bessell, 2022). The seroprevalence of CCHFV has been shown to be associated with many risk factors including the level of infestation and permanent bites by infected ticks (Adam et al., 2013; Mangombi et al., 2020). Animals may be infested by infected ticks once or recurrently during their lifetime, leading to a permanent activation of their immunity. However, little is known about the longevity of the anti-CCHFV antibody response in non-human vertebrates (Goedhals et al., 2017; Mangombi et al., 2020). This above-mentioned hypothesis is supported by the fact that we have previously reported high tick infestation rates in the cattle compared to small ruminants and a predominance of Hyalomma spp. ticks in cattle than small ruminants (Sado Yousseu et al., 2022). Unfortunately, juvenile cattle were not included in the study since sampling was done in markets. That information would contribute to broaden our understanding of recent virus transmission in the country (Nyakarahuka et al., 2018). Our seroprevalence results are higher than those obtained in the subregion in cattle and goats, even in countries with notified human CCHFV cases (Guilherme et al., 1996; Ibrahim et al., 2015; Sas et al., 2017; Mangombi et al., 2020; Oluwayelu et al., 2020; Lysholm et al., 2022). However, our results and those obtained in similar localities in Mali (Maiga et al., 2017; Balinandi et al., 2021) are comparable, supporting the suggestion that the circulation of CCHFV among ticks and vertebrate hosts varies significantly between different locations. Overall, these results may also signal a relatively higher risk of CCHF in Cameroon since CCHFV is mainly transmitted by tick bites and direct contact with bodily fluids on infected animals. This observation also shows the complexity of CCHFV epidemiology and the difficulties when comparing serological data over long periods and across countries.

CCHFV endemic regions overlap the geographic distribution of their Hyalomma vectors in Africa, Europe, and Asia. Indeed, Hyalomma marginatum is the most studied and known vector of CCHFV in Europe and Asia where it is responsible of the transovarian and transstadial transmission of the virus, ensuring, therefore, its maintenance in nature (Spengler et al., 2016). In Africa, the virus has been detected on numerous occasions in H. dromedarii, H. truncatum, H. rufipes and H. impeltatum (Swanepoel et al., 1983; Zivcec et al., 2017; Chitimia-Dobler et al., 2019; Kajihara et al., 2021; Schulz et al., 2021) but also in non Hyalomma species (Lule et al., 2022). In our study, CCHFV was found in H. truncatum, although at a low infection rate but not in H. rufipes, H. impeltatum, H. nitidum, H. detritum, and H. dromaderii. The detection of CCHFV genetic material in Hyalomma in Cameroon shows the current activity of the virus in the country. However, the role of these ticks in CCHFV maintenance and transmission is yet to be determined, especially their vector competence since we cannot conclude whether they became infected after feeding on these hosts or they were previously infected before infesting the livestock (genetic material present in the blood meal, or infectious viral particle) (Gargili et al., 2017). This concern can be at least partially addressed by screening unfed larvae, nymph, and adult ticks of this species to determine their role in CCHFV transmission in Cameroon.

In this study, we have determined that the CCHFV strain active in Cameroon belongs to the African III genotype on the L segment. Most of the phylogenetic analyses are currently done with the S or M segments for CCHFV classification into genotypes (Fakoorziba et al., 2015; Umair et al., 2020; Monsalve Arteaga et al., 2021). However, studies have also shown that genotypes obtained with the S segment are usually the same as with the L segment, corresponding to the geographic segregation of the virus (Bente et al., 2013). In Central Africa, a great CCHFV genetic diversity has been described, with the occurrence of African II and III genotypes (Grard et al., 2011). Here, we have analysed only a small portion of the L segment (445bp) representing very little genetic information on the virus. Indeed, CCHFV has a high level of genetic recombination and reassortments that can easily distort the classification into genotypes (Grard et al., 2011; Bente et al., 2013; Chinikar et al., 2015; Guo et al., 2017). Therefore, the most conclusive response to the genetic diversity of CCHFV in Cameroon can only be obtained by studying all the three genomic segments.

Given the high seroprevalence observed in animals and the detection of CCHFV in ticks, efforts should be done to increase the survey of CCHF in Cameroon. Indeed, CCHF should be included in the differential diagnosis of acute haemorrhagic fever among at-risk populations in the country. To the best of our knowledge, no human CCHF case has ever been recorded in Cameroon but the disease is known to suddenly emerge among humans in some locations 30 to 50 years apart (Grard et al., 2011).

In this study, we report high CCHFV seroprevalence in domestic ruminants and virus detection in Hyalomma ticks in Cameroon. Although CCHFV is asymptomatic in non-human vertebrates, the high seroprevalence and virus observed raise a public health concern about the occurrence of CCHF among humans, especially among at-risk occupational groups including abattoir workers, farmers, and veterinarians in Cameroon. These findings highlight the suitability of a One Health surveillance system in ticks, wild, and domestic animals that will guide the survey of the disease in humans and include CCHF in the differential diagnostic of acute fevers among at-risk groups in selected regions. Additionally, we have presented here the first genomic sequence of CCHFV in Cameroon, but longer sequences will be required if a greater insight into the genetic diversity and pathogenicity of the CCHFV strains active in the country, is sought. More remains to be done if we wish to understand the epidemiology of CCHFV in Cameroon.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material . The data are publicly available. Genbank accession number: ON564456 and OP292216. Other data can be found in the supplementary data.

The study protocol was implemented with approval from the Regional Delegation of the Ministry of Livestock, Fisheries, and Animal Industries (MINEPIA), authorizations N°000151/L/MINEPIA/SG/DREPIA-CE/SRDPIA and 00034/L/MINEPIA/SG/DREPIA-CE. Oral consent for blood and ticks sampling was obtained from the animal’s owners.

Conceptualization and study design: HST, PM, VL, CW. Supervision: BK, VL, F-LC, CW. Sample collection and Laboratory work: HST, FSY, NF. Data analysis: HST, FSY, F-LC. Writing- original draft: HST, FSY, VL. Writing – review & editing: HST, FSY, F-LC, NF, BK, PM, RN, VL, CW. All authors contributed to the article and approved the submitted version.